Article

The Roles of Harsh and Fluctuating Conditions in the Dynamics of Ecological Communities

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Harsh conditions (e.g., mortality and stress) reduce population growth rates directly; secondarily, they may reduce the intensity of interactions between organisms. Near-exclusive focus on the secondary effect of these forms of harshness has led ecologists to believe that they reduce the importance of ecological interactions, such as competition, and favor coexistence of even ecologically very similar species. By examining both the costs and the benefits, we show that harshness alone does not lessen the importance of species interactions or limit their role in community structure. Species coexistence requires niche differences, and harshness does not in itself make coexistence more likely. Fluctuations in environmental conditions (e.g., disturbance, seasonal change, and weather variation) also have been regarded as decreasing species interactions and favoring coexistence, but we argue that coexistence can only be favored when fluctuations create spatial or temporal niche opportunities. We argue that important diversity-promoting roles for harsh and fluctuating conditions depend on deviations from the assumptions of additive effects and linear dependencies most commonly found in ecological models. Such considerations imply strong roles for species interactions in the diversity of a community.

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... MCT has been widely successful. It has been the basis of important conceptual and theoretical advances (e.g., Chesson and Huntly, 1997;Stump and Chesson, 2015;Snyder and Chesson, 2003;Chesson and Kuang, 2008;Chesson and Kuang, 2010;Schreiber, 2021b), and several attempts to infer the mechanisms of coexistence in real communities (Cáceres, 1997;Venable et al., 1993;Pake and Venable, 1995;Pake and Venable, 1996;Adler et al., 2006;Sears and Chesson, 2007;Descamps-Julien and Gonzalez, 2005;Facelli et al., 2005;Angert et al., 2009;Adler et al., 2010;Usinowicz et al., 2012;Chesson et al., 2012;Chu and Adler, 2015;Usinowicz et al., 2017;Ignace et al., 2018;Hallett et al., 2019;Armitage and Jones, 2019;Armitage and Jones, 2020;Zepeda and Martorell, 2019;Zepeda and Martorell, 2019;Towers et al., 2020;Holt and Chesson, 2014;Ellner et al., 2016) or laboratory microcosms (Jiang and Morin, 2007;Letten et al., 2018). Additionally, MCT unifies seemingly dissimilar explanations for coexistence through categorization into coexistence mechanisms, thus organizing a scattered literature and highlighting similarities, such as the symmetrical role (with regards to coexistence) of resource specialization and specialist predators (Chesson and Kuang, 2008). ...
... The value of ∆E i does not depend on any species' density. This represented by the lack of a feedback loop in clearly in Figure 3. Consequentially, one species will have the largest ∆E i , regardless of which species is the invader; if all other terms in the invasion growth rate partition are zero, then all other species in the community will be excluded (Chesson and Huntly, 1997). This thought experiment demonstrates 1) that densitydependent factors are necessary for coexistence and therefore ∆E i might rightfully not deserve the title of "coexistence mechanism"; and 2) why all the Taylor series terms (of the average growth rate decomposition, Eq.6) containing only E j 's are shunted into ∆E i , while the growth rate components containing only C j 's are split between ∆ρ i and ∆N i : the density-independent effects are betweenspecies differences that cannot be responsible for coexistence, so it is often uninteresting to partition them further (but see Ellner et al., 2019). ...
... The idea is that individuals live longer than the timescale of environmental change, so they do not "feel" particular environmental states, but rather average over them. Here, there is still a single effective regulating factor, so the competitive exclusion principle for stochastic environments applies (Chesson and Huntly, 1997;Johnson and Hastings, 2022a;Hening and Nguyen, 2020). It is only when the timescales of competitive exclusion and environmental change are commensurable (i.e., tc ≈ te) that coexistence can be attained. ...
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In this paper, we discuss the conceptual underpinnings of Modern Coexistence Theory (MCT), a quantitative framework for understanding ecological coexistence. In order to use MCT to infer how species are coexisting, one must relate a complex model (which simulates coexistence in the real world) to simple models in which previously proposed explanations for coexistence have been codified. This can be accomplished in three steps: 1) relating the construct of coexistence to invasion growth rates, 2) mathematically partitioning the invasion growth rates into coexistence mechanisms (i.e., classes of explanations for coexistence), and 3) relating coexistence mechanisms to simple explanations for coexistence. Previous research has primarily focused on step 2. Here, we discuss the other crucial steps and their implications for inferring the mechanisms of coexistence in real communities. Our discussion of step 3 -- relating coexistence mechanisms to simple explanations for coexistence -- serves a heuristic guide for hypothesizing about the causes of coexistence in new models; but also addresses misconceptions about coexistence mechanisms. For example, the storage effect has little to do with bet-hedging or "storage" via a robust life-history stage; relative nonlinearity is more likely to promote coexistence than originally thought; and fitness-density covariance is an amalgam of a large number of previously proposed explanations for coexistence (e.g., the competition-colonization trade-off, heteromyopia, spatially-varying resource supply ratios). Additionally, we review a number of topics in MCT, including the role of "scaling factors"; whether coexistence mechanisms are approximations; whether the magnitude or sign of invasion growth rates matters more; whether Hutchinson solved the paradox of the plankton; the scale-dependence of coexistence mechanisms; and much more.
... The issue of stability in vegetation communities has long been debated (Broekman et al., 2019;Chesson, 2000;Connell & Slatyer, 1977), but what has been less investigated is whether the underlying mechanisms of stability are changing. Differing degrees of conspecific and heterospecific competitive responses during drought have the potential to alter community dynamics (Adler et al., 2006;Chesson & Huntly, 1997) and drive community change (Gilman et al., 2010;Lancaster et al., 2017). Theoretical studies have shown that increasing drought due to climate change may either increase community stability by reducing dominant competitive relationships (Adler et al., 2006;Lloret et al., 2012) or destabilize communities by increasing competitive exclusion of lessadapted species (Chesson & Huntly, 1997;Holt, 1985). ...
... Differing degrees of conspecific and heterospecific competitive responses during drought have the potential to alter community dynamics (Adler et al., 2006;Chesson & Huntly, 1997) and drive community change (Gilman et al., 2010;Lancaster et al., 2017). Theoretical studies have shown that increasing drought due to climate change may either increase community stability by reducing dominant competitive relationships (Adler et al., 2006;Lloret et al., 2012) or destabilize communities by increasing competitive exclusion of lessadapted species (Chesson & Huntly, 1997;Holt, 1985). Empirical studies of forests, however, have primarily focused on direct, physiological drought effects and drought-mediated disturbance processes (Furniss et al., 2020;Seidl et al., 2017). ...
... With climate change, however, limiting factors of the five species tested here appeared to converge on the singularly most limiting resource of water, producing elevated density-dependent mortality during drought on dry sites where water availability dropped most dramatically. Our forecasts suggest that the once stabilizing effect of climatic variability (Adler et al., 2006;Chesson & Huntly, 1997) may become weakened as climate change brings noveland increasingly extreme-ranges of drought variability (Germain & Lutz, 2020). This interpretation is in line with predictions that species will migrate to follow their moving climatic niche to track cooler/more moist sites as climate change brings warmer/dryer conditions (Chen et al., 2011;Root et al., 2003). ...
Article
Plant competition may intensify with climate warming, but whether this will occur equally for conspecific and heterospecific competition remains unknown. Competitive shifts have the potential to instigate community change because the relative strengths of conspecific and heterospecific negative density dependence mediate the stabilizing mechanisms underpinning species coexistence. We examined a mature temperate forest to assess both direct and indirect climate effects at multiple scales: individual species, interspecies relationships, and community stability mechanisms. Our coupled approach 1) quantified tree mortality risk dependence on the interactive effects of competition, climatic water deficit, snowpack, and soil moisture for 28,913 trees over eight years (3,149 mortalities), then 2) used a climate‐projection ensemble to forecast changes in conspecific and heterospecific competition from 2020 to 2100. We predict that projected climate warming will destabilize the foundational forest community by increasing the strength of heterospecific competition at a greater rate and to a greater degree than conspecific competition for four of five abundant tree species, particularly on dry microsites. Modeling showed that these findings were most pronounced after the year 2038, at which point snowpacks were projected to be too small to ameliorate the effects of drought on competitive interactions. Our finding that heterospecific competition is more sensitive than conspecific competition to climate warming may indicate impending loss of ecosystem functioning. We join the growing body of work showing a predominance of indirect drought effects, yet coupled climate models still fail to consider how changing community dynamics may impact forest cover and, in turn, disrupt forest–climate carbon feedbacks. Ecosystems sharing characteristics with our example forest – those with low species richness and thus a limited biodiversity insurance effect – may be similarly vulnerable to climate‐mediated destabilization. In such communities, increased heterospecific competition among even a small number of species can more easily destabilize communities without recourse from redundant species. This study of an overlooked but vital mechanism of community change can be adapted by researchers in a range of ecosystems to improve understanding of climate change consequences. This article is protected by copyright. All rights reserved.
... theory (Chesson and Huntly, 1997;Fox, 2013) and empirical findings (Violle et al., 2010) that harsher environments instead reinforce dependence on limiting factors. Without a quantitative framework that directly pairs theory and experiment, it has been difficult to determine the source of disagreement between the many conflicting predictions and observations surrounding DDRs. ...
... differently shaped growth curves) (Chesson, 1994;Letten et al., 2018). Yet, coexistence might also arise from the overall time-averaged disturbance intensity in a fluctuation-independent manner (Chesson and Huntly, 1997;Fox, 2013). To determine whether the effects of disturbance on diversity are truly fluctuation-dependent (Chesson, 2000), a disturbance should ideally be decomposed into distinct components of mean intensity (e.g. ...
... Notably, neither Lotka-Volterra nor linear consumer resource models predict differences in the DDR between fluctuation frequencies (Appendix 1-figures 1 and 2). The overlap of DDRs of different frequencies indicates that in these models the relevant metric is the time-averaged overall intensity, rather than the frequency, of disturbance (Chesson and Huntly, 1997;Fox, 2013). ...
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Environmental disturbances have long been theorized to play a significant role in shaping the diversity and composition of ecosystems. However, an inability to specify the characteristics of a disturbance experimentally has produced an inconsistent picture of diversity-disturbance relationships (DDRs). Here, using a high-throughput programmable culture system, we subjected a soil-derived bacterial community to dilution disturbance profiles with different intensities (mean dilution rates), applied either constantly or with fluctuations of different frequencies. We observed an unexpected U-shaped relationship between community diversity and disturbance intensity in the absence of fluctuations. Adding fluctuations increased community diversity and erased the U-shape. All our results are well-captured by a Monod consumer resource model, which also explains how U-shaped DDRs emerge via a novel 'niche flip' mechanism. Broadly, our combined experimental and modeling framework demonstrates how distinct features of an environmental disturbance can interact in complex ways to govern ecosystem assembly and offers strategies for reshaping the composition of microbiomes.
... This disproportionately affects species that would persist for the longest times in an undisturbed system, that is, the strong competitors that are able to displace weaker competitors (stronger colonizers). As a result, an intermediate disturbance range exists, that is, where neither strong competition nor strong colonization strategies dominate, which allows disturbances to act as an equalizing rather than a stabilizing mechanism (Chesson, 2000;Chesson & Huntly, 1997). In such regimes, both strategies have similar chances to succeed, and once species of either type drop to low levels of abundance they are unable to recover. ...
... In our model, fluctuations around the average per-capita mortality rates cancel out, so that, in the long term, growth rates depend only on the average values of these parameters. Such fluctuations in mortality can produce fluctuations in the abundances of species but, unless stronger competitors experience stronger fluctuations, do not create the stabilizing effect required for producing stable coexistence (i.e., fluctuation-dependent mechanism; Chesson & Huntly, 1997). Therefore, it is very important to distinguish between the effects of changes in the average long-term mortality rate from the effects of variation in mortality rates around a fixed mean. ...
... Relative nonlinearity, which acts when species' growth rates respond differently and nonlinearly to competition under a fluctuating environment, can allow multiple competitors to coexist stably, as the superior competitors' average population growth rates are significantly depressed as a result of disturbance (Chesson, 1994(Chesson, , 2000Roxburgh et al., 2004). The storage effect in turn can be conceptualized as temporal niche segregation: different species specialize in different phases of the environmental fluctuations, allowing them to survive unfavorable periods (Barab as et al., 2018;Chesson, 1994Chesson, , 2000Chesson & Huntly, 1997;Fox, 2013;Miller et al., 2011;Roxburgh et al., 2004;Shea et al., 2004). It is an open question whether there are circumstances under which the above fluctuation-dependent mechanisms have a strong influence on the DDR, and how common those circumstances are. ...
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Disturbance has long been recognized as a critical driver of species diversity in community ecology. Recently, it has been found that the well-known intermediate disturbance hypothesis, which predicts a unimodal diversity-disturbance relationship (DDR), fails to describe numerous experimental observations, as empirical DDRs are diverse. Consequently, the precise form of the DDR remains a topic of debate. Here we develop a simple yet comprehensive metacommunity framework that can account for complex competition patterns. Using both numerical simulations and analytical arguments, we show that strongly multimodal DDRs arise naturally, and this multimodality is quite robust to changing parameters or relaxing the assumption of a strict competitive hierarchy. Having multimodality as a robust property of DDRs in competition models suggests that much of the noise observed in empirical DDRs could be a critical signature of the underlying competitive dynamics.
... Another longstanding hypothesis regarding species' range limits is that competition is especially important for setting warm range edges, such as those at low elevation or latitude 1,2,6,7 . However, while some transplant studies have found patterns consistent with this hypothesis 5 , other studies indicate that competition is also important for setting cool range edges such as those at high elevation or latitude [8][9][10] . Thus, although the general importance of competition in shaping distributions is well established, how this varies across environmental gradients, as well as the underlying processes involved, remain unclear. ...
... Firstly, we focused on the population-level outcomes of competition rather than the individual-level strength of competition, as most previous empirical studies have done 5,25 . Both theoretical and empirical work demonstrate that competition can be equally important for population dynamics in harsh and benign environments 10 and its effects cannot be completely understood by focusing solely on the strength of competition 9,26 . For example, harsh environmental conditions could reduce population growth rates directly, making species less tolerant of interspecific competition and competitive exclusion correspondingly more likely to occur, despite any reduced intensity or prevalence of interspecific competition itself 9,27,28 . ...
... Both theoretical and empirical work demonstrate that competition can be equally important for population dynamics in harsh and benign environments 10 and its effects cannot be completely understood by focusing solely on the strength of competition 9,26 . For example, harsh environmental conditions could reduce population growth rates directly, making species less tolerant of interspecific competition and competitive exclusion correspondingly more likely to occur, despite any reduced intensity or prevalence of interspecific competition itself 9,27,28 . Secondly, we grew the same set of species in each experimental site, which allowed us to isolate changes in the intensity of competition from turnover in community composition across the environmental gradient 6 . ...
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Competition plays an important role in shaping species’ spatial distributions. However, it remains unclear where and how competition regulates species’ range limits. In a field experiment with plants originating from low and high elevations and conducted across an elevation gradient in the Swiss Alps, we find that both lowland and highland species can better persist in the presence of competition within, rather than beyond, their elevation ranges. These findings suggest that competition helps set both lower and upper elevation range limits of these species. Furthermore, the reduced ability of pairs of lowland or highland species to coexist beyond their range edges is mainly driven by diminishing niche differences; changes in both niche differences and relative fitness differences drive weakening competitive dominance of lowland over highland species with increasing elevation. These results highlight the need to account for competitive interactions and investigate underlying coexistence mechanisms to understand current and future species distributions.
... Perhaps we can arrive at a clearer justification of the scaling factors by studying papers in which the scaling factors played a crucial role. Chesson and Huntly (1997) analyzed a model where per capita growth rates responded linearly to environmental fluctuations and a single regulating factor. The scaling factors eliminated ∆ρ i , and the linear responses precluded the fluctuation-dependent mechanisms, ∆N i and ∆I i . ...
... Using this approach, Chesson (1994) showed that fluctuations are necessary for coexistence in the the lottery model and the annual plant model. Crucially, in both of the aforementioned papers (Chesson and Huntly, 1997;and Chesson, 1994), the cancelling of ∆ρ i is valuable because it tells us how species are not coexisting. ...
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How do species coexist? A framework known as Modern Coexistence Theory measures mechanisms of coexistence by comparing a species perturbed to low density (the invader) to other species that remain at their typical densities (the residents); this invader-resident comparison measures a rare-species advantage that results from specialization. However, there are several reasonable ways (i.e., methods) to compare invaders and residents, each differing in practicality and biological interpretation. Here, using theoretical arguments and case studies, we compare four such methods for calculating coexistence mechanisms: 1) Scaling factors, the traditional approach where resident growth rates are scaled by a measure of relative sensitivity to competition, obtained by solving a system of linear equations; 2) The simple comparison, which gives equal weight to all resident species; 3) Speed conversion factors, a novel method in which resident growth rates are scaled by a ratio of generation times, and; 4) The invader-invader comparison, another novel method in which a focal species is compared to itself at high vs. low density. We conclude that the conventional scaling factors can be useful in some theoretical research, but are not recommended for empirical applications, i.e., determining the mechanisms of coexistence in real communities. Instead, we recommend the simple comparison and speed conversion factor methods. The speed conversion factors are most useful when comparing species with dissimilar generation times. However, ecologists often study coexistence in guilds of species with similar life-histories, and therefore, similar generation times. In such scenarios, the easier-to-use simple comparison method is reasonable.
... Ecological and evolutionary theory recognizes seed banks as 'biodiversity reservoirs'. Indeed, seed banks support population persistence and biodiversity maintenance through temporal storage effects 3 and increasing the gene pool 4 , thereby maintaining a diverse but hidden species pool belowground that hedges against risk of environmental change 3 . Further, seed banks can affect the potential rate and even direction of evolutionary change because they increase the mean generation times of populations 5,6 . ...
... Ecological and evolutionary theory recognizes seed banks as 'biodiversity reservoirs'. Indeed, seed banks support population persistence and biodiversity maintenance through temporal storage effects 3 and increasing the gene pool 4 , thereby maintaining a diverse but hidden species pool belowground that hedges against risk of environmental change 3 . Further, seed banks can affect the potential rate and even direction of evolutionary change because they increase the mean generation times of populations 5,6 . ...
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Soil seed banks represent a critical but hidden stock for potential future plant diversity on Earth. Here we compiled and analyzed a global dataset consisting of 15,698 records of species diversity and density for soil seed banks in natural plant communities worldwide to quantify their environmental determinants and global patterns. Random forest models showed that absolute latitude was an important predictor for diversity of soil seed banks. Further, climate and soil were the major determinants of seed bank diversity, while net primary productivity and soil characteristics were the main predictors of seed bank density. Moreover, global mapping revealed clear spatial patterns for soil seed banks worldwide; for instance, low densities may render currently species-rich low latitude biomes (such as tropical rain-forests) less resilient to major disturbances. Our assessment provides quantitative evidence of how environmental conditions shape the distribution of soil seed banks, which enables a more accurate prediction of the resilience and vulnerabilities of plant communities and biomes under global changes.
... Such mechanisms of niche separation are profoundly affected by environmental variability, the magnitude of which strongly influences the strength and outcome of competitive interactions (Levins, 1979;Li & Chesson, 2016). For example, a harsh and fluctuating environment can slow the process of competitive exclusion (Chesson et al., 2004;Chesson & Huntly, 1997). In harsh environments, some argue that the intensity and importance of competitive interactions decrease in favor of positive interactions or facilitation (Barrio et al., 2013;Fugère et al., 2012;Kawai & Tokeshi, 2007, but see Hart & Marshall, 2013). ...
... In harsh environments, some argue that the intensity and importance of competitive interactions decrease in favor of positive interactions or facilitation (Barrio et al., 2013;Fugère et al., 2012;Kawai & Tokeshi, 2007, but see Hart & Marshall, 2013). However, such conditions may also make species less tolerant of competitive interactions and promote niche differentiation through environmental fluctuations (Chesson & Huntly, 1997). Gutt (2006) suggests that the ability of similar species to coexist, therefore, is dependent on minimizing competitive displacement by maximizing utilization of environmental variability. ...
Article
Coexistence of similar species can be influenced by the intensity of interspecific interactions, which often depends on the availability of limiting resources. Habitat availability varies strongly with tidal phase in many intertidal ecosystems, potentially affecting interspecific interaction strength, particularly for mobile species. Four closely related species of highly mobile intertidal detritivores (talitrid amphipods Megalorchestia californiana, M. corniculata, M. benedicti, M. minor) inhabit sandy beaches in southern California, where they consume wave‐cast macroalgal wrack originating on coastal reefs. Their coexistence suggests that mechanisms, such as niche separation, are operating to weaken competition among these species. To evaluate this possibility, we explored how tidal phase may mediate temporal and spatial patterns of habitat use among these closely related congeners. We hypothesized that neap tides that reduce intertidal habitat would strengthen temporal separation between species, whereas spatial separation would be greater during spring tides when more habitat is available. We investigated these questions during spring and neap tide phases using (1) comparisons of intertidal distributions of burrowed amphipods and (2) observations of surface activity of amphipods from pitfall traps and mesocosms. We found significant effects of tide phase and species identity on mean intertidal positions and separation of burrowed amphipods. Intertidal distributions of the four species overlapped during neap tide and were significantly separated during spring tide when more intertidal habitat was available. Surface activity patterns differed among species and were more widely separated in time during neap tide than during spring tide. Consequently, the cumulative activity time of all species on neap tides was twice that observed during spring tides. Our findings suggest that mobile intertidal species, like these sympatric talitrid amphipods, can avoid interspecific competition by shifting their activity patterns with tide phase and beach condition. As rising sea levels reduce beach habitat, interspecific competition among these important intertidal consumers may increasingly influence their behavior and coexistence.
... Community ecology provides multiple hypotheses for predicting how disturbance should affect biodiversity. For example, the intermediate disturbance hypothesis suggests that moderate levels of disturbance increase richness by delaying extinction from competitive exclusion and facilitating colonization (sensu Connell 1978, Sousa 1979, Chesson and Huntly 1997. However, disturbance can reduce diversity under conditions of limited colonization where disturbance-driven extinctions dominate, such that only species tolerant of, or resistant to, the disturbance persist (e.g., Tilman and El Haddi 1992). ...
... Conversely, disturbance can enhance diversity if successful colonization is limited by resource availability and disturbance opens space for the successful establishment of migrants (e.g., Goodsell and Connell 2005 and references therein). Finally, multiple studies in community ecology reveal no effect of disturbance on richness (reviewed in Mackey andCurrie 2001, Hughes et al. 2007), especially in systems not structured by competitive hierarchies (Chesson and Huntly 1997) or where several of the above mechanisms counteract each other. ...
Article
One objective of eco‐evolutionary dynamics is to understand how the interplay between ecology and evolution on contemporary timescales contributes to the maintenance of biodiversity. Disturbance is an ecological process that can alter species diversity through both ecological and evolutionary effects on colonization and extinction dynamics. While analogous mechanisms likely operate among genotypes within a population, empirical evidence demonstrating the relationship between disturbance and genotypic diversity remains limited. We experimentally tested how disturbance altered the colonization (gain) and extinction (loss) of genets within a population of the marine angiosperm Zostera marina (eelgrass). In a 2‐year field experiment conducted in northern California, we mimicked grazing disturbance by migratory geese by clipping leaves at varying frequencies during the winter months. Surprisingly, we found the greatest rates of new colonization in the absence of disturbance and that clipping had negligible effects on extinction. We hypothesize that genet extinction was not driven by selective mortality from clipping or from any stochastic loss resulting from the reduced shoot densities in clipped plots. We also hypothesize that increased flowering effort and facilitation within and among clones drove the increased colonization of new genets in the undisturbed treatment. This balance between colonization and extinction resulted in a negative relationship between clipping frequency and net changes in genotypic richness. We interpret our results in light of prior work showing that genotypic diversity increased resistance to grazing disturbance. We suggest that both directions of a feedback between disturbance and diversity occur in this system with consequences for the maintenance of eelgrass genotypic diversity.
... Contrary to conclusions drawn in early and still 152 influential literature (e.g. Hutchinson 1961), environmental fluctuations themselves are not 153 sufficient to support coexistence of competing species (Chesson & Huntly 1997;Fox 2013) and can 154 hinder as much as facilitate coexistence. The framework of modern coexistence theory (MCT, 155 Chesson 2000) has been developed to robustly understand the impact of variability on coexistence. ...
... In order to simplify the following discussion, we assume that only the ability of a particular focal 163 species to persist at a site alongside one or more competitor species is in question. Through MCT, 164 precise principles have been developed to identify how temporal variability influences coexistence 165 by quantifying the effects of temporal variability on the long-term average growth rate when the 166 focal species is at low densities ( , Chesson & Warner 1981;Chesson & Huntly 1997;Amarasekare 167 et al. 2004;Snyder 2008). 168 ...
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Recent developments in understanding and predicting species responses to climate change have emphasised the importance of both environmental variability and consideration of the wider biotic community. To date, the interaction between the two has received less attention. However, considerable bodies of theory and empirical results suggest that multi-species consequences of variability can have strong impacts on range limits and the speed of range shifts. Here we demonstrate how biotic interactions and temporal variability can act together to influence range shift dynamics and highlight the need to understand these interactions in order to predict how species will respond to global change. We emphasise the value and utility of partitioning approaches applied to parameterised models to determine the direction and relative importance and direct of these forces in empirical systems. Authorship JCDT wrote the manuscript and built the models. All authors contributed significantly to the editing and manuscript development. Funding The work was supported by NERC grant NE/T003510/1 Data Sharing and Data Accessibility Code to generate all results is publicly available at https://github.com/jcdterry/ClimateVar_BioticInts and should the manuscript be accepted will be permanently archived. The paper contains no new datasets.
... These stabilising niche differences among species contribute to the maintenance of diversity by causing species to limit their own growth more than the growth of other species, thereby preventing competitive exclusion and allowing populations to recover from low abundances (Adler et al., 2007;Chase & Leibold, 2003;Chesson, 2000). Some stabilising mechanisms of coexistence rely on environmental fluctuations and can further increase the number of species in a community (Chesson, 1994;Chesson & Huntly, 1997;Descamps-Julien & Gonzalez, 2005). For example, in seasonal environments, species that are favoured at different times of the year may be able to coexist in the community if they can survive through periods of unfavourable environmental conditions (Pake & Venable, 1996). ...
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Coexisting species often exhibit negative frequency dependence due to mechanisms that promote population growth and persistence when rare. These stabilising mechanisms can maintain diversity through interspecific niche differences, but also through life-history strategies like dormancy that buffer populations in fluctuating environments. However, there are few tests demonstrating how seed banks contribute to long-term community dynamics and the maintenance of diversity. Using a multi-year, high-frequency time series of bacterial community data from a north temperate lake, we documented patterns consistent with stabilising coexistence. Bacterial taxa exhibited differential responses to seasonal environmental conditions, while seed bank dynamics helped maintain diversity over less-favourable winter periods. Strong negative frequency dependence in rare, but metabolically active, taxa suggested a role for biotic interactions in promoting coexistence. Together, our results provide field-based evidence that niche differences and seed banks contribute to recurring community dynamics and the long-term maintenance of diversity in nature.
... Patricia hurricane affected the primary tropical dry forest, creating mosaic gaps (Álvarez-Yépiz, 2020) with high content of soil organic matter, more sun radiation at ground level, less soil humidity, etc.; this environmental heterogeneity opened new niches and increased fungal richness. The mosaic patches yield successional states with high diversity through competition at landscape scale (Chesson and Huntly, 1997). Disturbance can have a positive effect on richness, while undisturbed sites limit the establishment of new species (Wohlgemuth et al., 2002;Tanentzap et al., 2013;Farrior et al., 2016). ...
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Climate change is increasing the frequency and intensity of catastrophic events such as hurricanes. Soil microbial communities regulate geochemical cycles and other important ecosystem processes. How the hurricanes affect soil microbial communities and their function, however, is largely unknown. Our aim was to describe the impact of a category 4 hurricane (Patricia) on the soil fungal community structure and diversity in a Mexican tropical dry forest. Soil fungal community was inferred sequencing the ITS2 rDNA of composite soil samples taken in a time series ranging one year before and two years after the hurricane. OTU richness before the hurricane and in the first sampling after were comparable, however a 20–40% decrease in richness was observed in later sampling times. There were also taxonomic shifts associated with the disturbance, changing from a higher richness and abundance of Ascomycota fungi to greater dominance of Basidiomycota and Glomeromycota fungi post-hurricane. Arbuscular mycorrhizal fungi and plant pathogens diversity increased immediately after the hurricane but decreased in subsequent years. Approximately 7% of the soil community, primarily composed of saprotrophic fungi, persisted across the hurricane and the harsh seasonality of this ecosystem. Soil fungal community was affected by the hurricane but had function stability and resilience through years.
... These features allow environmental variation to enable coexistence by the storage effect (Chesson 1994(Chesson , 2003. In the absence of environmental variation, coexistence at a stable equilibrium is not possible due to the fact that all species share the same density response variable D(t) (Chesson and Huntly 1997). However, sustained deterministic fluctuations are a possibility in this model, and can support coexistence under some circumstances by a mechanism termed "relative nonlinearity" (Kuang and Chesson 2008). ...
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We analyze a general theory for coexistence and extinction of ecological communities that are influenced by stochastic temporal environmental fluctuations. The results apply to discrete time (stochastic difference equations), continuous time (stochastic differential equations), compact and non-compact state spaces and degenerate or non-degenerate noise. In addition, we can also include in the dynamics auxiliary variables that model environmental fluctuations, population structure, eco-environmental feedbacks or other internal or external factors. We are able to significantly generalize the recent discrete time results by Benaim and Schreiber (J Math Biol 79:393–431, 2019) to non-compact state spaces, and we provide stronger persistence and extinction results. The continuous time results by Hening and Nguyen (Ann Appl Probab 28(3):1893–1942, 2018a) are strengthened to include degenerate noise and auxiliary variables. Using the general theory, we work out several examples. In discrete time, we classify the dynamics when there are one or two species, and look at the Ricker model, Log-normally distributed offspring models, lottery models, discrete Lotka–Volterra models as well as models of perennial and annual organisms. For the continuous time setting we explore models with a resource variable, stochastic replicator models, and three dimensional Lotka–Volterra models.
... From a population control perspective, this implies that adding an extrinsic source of mortality to most larval populations of A. nigripes, for example a larvicide, would be effective at reducing population density. As Chesson and Huntly (1997) point out, low population densities do not necessarily imply that competition does not have appreciable effects on population and community dynamics, but rather that multiple mechanisms are involved. It seems likely that in this system, other processes such as predation or egg-mortality, could play an important role in lowering A. nigripes larval densities in certain years to the point where little or no intraspecific competition occurs. ...
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Processes that change with density are inherent in all populations, yet quantifying density dependence with empirical data remains a challenge. This is especially true for animals recruiting in patchy landscapes because heterogeneity in habitat quality in combination with habitat choice can obscure patterns expected from density dependence. Mosquitoes (Diptera: Culicidae) typically experience strong density dependence when larvae compete for food, however, effects vary across species and contexts. If populations experience intense intraspecific density-dependent mortality then overcompensation can occur, where the number of survivors declines at high densities producing complex endogenous dynamics. To seek generalizations about density dependence in a widespread species of Arctic mosquito, Aedes nigripes, we combined a laboratory experiment, field observations, and modeling approaches. We evaluated alternative formulations of discrete population models and compared best-performing models from our lab study to larval densities from ponds in western Greenland. Survivorship curves from the lab were the best fit by a Hassell model with compensating density dependence (equivalent to a Beverton-Holt model) where peak recruitment ranged from 8 to 80 mosquitoes per liter depending on resource supply. In contrast, our field data did not show a signal of strong density dependence, suggesting that other processes such as predation may lower realized densities in nature, and that expected patterns may be obscured because larval abundance covaries with resources (cryptic density dependence). Our study emphasizes the importance of covariation between the environment, habitat choice, and density dependence in understanding population dynamics across landscapes, and demonstrates the value of pairing lab and field studies.
... On the other hand, generalist species -those with characteristics that allow their persistence across a range of ecological conditions -will have broad distributions and low realized niche specialization but are unlikely to be found in stressful ecological conditions. While stressful ecological conditions might limit species richness, moderately harsh conditions, such as those with multiple limiting resources or intermittent disturbances, may promote high species richness by creating heterogeneous microhabitats (Chesson, 2000;Chesson & Huntly, 1997) or increasing the number of niche axes on which a variety of species can coexist (Harpole & Tilman, 2007;Hutchinson, 1957). ...
Article
Aim Humans impact biodiversity by altering land use and introducing nonnative species. Yet the extent to which coexistence processes, such as competition and niche shifts, mediate these relationships is not clear. This study compares how human development influences wetland plant diversity by examining patterns of species richness, niche specialization and nonnative species occurrences along a human development gradient. Location Alberta, Canada. Taxon Plants. Methods We computed species richness and niche specialization (a measure of the range of human development extents over which a species occurs) from species occurrence data across 1582 wetlands. We tested associations between human development extent and species richness, niche specialization and nonnative species using linear mixed models. We used nonmetric multidimensional scaling ordination to examine whether community composition differed among wetlands surrounded by different human development extents. Results Species richness and niche specialization show contrasting relationships with human development: richness was highest and niche specialization was lowest at intermediate human development extents, suggesting that competitive ability and environmental filtering may contribute to low richness at low and high development extents, respectively. Wetlands surrounded by the highest and lowest human development extents had similar levels of richness and niche specialization, but differed in community composition. The proportion of nonnative species increased with increasing human development, alternatively suggesting that the substitution of native species by nonnatives in developed areas may contribute to reduced richness and influence community assembly. Main conclusions These findings demonstrate that human land development plays a major role in shaping species richness by influencing the number of nonnative species and the niche specialization of species inhabiting a wetland. Furthermore, these findings suggest that the proportion of nonnative species is an overlooked factor potentially influencing plant richness; including this variable may help clarify the inconsistent responses of diversity to human development over large spatiotemporal scales.
... Moreover, more diverse communities usually have greater temporal stability in species composition (McCann, 2000) as evidenced across a range of taxa (see Shurin, 2007). It appears that, in variable environments, changing conditions selectively favour species with different affinities (Chesson & Huntly, 1997). Moreover, large environmental fluctuations might lead to patchier species occurrence, resulting in lower species richness and a temporal turnover increase (White et al., 2006). ...
Article
• Most ecological studies involving insects are based on medium‐ and short‐term observations; however, the extent to which such data captures reality remains unclear. • We investigated the long‐term dynamics of two Odonata communities (disturbed and undisturbed sites) over 18 years and analysed the differences in the short‐ and long‐term results. We also focused on the sampling methodology to enhance the efficacy and objectivity of long‐term monitoring involving Odonata. • During one year, we captured only 53% of the overall species richness; during three consecutive sampling years, it was 65%. To capture 95%, we needed 16 years. Changes in quantitative similarity (Renkonen index, P) were more pronounced within sites over time than between sites. Species constancy significantly increased with the maximum abundance class but decreased with increasing fluctuation ratio and specialisation (Dragonfly Biotic Index). Based on exuviae, we detected half of the species compared to adults, but the species accumulation curves peaked after a few sampling years. • Long‐ and short‐term monitoring yield different results, both qualitatively (species richness, specialisation) and quantitatively (abundance, dominance). Ideal sampling should be sequential, lasting at least 10 years (capturing >80% of species). Intermittent sampling (one‐year interspersed with pauses), allowing the inclusion of multiple sites in monitoring programme, may also provide satisfactory results when performed over a longer period. • Over the long term, sampling adults semi‐quantitatively and exuviae qualitatively provided sufficient information, while being feasible in terms of both personnel and costs, thereby overcoming the main pitfalls of long‐term monitoring programmes.
... First, founder-controlled communities in spatially explicit systems can, in contrast to nonspatial systems, remain diverse in spite of potential competitive exclusion of one species by the other, essentially due to an equalizing mechanism that prevents fitness differences from prevailing (Molofsky et al., 2002;Bolker et al., 2003). Although this mechanism does not account for stable-state coexistence (Chesson and Huntly, 1997), time to extinction is far too long in realistic settings, making it effectively a mechanism of coexistence. Second, clonality may strongly contribute to fast occupancy of space in open patches in the community by a process that has been termed spatial successional niches (Bolker et al., 2003; see e.g. ...
Article
Plants with clonal growth can produce multiple potentially independent units, termed ramets. Clonal growth can have important ecological and evolutionary consequences, such as by increasing probability of reproduction, space monopolization, and regeneration after injury; and by permitting physiological integration of connected ramets. Although clonal growth is widespread among species and habitats, it has received relatively little attention in plant ecology. To introduce this special issue on clonal plant ecology, we first provide a brief background on the topic, noting its importance in areas ranging from evolution to the impacts of climate change on plant communities. We then focus on a set of pressing questions, to highlight both the obstacles and opportunities to more explicitly incorporate clonal growth in research on plant ecology and evolution.
... Their occurrence dynamics are mediated by stochastic events of dispersal and extinctions in space and time (Rosindell et al., 2011). According to its creator, neutral ecological theories of that kind have encountered no small resistance in ecologye.g., Chesson and Huntly (1997). ...
Article
Ecologists have long had a "love-hate" relationship with the niche concept. Sometimes referred to as a term best left undefined, the niche concept nonetheless spans ecology. Deeply rooted in the Darwinian struggle for survival , "niche" has been a core, although slippery, idea in ecology since its origins. What ecologists mean by niche has changed semantically over time. In this paper, we review the history of the term, focusing on its uses and the disagreements that have arisen over it within ecology. Because classic niche concepts are not exclusive and share some similarities, we disentangle them into key theoretical components to create a heuristic classification scheme for niche concepts. We, therefore, analyze coherence on rhetoric within the ecological literature, by classifying how ecologists use niche concepts in their writing, aiming at clarifying communication on what is being studied. To assess if modern ecological theories are coherent in their usage of the niche concept, we surveyed a sample of three research areas: ecological niche modeling, coexistence between species and meta-communities. We found that research agendas are segregated when it comes to rhetoric about niches. Ecologists have long tried to achieve a truly unifying biodiversity theory, or at least a universal definition of niche. We, however, move in the opposite direction and suggest that the niche concept should be dismembered into its key components, highlighting which elements of the concept are being addressed and analyzed. Explicitly stating to which niche concept a study is referring may enhance communication among researchers from different backgrounds and perhaps alleviate this century-old dilemma.
... Hutchinson [4] suggested that fluctuations in environmental conditions may allow for the coexistence of species competing for limiting resources. For example, one of the key arguments behind the intermediate disturbance hypothesis is that species can reach an equilibrium state and exclude other competing species under reduced environmental fluctuations, whereas increased fluctuations prevent species from reaching equilibrium and therefore prevent competing species from excluding others [4][5][6][7][8] (but see [9]). By contrast, May [10][11][12] argued that for competing species to coexist in a fluctuating environment, niche differences need to be greater because random environmental fluctuations may favour one species while occasionally excluding another. ...
Article
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Assessing the impact of environmental fluctuations on species coexistence is critical for understanding biodiversity loss and the ecological impacts of climate change. Yet determining how properties like the intensity, frequency or duration of environmental fluctuations influence species coexistence remains challenging, presumably because previous studies have focused on indefinite coexistence. Here, we model the impact of environmental fluctuations at different temporal scales on species coexistence over a finite time period by employing the concepts of time-windowed averaging and performance curves to incorporate temporal niche differences within a stochastic Lotka–Volterra model. We discover that short- and long-term environmental variability has contrasting effects on transient species coexistence, such that short-term variation favours species coexistence, whereas long-term variation promotes competitive exclusion. This dichotomy occurs because small samples (e.g. environmental changes over long time periods) are more likely to show large deviations from the expected mean and are more difficult to predict than large samples (e.g. environmental changes over short time periods), as described in the central limit theorem. Consequently, we show that the complex set of relationships among environmental fluctuations and species coexistence found in previous studies can all be synthesized within a general framework by explicitly considering both long- and short-term environmental variation.
... Of these interactions, competition has been shown to alter population densities (Hairston 1951), foraging efficiency, growth, age structure (Cameron et al. 2007), habitat use (Creel and Creel 1996), and activity patterns (Ziv et al. 1993;Jones et al. 2001), and hence holds a central place in ecological and evolutionary theory (MacArthur and Levins 1964Levins , 1967Abrams 1980;Gurevitch et al. 1992). Ecological competition is perceived as an impediment to species coexistence and community diversity and hence has been the catalyst for studies examining the conditions under which coexistence between interacting species is possible (Chesson and Huntly 1997;Gordon 2000). Several conditions, such as spatial heterogeneity (Atkinson and Shorrocks 1981;Hanski and Beverton 1994;Rees et al. 1996;Questad and Foster 2008), temporal heterogeneity (Huston 1979;Menge 1979;Chesson 1985) and resource partitioning (MacArthur and Levins 1967), have been proposed to explain species coexistence and community diversity. ...
Article
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Understanding the conditions under which interacting species can persist is a major goal in ecology. Dietary partitioning is one of the major strategies that enables ecologically similar species to coexist in communities. In this study, we examined the dietary patterns of a selected group of amphibians in an amphibian community in northern Sri Lanka to understand differential resource use by coexisting species. The stomach flushing method was used to examine the diet of amphibians to study the niche breadth and pairwise species dietary niche overlap. Seventeen different prey categories were identified from the diet of six species of amphibians in the community. The most frequently used prey category by all amphibians was hymenoptera. Among the amphibians, some consumed several different prey categories (8–9 prey categories), while some were more specialised (e.g. Uperodon rohani fed only on ants), consuming only one or two different prey categories. The average niche overlap among the species in the community was 0.392 indicating low trophic niche overlap. This study indicates a low level of dietary niche overlap between the selected amphibian species and hence, a high degree of dietary niche partitioning. The findings also provide valuable insights into the dietary ecology of these amphibians, which will be invaluable for the formulation of conservation strategies.
... Major sources of natural regeneration are seed rain, seedling banks, coppices and soil seed banks (Garwood, 1989;Argaw et al., 1999;Teketay, 2005a;Kebede et al., 2012). Seed banks are critical repositories of woodland diversity that can contribute to local population persistence and biodiversity maintenance through temporal storage effects (Chesson and Huntly, 1997). ...
Article
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Soil seed banks serve as reservoirs of seeds for subsequent regeneration of plants. Soil seed banks were investigated along the Thamalakane Riparian Woodlands (hereafter referred to as TRWs) of the Okavango Delta, northern Botswana, from January-July 2015 and January-July 2016. The study aimed at examining species richness and diversity, determining densities, assessing the spatial distribution of seeds in the soil and comparing the similarity in species composition between the standing vegetation and soil seed bank flora. The vegetation was sampled in 71 plots (20 × 50 m) and soil samples were collected from 568 subplots. Agglomerative hierarchical cluster analysis was used to determine soil seed bank communities. Indicator species analysis was used to calculate indicator values for species in each community of germinated seeds and across different soil layers. Multi-response permutation procedures (MRPP) were used to compare similarity in soil seed bank composition. Bray-Curtis ordination was used to infer spatial relationships of soil layers in terms of soil seed bank composition. A total of 109 plant species were identified in the litter and top 9 cm soil layers with a mean density of 2101 seeds m−2. Herbs, grasses, sedges and woody plants were represented by 68, 19, 9 and 13 species, respectively, in 30 families and 87 genera. The overall total diversity and evenness of the soil seed bank in the TRWs was 3.25 and 0.69, respectively. Four plant communities were identified from the soil seed bank, namely Setaria verticilata-Amaranthus hybridus, Acanthospermum hispidum-Setaria sagittifolia, Digitaria eriantha-Eclipta prostrata and Cyperus longus-Fimbristylis dichotoma. Bray-Curtis ordination showed that there was overlap between these communities in terms of seed bank composition. However, MRPP analysis showed that there was significant (P
... On the scale of the whole of Finland, the regional pool thus has more candidate colonizers to offer for fertile sites than for nutrient-poor sites. Moreover, a large species pool is likely to include species that are sufficiently different from the resident species to enable coexistence (Chesson, 2000;Chesson & Huntly, 1997 (Brown & Parker, 1994;Hart & Chen, 2006;Kuuluvainen et al., 1993 shifted their identity ( Figure 3). This finding implies that in the young and fertile stands the newly gained species rapidly reached high dominance making previously common species less abundant or locally extinct. ...
Article
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Aim The diversity and composition of natural communities are rapidly changing due to anthropogenic disturbances. Magnitude of this compositional reorganization varies across the globe, but reasons behind the variation remain largely unknown. Disturbances induce temporal turnover by stimulating species colonizations, causing local extinctions, altering dominance structure, or all of these. We test which of these processes drive temporal community changes, and whether they are constrained by natural environmental gradients. Moreover, we assess to what degree identity shifts translate to changes in dominance structure. Location Finland. Time period Observations 1985–2006, disturbance history > 140 years. Major taxa studied Vascular plants. Methods We investigated temporal turnover of boreal forest understorey in response to disturbance, here forest management, along a soil fertility gradient. We disentangle the roles of species gains, losses and abundance changes in driving temporal turnover in response to and after disturbance by comparing turnover rates in different forest age categories along a fertility gradient. We quantify temporal turnover using richness‐based complement of Jaccard’s similarity index and proportional‐abundance based dissimilarity index. We also test whether disturbance history or fertility influence the relationship between identity shifts and dominance structure. Results We found that the impact of disturbance on temporal turnover depends on soil fertility. The greatest turnover occurred in the most fertile forests immediately after disturbance. There, species gains and losses strongly altered dominance structure leading to high turnover, whereas undisturbed old forests and nutrient‐poor habitats were characterized by stable dominant species even when the majority of species shifted their identity. Main conclusions Our results suggest that human impacts on temporal biodiversity change vary along environmental gradients. In boreal forests, the fertile habitats have a higher probability than nutrient‐poor sites of changing their composition in response to anthropogenic disturbances. Resource availability and disturbance history may thus influence consequences of temporal turnover for ecosystem functioning.
... It is also often posited that a successful invader is a superior competitior compared to its native neighbours, though this advantage can be transient or might work in concert with other environmental variables (Gioria and Osborne 2014). For example, the interactions between environmental and biological components of a system will influence community composition and species establishment (e.g., Kraft et al. 2015), while fluctuations in spatial or temporal niche opportunities (e.g., Chesson and Huntly 1997) can favour the coexistence of seemingly similar species. The complexities of biological communities can make it difficult to predict the effects of invasive species. ...
Article
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Identifying the mechanisms that result in a “high impact” invasive species can be difficult. Coexistence theory suggests that detrimental invasive species can be better predicted by incorporating both niche differences and fitness differences than examining niche overlap alone. Specifically, detrimental invasive species should take up shared limited resources more efficiently than their neighbouring resident species. While there is clear evidence that invasive Phragmites australis is successfully displacing resident species, there remains few field studies that attempt to quantify the niche overlap and fitness difference between P. australis and the species it is displacing in the field. We measured differences in photosynthetic performance (carbon assimilation rate, δ¹³C, photosynthetic water use efficiency, biomass, light compensation point, light saturation point), canopy height and interception of photosynthetically active radiation, and niche overlap between P. australis and three resident freshwater wetland species (Calamagrostis canadensis, Carex aquatilis, and Typha spp.) growing with or without aboveground interspecific competition. Invasive P. australis intercepted more photosynthetically active radiation, had higher photosynthetic water use efficiency, a higher average light saturation point, and had a larger niche region compared to resident species. Resident plant species showed a significant decrease in photosynthetic performance when growing in competition with P. australis and had a high probability of overlap onto the niche space of P. australis. These results provide evidence that the ability of P. australis to reduce the availability of a required resource and more efficiently use it over the growing season, while exhibiting high niche overlap with resident species, likely contributes directly to its success in North American freshwater wetlands.
... Although competitive interactions are often considered less important in harsh or stressful ecosystems (Grime 1977), native species competition can determine whether the establishment of potential invader is possible or not (Chesson and Huntly 1997). In harsh environments, resource scarcity may limit the competitive advantage conferred to fast-growing potential invaders, and biotic resistance may additionally prevent their invasion. ...
Article
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The diversity of a native community is not the sole driving factor determining the success of an invader; abiotic factors can also play a role, making it important to understand their contributions in modifying the expected outcome of biotic and abiotic resistance to invasion. In order to test the contributions of native diversity, relatedness between native and alien plants, and abiotic factors in resisting alien invasion, we used a trait–environment data set of 33 alien- and 130 native plants in 166 invaded plots, covering a gradient of soil resources in Saint-Katherine-Protectorate, an arid-protected area in Egypt. We measured the native diversity components as predictors of native biotic resistance. We quantified the success of alien plants based on their abundance and their performance traits. Additionally, we calculated the mean functional and phylogenetic distance between aliens and natives in each plot. We tested all plausible influences of native diversity components, relatedness, and soil resources on alien abundance and performance first using mixed-effects models and second with a structural equation model (SEM). Mixed-effect models revealed that, in the resource-rich environments, alien abundance increased even though native plants were more productive, but alien performance decreased. SEM direct pathways revealed that native communities with high functional dispersion and richness and high mean-weighted performance traits repelled alien plants by lowering their success. Simultaneously, in line with mixed-effect models results, the effect of soil resources on alien success was found to be indirect, being mediated by the mean functional and phylogenetic distance between aliens and natives: The success of alien plants increased in resourcerich environments when they were dissimilar to the natives, while it decreased for aliens similar to natives in resource-limited environments. Our findings provide new insights on native community resistance to invasion, with resistance being higher in resource-limited environments and when aliens are functionally and phylogenetically similar to resident natives.
... This feature of MCT stands in contrast to several big theories in community ecology -such as neutral theory, maximum entropy, and metacommunity theory -in which a small number of models are used to make inferences about many communities. MCT has been successfully used to derive theoretical insights (e.g., Chesson and Huntly, 1997;Stump and Chesson, 2015;Li and Chesson, 2016;Snyder and Chesson, 2003;Chesson and Kuang, 2008;Kuang and Chesson, 2010;Schreiber, 2021), and has been used to determined the mechanisms of coexistence in real communities (Caceres, 1997;Adler et al., 2006;Angert et al., 2009;Sears and Chesson, 2007;Usinowicz et al., 2012;Descamps-Julien and Gonzalez, 2005;Chu and Adler, 2015;Usinowicz et al., 2017;Ignace et al., 2018;Towers et al., 2020). Despite MCT's successes, there are a handful of problems that limit its applicability. ...
Preprint
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Ecologists have put forward many explanations for coexistence, but these are only partial explanations; nature is complex, so it is reasonable to assume that in any given ecological community, multiple mechanisms of coexistence are operating at the same time. Here, we present a methodology for quantifying the relative importance of different explanations for coexistence, based on an extension of Modern Coexistence Theory. Current versions of Modern Coexistence Theory only allow for the analysis of communities that are affected by spatial or temporal environmental variation, but not both. We show how to analyze communities with spatiotemporal fluctuations, how to parse the importance of spatial variation and temporal variation, and how to measure everything with either mathematical expressions or simulation experiments. Our extension of Modern Coexistence Theory allows empiricists to use realistic models and more data to better infer the mechanisms of coexistence in real communities.
... In the study region, these species are commonly found in forest-edge communities and advanced stages of ecological succession in hay meadows (Ferrez et al. 2011). Disturbances are known to shift back the stage of succession (Huston 1994;Chesson & Huntly 1997), which could explain why these species presented lower relative cover in highly disturbed plant communities. It is also possible that Anthriscus sylvestris ...
Thesis
En Europe, les prairies semi-naturelles de moyenne montagne sont principalement des écosystèmes ayant évolués au cours de plusieurs décennies d’activité humaine. Ces écosystèmes présentent une biodiversité remarquable et dépendent de régimes traditionnels de perturbations par la fauche ou le pâturage. Cependant, dans l’objectif d’augmenter leur production de fourrage, les prairies semi-naturelles sont soumises à des régimes de perturbations de plus en plus importants ainsi qu’à de nouveaux types de perturbations. Ce travail de thèse propose d’apporter de nouveaux éléments pour suivre et comprendre l’impact des perturbations sur la diversité des communautés végétales des prairies semi-naturelles.Dans un premier temps, la comparaison de relevés de végétation anciens (2005 à 2009) avec des relevés récents (2019) a été réalisée dans des prairies de fauche de moyenne montagne. Cette comparaison a permis de mettre en évidence des évolutions contrastées de la diversité végétale et des régimes de perturbations entre deux massifs. Dans le massif des Vosges, la diversité végétale ainsi que les régimes de perturbations ne semblent pas avoir évolué. A l’inverse, dans le massif du Jura, la diversité végétale a fortement diminué, probablement en association avec une augmentation de la fréquence des régimes de perturbations et de la fertilisation.Dans un second temps, l’impact de perturbations de forte intensité sur la diversité végétale a été évalué. Dans les prairies de fauche, les perturbations par les pullulations de campagnols terrestres semblent permettre une augmentation de la richesse spécifique par la réduction de la compétition pour la lumière. A l’inverse, ces perturbations semblent favoriser des espèces proches phylogénétiquement et entraîner une diminution de l’équitabilité phylogénétique. Dans les pelouses sèches, les perturbations par l’utilisation de broyeurs de pierres ne semblent pas impacter la diversité végétale. En revanche, la composition en espèces des milieux perturbés évolue vers des végétations de prairies productives suite à la perte des espèces typiques des pelouses.Dans un troisième temps, l’utilisation d’espèces diagnostiques comme indicateurs des régimes de perturbations et de la diversité végétale dans les prairies pâturées du massif du Jura a été testée. Le nombre d’espèces diagnostiques dans un relevé de végétation s’est révélé être un bon indicateur de la diversité végétale et des régimes de fertilisation. Cependant, les espèces diagnostiques ne semblent pas être de meilleurs indicateurs que des espèces généralistes des prairies pour évaluer l’intensité des régimes de perturbations.Nos résultats confirment que les changements de pratiques agricoles sont une menace majeure pour la diversité végétale des prairies semi-naturelles de moyenne montagne, en particulier dans le massif du Jura. Nos travaux mettent également en avant que l’augmentation de la fréquence des régimes de perturbations est susceptible d’avoir davantage d’effets négatifs sur la diversité végétale que des perturbations de forte intensité mais peu fréquentes. Néanmoins, certaines perturbations de forte intensité, comme l’utilisation de broyeurs de pierres, peuvent entraîner des modifications très importantes et irréversibles de la composition en espèces des milieux perturbés. Dans l’objectif de concilier enjeux sociétaux et environnementaux, il convient de maintenir des parcelles productives ou les régimes de perturbations par la fauche ou le pâturage sont fréquents, ce qui permet d’assurer une production fourragère importante. Cependant, Il est également nécessaire de limiter la fréquence et l’intensité des perturbations dans des parcelles encore peu intensifiées afin de protéger leur composition en espèces ainsi que leur diversité végétale.
... The ecological stress in the headwaters of the Yangtze River represents a natural disturbance to the structure and function of the ecosystem caused by human action, and this direct or indirect interference often exceeds the scope of ecological rehabilitation [84]. In this paper, the ecological landscape was introduced in order to scientifically define ecological stress [85,86]. Ecologically, the landscape is a spatially heterogeneous region that repeatedly appears in a similar form, and it is a natural complex with classification significance [87]. ...
Article
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The headwaters of the Yangtze River are a complicated system composed of different eco-environment elements. The abnormal moisture and energy exchanges between the atmosphere and earth systems caused by global climate change are predicted to produce drastic changes in these eco-environment elements. In order to study the dynamic effect and ecological stress in the eco-environment, we adapted the Double Attention Mechanism (DAM) to improve the performance of the DeepLab V3+ network in large-scale semantic segmentation. We proposed Elements Fragmentation (EF) and Elements Information Content (EIC) to quantitatively analyze the spatial distribution characteristics and spatial relationships of eco-environment elements. In this paper, the following conclusions were drawn: (1) we established sample sets based on “Sentinel-2” remote sensing images using the interpretation signs of eco-environment elements; (2) the mAP, mIoU, and Kappa of the improved DeepLab V3+ method were 0.639, 0.778, and 0.825, respectively, which demonstrates a good ability to distinguish the eco-environment elements; (3) between 2015 and 2021, EF gradually increased from 0.2234 to 0.2394, and EIC increased from 23.80 to 25.32, which shows that the eco-environment is oriented to complex, heterogeneous, and discontinuous processes; (4) the headwaters of the Yangtze River are a community of life, and thus we should build a multifunctional ecological management system with which to implement well-organized and efficient scientific ecological rehabilitation projects.
... Depending on longevity, transient seed banks are seeds that live for a short period and persistent seed banks are seeds that can survive in the soil for long periods (Gulden & Shirtliffe, 2009). A wide range of values provided by seed banks have been recorded, including offering the plants the ability to migrate to new sites, contributing to the stability of plant populations and conservation of biodiversity (Chesson & Huntly 1997, Faist et al., 2013, Plue & Cousins, 2013Cabin et al., 2000, Ayre et al., 2009, Lundemo et al., 2009, Mandak et al., 2012. Also, increasing the lifespan of plant populations and thus influencing the rate and direction of evolution (Brown & Venable, 1986, Evans & Cabin, 1995, Evans & Dennehy, 2005, gives the plant populations the ability to cope with environmental change. ...
Article
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We explored the species composition, diversity, and spatial distribution of soil seed banks and plant litter at two sites within Al Wadi Al Akhdar 'the green valley' area of Tabuk region, KSA, the main valley, and an adjoining shallow stream which are in essence different in elevation, slope and amount of overflow. The objectives were to evaluate and detect the species diversity and spatial variability in soil seed banks and plant litter. We used the systematic sampling procedure to collect soil samples, and the flotation method to extract seeds and plant litter. We employed Paleontological Statistics (PAST) version 4.03 to achieve summary statistics, diversity indices, and graphs and drive appropriate statistical tests that are crucial to answering research questions. Seed banks and plant litter exhibited abnormal spatial distributions. Differences in the mean number of seeds and plant litter contents between sampling points and between the two sites were not significant (p≤0.05). The shallow stream was more diverse in seed species with a high evenness of distribution between species compared to the valley. The valley possessed a higher number of seeds with a higher dominance _ D value. Species within the shallow site varied significantly (p≤0.05) in their mean number of seeds, but the difference between the valley species was not significant. Senecio sp. seeds dominated the seed bank of the valley, while Brassica sp. seeds dominated the shallow stream seed bank. The seed banks of both sites were made of herbaceous species. The parent plants of seed banks do not belong to the standing vegetation. We concluded that elevation and slope influenced soil seed banks' spatial distribution, diversity, and plant litter.
... Here we tested the effects of potential anthropogenic stressors -roads and The distribution of anurans is conditioned by the adequacy of their morphological characteristics and physiological functions to environmental conditions and, especially, by their dispersion abilities, which may be limited for some groups (Semlitsch, 2008;Oliveira et al., 2016 (Chesson and Huntly, 1997;Roxburgh et al., 2004), favoring the coexistence of a larger number of evolutionary lineages (Yuan et al., 2016). However, anthropogenic effects on functional composition may be minor. ...
Thesis
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The communities present in a given region make up a larger entity: the metacommunitiy. In these large ecological entities, the structuring processes operate on large spatial and even temporal scales, so that no community is seen as an isolated entity. Still, these sets of communities would be united with each other through the dispersion of the organisms that compose them. The Metacommunity Theory postulates that metacommunities would be structured according to four major paradigms associated with different processes: neutral processes; niche-based processes; patch dynamics; mass effects. The distinction between these processes is in the relative importance they attach to environmental and spatial predictors. In the last decades, studies on metacommunities have promoted a great advance in knowledge by incorporating different facets of diversity in the process of evaluating their structure. Thus, in addition to taxonomic diversity (that is, diversity of species and/or taxa), functional (that is, diversity of traits) and phylogenetic diversity (diversity of evolutionary lineages) became part of the theoretical-analytical framework in metacommunities . Therefore, in this thesis I tried to elucidate what were the relative influences of metacommunity processes in anuran communities in the extreme south of Brazil. For this, I evaluated the taxonomic, functional and phylogenetic structure of adult and tadpole metacommunities. My main findings were that the influence of each metacommunity process depends on: (I) the facet of diversity in question; (II) the season; (III) the set of evaluated functional traits.
... local species richness). A low alpha diversity is often associated with a low productivity because of resource shortening and physical stress (Adler, Seabloom, Borer, Hillebrand, Hautier, Hector, Harpole, O'Halloran, et al., 2011;Chesson & Huntly, 1997;Gillman & Wright, 2006). In addition, less productive sites are associated with less competition events and more facilitation relationships compared to more productive sites (Fig 1-8A). ...
Thesis
The absence of a species from a focal community may be due to either dispersal limitation or recruitment limitation. The present work aims to assess species behavior in different types of landscapes. I first investigated the relationship between alpha and gamma diversity at different scales and for different fragmentation levels in forest fragments embedded in an agricultural landscape matrix. Results highlight the importance of considering species identity when studying the effect of fragmentation on the plant community structure. I then assessed the germination success and persistence of vascular plant species (weeds and herbaceous plants), using semi-controlled experiments. In the first experiment, I studied the germination and persistence success of sown weed species, and their subsequent effect on crop yield, under contrasted agricultural practices. We evidenced a sorting effect of agricultural practices on local plant species composition. In the second experiment, I assessed the potential of hedgerows to serve as habitats for forest plant species. We sought seeds and transplanted seedlings in both hedgerows and forest to monitor germination and persistence, respectively, each time with and without resident vegetation removal to further assess the role of competition. Our preliminary results show that few species germinate and survive in hedgerows, suggesting that both dispersal and recruitment limitations are at play. Results from this work emphasize the importance of community assembly processes in ecosystem conservation and maintaining its delivered services
... Coexistence requires negative feedback loops where species demonstrate some degree of specialization on density-dependent factors (Meszéna et al., 2006), but some communities do not have enough density-dependent factors to be specialized upon (Hutchinson, 1959;Hutchinson, 1961; but see Levin, 1970;Haigh and Smith, 1972;Abrams, 1988). On the other hand, species may readily specialize on different environmental states, but environmental variation alone cannot promote coexistence (Chesson and Huntly, 1997). The interaction effect combines the competition parameter with the environmental parameter to get the best of both worlds: the density-dependent factors (implicit in the competition parameter) provide the negative feedback while the species-specific environmental parameter provides the specialization. ...
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The storage effect is a well-known explanation for the coexistence of competing species in temporally varying environments. Like many complex ecological theories, the storage effect is often used as an explanation for observed coexistence on the basis of heuristic understanding, rather than careful application of a detailed model. But, a careful examination of some widely employed heuristic representations of the storage effect shows that they can be misleading. One interpretation of the storage effect states that species coexist by specializing on a small slice of all environmental states, and therefore must have a robust life-stage (e.g., long-lived adults, a seedbank) in order to "wait it out" for a good year. Another more general interpretation states that "buffering" helps species "wait it out", where "buffering" means populations are protected from large losses when the environment is poor and competition is high. Here, we show that both of these conventional interpretations are imperfect. Multiple models show that stage-structure, long lifespans, and overlapping generations are not required for the storage effect. Further, a species that experiences buffering necessarily grows poorly when the environment is favorable and competition is high. We review the empirical literature and conclude that species are likely to find themselves in this good-environment/high-competition scenario, and therefore, that buffering tends to hurt individual species. The buffering interpretation of the storage effect can be thought of as conflating a community-level criterion for coexistence with a population-level criterion for persistence; it is like claiming that species can persist by limiting their own growth rates, since the Lotka-Volterra model tells us that intraspecific competition must be greater than interspecific competition.
Preprint
Plant competition experiments commonly suggest that larger species have an advantage, especially in light acquisition. However, within crowded natural vegetation, where competition evidently impacts fitness, most resident species are relatively small. It remains unclear, therefore, whether the size-advantage observed in controlled experiments is realized in habitats under intensive competition. We tested for evidence of a size-advantage in competition for light in an old-field plant community composed of herbaceous perennial species. We investigated whether larger species contributed to reduced light penetration (i.e., greater shading), and examined the impact of shade on smaller species by testing whether their abundance and richness were lower in plots with less light penetration. Light penetration in plots ranged from 0.3-72.4%. Plots with greater mean species height had significantly lower light penetration. Plots with lower light penetration had significantly lower small species abundance and richness. However, the impact of shade on small species abundance and richness was relatively small (R values between 8% and 15%) and depended on how we defined “small species”. Significant effects were more common when analyses focused on individuals that reached reproduction; focusing on only flowering plants can clarify patterns. Our results confirm that light penetration in herbaceous vegetation can be comparable to levels seen in forests, that plots with taller species cast more shade, and that smaller species are less abundant and diverse in plots where light penetration is low. However, variation in mean plot height explained less than 10% of variation in light penetration, and light penetration explained 5-15% of variation in small species abundance and richness. Coupled with the fact that reproductive small species were present even within the most heavily shaded plots, our results suggest that any advantage in light competition by large species is limited. One explanation is that some small species in these communities are shade tolerant.
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To illuminate mechanisms supporting diversity in plant communities, we construct two-dimensional cellular automata and ‘grow’ virtual plants in real experiments. The plants are nineteen different, fully validated functional types drawn from universal adaptive strategy theory. The scale of approach is far beyond that of even the most ambitious investigations in the physical world. By simulating 496 billion plant-environment interactions, we succeed in creating conditions that sustain high diversity realistically and indefinitely. Our simulations manipulate the levels of, and degree of heterogeneity in the supply of, resources, external disturbances, and invading propagules. We fail to reproduce this outcome when we adopt the assumptions of unified neutral theory. The nineteen functional types in our experiments respond in complete accordance with universal adaptive strategy theory. We find that spatial heterogeneity is a strong contributor to long-term diversity, but temporal heterogeneity is less so. The strongest support of all comes when an incursion of propagules is simulated. We enter caveats and suggest further directions for working with cellular automata in plant science. We conclude that although (i) the differentiation of plant life into distinct functional types, (ii) the presence of environmental heterogeneity, and (iii) the opportunity for invasion by propagules can all individually promote plant biodiversity, all three appear to be necessary simultaneously for its long-term maintenance. Though further, and possibly more complex, sets of processes could additionally be involved, we consider it unlikely that any set of conditions more minimal than those described here would be sufficient to deliver the same outcome.
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One of the key questions in ecology is to understand the mechanisms that modulate the coexistence between syntopic species. Climate and habitat perturbation gradients have been proposed as moderators of species coexistence. The first is related to time availability and the diversity of food resources. The second is related to change in habitat structure that promotes changes in plant structure and diversity that impacts the diversity of other organisms. Although there is empirical evidence that supports these mechanisms to explain coexistence, they have not been evaluated quantitatively and on a wide geographic scale. Using phytophagous bat species of the genus Sturnira we evaluated both mechanisms. These bats are morphologically similar and are key organisms for the seed dispersal of Neotropical Forest plants. Using systematic review protocols, we obtained data of the occurrence and relative abundance of syntopic Sturnira species of different Neotropical forests. We used generalized linear models to evaluate the predictor power of Holdridge's life zones and habitat disturbance type to explain the evenness of syntopic Sturnira species. We found a highly Sturnira species evenness across life zones, going from 0.68–0.92, and found life zones with a maximum of eight Sturnira species coexisting in the same type of forest. The life zones and the type of disturbance do not explain the variation of evenness in Sturnira species. These results suggest that there is not a competitive exclusion among Sturnira species in tropical forests, and despite their ecological niche similarity, the environment allows the coexistence among these highly related species.
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Perhaps more than any other ecological discipline, invasion biology has married the practices of basic science and the application of that science. The conceptual frameworks of population regulation, metapopulations, supply-side ecology, and community assembly have all to some degree informed the regulation, management, and prevention of biological invasions. Invasion biology needs to continue to adopt emerging frameworks and paradigms to progress as both a basic and applied science. This need is urgent as the biological invasion problem continues to worsen. The development of metacommunity theory in the last two decades represents a paradigm-shifting approach to community ecology that emphasizes the multi-scale nature of community assembly and biodiversity regulation. Work on metacommunities has demonstrated that even relatively simple processes at local scales are often heavily influenced by regional-scale processes driven primarily by the dispersal of organisms. Often the influence of dispersal interacts with, or even swamps, the influence of local-scale drivers like environmental conditions and species interactions. An emphasis on dispersal and a focus on multi-scale processes enable metacommunity theory to contribute strongly to the advancement of invasion biology. Propagule pressure of invaders has been identified as one of the most important drivers facilitating invasion, so the metacommunity concept, designed to address how dispersal-driven dynamics affect community structure, can directly address many of the central questions of invasion biology. Here we revisit many of the important concepts and paradigms of biological invasions—propagule pressure, biotic resistance, enemy release, functional traits, neonative species, human-assisted transport,—and view those concepts through the lens of metacommunity theory. In doing so, we accomplish several goals. First, we show that work on metacommunities has generated multiple predictions, models, and the tools that can be directly applied to invasion scenarios. Among these predictions is that invasibility of a community should decrease with both local controls on community assembly, and the dispersal rates of native species. Second, we demonstrate that framing biological invasions in metacommunity terms actually unifies several seemingly disparate concepts central to invasion biology. Finally, we recommend several courses of action for the control and management of invasive species that emerge from applying the concepts of metacommunity theory.
Article
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Plant competition experiments commonly suggest that larger species have an advantage, primarily in terms of light acquisition. However, within crowded natural vegetation, where competition evidently impacts fitness, most resident species are relatively small. It remains unclear, therefore, whether the size advantage observed in controlled experiments is normally realized in habitats where competition is most intense. We characterized the light environment and tested for evidence of a size advantage in competition for light in an old-field plant community composed of perennial herbaceous species. We investigated whether larger species contributed to reduced light penetration (i.e., greater shading), and examined the impact of shade on smaller species by testing whether their abundance and richness were lower in plots with less light penetration. Light penetration in plots ranged from 0.3% to 72.4%. Significant effects were more common when analyses focused on small plants that reached reproduction (i.e., flowering rooted units); focusing on only flowering plants (i.e., excluding nonflowering rooted units) can clarify community patterns. Plots with a greater mean species height had significantly lower light penetration, and plots with lower light penetration had significantly lower flowering abundance and richness of small species. However, the impact of shade on the flowering abundance and richness of small species was relatively small (R2 values between 8% and 15%) and depended on how we defined “small species.” Synthesis: Our results confirm that light penetration in herbaceous vegetation can be comparable to levels seen in forests, that plots with taller species cast more shade, and that flowering smaller species are less abundant and diverse in plots where light penetration is low. However, variation in mean plot height explained less than 10% of variation in light penetration, and light penetration explained between 5 and 15% of variation in the flowering abundance and richness of small species. Coupled with the fact that flowering small species were present even within the most heavily shaded plots, our results suggest that any advantage in light competition by large species is limited. One explanation is that at least some small species in these communities are shade-tolerant. Shade tolerance in predominantly herbaceous communities, particularly among small plant species, requires further research.
Article
Tackling antibiotic resistance necessitates deep understanding of how resource competition within and between species modulates the fitness of resistant microbes. Recent advances in ecological coexistence theory offer a powerful framework to probe the mechanisms regulating intra- and interspecific competition, but the significance of this body of theory to the problem of antibiotic resistance has been largely overlooked. In this Perspective, we draw on emerging ecological theory to illustrate how changes in resource niche overlap can be equally important as changes in competitive ability for understanding costs of resistance and the persistence of resistant pathogens in microbial communities. We then show how different temporal patterns of resource and antibiotic supply, alongside trade-offs in competitive ability at high and low resource concentrations, can have diametrically opposing consequences for the coexistence and exclusion of resistant and susceptible strains. These insights highlight numerous opportunities for innovative experimental and theoretical research into the ecological dimensions of antibiotic resistance. This Perspective draws on emerging ecological coexistence theory to illustrate how changes in both competitive ability and niche overlap are critical for understanding the costs of antibiotic resistance and the persistence of pathogens in microbial communities.
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Coexistence theories develop rapidly at the ecology forefront suffering from interdisciplinary gaps and a lack of universality. The modern coexistence theory (MCT) was developed to address these deficiencies by formulating the universal conditions for coexistence. However, despite this theory's mechanistic foundation, initially, it has only rarely been used to determine the exact mechanisms that govern the competitive outcome. Recent theoretical developments have made MCT more accessible to experimentalists, but they can be challenging in practice. We propose that a comprehensive understanding of species co-occurrence patterns in nature can be reached by complementing the phenomenological approach with both the mechanistic view of MCT and coexistence experiments of the type that prevailed from the 1970s to the 2010s, which focused on specific mechanisms (designated the "mechanistic approach"). As a first step in this direction, we conducted a systematic review of the literature from 1967 to 2020, covering mechanistic experiments for invasibility-the criterion for species coexistence-and the best-studied classical coexistence mechanisms, namely, resource-ratio, natural enemy partitioning, frequency-dependent exploitation by generalist enemies, and the storage effect. The goals of the review were to evaluate (i) the percentage of the abovementioned mechanistic experiments that satisfy the theoretical criteria (designated "eligible studies"), (ii) the scope of these eligible studies, and (iii) their level of support for the theoretical predictions, and to identify their (iv) overarching implications and (v) research gaps. Through examination of 2,510 publications, the review reveals that almost 50 years after the theoretical formulations of the above four coexistence mechanisms, we still lack sufficient evidence to reveal the prevalence of coexistence and of each of the coexistence mechanisms, and to assess the dependency of the mechanisms on the natural history of the competing organisms. By highlighting, on the one hand, the overarching implications of the mechanistic approach to coexistence, and on the other hand, current research gaps, and by offering ways to bridge these gaps in the future, we seek to bring the mechanistic approach back to life.
Article
Seasonal patterns of wood formation (xylogenesis) remain understudied in mixed pine–oak forests despite their contribution to tree coexistence through temporal niche complementarity. Xylogenesis was assessed in three pine species ( Pinus cembroides , Pinus leiophylla , Pinus engelmannii ) and one oak ( Quercus grisea ) coexisting in a semi-arid Mexican forest. The main xylogenesis phases (production of cambium cells, radial enlargement, cell-wall thickening and maturation) were related to climate data considering 5–15-day temporal windows. In pines, cambium activity maximized from mid-March to April as temperature and evaporation increased, whereas cell radial enlargement peaked from April to May and was constrained by high evaporation and low precipitation. Cell-wall thickening peaked from June to July and in August–September as maximum temperature and vapour pressure deficit (VPD) increased. Maturation of earlywood and latewood tracheids occurred in May–June and June–July, enhanced by high minimum temperatures and VPD in P. engelmannii and P. leiophylla . In oak, cambial onset started in March, constrained by high minimum temperatures, and vessel radial enlargement and radial increment maximized in April as temperatures and evaporation increased, whereas earlywood vessels matured from May to June as VPD increased. Overall, 15-day wet conditions enhanced cell radial enlargement in P . leiophylla and P. engelmannii , whereas early-summer high 15-day temperature and VPD drove cell-wall thickening in P. cembroides . Warm night conditions and high evaporation rates during spring and summer enhanced growth. An earlier growth peak in oak and a higher responsiveness to spring–summer water demand in pines contributed to their coexistence.
Article
Local coexistence of species in large ecosystems is traditionally explained within the broad framework of niche theory. However, its rationale hardly justifies rich biodiversity observed in nearly homogeneous environments. Here we consider a consumer-resource model in which a coarse-graining procedure accounts for a variety of ecological mechanisms and leads to effective spatial effects which favor species coexistence. Herein, we provide conditions for several species to live in an environment with very few resources. In fact, the model displays two different phases depending on whether the number of surviving species is larger or smaller than the number of resources. We obtain conditions whereby a species can successfully colonize a pool of coexisting species. Finally, we analytically compute the distribution of the population sizes of coexisting species. Numerical simulations as well as empirical distributions of population sizes support our analytical findings.
Chapter
Global warming and climate change are the key issues of ongoing environmental crisis and have consequently reached the planetary tipping point. This huge environmental heterogeneity poses an evolutionary challenge for organisms. It is of paramount importance to understand and examine the molecular bases of adaptability as it will play a crucial role in shaping the course of evolution. This review exploits the diversity of adaptive mechanisms and discusses how these microbial systems have benefitted the hype of microbial genetic engineering and environmental biotechnology. Diverse microbial communities have been a key component of carbon, nitrogen, and other biogeochemical cycles. As in nature bacteria have been designed to be adaptable, so, the complexity of the environment influences the course of adaptation, therefore, also accounts for the molecular relationships among the genes involved and between the genes and the phenotypes under selection. Several mechanisms that are responsible for these phenomena are shift in community composition or abundances, mutations within populations, horizontal gene transfer, or recombination events. By modeling microbial population responses to environmental change here this review attempts to emphasize the importance of microbes in the biosphere as the cause and “victim” of climate change.
Article
Climate change alters the environments of all species. Predicting species responses requires understanding how species track environmental change, and how such tracking shapes communities. Growing empirical evidence suggests that how species track phenologically - how an organism shifts the timing of major biological events in response to the environment - is linked to species performance and community structure. Such research tantalizingly suggests a potential framework to predict the winners and losers of climate change, and the future communities we can expect. But developing this framework requires far greater efforts to ground empirical studies of phenological tracking in relevant ecological theory. Here we review the concept of phenological tracking in empirical studies and through the lens of coexistence theory to show why a community-level perspective is critical to accurate predictions with climate change. While much current theory for tracking ignores the importance of a multi-species context, basic community assembly theory predicts that competition will drive variation in tracking and trade-offs with other traits. We highlight how existing community assembly theory can help understand tracking in stationary and non-stationary systems. But major advances in predicting the species- and community-level consequences of climate change will require advances in theoretical and empirical studies. We outline a path forward built on greater efforts to integrate priority effects into modern coexistence theory, improved empirical estimates of multivariate environmental change, and clearly defined estimates of phenological tracking and its underlying environmental cues.
Article
Seeds and seed banks are fundamental components of plant community theory and experimental design in desert ecosystems. Here, we made two simple observations from previous research that used seed sowing as a first step and proposed a simple workflow comprised of 5 steps to facilitate more replicable science and control for potential confounding effects of density that can be introduced between seeds and seedlings. The first observation was that choice of seeding density was relatively arbitrary and based primarily on convenience, i.e. 10 seeds per pot was common in the seeding literature. Second, reporting of the key components needed to calculate seed density in desert experiments, wherein the primary focus was not density, was sparse and limited reproducibility and contrasts between studies for the same species. The 5 steps proposed to resolve these challenges focussed on data-checking in different categories of resources from texts to bibliometrics resources that index peer-reviewed publications and datasets archived in established data repositories. The implications of adopting this process include compiling and providing evidence justifies the choice of a reasonable and representative density for most species with an arid system in a given study and this improves replication science and higher levels of consistency in experimental design. This process will also improve the reporting in the literature that can enable novel scientific syntheses for arid ecosystems.
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Over 40 years ago, seminal papers by Armstrong and McGehee and by Levins showed that temporal fluctuations in resource availability could permit coexistence of two species on a single resource. Such coexistence results from non-linearities or non-additivities in the way resource supply translates into fitness. These reflect trade-offs where one species benefits more than the other during good periods and suffers more (or does less well) than the other during less good periods, be the periods stochastic, unstable population dynamics, or seasonal. Since, coexistence based on fluctuating conditions has been explored under the guises of “grazers” and “diggers,” variance partitioning, relative non-linearity, “opportunists” and “gleaners,” and as the storage effect. Here we focus on two phenotypes, “cream skimmers” and “crumb pickers,” the former having the advantage in richer times and the latter in less rich times. In nature, richer and poorer times, with regular or stochastic appearances, are the norm and occur on many time scales. Fluctuations among richer and poorer times also appear to be the norm in cancer ecosystems. Within tumors, nutrient availability, oxygen, and pH can fluctuate stochastically or periodically, with swings occurring over seconds to minutes to hours. Despite interest in tumor heterogeneity and how it promotes the coexistence of different cancer cell types, the effects of fluctuating resource availability have not been explored for cancer. Here, in the context of pulsed resources, we (1) develop models of foraging consumers who experience pulsed resources to examine four types of trade-offs that can promote coexistence of phenotypes that do relatively better in richer versus in poorer times, (2) establish that conditions in tumors are conducive for this mechanism, (3) propose and empirically explore biomarkers indicative of the two phenotypes (HIF-1, GLUT-1, CA IX, CA XII), and (4) and compare cream skimmer and crumb picker biology and ecology in nature and cancer to provide cross-disciplinary insights into this interesting, and, we argue, likely very common, mechanism of coexistence.
Thesis
One of the most enduring questions in ecology is what accounts for coexistence among trophically similar organisms. Niche differentiation is one answer, but so is neutrality: species can coexist either because of ecological differences or ecological similarities. Despite being diametrically opposite, these two theories can be difficult to separate in nature. Because neutral theory posits that species traits are irrelevant for ecological performance, trait patterns are commonly used in niche inference, but results are mixed. This dissertation argues that widely accepted ideas about trait pattern driven by niche differentiation must be updated in light of recent findings, and takes the first steps in that direction. We contend that the current theory of trait patterning is incomplete, and progress requires exploring patterns across a variety of niche models. It emerges from this exploration that stochastic niche dynamics may result in the spontaneous formation of species clusters, under the qualification that pattern will not form if idiosyncratic regulation mechanisms allow arbitrarily similar species to maintain distinct ecological strategies. We provide a new metric for identifying and quantifying species clusters, which outperforms existing metrics in rejecting neutrality in our pseudodata. Another major theme is that the null hypothesis is critical in inference tests. Process-based null models are superior to statistical null hypotheses based on randomization of data because the latter destroys pattern caused by forces unrelated to niche differentiation. For example, we show that when species can randomly mutate into similar species, clusters may occur even in the absence of niche differentiation. A final key theme is that the effect of niche differentiation on species dynamics and pattern may be more complex than is currently appreciated when stochastic dynamics under immigration is considered. Species richness, lifetimes, and extinction rates in niche-differentiated assemblages may be lower or higher than neutral assemblages, depending on the ratio between regional diversity and the number of niches available. The findings of this dissertation contribute to our theoretical understanding of niche differentiation as an important coexistence mechanism, and to realizing the potential of trait patterns in assessing its prevalence in nature.
Article
Soil seed banks represent reservoirs of diversity in the soil that may increase resilience of communities to global changes. Two global change factors that can dramatically alter the composition and diversity of aboveground communities are nutrient enrichment and increased rainfall. In a full‐factorial nutrient and rainfall addition experiment in an annual Californian grassland, we asked whether shifts in aboveground composition and diversity were reflected in belowground seed banks. Nutrient and rainfall additions increased exotic and decreased native abundances, while rainfall addition increased exotic richness, both in aboveground communities and seed banks. Under nutrient addition, forbs and short‐statured plants were replaced by grasses and tall‐statured species, both above and below ground, and whole‐community responses to the treatments were similar. Structural equation models indicated that especially nutrient addition effects on seed banks were largely indirect via aboveground communities. However, rainfall addition also had a direct negative effect on native species richness and abundance of species with high specific leaf area (SLA) in seed banks, showing that seed banks are sensitive to the direct effects of temporary increases in rainfall. Our findings highlight the vulnerability of seed banks in annual, resource‐poor grasslands to shifts in compositional and trait changes in aboveground communities and show how invasion of exotics and depletion of natives are critical for these above‐belowground compositional shifts. Our findings suggest that seed banks have limited potential to buffer resource‐poor annual grasslands from the community changes caused by resource enrichment.
Article
Recent developments in understanding and predicting species responses to climate change have emphasised the importance of both environmental variability and consideration of the wider biotic community. However, to date, the interaction between the two has received less attention. Both theoretical and empirical results suggest that the combined effect of environmental variability and interspecific interactions can have strong impacts on existing range limits. Here we explore how competitive interactions and temporal variability can interact with the potential to strongly influence range shift dynamics. We highlight the need to understand these between‐process interactions in order to predict how species will respond to global change. In particular, future research will need to move from evaluating possibilities to quantifying their impact. We emphasise the value and utility of empirically parameterised models to determine the direction and relative importance of these forces in natural systems.
Article
Generalist predators whose primary prey undergoes cyclic fluctuations, will predate on alternate food sources when the abundance of their primary prey is low. In this paper we have developed a general model of a predator that switches predation between its primary prey and two alternative, competing, prey species. When the predators primary prey is at high abundance, predation of the alternate, competing, prey species is low, which provides periods of temporal refuge for the alternate prey from predation. When the inter‐specific interactions between the competing prey species lead to different dynamical outcomes in the presence and absence of predation, increasing the duration of the temporal refuge promotes dominance of a competitively superior species that is vulnerable to predation. The general theoretical framework was extended to consider a key case study system of pine marten predation on red and grey squirrels. In the absence of predation, grey squirrels out‐compete red squirrels but preferential predation by pine marten on grey squirrels can suppress grey squirrel density and allow red squirrel recovery. A temporal refuge for both squirrel species can arise due to prey switching by pine marten in years when field voles, their primary prey in the UK, are abundant. The duration of the temporal refuge, quantified as the relative length of the multi‐annual vole population cycle where vole density is above a population threshold, is a critical factor determining the persistence of red and grey squirrels. Our findings therefore provide insights for the conservation of the endangered red squirrel in the UK and the Republic of Ireland and more generally on the influence of the population dynamics of primary prey species in determining community composition.
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As a partial explanation for the maintenance of high tree diversity in wet tropical forests, Janzen (1970) and Connell (1971) independently hypothesized that natural enemies act to increase spacing within these tree populations through disproportionately high attack on progeny near adults. A minimum critical distance effect occurs because of 100% progeny mortality within a given distance of adults. Data describing the spacing dynamics of Dipteryx panamensis support both hypotheses. From 7 mo to 2 yr postgermination, seedling survival was positively correlated with distance to adult and negatively correlated with local conspecific seedling density. Partial correlation was used to separate the effects of density and distance. Seedling density was the only significant factor in this case. No seedlings or juveniles survived within 8 m of an adult bole. A review of 24 data sets on tropical woody plants showed that most evidence indicates either density-dependence or distance-dependence in progeny mortality. Some positive evidence also exists for the minimum critical distance effect for tropical trees. -from Authors
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Field experiments demonstrate that the herbivorous marine snail Littorina littorea controls the abundance and type of algae in high intertidal tide pools in New England. Here the highest species diversity of algae occurs at intermediate Littorina densities. This unimodal relationship between algal species diversity and herbivore density occurs because the snail's preferred food is competitively dominant in tide pool habitats. Moderate grazing allows inferior algal species to persist and intense grazing eliminates most individuals and species. In contrast to pools, on emergent substrata where the preferred food is competitively inferior, this herbivore decreases algal diversity. Thus, the effect of this consumer on plant species diversity depends on the relationship between herbivore food preference and competitive abilities of the plants. These results may apply to most generalized consumers and provide a framework within which previously confusing results can be understood. Thus predators or herbivores d...
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trophic level, explanations for coexistence are usually framed in terms of competition by asking what properties of the species, and of their environment, prevent competition from excluding some members of the assemblage. Predation, impinging on a group of (presumably) competing species, has been implicated as a potentially important factor which might allow coexistence. Observations of such "predator-mediated coexistence" are by now commonplace, including Darwin (1859) on mowing of grassland plants, Summerhayes (1941) on voles and grassland plants, Paine (1966, 1971) and Dayton (1971) on starfish and intertidal invertebrates, Harper (1969) on grazers and plants, Slobodkin (1964) on harvesting of laboratory hydrids, Neill (1972) on fish and laboratory zooplankton, and Porter (1972) on starfish and corals. Brooks and Dodson (1965), Wells (1970), and Hall et al. (1970) showed that fish predation could mediate zooplankton coexistence, although it is not certain that the interaction being affected is purely competitive (Dodson 1974). Finally, predation has also been shown to contribute to the coexistence of different genetic morphs within single species, e.g., zooplankton (Zaret 1972) and moths (Kettlewell 1955; Lees and Creed 1975). There are also studies that have failed to demonstrate predator-mediated coexistence or actually have shown a decrease in the number of coexisting species under the impact of predation (e.g., Harper 1969; Paine and Vadas 1969; Hurlbert et al. 1972; Adicott 1974; Janzen 1976). These studies are particularly useful as tests of any theory devised to explain predator-mediated coexistence. Not only must such a theory be able to generate the observed positive effect of predation on coexistence, but it should also explain the conditions under which the effect is not seen or is negative. The empirical observations of predator-mediated coexistence have led naturally to the suggestion that it may play a major role in structuring communities (Paine 1966; Janzen 1970; Connell 1970) and thus to attempts to incorporate it into the mathematical framework of population biology. These attempts to date have been frustrating. While the possibility of predatormediated coexistence has been demonstrated, most analyses make its occur
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A high number of tree species, low density of adults of each species, and long distances between conspecific adults are characteristic of many low-land tropical forest habitats. I propose that these three traits, in large part, are the result of the action of predators on seeds and seedlings. A model is presented that allows detailed examination of the effect of different predators, dispersal agents, seed-crop sizes, etc. on these three traits. In short, any event that increases the efficiency of the predators at eating seeds and seedlings of a given tree species may lead to a reduction in population density of the adults of that species and/or to increased distance between new adults and their parents. Either event will lead to more space in the habitat for other species of trees, and therefore higher total number of tree species, provided seed sources are available over evolutionary time. As one moves from the wet lowland tropics to the dry tropics or temperate zones, the seed and seedling predators in a habitat are hypothesized to be progressively less efficient at keeping one or a few tree species from monopolizing the habitat through competitive superiority. This lowered efficiency of the predators is brought about by the increased severity and unpredictability of the physical environment, which in turn leads to regular or erratic escape of large seed or seedling cohorts from the predators.
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We define 'enemy free space' as ways of living that reduce or eliminate a species' vulnerability to one or more species of natural enemies. Many aspects of species' niches, in ecological and evolutionary time have apparently been moulded by interactions with natural enemies for enemy free space. We review a large number of examples. Yet many ecologists continue to think and write as though classical resource based competition for food or space is the primary determinant of species' niches. Often it is not. The recognition that the struggle for enemy free space is an important component of many species' ecologies may have important consequences for studies of community convergence, limits to species packing, and the ratio of predator species to prey species in natural communities. © 1984.
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In deterministic approaches to modeling, two species are generally regarded as capable of coexistence if the model has a stable equilibrium with both species in positive numbers. Temporal environmental variability is assumed to reduce the likelihood of coexistence by keeping species abundances away from equilibrium. Here we present a contrasting view based on a model of competition for space among coral reef fishes, or any similarly territorial animals. The model has no stable equilibrium point with both species in positive abundance, yet both species persist in the system provided environmental variability in birth rates is sufficiently high. In general the higher the environmental variability the more likely it is that coexistence will occur. This conclusion is not affected by one species having a mean advantage over the other. Not all kinds of environmental variability necessarily lead to coexistence, however, for when the death rates of the two species are highly variable and negatively correlated, the extinction of one species, determined by chance, is likely to occur. The results in this paper are shown to depend on the nonlinearity of the dynamics of the system. This nonlinearity arises from the simple fact that the animals have overlapping generations. When applied to the coral reef fish setting, our analysis confirms the view that coexistence can occur in a system where space is allocated largely at random, provided environmental variability is sufficiently great (Sale 1977); but our explanations and predictions differ in detail with those of Sale.
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Many explanations for diversity patterns have been proposed, and there have been several recent reviews of the subject (Pianka 1966, 1974; Ricklefs 1973; Pielou 1975). High diversity has been attributed both to intense competition which forces niche restriction (Dobzhansky 1950; MacArthur and Wilson 1967)and reduced competition resulting from predation (Paine 1966; Harper 1969; Janzen 1970; Connell 1975). Diversity has been positively correlated with productivity (Connell and Orias 1964; Pianka 1966; MacArthur 1969) and negatively correlated with productivity (Yount 1956; Margalef 1969). The question is far from settled. This paper develops an approach to the problem of species diversity based on the nonequilibrium interactions of competing populations. Under nonequilibrium conditions, differences in diversity are strongly influenced by variations in the rates of competitive displacement between communities, and such factors as relative competitive abilities, niche partitioning, etc., may not be particularly important. This approach deals primarily with the maintenance of diversity, as opposed to the generation of diversity. While most of the current diversity hypotheses have some
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Patterns of tree abundance and dispersion in a tropical deciduous (dry) forest are summarized. The generalization that tropical trees have spaced adults did not hold. All species were either clumped or randomly dispersed, with rare species more clumped than common species. Breeding system was unrelated to species abundance or dispersion, but clumping was related to mode of seed dispersal. Juvenile densities decreased approximately exponentially away from adults. Rare species gave evidence of poor reproductive performance compared with their performance when common in nearby forests. Patterns of relative species abundance in the dry forest are compared with patterns in other forests, and are explained by a simple stochastic model based on random-walk immigration and extinction set in motion by periodic community disturbance.
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@'Switching@' in predators which attack several prey species potentially can stabilize the numbers in prey populations. In switching, the number of attacks upon a species is disproportionately large when the species is abundant relative to other prey, and disproportionately small when the species is relatively rare. The null case for two prey species can be written: P"1/P"2 = cN"1/N"2, where P"1/P"2 is the ratio of the two prey expected in the diet, N"1/N"2 is the ratio given and c is a proportionality constant. Predators were sea-shore snails and prey were mussels and barnacles. Experiments in the laboratory modelled aspects of various natural situations. When the predator had a strong preference (c) between prey the data and the @'null case@' model were in good agreement. Preference could not altered by subjecting predators to training regimens. When preference was weak the data did not fit the model replicates were variable. Predators could be trained easily to one or other prey species. From a number of experiments it was concluded that in the weak-preference case no switch would occur in nature except where there is an opportunity for predators to become trained to the abundant species. A patchy distribution of the abundant prey could provide this opportunity. Given one prey species, snails caused a decreasing percentage mortality as prey numbers increased. This occurred also with 2 prey species present when preference was strong. When preference was weak the form of the response was unclear. When switching occurred the percentage prey mortality increased with prey density, giving potentially stabilizing mortality. The consequences of these conclusions for prey population regulation and for diversity are discussed.
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One general hypothesis to explain how forest tree diversity is maintained is that rarer species are favored over commoner species in their reproduction, growth, and/or mortality. Mechanisms acting in this way would continually compensate for the tendency of some species to increase at the expense of others, and would reduce the chance of local extinction of rare species. Two hypotheses concerning such compensatory mechanisms were tested in subtropical and tropical evergreen rain forests in Queensland, Australia. Hypothesis 1: on the scale of 1-2 ha, commoner species have lower rates of recruitment and growth and higher rates of mortality than do rarer ones. This hypothesis was tested using abundances either of adults or of members of the same size-class, and was rejected for growth and mortality and for recruitment of over-story species, but not rejected for recruitment in subcanopy and understory species in either forest. Hypothesis 2: the close proximity of other individuals is more deleterious (i.e., causes slower growth or higher mortality) if they are the same species than if they are different species. This hypothesis was accepted for growth or survival of nearest neighbors in several of the seedling size-classes in both forests. In contrast, increased densities of the same species in quadrats had no more deleterious effect on growth and survival than did increased densities of different species. At the scale of proximity to adults, hypothesis 2 predicts that young trees have higher mortality nearer conspecific adults than farther away. In both forests, 90% of the species tested showed no such pattern of mortality of seedlings or saplings, nor was the strength or direction of the deviation from equal mortality correlated with the abundance of adults of that species. Field experiments gave the same results. In summary, tests of both hypotheses showed that some compensatory trends occurred and that these were very similar in the two forests. The mechanisms producing these compensatory trends may be attacks by natural enemies (grazers, pathogenic fungi, etc.), interference, or, less likely, competition for resources.
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It is argued that the problem of pattern and scale is the central problem in ecology, unifying population biology and ecosystems science, and marrying basic and applied ecology. Applied challenges, such as the prediction of the ecological causes and consequences of global climate change, require the interfacing of phenomena that occur on very different scales of space, time, and ecological organization. Furthermore, there is no single natural scale at which ecological phenomena should be studied; systems generally show characteristic variability on a range of spatial, temporal, and organizational scales. The observer imposes a perceptual bias, a filter through which the system is viewed. This has fundamental evolutionary significance, since every organism is an "observer" of the environment, and life history adaptations such as dispersal and dormancy alter the perceptual scales of the species, and the observed variability. It likewise has fundamental significance for our own study of ecological systems, since the patterns that are unique to any range of scales will have unique causes and biological consequences. The key to prediction and understanding lies in the elucidation of mechanisms underlying observed patterns. Typically, these mechanisms operate at different scales than those on which the patterns are observed; in some cases, the patterns must be understood as emerging form the collective behaviors of large ensembles of smaller scale units. In other cases, the pattern is imposed by larger scale constraints. Examination of such phenomena requires the study of how pattern and variability change with the scale of description, and the development of laws for simplification, aggregation, and scaling. Examples are given from the marine and terrestrial literatures.
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This chapter discusses the marine studies of community regulation to evaluate relevant community theory. The conceptual framework of community theory essentially deals with community regulation. The chapter considers a model that proposes variation in community structure depends directly on variation in the effects of abiotic disturbance, competition, and predation; and indirectly on variation in recruitment density and environmental stress. The major goal of community ecology is to determine the causes of spatial and temporal variation in community structure. The components of community structure include species diversity, species composition, relative abundance, trophic complexity, size structure, and spatial structure. Differences in community structure can occur at several scales in space and time. Community patterns can generally vary on spatial scales ranging from centimetres to hundreds of kilometers. On rocky shores, these spatial scales correspond to microspatial variation within site variation and regional and global variation. Community structure can also vary due to physical, physiological stress, and biological factors.
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We present a model of community regulation that incorporates the effects of abiotic disturbance, predation, competition, and recruitment density. We assume that mobile organisms (i.e., consumers) are more strongly affected by environmental stress than are sessile organisms and that food-web complexity decreases with increasing stress. The model makes three predictions under conditions of high recruitment. First, in stressful environments, consumers have no effect because they are absent or inactive, and competition for space is prevented. Both mobile and sessile organisms are regulated directly by environmental stress. Second, in moderate environments, consumers are still ineffective, but sessile organisms are less affected by stress and frequently attain high densities, leading to competition for space. Finally, in benign environments, consumers prevent competition for space unless the prey can escape a predation bottleneck and reach a high abundance. A reduction in recruitment density reduces the import...
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Adversity or A-selection favors conservation of adaptation in severe, but stable and predictable, environments. It is contrasted with r- or exploitation selection and K- or interaction selection. The correlates of A-selection (eg. parthenogenesis, poor migratory ability, long life histories, and low reproductive rates in simple communities in harsh environments) are derived from brief accounts of a genus of tropical log-inhabiting staphylinid beetles and terrestrial invertebrates at high latitudes. The 3 selection types are related to each other and the environment by means of Southwood's habitat templet with its 2 axes, habitat predictability and favorableness. The potential usefulness of the habitat templet when partitioned according to predominant selection process is illustrated by applying it to the evolution of ecological strategies in deserts and the course of decomposition of organic matter.- Author
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It is shown in this paper that no stable equilibrium can be attained in an ecological community in which some r of the components are limited by less than r limiting factors. The limiting factors are thus put forward as those aspects of the niche crucial in the determination of whether species can coexist. For example, consider the following simple food web: Despite the similar positions occupied by the two prey species in this web, it is possible for them to coexist if each is limited by an independent combination of predation and resource limitation, since then two independent factors are serving to limit two species. On the other hand, if two species feed on distinct but superabundant food sources, but are limited by the same single predator, they cannot continue to coexist indefinitely. Thus these two species, although apparently filling distinct ecological niches, cannot survive together. In general, each species will increase if the predator becomes scarce, will decrease where it is abundant, and wi...
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The relationship between persistent, small to moderate levels of random environmental fluctuations and limits to the similarity of competing species is studied. The analytical theory hinges on deriving conditions under which a rare invading species will tend to increase when faced with an array of resident competitors in a fluctuating environment. A general approximation scheme predicts that the effects of low levels of stochasticity will typically be small. The technique is applied explicitly to a class of symmetric, discrete-time stochastic analogs of the Lotka-Volterra equations that incorporate cross-correlation but no autocorrelation. The random environment limits to similarity are always very close to the corresponding constant environment limits. However, stochasticity can either facilitate or hinder invasion. The exact limits to similarity are extremely model-dependent. In addition to the symmetric models, an analytically tractable class of models is presented that incorporates both auto- and cross-correlation and no symmetry assumptions. For all of the models investigated, the analytical theory predicts that small-scale stochasticity does little, if anything, to limit similarity. Extensive Monte Carlo results are presented that confirm the analytical results whenever the dynamics of the discretetime models are biologically reasonable in the sense that trajectories do not exhibit unrealistic crashes. Interestingly, the class of stochastic models that is well behaved in this sense includes models whose deterministic analogs are chaotic. The qualitative conclusion, supported by both the analytical and simulation results, is that for competitive guilds adequately modeled by Lotka-Volterra equations including small to moderate levels of random fluctuations, practical limits to similarity can be obtained by ignoring the stochastic terms and performing a deterministic analysis. The mathematical and biological robustness of this conclusion is discussed.
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The development of mechanistic, predictive ecological theory will entail the explicit inclusion of organismal tradeoffs, of environmental constraints, and of the basic mechanisms of interspecific interaction. This approach was used to address the causes of species dominance and successional dynamics in sandplain vegetation in Minnesota. The major constraints on plants were soil N and disturbance, with N competition being a major force. Nutrients other than N, herbivory and light were of minor importance. As predicted by theory, the superior N competitors were the species that, when growing in long-term monocultures in the field, lowered soil extractable N the most. These species had high root biomass and low tissue N levels. Seven alternative hypotheses of succession, each named after its underlying tradeoff, were tested. The colonization-nutrient competition hypothesis provided the best explanation for the initial dominance (years 0-40) of herbs, whereas the nutrient versus light competition hypothesis best explained the long-term dominance by woody plants. Hypotheses involving transient dynamics caused by differences in maximal growth rates were rejected. -from Author
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The effects of environmental fluctuations on coexistence of competing species can be understood by a new geometric analysis. This analysis shows how a species at low density gains an average growth rate advantage when the environm ent fluctuates and all species have growth rates of the particular geometric form called subadditive. This low density advantage opposes competitive exclusion. Additive growth rates confer no such low density advantage, while superadditive growth rates promote competitive exclusion. Growth-rate geometry can be understood in terms of heterogeneity within populations. Total population growth is divided into different components, such as may be contributed by different lifehistory stages, phenotypes, or subpopulations in different microhabitats. The relevant aspects of such within-population heterogeneity can be displayed as a scatter plot of sensitivities of different components of population growth to environm ental and competitive factors, and can be measured quantitatively as a covariance. A three-factor model aids the conceptual division of population growth into suitable components
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1. Are local ecological communities ever saturated with species? That is, do they ever reach a point where species from the regional pool are unable to invade the local habitat because of exclusion by resident species? 2. We review the theoretical evidence for saturation in various community models and find that non-interactive models predict the absence of saturation as expected, but that interactive models do not uniformly predict saturation. 3. Instead, models where coexistence is based on niche space heterogeneity predict saturation, whereas those where coexistence is based upon spatial heterogeneity yield mixed predictions. 4. Thus, theory says that species interactions are a necessary but not sufficient condition for local saturation in ecological time. 5. We then argue that unsaturated (Type I) assemblages are likely to be ubiquitous in nature and that even saturated (Type II) assemblages may not show hard limits to richness over evolutionary time-scales. 6. If local richness is not often saturated, then regional richness is freed from local constraint, and other limits on regional richness (which, in turn, limit local richness) become important, including phylogenetic diversification over evolutionary time-scales. 7. Our speculations inevitably suggest that the principal direction of control for species richness is from regional to local. If correct, then the key to community structure may lie in extrinsic biogeography rather than in intrinsic local processes, making community ecology a more historical science.
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The influence of density-independent mortality on the coexistence of competing species is discussed. It is shown that equivalent mortality—an increase in mortality which reduces the intrinsic growth rate of all species by an equal proportional amount—does not affect the conditions for coexistence in the n-species Lotka-Volterra model. In this model the per capita growth rate of each species declines linearly with the population size of each competitor. In more general models, which incorporate nonlinearities in competitive interactions, it is shown that equivalent mortality may substantially change the conditions for competitive coexistence. A graphical model of the conditions for invasion shows that equivalent mortality can either reduce or increase the likelihood of coexistence for competing species, depending upon the kind of non-linearity built into the competition model. Both outcomes are illustrated by the Ayala-Gilpin-Ehrenfeld competition model, which incorporates a non-linear term for intraspecific competition.
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We compared the spatial distribution of stem cankers on the canopy tree Ocotea whitei (Lauraceae) in a 20-ha plot on Barro Colorado Island, Panama, with spatial and temporal patterns of mortality in this host over the previous decade. The cankers occur both on adult and juvenile individuals, aothough juveniles are much more likely the adults to show symptoms. Disease incidence is host-density dependent, and both the presence of the disease and host mortality are more likely close to than far from a conspecific adult, which resulted in a net spatial shift of the juvenile population away from conspecific adults through time. Disease incidence is lower than expected among juveniles of O. whitei growing near to adults of the non-susceptible canopy tree Beilschmiedia pendula. The coincidence of spatial patterns of canker incidence and host mortality suggest a role for the disease in regulating host spatial distribution, in agreement with predictions of the Janzen-Connell hypothesis.
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It is shown that the lottery competition model permits coexistence in a stochastic environment, but not in a constant environment. Conditions for coexistence and competitive exclusion are determined. Analysis of these conditions shows that the essential requirements for coexistence are overlapping generations and fluctuating birth rates which ensure that each species has periods when it is increasing. It is found that a species may persist provided only that it is favored sufficiently by the environment during favorable periods independently of the extent to which the other species is favored during its favorable periods. Coexistence is defined in terms of the stochastic boundedness criterion for species persistence. Using the lottery model as an example this criterion is justified and compared with other persistence criteria. Properties of the stationary distribution of population density are determined for an interesting limiting case of the lottery model and these are related to stochastic boundedness. An attempt is then made to relate stochastic boundedness for infinite population models to the behavior of finite population models.
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The interaction between the sympatric, predaceous seastars, Asterias forbesi and A. vulgaris was studied for five years at eight study sites in northern New England. These species range in depth from the low intertidal to at least 50 m and cooccur over a broad geographic range from central Maine to Cape Hatteras. Both overlap greatly in times and intensity of feeding, body size, diet composition and size of prey consumed. Variations occur in these characteristics from site to site but are always positively correlated. Such similarity along resource dimensions is generally taken to indicate that species compete for resources. In this study, interspecific competition does not seem to occur. Though these seastars are generally smaller than their potential size, and food seems in short supply in some subhabitats, food seems unlimited in other subhabitats. Hence, exploitation competition probably occurs sporadically, not chronically, and is probably a weak selective agent. Laboratory experiments suggest that neither intra- nor interspecific aggression occurs between these seastars. Hence, interference competition seems non-existent in this case. Observations of massive mortality from disease and storms, large variations in seastar density, and a patchy food supply suggests that these populations are generally held below carrying capacity by a kaleidoscopic suite of selective agents. Under such conditions resource shortage would be unlikely to exert strong selective pressure. The high overlaps are thus most likely a reflection of the general absence of pressure to subdivide resources rather than an indication of severe competition. In studies of competition, ecological overlaps should be supplemented by other evidence, including experiments before they can be used as indications of competitive pressure.
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Most theories of plant species coexistence assume the presence of diversity maintenance mechanisms, i.e. mechanisms enhancing species richness in a community. We wished to determine whether such a mechanism was operating by establishing a field experiment in the species-rich wooded meadow of Laelatu, western Estonia. Ten to seventeen subordinate species were removed periodically (for 4 years) from 10 permanent plots of 1×1 m (each plot had its specific list of excluded species; 10 plots served as control). Since the removed species were all subordinate ones, very little biomass was removed, but at the same time richness was reduced by 25–33%. If some diversity maintenance mechanism was operating, we would expect that immigration of other subordinate species would restore the original species richness. It was not possible to reject the null hypothesis of an identical immigration rate of new species into manipulated and control plots. The rate of small-scale species turnover was not affected by the removal of subordinate species. Interrelations of five richness characteristics were studied, by comparing empirical correlations among them, with those expected from a null model of random migration of species. The immigration rate of new species appeared to be related to the number of constant species, and immigration/extinction balance related to initial richness, more strongly than predicted by the null model. In the manipulated series these relationships matched the expectation from the null model. While the results generally support the so-called species pool hypothesis (and the carousel model), it seems that species small-scale turnover depends on the richness pattern in the studied grassland. In the case of plots with artificially reduced richness no departures can be detected from the random migration hypothesis.
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A model which incorporates random temporal variation in resource consumption rates is used to investigate the effects that such variation has on the coexistence of competitors. The analysis of the model and several extensions of it suggests that such variation in consumption rates will often allow two or more competitors to coexist while limited by the same resource. For variability to promote coexistence, it is necessary that the time scale of resource population dynamics be fast relative to the time scale of environmental change. Variability is especially likely to promote coexistence if there is a large variance in consumption rates, negative correlation between the consumption rates of different species, and a linear or concave relationship between resource consumption and per capita population growth. Many previous studies which have found coexistence of two or more species on one resource can be interpreted as examples of coexistence due to varying resource consumption rates.
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The idea that different species must have distinct ecologies if they are to coexist has been challenged recently by the claim that some models involving stochastic factors or clumped spatial distributions permit stable coexistence of species that are identical or differ only in competitive ability. However, these models have been misinterpreted; except in rather limited circumstances, they provide further support for the notion that species must be sufficiently ecologically distinct to coexist stably. The possible, limited, exceptions to this rule involve social factors by which individuals of a species discriminate between heterospecifics and conspecifics without there being any true ecological differences between species.
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MacArthur's consumer-resource model is reviewed and new ways of understanding it are presented. Statistical measures of association between the utilization functions of different species are developed to show how coexistence conditions can be expressed in simple and understandable ways without the need to introduce strong symmetry assumptions. It is hoped that this new analysis will encourage both the use of the model in its full form without special simplifying assumptions, and the development of competition models of similar biological richness but different basic assumptions.
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1. There is a limit to the similarity (and hence to the number) of competing species which can coexist. The total number of species is proportional to the total range of the environment divided by the niche breadth of the species. The number is reduced by unequal abundance of resources but increased by adding to the dimensionality of the niche. Niche breadth is increased with increased environmental uncertainty and with decreased productivity. 2. There is a different evolutionary limit, L, to the similarity of two coexisting species such that a) If two species are more similar than L, a third intermediate species will converge toward the nearer of the pair. b) If two species are more different than L, a third intermediate species will diverge from either toward a phenotype intermediate between the two.
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A simple model to elucidate the effect of disturbance on a large number of competitors that compete for space and exhibit a competitive hierarchy is developed. Conditions are derived that determine presence of species, and diversity is calculated as a function of percentage cover. The model is compared to data from coral reefs collected by J. W. Porter (1974, Science 186, 543–545). Using parameter values in the model that allow a fit to Porter's data, the response of an individual species to changes in disturbance becomes quite complex, depending on the position (odd or even) of the species in the competitive hierarchy. For these same parameter values, the system is interactive: the existence of a particular species may effect the presence of another. Different parameter values would lead to a noninteractive system.
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It is suggested that local animal species diversity is related to the number of predators in the system and their efficiency in preventing single species from monopolizing some important, limiting, requisite. In the marine rocky intertidal this requisite usually is space. Where predators capable of preventing monopolies are missing, or are experimentally removed, the systems become less diverse. On a local scale, no relationship between latitude (10⚬ to 49⚬ N.) and diversity was found. On a geographic scale, an increased stability of annual production may lead to an increased capacity for systems to support higher-level carnivores. Hence tropical, or other, ecosystems are more diverse, and are characterized by disproportionately more carnivores.
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A community which would not reach a stable equilibrium may nevertheless persist if there is temporal variation and nonlinear dynamics. A procedure is introduced for taking time averages of the rates of change. Since the average of a nonlinear function is not the function of the average, higher terms such as the variances of resources or covariances among species and environmental factors enter into the coexistence conditions. These measures behave as if they were resources. Therefore the number of consumer species cannot exceed the number of resources plus distinct nonlinearities. The nonlinearities arise from predator saturation, learning, group hunting, multiple nutritional requirements, or seasonally variable feeding rates. It is shown that there is no long term correlation between the abundance of a species and its rates of increase.
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A general model of competition between several species in a variable environment is presented and analyzed using a general method that unifies treatment of different specific models. This method yields broad conclusions that are independent of the details of a model. It is used here to show that mechanisms of coexistence and competitive exclusion are largely restricted to three broad categories. One of these categories includes classical mechanisms that do not depend on fluctuations over time. Another category includes mechanisms which may be referred to collectively as the storage effect. These mechanisms involve species-specific responses to environmental fluctuations, a relationship between fluctuations in competition and fluctuations in the environment, and an interaction between environment and competition. The final category depends on fluctuating competition and nonlinear responses to competition that differ between species. These general results are illustrated with analyses of several specific models, including a Lotka-Volterra model, a model of nonlinear resource consumption, and models of recruitment fluctuations for iteroparous organisms and for annual plants.
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Among ecologists, there has been a growing recognition of the importance of long-term correlations In environmental time series. The family of 1 f -noises - fluctuations defined in terms of the different timescales present - is a useful approach to this problem. White noise and the random walk, the two currently favoured descriptions of environmental fluctuations, lie at extreme ends of this family of processes. Recent analyses of data, results of models, and examination of basic 1 f -noise properties, suggest that pink 1 < f noise, which lies midway between white noise and the random walk, might be the best null model of environment variation. If true, this would have important consequences for the interpretation of ecological time series and for ecological and evolutionary modelling.
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to account for the fact that tropical rain forests are vastly more diverse in species than temperate forests. A particularly popular one in recent years has been the refuge theorylw4. This pos- tulates that during major adverse cli- matic phases of the Pleistocene, populations of plants and animals were divided into subpopulations in refugia, thus permitting allopatric speciation. Reunited by climatic amelioration, the new species re- mained genetically isolated. Repetition of this process by repeated climatic oscillation led to a ‘ species pump’ and hence to the present high diver- sity. As a corollary to the theory, areas of present exceptionally high diversity are thought to indicate the locations of the Pleistocene refugia. Attractive as it is, the theory and its corollary have nevertheless received some criticism. It has not yet been clearly demonstrated that the climatic oscillations in the tropics restrict rain forests in the way required 5-g. Temperate forest popu- lations were also restricted to refu- gia in adverse climatic phases”. Why, therefore, does the species pump concept not apply equally in temperate regions? Some of the areas of present high diversity have been shown to be at-tefacts of collec- tion density ‘ I. Some of the proposed refugia have been shown by palyn- ology not to have borne forest during the last adverse climatic phase’ 2f’ 3. A new paper by Bush
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Since Darwin accepted the Malthusian population theory to solve the demographic problems he thought to be logically connected with the universal operation of natural selection, the numerical processes in both populations and communities were generally supposed to be governed by competition. For interspecific relations this found expression in the 'competitive exclusion principle'. After it was shown that coexistence rather than exclusion of closely related species is the rule, this principle gradually changed into the 'competitive niche shift principle'. Recently the universality of competition has been increasingly questioned, so that other interspecific relationships (especially predation) are revaluated as possibly governing many natural population and inter-population processes.
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It is argued that alternate prey species in the diet of a food-limited generalist predator should reduce each