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The Roles of Harsh and Fluctuating Conditions in the Dynamics of Ecological Communities
Abstract
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.
... between growth on different carbon sources were observed in classical studies of diauxic shifts 23 , 44 and have more recently been associated with growth tradeoffs [24][25][26] and specificity of gene 45 expression 27 . Even static environments with a single carbon source can introduce transitions, 46 such as in batch culture experiments where microbes cease growth after depleting the resource, 47 after which a small amount of culture is diluted into fresh medium, and growth begins again after 48 the lag time. Experimental evolution of yeast in this sort of environment led to the emergence of 49 adaptive mutants that gained fitness advantage in the lag phase, and the magnitude of this 50 advantage was determined by the growth parameters of the previous cycle 28 . ...
... whether the timing and order of fluctuations affects the final results, nor did we examine 55 environments fluctuating between more than two conditions. In all cases, mathematical models 56 offer a promising tool for interpreting how outcomes may be altered by fluctuation-and 57 frequency-dependent effects 46 . (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. ...
Evolution in a static environment, such as a laboratory setting with constant and uniform conditions, often proceeds via large-effect beneficial mutations that may become maladaptive in other environments. Conversely, natural settings require populations to endure fluctuating conditions. A naive assumption is that the fitness of a lineage in a fluctuating environment is the time-average of its fitness over the sequence of static conditions it encounters. However, transitions between conditions may pose entirely new challenges, which could cause deviations from this time-average. To test this, we tracked hundreds of thousands of barcoded yeast lineages evolving in static and fluctuating conditions and subsequently isolated 900 mutants for pooled fitness measurements in 15 environments. We find that fitness in fluctuating environments indeed often deviates from the expectation based on static components, leading to fitness non-additivity. Moreover, closer examination reveals that fitness in one component of a fluctuating environment is often strongly influenced by the previous component. We show that this environmental memory is especially common for mutants with high variance in fitness across tested environments, even if the components of the focal fluctuating environment are excluded from this variance. We employ a simple mathematical model and whole-genome sequencing to propose mechanisms underlying this effect. Our results demonstrate that transitions in fluctuating environments have large impacts on fitness and suggest a framework for interpreting change in unpredictable conditions.
... Recent increases in the availability of animal diet and climate data now allow scientists to examine patterns and ecological drivers in an understudied area of diversity, trophic diversity. In this study, we focus on three longstanding ecological-diversity hypotheses that can be tested using global-scale climate data to explain patterns in trophic richness in mammals: the species-energy hypothesis (Willig et al., 2003), the environmental heterogeneity hypothesis and the environmental harshness hypothesis (Chesson & Huntly, 1997). ...
... The environmental harshness hypothesis also predicts that the environment operates on diversity by influencing niche differentiation and competition (Chesson & Huntly, 1997) and could be assessed using extremes in mean annual temperature and precipitation. Similar to the effects of environmental variability on trophic structure diversity, omnivores may be more tolerant of harsh environments than herbivores and predators. ...
Understanding environmental drivers of species diversity has become increasingly important under climate change. Different trophic groups (predators, omnivores and herbivores) interact with their environments in fundamentally different ways and may therefore be influenced by different environmental drivers. Using random forest models, we identified drivers of terrestrial mammals' total and proportional species richness within trophic groups at a global scale. Precipitation seasonality was the most important predictor of richness for all trophic groups. Richness peaked at intermediate precipitation seasonality, indicating that moderate levels of environmental heterogeneity promote mammal richness. Gross primary production (GPP) was the most important correlate of the relative contribution of each trophic group to total species richness. The strong relationship with GPP demonstrates that basal-level resource availability influences how diversity is structured among trophic groups. Our findings suggest that environmental characteristics that influence resource temporal variability and abundance are important predictors of terrestrial mammal richness at a global scale.
... Interspecific competition between birds in which feeding niches overlap is another factor that can affect winter survival [32,33]. Closely related species of animals often share resources such as space and food to minimize competition and enable the divergence of their ecological niches [34,35]. The two species of wintering buzzards, which employ the same hunting strategy for the same source of food, provide a good example of these processes [36]. ...
... We also expected that species and time spent on different hunting sites, as well as the number of changes in hunting sites and weather conditions, would affect the number of attacks on prey. Knowledge of the differences in hunting techniques and the factors that improve foraging success between these two buzzard species may contribute to a better understanding of how competition is reduced and how niche differentiation strategies develop in morphologically and ecologically similar species [35,38,39]. ...
Birds wintering in the northern Palearctic compensate for substantial energy losses and prepare for a food deficit in winter by adjusting their foraging behavior. Apart from weather conditions, interspecific competition also drives hunting strategies. To describe this phenomenon, we observed the behavior of two sympatrically wintering raptor species: the Common Buzzard and the Rough-legged Buzzard. The study was carried out in east-central Poland during four seasons on a study plot where the densities of both species were high. Interspecific differences were detected in the use of available hunting sites. Rough-legged Buzzards conspicuously avoided using fence posts for scanning the surroundings and spent the most time standing on the ground. Common Buzzards more often used trees for this purpose when the snow cover was thick. Thicker snow cover resulted in fewer attempted attacks on prey in both species and caused Common Buzzards to change their hunting sites less frequently. The study also showed that the more often a bird changed its hunting site, the greater the number of attempted attacks. The outcome is that the ultimate effectiveness of hunting is mediated by the overview of the foraging area from different heights and perspectives, not by the type of hunting site. Snow cover was the most important factor in modifying foraging behavior and possibly intensifying interspecific competition.
... Despite considerable empirical evidence in support of the role that surface fires and other low-intensity disturbances appear to play in maintaining high plant species diversity, the concept of disturbance-mediated, nonequilibrium species coexistence has received sharp criticism on theoretical grounds (Chesson & Huntly, 1997;Fox, 2013; but see Huston, 2014). According to modern coexistence theory (MCT), stabilizing niche differences among species is necessary to explain the maintenance of species diversity over the long term, even when fitness similarity, disturbances, and dispersal limitation are sufficient to maintain short-term species coexistence (Chesson & Huntly, 1997;Fox, 2013;Levine & HilleRisLambers, 2009). ...
... Despite considerable empirical evidence in support of the role that surface fires and other low-intensity disturbances appear to play in maintaining high plant species diversity, the concept of disturbance-mediated, nonequilibrium species coexistence has received sharp criticism on theoretical grounds (Chesson & Huntly, 1997;Fox, 2013; but see Huston, 2014). According to modern coexistence theory (MCT), stabilizing niche differences among species is necessary to explain the maintenance of species diversity over the long term, even when fitness similarity, disturbances, and dispersal limitation are sufficient to maintain short-term species coexistence (Chesson & Huntly, 1997;Fox, 2013;Levine & HilleRisLambers, 2009). Hence, given that numerous long-term fire experiments have shown a positive relationship between fire frequency and plant diversity (Brockway & Lewis, 1997;Glitzenstein et al., 2003Glitzenstein et al., , 2012Palmquist et al., 2014;Tester, 1989), either (1) MCT is inadequate to explain species diversity in these systems, (2) existing long-term studies have not run long enough for species losses to become apparent, or ...
Temperate savannas and grasslands maintained by frequent, low‐intensity disturbances such as fire contain among the most species‐rich plant communities in the world. Precisely how these disturbances maintain such high fine‐scale diversity is poorly understood. This study examined the effects of the frequency of simulated fire (clipping combined with litter removal) and the relative importance of recruitment and survival on species diversity and trait and species composition at each of two pine savannas in southeastern Mississippi (USA) that had not been recently burned. Ten 2 × 2 m plots at each site were clipped/cleared annually from 2014 to 2019 and again in spring 2021 (annual frequency). The other 10 clipping plots were not clipped from 2018 to 2020 (reduced frequency). Vegetation in small subplots in annual frequency and reduced frequency plots was compared in August 2021 to test the effects of a short period without clipping on diversity and composition. To test the relative importance of recruitment and survival on diversity and composition, four 0.25 × 0.25 m quarter plots were established within each of 10 annual frequency plots per site following a clipping treatment in fall 2019 and assigned a 2 × 2 factorial arrangement of transplantation of sods from long‐unburned areas and herbicide application. Reducing the frequency of clipping reduced plant diversity and altered composition at both sites. A comparison of diversity and trait composition responses to transplant and herbicide treatments revealed how recruitment and survival combined to affect species diversity. Partial or complete recovery of diversity following clipping and litter removal at both sites was driven by rapid increases in short‐lived, resilient species that show fire‐stimulated emergence from a seed bank and the persistence of long‐lived species capable of surviving the prolonged period without fire or clipping. Species with reduced resilience and persistence were more likely to be lost in the reduced frequency treatment. Results are consistent with a model of short‐term coexistence of maximum species diversity maintained by the most frequent fire regime fuels will permit.
... 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 Δρ i , and the linear responses precluded the fluctuation-dependent mechanisms, ΔN i and ΔI i . ...
... Using this approach, Chesson (1994) showed that fluctuations are necessary for coexistence in the lottery model and the annual plant model. Crucially, in both of the aforementioned papers (Chesson 1994, Chesson andHuntly 1997), the cancelling of Δρ i is valuable because it tells us how species are not coexisting. ...
How do species coexist? A framework known as modern coexistence theory can ‘measure coexistence' by partitioning invasion growth rates into coexistence mechanisms, terms which correspond to classes of explanations for coexistence. There are several reasonable ways to define coexistence mechanisms, each depending on exactly how a species perturbed to low density (the invader) is compared to other species that remain at their typical densities (the residents). Using conceptual arguments and two case studies, we compare five methods for calculating coexistence mechanisms: 1) scaling factors, the traditional approach which attempts to eliminate the linear effects of regulating factors; 2) the simple comparison, which gives equal weight to all resident species; 3) generation time scaling, a novel method which corrects for intrinsic differences in population‐dynamical speed; 4) β scaling, where resident growth rates are scaled by a measure of relative sensitivity to competition; and 5) the invader–invader comparison, a previously obscure method in which a focal species is compared to itself at high versus low density. We find that the conventional scaling factors can lead to nonsensical results when species have strong and asymmetric niche differences; though scaling factors can be useful in certain theoretical studies, they are not recommended for explaining coexistence in real communities. Invader–invader comparisons are also problematic, as they do not effectively measure specialization or niche differentiation. The universally‐applicable simple comparison often works well, but can give counterintuitive results when species have disparate generation times. The β scaling method often works well in simple models, but faces implementation problems in complex models. We tentatively recommend generation time scaling as the all‐purpose method for calculating coexistence mechanisms.
... Algunas montañas tienen la particularidad de ser volcanes activos, lo que agrega al sistema factores adicionales como el disturbio natural por la deposición de diferentes materiales volcánicos como lava, ceniza o flujo piroclástico (Crisafulli et al., 2015;Dalsgaard et al., 2007;Sergio et al., 2018). Desde el punto de vista antropogénico, es posible distinguir también diferentes tipos de perturbación a lo largo de gradientes altitudinales, los cuales intervienen en la estructuración de los ensamblajes de aves y otros organismos (e.g., agricultura en las zonas bajas y extracción de madera en las zonas altas; Schnell et al., 1974), y se encuentran frecuentemente distribuidos en un mosaico de parches de vegetación en diferentes estadios de sucesión ecológica (Chesson y Huntly, 1997). ...
... El disturbio en combinación con los tipos de vegetación, el efecto de borde y las variables correlacionadas de la altitud forman un mosaico de parches con diferentes tipos de hábitat (polígonos). En este mosaico encontramos vegetación conservada y secundaria, suelos desnudos y perturbación por disturbio volcánico, es decir, encontramos un mosaico con diferentes fases de sucesión ecológica (Chesson y Huntly, 1997). La interacción de los parches en un mosaico sucesional en conjunto con los campos agrícolas y establecimientos humanos conlleva a la coexistencia de especies con preferencia a diferentes tipos de hábitat (Connell, 1978). ...
Deterministic processes are fundamental in structuring the bird assemblages of the Colima Volcanic Complex, an area of high endemism, migration, and taxonomic turnover, but also for its volcanic activity, complex elevational gradient, and vegetation types. Herein we describe the bird assemblages of Colima Volcanic Complex and the ecological and biogeographical mechanisms that structure the species composition, based on incidence data and environmental variables. The results show that volcanic risk, elevational gradient, and cardinal orientation, are the explicative variables of the species richness patterns. A total of 397 species were recorded; 248 (62.5%) of which are residents, 149 (37.5%) are migratory, and 89 (22.4%) are in some category of endemism. There is a high dissimilarity (βSOR = 0.907) caused by a high taxonomic turnover (βSIM = 0.749). The most represented biogeographical affinity is the Mesoamerican domain with 41-47% of the resident species. We conclude that ecological and biogeographical processes structure the regional avifauna, providing a first approach to the understanding of volcanic activity as an active mechanism in the regional biological dynamics.
... Community ecology posits 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], Chesson & Huntly [1997], and Sousa [1979]). 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 & 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 & Connell [2005] and references therein). Finally, multiple studies in community ecology reveal no effect of disturbance on richness (reviewed in Hughes et al., [2007] and Mackey & Currie [2001]), especially in systems not structured by competitive hierarchies (Chesson & Huntly, 1997) or where several of the aforementioned mechanisms counteract each other. ...
... Central to these studies is the notion that environmental fluctuations provide opportunities for temporal niche partitioning among species (e.g. Chesson, 2000;Chesson & Huntly, 1997). Within plant communities, intra-annual (i.e. ...
Intra‐annual (i.e. seasonal) temporal niche partitioning is essential to the maintenance of biodiversity in many plant communities. However, understanding of how climate and global change drivers such as eutrophication influence seasonal niche partitioning in plant assemblages remains limited.
We used early‐season and late‐season compositional data collected from 10 grassland sites around the world to explore relationships between climate variability and intra‐annual species segregation (i.e. seasonal β ‐diversity) and to assess how nutrient enrichment alters seasonal β ‐diversity in plant communities. We then assessed whether changes in seasonal β ‐diversity in response to nutrient enrichment are underpinned by species turnover or nestedness and determined how specific functional groups (i.e. annual forbs, perennial forbs, C3 and C4 graminoids and legumes) respond to eutrophication within and across early and late sampling dates.
We found a positive relationship between intra‐annual temperature variability and seasonal β ‐diversity but observed no relationship between intra‐annual precipitation variability and seasonal β ‐diversity. Nutrient enrichment increased seasonal β ‐diversity and increased turnover of species between early‐ and late‐season communities. Nutrient enrichment reduced the abundance of C4 graminoids and legumes within and across sampling timepoints and eliminated intra‐annual differences in these groups. In contrast, nutrient enrichment resulted in seasonal differences in C3 graminoids, which were not observed in control conditions and increased abundance of C3 graminoids and annual forbs within and across early and late sampling dates.
Synthesis : Our understanding of how grasslands respond to various components of global change is primarily based on studies that document community changes at inter‐annual scales. Using early‐season and late‐season compositional data from 10 grassland sites around the world, we show that nutrient enrichment increases seasonal β ‐diversity and alters intra‐annual dynamics of specific functional groups in unique ways.
... However, these studies tend to overlook an important ecological fact: in nature, species must coexist and compete for the same resources [2,5]. The ecological implications of various metabolic strategies, particularly in environments where resource availability fluctuates over time, remain largely unexplored [3,6,7]. More specifically, we do not understand how likely communities of organisms with different metabolic strategies are to successfully and stably assemble (feasibility and dynamical stability) [8,9,10,11,12], and whether there are any trade-offs or synergies between assembly success and resilience against environmental changes (i.e., structural stability). ...
Microbes adopt a variety of metabolic strategies to consume resources in fluctuating environments, but most work has focused on understanding these strategies in the context of isolated species, rather than diverse natural communities. We systematically measure the feasibility, dynamical and structural stability of multispecies microbial communities adopting different metabolic strategies. Our results reveal key distinctions between the ecological properties of different metabolic strategies, showing that communities containing sequential utilizers are more resilient to resource fluctuations, but are less feasible than co-utilizing communities.
... 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 highly constrained 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 2008;Kuang and Chesson 2010;Schreiber 2021), and to infer the mechanisms of coexistence in real communities (Cáceres 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). ...
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 the 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 shows that many more species can coexist than originally thought. More importantly, it allows empiricists to use realistic models and more data to better infer the mechanisms of coexistence in real communities.
... Periodic antimicrobial treatment facilitates the co-existence of the susceptible and the resistant strain due to temporal niche partitioning, which creates a temporal storage effect (Chesson and Huntly, 1997). This result is in agreement with the within-host findings of Letten et al. (2021). ...
The steep rise of infections caused by bacteria that are resistant to antimicrobial agents threatens global health. However, the association between antimicrobial use and the prevalence of resistance is not straightforward. Therefore, it is necessary to quantify the importance of additional factors that affect this relationship. We theoretically explore how the prevalence of resistance is affected by the combination of three factors: antimicrobial use, bacterial transmission, and fitness cost of resistance. We present a model that combines within-host, between-hosts and between-populations dynamics, built upon the competitive Lotka-Volterra equations. We developed the model in a manner that allows future experimental validation of the findings with single isolates in the laboratory. Each host may carry two strains (susceptible and resistant) that represent the host’s commensal microbiome and are not the target of the antimicrobial treatment. The model simulates a population of hosts who are treated periodically with antibiotics and transmit bacteria to each other. We show that bacterial transmission results in strain co-existence. Transmission disseminates resistant bacteria in the population, increasing the levels of resistance. Counterintuitively, when the cost of resistance is low, high transmission frequencies reduce resistance prevalence. Transmission between host populations leads to more similar resistance levels, increasing the susceptibility of the population with higher antimicrobial use. Overall, our results indicate that the interplay between bacterial transmission and strain fitness affects the prevalence of resistance in a non-linear way. We then place our results within the context of ecological theory, particularly on temporal niche partitioning and metapopulation rescue, and we formulate testable experimental predictions for future research.
... First, bee abundance and richness depend on the level of disturbance as in some findings bee species richness was higher at intermediate level of human disturbance (37) . Second, disturbance followed different succession stage, each succession stage contributes to increase species richness over time (38) . In Europe, for example, anthropogenic disturbance has replaced river flood plains creating early successional habitats used by many bee species (39) . ...
This study analyses the effects of land use in Hymenopterans diversity in Unguja Island, Zanzibar. Hymenopterans were sampled in five forms of land-use. In each study site, four linear transects of 50m long were established, three pan traps and sweep-nets were used to capture Hymenopterans. A total of 734 Hymenopterans consisting of 60 species were sampled. Kruskal Wallis test shows that species richness and diversity differ significantly among different land-use. There was no difference in efficiency between nets and pan traps in assessing Hymenopterans diversity (p > 0.1). Home gardens showed higher species richness than other study sites. There was no significance difference in Hymenopterans species richness and diversity captured by traps. This study concludes that home garden and mixed farming attract more Hymenopterans species than natural forest. The study recommends the conservations of Hymenopterans should be by establishment of home gardens and mixed crops farming practices.
... Originally, modern coexistence theory was developed to better understand the importance of fluctuation-dependent coexistence mechanisms (Chesson, 1994;Chesson and Huntly, 1997). ...
Many potential mechanisms promote species coexistence, but we know little about their relative importance. To compare multiple mechanisms, we modeled a two-trophic planktonic food web based on mechanistic species interactions and empirically measured species traits. We simulated thousands of possible communities under realistic and altered interaction strengths to assess the relative importance of three potential drivers of phytoplankton and zooplankton species richness: resource-mediated coexistence mechanisms, predator-prey interactions, and trait trade-offs. Next, we computed niche and fitness differences of competing zooplankton to obtain a deeper understanding of how these mechanisms determine species richness. We found that predator-prey interactions were the most important driver of phytoplankton and zooplankton species richness and that large zooplankton fitness differences were associated with low species richness, but zooplankton niche differences were not associated with species richness. However, for many communities we could not apply modern coexistence theory to compute niche and fitness differences of zooplankton because of conceptual issues with the invasion growth rates arising from trophic interactions. We therefore need to expand modern coexistence theory to fully investigate multitrophic-level communities.
... So, we predict stronger effects of anthropogenic variables on functional and phylogenetic composition than on taxonomic composition. Indeed, up to a certain level, environmental disturbance allows for the coexistence between dominant competitors and fast colonisers (Chesson and Huntly 1997;Roxburgh et al. 2004), favouring the coexistence of a larger number of evolutionary lineages (Yuan et al. 2016). We also expect patterns of functional dispersion to be associated to different stressors. ...
The loss of species with unique traits and evolutionary histories resulting from anthropogenic land-use changes poses a risk to the maintenance of ecosystems. We investigated the influence of Pinus monocultures and roads on functional, phylogenetic, and taxonomic composition, as well as on functional dispersion of an anuran metacommunity in Brazil. We expected the influence of anthropogenic stressors to (1) vary according to the diversity component, with greater influence on the functional and phylogenetic components than on the taxonomic component; and (2) traits related to habitat exploration to be more influenced by monocultures, while the traits related to dispersal and physiological control to be influenced by roads and monocultures, especially for species with smaller body size and low dispersal potential. We found that, while the distance from ponds to monocultures influences the phylogenetic composition, distance to roads influences functional composition and dispersion. The anthropogenic stressors affect the structure of these communities, even those formed by generalist species in terms of habitat use. The phylogenetic composition shows that the distribution of evolutionary clades in human-modified areas is linked to the preferred habitat of species, while functional composition is related to body size. Furthermore, functional dispersion was higher in areas closer to roads, contrary to our expectations. Our results represent new evidence for the conservation of anurans in modified landscapes reinforcing the importance of including functional and phylogenetic information in conservation-driven studies.
... These patterns of between-year partitioning may suggest coexistence via the storage effect. The storage effect hypothesis posits that competitors may coexist if overlapping generations experience temporal fluctuations in the recruitment of individuals due to species-specific responses to the environment (Chesson, 2000b;Chesson & Huntly, 1997). Yearly fluctuations in environmental conditions may have differentially benefited one species over the other, resulting in patterns of alternating dominance. ...
Biological invasions are usually examined in the context of their impacts on native species. However, few studies have examined the dynamics between invaders when multiple exotic species successfully coexist in a novel environment. Yet, long‐term coexistence of now established exotic species has been observed in North American lady beetle communities. Exotic lady beetles Harmonia axyridis and Coccinella septempunctata were introduced for biological control in agricultural systems and have since become dominant species within these communities. In this study, we investigated coexistence via spatial and temporal niche partitioning among H. axyridis and C. septempunctata using a 31‐year dataset from southwestern Michigan, USA. We found evidence of long‐term coexistence through a combination of small‐scale environmental, habitat, and seasonal mechanisms. Across years, H. axyridis and C. septempunctata experienced patterns of cyclical dominance likely related to yearly variation in temperature and precipitation. Within years, populations of C. septempunctata peaked early in the growing season at 550 degree days, while H. axyridis populations grew in the season until 1250 degree days, and continued to have high activity after this point. Coccinella septempunctata was generally most abundant in herbaceous crops, whereas H. axyridis did not display strong habitat preferences. These findings suggest that within this region H. axyridis has broader habitat and abiotic environmental preferences, while C. septempunctata thrives under more specific ecological conditions. These ecological differences have contributed to the continued coexistence among these two invaders. Understanding mechanisms that allow coexistence of dominant exotic species contributes to native biodiversity conservation management of invaded ecosystems. This article is protected by copyright. All rights reserved.
... In these conditions, competitive exclusion guarantees that only the strain best adapted to the steadystate concentration will be able to survive [13]. In contrast, in fluctuating environments where nutrient concentrations change in time, the existence of satellite niches remains a possibility [18][19][20]. Indeed, in the extreme case of environmental fluctuations, i.e., in boom and bust cycles, where resource concentrations may vary over orders of magnitude, there is ample opportunity for several strains to coexist [21,22]. ...
Recent observations have revealed that closely related strains of the same microbial species can stably coexist in natural and laboratory settings subject to boom and bust dynamics and serial dilutions, respectively. However, the possible mechanisms enabling the coexistence of only a handful of strains, but not more, have thus far remained unknown. Here, using a consumer-resource model of microbial ecosystems, we propose that by differentiating along Monod parameters characterizing microbial growth rates in high and low nutrient conditions, strains can coexist in patterns similar to those observed. In our model, boom and bust environments create satellite niches due to resource concentrations varying in time. These satellite niches can be occupied by closely related strains, thereby enabling their coexistence. We demonstrate that this result is valid even in complex environments consisting of multiple resources and species. In these complex communities, each species partitions resources differently and creates separate sets of satellite niches for their own strains. While there is no theoretical limit to the number of coexisting strains, in our simulations, we always find between 1 and 3 strains coexisting, consistent with known experiments and observations.
... Such topics have been investigated with macro- (Fried-Petersen et al., 2020) and micro-scale (Leary et al., 2012) approaches, and tackled at the population (Gonzalez & Holt, 2002) and community level (Shurin et al., 2010). To date, the idiosyncrasy of how fluctuating conditions affect the coexistence of species in communities has resulted in mixed findings (Adler & Drake, 2008;Chesson & Huntly, 1997;Connell, 1978). Environmental variation has been shown to lower species fitness and diversity due to differential responses to new conditions and increased risk of stochastic extinctions (Adler & Drake, 2008;Rasconi et al., 2015). ...
Temperature is a fundamental driver of species' vital rates and thus coexistence, extinctions and community composition. While temperature is neither static in space nor in time, little work has incorporated spatiotemporal dynamics into community‐level investigations of thermal variation.
We conducted a microcosm experiment using ciliate protozoa to test the effects of temperatures fluctuating synchronously or asynchronously on communities in two‐patch landscapes connected by short or long corridors.
We monitored the abundance of each species for 4 weeks—equivalent to ~28 generations—measuring the effects of four temperature regimes and two corridor lengths on community diversity and composition.
While corridor length significantly altered the trajectory of diversity change in the communities, this did not result in different community structures at the end of the experiment. The type of thermal variation significantly affected both the temporal dynamics of diversity change and final community composition, with synchronous fluctuation causing deterministic extinctions that were consistent across replicates and spatial variation causing the greatest diversity declines.
Our results suggest that the presence and type of thermal variation can play an important role in structuring ecological communities, especially when it interacts with dispersal between habitat patches.
... How species or close genera can coexist together in spite of competition is one of the main puzzles of community ecology, especially for primary producers that seemingly share the same basic resources [168]. Many theoretical studies of competition models have shown that competitive exclusion is likely in those circumstances, unless mechanisms involving spatial or temporal variation are at play [15,16,78,166,211,76]. Neutral theory models, assuming that all individuals have equal birth and death rates and exert equal competitive pressure on conspecifics and heterospecifics alike, produce instead a non-equilibrium coexistence maintained by dispersal from a regional pool. ...
Phytoplankton communities, made of photosynthetic algae, can include up to hundreds of species requiring similar resources. Classical population dynamics models, however, often predict that the number of coexisting species cannot be much larger than the number of resources. Numerous explanations to this "paradox of the plankton" have been proposed, often based on the same hypotheses: interactions are competitive, population dynamics are based on a single life stage, corresponding to the organism floating in the water column (the pelagic stage), and these organisms are distributed homogeneously in space, all species being perfectly mixed in the environment. In this thesis, we build two independent models which enable us to relax these hypotheses.Firstly, we establish a community dynamics model with two life stages, involving a dormant one (a ‘seed’), which has a higher survival probability than the pelagic stage, especially in adverse environmental conditions. In this model, pelagic organisms can move between the ocean and the coast while dormant individuals remain in a coastal seed bank. The structure of interactions is inspired by field data, and comprises facilitation in addition to competition. The presence of a seek bank allows specialist species to survive in the community, and avoid the extinction of all species in harsh environmental conditions. Facilitation does not seem to promote coexistence.In the spatial section of the thesis, we present an individual-based model including hydrodynamic and demographic processes at the microscale. The replication of an existing single-species model in two dimensions allowed us to develop the numerical and analytical methods which serve as a foundation for a three-dimensional, multispecies model. In this model, birth and death events are modeled by a branching process, organisms are displaced by a random walk representing diffusion, and by a simplified model of turbulence. Parameter values are based on phytoplankton characteristics. We show that, for distances between individuals allowing interactions to happen, small organisms (nanophytoplankton) are mostly surrounded by individuals of the same species, which can favour coexistence, while larger species (microphytoplankton) are more mixed, which favours interspecific competition. We then discuss other potential mechanisms that could explain microphytoplankton diversity maintenance.
... Classic examples include species using different resources (Abrams and Rueffler, 2009;Macarthur and Levins, 24 1967; Roughgarden, 1976). However, any trait affecting a population's growth rate can serve as the basis for niche partitioning, including interactions with natural enemies (Abrams and Chen, 2002;Ehrlich et al., 2017;Grover and Holt, 1998;Vandermeer and Maruca, 1998) and responses 27 to environmental change (Armstrong and McGehee, 1976;Chesson, 1994;Chesson and Huntly, 1997;Kremer and Klausmeier, 2017;Loreau, 1992;Pacala and Tilman, 1994). Alternatively, traits that increase species' per capita effects on their competitors should inhibit coexistence. ...
Coevolution of competitors can lead to niche partitioning promoting coexistence or to heightened conflicts promoting competitive exclusion. If both are possible, when should coevolution favor coexistence versus exclusion? We investigated this question with a general eco-evolutionary model in which species can reduce the interspecific competition they experience through evolutionary investments in two types of competitive traits: partitioning traits that promote coexistence and conflict traits that promote exclusion. We found that communities were generally mixed, consisting of species investing in both trait types or mixtures of species specializing in one type. For each species, its competitors' abundances and investments determined its experienced competition, and stronger competition begot greater competitive trait investment. Species investing in conflict traits strengthened competition for other species both directly and indirectly, whereas partitioning traits only weakened competition via direct effects. Conflict traits were therefore the stronger driver of community-wide investments in all traits. However, species investing most in conflict traits experienced less competition, so they ultimately evolved least investment, making them most likely to be excluded by the next invader. Thus, coevolution may provide an open door for species that play nice and a revolving door of exclusion for those that do not.
... With one limiting resource in a constant environment, interspecific competition should result in competitive exclusion [4,5], and there are numerous instances where a competitively superior invader has displaced resident species' [6,7]. However, there is also evidence of competitively inferior residents escaping competitive exclusion via a number of mechanisms, including differential resource use [4,8], habitat segregation [9,10], and trade-offs between competitive ability and environmental tolerances [11,12]. Condition-specific competition, whereby abiotic conditions reduce or reverse the outcome of competition between two species, can facilitate coexistence when there are temporal or spatial changes in conditions [13][14][15]. ...
(1) Background: Condition-specific competition, when the outcome of competition varies with abiotic conditions, can facilitate species coexistence in spatially or temporally variable environments. Discarded vehicle tires degrade to leach contaminants into collected rainwater that provide habitats for competing mosquito species. We tested the hypothesis that more highly degraded tires that contain greater tire leachate alters interspecific mosquito competition to produce a condition-specific advantage for the resident, Culex pipiens, by altering the outcome of competition with the competitively superior invasive Aedes albopictus. (2) Methods: In a competition trial, varying densities of newly hatched Ae. albopictus and Cx. pipiens larvae were added to tires that had been exposed to three different ultraviolet (UV)-B conditions that mimicked full-sun, shade, or no UV-B conditions in the field. We also measured Cx. pipiens and Ae. albopictus oviposition preference among four treatments with varying tire leachate (high and low) and resources (high and low) amounts to determine if adult gravid females avoided habitats with higher tire leachate. (3) Results: We found stronger competitive effects of Cx. pipiens on the population performance and survival of Ae. albopictus in tires exposed to shade and full-sun conditions that had higher concentrations of contaminants. Further, zinc concentration was higher in emergent adults of Ae. albopictus than Cx. pipiens. Oviposition by these species was similar between tire leachate treatments but not by resource amount. (4) Conclusions: These results suggest that degraded tires with higher tire leachate may promote condition-specific competition by reducing the competitive advantage of invasive Ae. albopictus over resident Cx. pipiens and, combined with Cx. pipiens’ preferential oviposition in higher resource sites, contribute to the persistence of the resident species.
... Reduced access to resources such as prey and water during periods of drought could lead to increased competition among predator species. However, it is unclear how disturbances such as drought affect competition within a guild; competition could be greater under drought-induced resource shortage, but alternatively, such resource shortages could result in competitive release when more dominant species decline in numbers (Chesson & Huntly, 1997;McCluney et al., 2012;Prugh et al., 2018;Tilman, 1982). ...
Mesopredators in western North America are facing major changes to their ecosystems, including drought and the expansion of human disturbance. To balance resource needs and risk-taking on the landscape, mesopredators are likely to shift their habitat use and interspecies interactions. As part of a large-scale study to help evaluate responses of terrestrial wildlife to severe drought, the California Department of Fish and Wildlife surveyed mesopredator presence across 585 sites in the Mojave Desert (MD) and Central Valley (CV) ecoregions of California. This study spanned a drought year (2016) and a post-drought year (2017), providing the opportunity to investigate how drought and interspecific interactions may mediate spatial patterns of mesopredator occurrence across a continuum of human disturbance levels. We used single-season, single-species, and conditional two-species occupancy models to elucidate these relationships in both ecoregions. We examined the occupancy and detection of coyotes (Canis latrans) and smaller mesopredators, including bobcats (Lynx rufus) in both ecoregions, raccoons (Procyon lotor) in the CV, and desert kit foxes in the MD (Vulpes macrotis arsipus). The presence of coyotes influenced the detection probability of all other mesopredator species , and the influence of drought varied by species and ecoregion. Detection of mesopredators in the CV was typically higher in 2016, especially at low disturbance sites, suggesting species may have become more active during the drought to meet resource needs. However, detection of mesopredators in the MD tended to increase after the drought, suggesting a response to an increase in resources (e.g., prey). Coyotes in the MD became more detectable in high human disturbance in 2016 and less detectable in 2017, possibly increasing activity during the drought in human-disturbed areas to obtain anthropogenic resources. Drought not only affects individual species and their relationships to human disturbance, but it can also influence their interspecies interactions and use of different landscape features.
... However, species may coexist if they occupy different habitats (spatial niche partitioning; Macarthur, 1958) or are favored by the physical environment at different times (temporal niche partitioning; Hutchinson, 1961). Species responses to spatial variation has received substantially more research attention than responses to temporal variation, despite the recognition that species can use different resources and be active at different times of the year (Chesson and Huntly, 1997;Shuter et al., 2012;McMeans et al., 2015). ...
Seasonality could play a crucial role in structuring species interactions. For example, many ectotherms alter their activity, habitat, and diet in response to seasonal temperature variation. Species also vary widely in physiological traits, like thermal preference, which may mediate their response to seasonal variation. How behavioral responses to seasonality differ between competing species and alter their overlap along multiple niche axes in space and time, remains understudied. Here, we used bulk carbon and nitrogen stable isotopes combined with stomach content analysis to determine the seasonal diet overlap between a native cold-water species [lake trout (Salvelinus namaycush)] and a range-expanding warm-water species [smallmouth bass (Micropterus dolomieu)] in two north-temperate lakes over 2 years. We coupled these analyses with fine-scale acoustic telemetry from one of the lakes to determine seasonal overlap in habitat use and activity levels. We found that dietary niche overlap was higher in the spring, when both species were active and using more littoral resources, compared to the summer, when the cold-water lake trout increased their reliance on pelagic resources. Telemetry data revealed that activity rates diverged in the winter, when lake trout remained active, but the warm-water smallmouth bass reduced their activity. Combining stable isotopes and stomach contents with acoustic telemetry was a powerful approach for demonstrating that species interactions are temporally and spatially dynamic. In our case, the study species diverged in their diet, habitat, and activity more strongly during certain times of the year than others, in ways that were consistent with their thermal preferences. Despite large differences in thermal preference, however, there were times of year when both species were active and sharing a common habitat and prey source (i.e., resource overlap was greater in spring than summer). Based on our findings, important ecological processes are occurring during all seasons, which would be missed by summer sampling alone. Our study stresses that quantifying multiple niche axes in both space and time is important for understanding the possible outcomes of altered seasonal conditions, including shorter winters, already arising under a changing climate.
... The interaction effect breaks this theoretical limit to coexistence by combining environment and competition into a large number of effective regulating factors. The species-specific environmental responses explain the preponderance of these factors, whereas competition provides the density-dependence inherent in a regulating factor (environmental niche differentiation alone is incapable of promoting coexistence; Chesson & Huntly, 1997). ...
The storage effect is a general explanation for coexistence in a variable environment. Unfortunately, the storage effect is poorly understood, in part because the generality of the storage effect precludes an interpretation that is simultaneously simple, intuitive and correct. Here, we explicate the storage effect by dividing one of its key conditions—covariance between environment and competition—into two pieces, namely that there must be a strong causal relationship between environment and competition, and that the effects of the environment do not change too quickly. This finer‐grained definition can explain a number of previous results, including (1) that the storage effect promotes annual plant coexistence when the germination rate fluctuates, but not when the seed yield fluctuates, (2) that the storage effect is more likely to be induced by resource competition than the apparent competition, and (3) why the storage effect arises readily in models with either stage structure or environmental autocorrelation. Additionally, our expanded definition suggests two novel mechanisms by which the temporal storage effect can arise—transgenerational plasticity and causal chains of environmental variables—thus suggesting that the storage effect is a more common phenomenon than previously thought.
... 1 | INTRODUCTION Animals are exposed to various physiological challenges throughout the year which are caused by seasonal changes in environmental conditions (Chesson & Huntly, 1997;Ruf et al., 2006;Varpe, 2017). These challenges can be extrinsic, such as changes in resource availability (Chapman et al., 2015;Ruf et al., 2006) or seasonally occurring weather events (e.g., droughts and cyclones) (Fardi et al., 2018;Sapolsky, 1986). ...
Throughout the year, wild animals are exposed to a variety of challenges such as changing environmental conditions and reproductive activity. These challenges may affect their stress hormone levels for varying durations and in varying intensities and impacts. Measurements of the glucocorticoid hormone cortisol in the hair of mammals are considered a good biomarker for measuring physiological stress and are increasingly used to evaluate stress hormone levels of wild animals. Here, we examined the influence of season, reproductive activity, sex, as well as body condition on hair cortisol concentrations (HCC) in Lepilemur edwardsi, a small Malagasy primate species. L. edwardsi lives in the seasonal dry forests of western Madagascar, which are characterized by a strongly changing resource availability throughout the year. We hypothesized that these seasonal changes of resource availability and additionally the reproductive cycle of this species would influence HCC of L. edwardsi. Results revealed that hair cortisol concentration of females did not change seasonally or with the reproductive cycle. However, we found a significant increase of hair cortisol levels in males from the early wet season during the early dry season (mating season). This increase is presumably due to changed behavior during the mating season, as sportive lemurs travel more and show aggressive behavior during this time of the year. This behavior is energy-costly and stressful, and presumably leads to elevated HCC. As elevated cortisol levels may impair immune function, L. edwardsi males might also be more susceptible to parasites and diseases, which is unfavorable in particular during a period of low resource availability (dry season).
... The effect of the season was comparably important for β-diversity as was the spatial distance (Fig. 3). Therefore, seasonally fluctuating environmental conditions favor niche partitioning based on species' thermal constraints (Albrecht and Gotelli, 2001;Chesson and Huntly, 1997). Indeed, many dung beetle species (including red-listed ones) are largely seasonal, such as Euorodalus coenosus peaking in spring, Onthophagus illyricus or O. furcatus peaking in summer, Sigorus porcus peaking in autumn, or Melinopterus consputus having two peaks in early spring and late autumn. ...
The conversion and degradation of natural habitats has caused global declines in terrestrial insect diversity. Conserving insect diversity has therefore gained increasing attention in international conservation agendas. Dung beetles (Geotrupidae, Scarabaeidae) are diverse and abundant coprophagous insects who remove dung and enhance nutrient cycling. Today, half of Central European dung beetle species are threatened. While past research has concentrated on local (α-diversity) of dung beetles, the compositional differences between sites (β-diversity) have been largely neglected. However, maximizing the overall outcome of conservation measures requires a detailed understanding of the factors determining compositional differences between sites. We separated β-diversity of dung beetle communities, sampled across the Czech Republic, into independent components of spatial turnover and nestedness. We tested the relative importance of space, temperature, precipitation , elevation, season, grazing herbivore and antiparasitic treatment via multiple regressions for distance matrices to reveal which factors drive the β-diversity of dung beetle communities. Our results show that β-diversity of dung beetle communities is mostly driven by spatial distance, followed by similarly strong effects of season and mean temperature. Herbivore type and antiparasitic treatment had lesser influence than environment but can be influenced by conservation management. Nevertheless, antiparasitic treatment increased nestedness and resulted in an overall lower γ-diversity. Based on the principle of complementarity, we recommend-in order of decreasing importance-giving (1) conservation priority to the most distant sites, under (2) the largest differences in mean annual temperature, ensure (3) year-round grazing by (4) multiple herbivore species, and (5) avoid using antiparasitics.
... In allowing coexistence, harshness creates the potential for co-evolutionary adjustments such as character displacement among the coexisting species. Other density-dependent limitations also affect coexistence, for example, responses to predation and to the physical environment, and coexistence may be facilitated by interactions between these factors under environmental fluctuations (Chesson and Huntly 1997). ...
p>The aim of this thesis was to investigate ecological conditions under which the genetic variation inherent to sexual reproduction may confer immediate competitive advantages over asexual reproduction. The model species for these tests were the freshwater crustacean Daphnia , with seasonally sexual and obligately parthenogenetic forms, and the littoral enchytraeid worm Lumbricillus lineatus , which has sexual diploid and pseudogamous triploid reproductive systems.
Laboratory experiments with Daphnia pulex during their parthenogenetic phase set genetically varied and genetically uniform populations in competition for an impure food mix, simulating sexual-asexual coexistence. Genetically varied populations had significantly higher birth rates in competition with genetically uniform populations than in competition with themselves. This small competitive release was predicted to be sufficient for immediate coexistence of sexual and asexual populations of Daphnia pulex . Further laboratory experiments instigated invasions by small and large groups of genetically varied Daphnia obtusa into large genetically uniform populations of the same species, and vice versa. Genetically varied invaders of initially large group size increased their representation by more than those of initially small size; in contrast, genetically uniform invaders of large group size diminished on average by more than those of small size, supporting the hypothesis that larger genetically varied groups, with greater genetic variation, had greater competitive advantage than smaller groups with less genetic variation.
The population dynamics of the pseudogamous Lumbricillus lineatus system were investigated by studying life-history characteristics at different starting ratios of triploids to diploids. At lower temperatures and higher food quality, reproductive outputs of triploids and diploids were each inversely proportional to the abundance of the other. This dynamic appeared to favour eventual reversion to diploid-only populations as a result either of diploids out-performing triploids, or of triploids out-performing diploids and crashing in the absence of diploid sperm.</p
... In addition to flaws in the theoretical underpinnings of the IDH, empirical studies have shown that relationships between species diversity and natural disturbance are often not hump-shaped but can have various shapes and strengths (Mackey & Currie, 2001;Bongers et al., 2009). It has been hypothesised that disturbance intensity and frequency may have negligible effects on species diversity in communities where succession is driven by ecological processes other than competition (Chesson & Huntly, 1997). The diversitydisturbance relationship may differ depending on the organismal group, with hump-shaped responses for primary producers or sessile organisms, but non-hump-shaped responses for mobile organisms at higher trophic levels (Wootton, 1998). ...
Disturbances alter biodiversity via their specific characteristics, including severity and extent in the landscape, which act at different temporal and spatial scales. Biodiversity response to disturbance also depends on the community characteristics and habitat requirements of species. Untangling the mechanistic interplay of these factors has guided disturbance ecology for decades, generating mixed scientific evidence of biodiversity responses to disturbance. Understanding the impact of natural disturbances on biodiversity is increasingly important due to human-induced changes in natural disturbance regimes. In many areas, major natural forest disturbances, such as wildfires, windstorms, and insect outbreaks, are becoming more frequent, intense, severe, and widespread due to climate change and land-use change. Conversely, the suppression of natural disturbances threatens disturbance-dependent biota. Using a meta-analytic approach, we analysed a global data set (with most sampling concentrated in temperate and boreal secondary forests) of species assemblages of 26 taxonomic groups, including plants, animals, and fungi collected from forests affected by wildfires, windstorms, and insect outbreaks. The overall effect of natural disturbances on α-diversity did not differ significantly from zero, but some taxonomic groups responded positively to disturbance, while others tended to respond negatively. 2 Mari-Liis Viljur and others Disturbance was beneficial for taxonomic groups preferring conditions associated with open canopies (e.g. hymenopterans and hoverflies), whereas ground-dwelling groups and/or groups typically associated with shady conditions (e.g. epigeic lichens and mycorrhizal fungi) were more likely to be negatively impacted by disturbance. Across all taxonomic groups, the highest α-diversity in disturbed forest patches occurred under moderate disturbance severity, i.e. with approximately 55% of trees killed by disturbance. We further extended our meta-analysis by applying a unified diversity concept based on Hill numbers to estimate α-diversity changes in different taxonomic groups across a gradient of disturbance severity measured at the stand scale and incorporating other disturbance features. We found that disturbance severity negatively affected diversity for Hill number q = 0 but not for q = 1 and q = 2, indicating that diversity-disturbance relationships are shaped by species relative abundances. Our synthesis of α-diversity was extended by a synthesis of disturbance-induced change in species assemblages, and revealed that disturbance changes the β-diversity of multiple taxonomic groups, including some groups that were not affected at the α-diversity level (birds and woody plants). Finally, we used mixed rarefaction/extrapolation to estimate biodiversity change as a function of the proportion of forests that were disturbed, i.e. the disturbance extent measured at the landscape scale. The comparison of intact and naturally disturbed forests revealed that both types of forests provide habitat for unique species assemblages, whereas species diversity in the mixture of disturbed and undisturbed forests peaked at intermediate values of disturbance extent in the simulated landscape. Hence, the relationship between α-diversity and disturbance severity in disturbed forest stands was strikingly similar to the relationship between species richness and disturbance extent in a landscape consisting of both disturbed and undisturbed forest habitats. This result suggests that both moderate disturbance severity and moderate disturbance extent support the highest levels of biodiversity in contemporary forest landscapes.
While the mechanisms that govern disease emergence and spread among hosts are increasingly well-described, the mechanisms that promote parasite diversity within-hosts, affecting host outcomes and spillover potential, have been comparatively understudied. Furthermore, while attention has been paid to the effects of increasing temperatures on disease systems, the effects of environmental variability have been left underexplored, despite rising climatic variability and internal temperature variability in a prolific reservoir for disease. To investigate the impacts of environmental variability on parasite diversity within-hosts, we analyzed a model of within-host population dynamics wherein two parasites indirectly compete through the host's immune response. We simulated the model under constant, demographically stochastic, environmentally stochastic, and demographically and environmentally stochastic conditions, and analysed the viability and longevity of non-equilibrium parasite co-occurrence. We found that environmental stochasticity increased the viability and longevity of parasite co-occurrence, suggesting that thermal variability arising from climatic change and as a physiological trait may promote parasite diversity within ectotherms and help explain bats' propensity to support diverse communities of parasites. Further, we found that under certain conditions, the transmissibility of co-occurring parasites can surpass the transmissibility of single parasites, suggesting that thermal variability may increase the transmission potential of co-occurring parasites.
Global warming generates changes in environmental conditions, affecting the spatial-temporal dynamics of precipitation and temperature. Droughts, events of low rainfall, are becoming more frequent and severe. In central Chile, from 2010 to date, an unprecedented drought event has developed, affecting the ecosystem and creating pressure on the dynamics of food webs. The present study analysed the trophic ecology of Bubo magellanicus, a top predator in the Mediterranean region of Chile, between 2019 and 2020 a period with a rainfall deficit of 72.6%. Our results established a diet mainly described by invertebrates (97.75%), in particular by the Gramnostola rosea spider (87.86%), and a low contribution of small vertebrates (2.24%). The trophic niche breadth (B = 0.37) and the standardised Levin's index (B STA = 0.01) are the lowest recorded in the species B. magellanicus. A comparative analysis of trophic ecology with other studies developed in the same region established significant differences in the composition of the diet (frequency of occurrence of prey unit). This work provides evidence that droughts and other extreme environmental scenarios restructure the food webs of an ecosystem, with direct consequences on the trophic niche of the species, specifically top predators.
The extremely high species diversity of soil bacterial community has fascinated and puzzled community ecologists. Although theory predicts that fluctuations in environments can facilitate diversity maintenance, the effects of fluctuating temperature on species diversity have rarely been investigated in species-rich microbial communities. Here, we examined whether fluctuating temperature had positive effects on species diversity relative to constant temperatures in soil bacterial communities, and investigated the effects of fluctuating temperature on bacterial performances (changes in relative abundance). We performed a temperature manipulation experiment with soils collected from temperate and subtropical zones, where the soils were subjected to constant high, low or fluctuating temperatures. We found that fluctuating temperatures showed significant positive effects on species diversity. The time-averaged effect of fluctuating temperatures (i.e. averaging out the differences between species in their environment-dependent performances) appeared to delay species loss in both the temperate and the subtropical communities. In addition, we found that the performances of temperature-responsive species at fluctuating temperatures significantly deviated from their time-weighted average performances at constant high and low temperatures, which was defined as fluctuation-dependent effects in our study. Intriguingly, fluctuation-dependent effects beyond time-averaged effect led to an opposite trend: differences in temperature-responsive species' performances decreased in the temperate communities, but increased in the subtropical communities. Our findings provide new insights into diversity maintenance in soil bacterial communities, and imply that the effects of fluctuating temperature on species diversity in soil bacterial community might vary across latitude.
This is the new version of the ms submitted last September. There are minor changes in the main text, but we completely rewrote the discussion. I think that it is worth checking out! We submitted the ms to Quarterly Review of Biology. I would like to thank you for the comments and suggestions.
Successful fundamental theories are built on verifiable principles that include measurable variables. This paper shows that Darwin's inclusive theory is built on such principles and follows their rocky road into modern operational theories. Besides reproduction, variation, and heredity, Darwin's conditions of diversification also include the potential for exponential (geometric) population growth and its necessarily limited nature. The Struggle for Existence (Malthus Doctrine), the Principles of Natural Selection, Competitive Exclusion (Rule of Similar Checks), and Divergence are mere deductions from these conditions. The dynamic system theory of robust coexistence, the theory of adaptive dynamics, and the extended theory of evolution all assume Darwin's inclusive principles as essentials. Incorporating the feedbacks controlling population growth and the tradeoffs between fitness components into the core of evolutionary theory leads to the conclusion that diversification is a fundamental, inherent feature of life and provides laws that support the determination of the expected direction of evolution in any particular case. Content
The invasive planthopper, spotted lanternfly (SLF), Lycorma delicatula (White) (Hemiptera: Fulgoridae), feeds on a broad range of plants including species of economic importance such as grape. Although SLF feeds on wild and cultivated grape, the effect of grapevines on the insect’s life history traits is unknown. This study examined the effect of cultivated Concord grapevines ( Vitis labrusca ) and the insect’s preferred host tree of heaven (TOH), Ailanthus altissima , on SLF development, survival, reproduction, and body mass. Newly emerged nymphs were allowed to feed on either TOH, Concord grapevines or a mixed diet of Concord grapevines plus TOH through adulthood until death. Development, mortality, and oviposition of paired adults were tracked daily to calculate the SLF rate of development, survival, and reproduction among treatments. When feeding exclusively on Concord grapevines, SLF was able to develop and reproduce but had higher mortality, slower development, and produced fewer eggs. SLF fed on the mixed diet of grapevines plus TOH exhibited faster nymphal development, laid more eggs, and had higher body mass compared with those fed only on grape or TOH. SLF had greater survival when fed on either the mixed diet or on TOH alone. We conclude that Concord grapevines are a poor-quality host for SLF, but when combined with TOH, SLF fitness increases above that of feeding on TOH alone. This study supports the elimination of TOH as a part of SLF vineyard management practices.
A central goal of ecology is identifying the mechanisms that allow large, complex food webs to persist. Spatial mechanisms resulting from dispersal connections among local food webs are one factor shown to play a significant role in enabling species persistence, particularly by driving asynchrony in the dynamics among local food webs. However, it is still unknown how these spatial persistence mechanisms operate across food webs. Using simulations of full non-linear food web models, we investigate how spatial persistence mechanisms emerge in multi-species food webs that possess different structural metrics. Specifically, we ask whether 1) spatial persistence mechanisms work similarly across food webs, and 2) if differences can be explained by food web features influencing stability in the absence of dispersal, particularly trophic structure. Food web structures are generated using the allometric niche model that is capable of reproducing realistic feeding patterns and interaction strengths. Our analyses quantify the tendency of modeled food webs to achieve asynchrony in the presence of dispersal and show that this positively affects the ability of species in the food web to persist. We observe an inverse relationship between the ability of food webs to persist when isolated and their tendency to be asynchronous when spatial, indicating a limited ability of food webs that persist when isolated to benefit from spatial persistence mechanisms. Our results demonstrate a relatively unexplored layer of food web properties which determine the ability of a food web to capitalize on the stabilizing opportunities created by dispersal, specifically those that influence the tendency for dispersal-linked food webs to be asynchronous. Future studies should expand on our results by examining how properties of spatial connections and food webs influence the ability of food webs to achieve asynchrony.
The McMurdo Dry Valleys form the largest relatively ice-free area on the Antarctic continent. The perennially ice-covered lakes, ephemeral streams and extensive areas of exposed soil are subject to low temperatures, limited precipitation and salt accumulation. The dry valleys thus represent a region where life approaches its environmental limits. This unique ecosystem has been studied for several decades as an analog to environments on other planets, particularly Mars. For the first time, the detailed terrestrial research of the dry valleys is brought together here, presented from an astrobiological perspective. Chapters include a discussion on the history of research in the valleys, a geological background of the valleys, setting them up as analogs for Mars, followed by chapters on the various sub-environments in the valleys such as lakes, glaciers and soils. Includes concluding chapters on biodiversity and other analog environments on Earth.
It is one of the central goals of microbial ecology to understand the factors that drive the succession of communities. Abiotic environmental factors affect the growth of populations and the interactions between them. Microbial communities are very sensitive and easy to respond to changes in abiotic environmental factors such as disturbance. Studying how different types of disturbance have various effects will enhance the understanding of the relationship between diversity, structure and function of the microbial community. Based on the classification criteria of the action form, this review sorts out the influence of various abiotic environmental factors on the succession of microbial communities, including non-specific, specific, and special (e.g. space) environments’ abiotic environmental factors. Although studies have been conducted on the succession of microbial communities in various forms due to disturbance, there is still a long way for related exploration. The quantification, multi-factor effect of disturbance, and the time course of succession have yet to be studied enough.
The McMurdo Dry Valleys form the largest relatively ice-free area on the Antarctic continent. The perennially ice-covered lakes, ephemeral streams and extensive areas of exposed soil are subject to low temperatures, limited precipitation and salt accumulation. The dry valleys thus represent a region where life approaches its environmental limits. This unique ecosystem has been studied for several decades as an analog to environments on other planets, particularly Mars. For the first time, the detailed terrestrial research of the dry valleys is brought together here, presented from an astrobiological perspective. Chapters include a discussion on the history of research in the valleys, a geological background of the valleys, setting them up as analogs for Mars, followed by chapters on the various sub-environments in the valleys such as lakes, glaciers and soils. Includes concluding chapters on biodiversity and other analog environments on Earth.
The neotropical primate family Pitheciidae consists of four genera Cacajao (uacaris), Callicebus (titis), Chiropotes (bearded sakis) and Pithecia (sakis), whose 40+ species display a range of sizes, social organisations, ecologies and habitats. Few are well known and the future survival of many is threatened, yet pitheciines have been little studied. This book is the first to review the biology of this fascinating and diverse group in full. It includes fossil history, reviews of the biology of each genus and, among others, specific treatments of vocalisations and foraging ecology. These studies are integrated into considerations of current status and future conservation requirements on a country-by-country basis for each species. A state-of-the-art summary of current knowledge, Evolutionary Biology and Conservation of Titis, Sakis and Uacaris is a collective effort from all the major researchers currently working on these remarkable animals.
The neotropical primate family Pitheciidae consists of four genera Cacajao (uacaris), Callicebus (titis), Chiropotes (bearded sakis) and Pithecia (sakis), whose 40+ species display a range of sizes, social organisations, ecologies and habitats. Few are well known and the future survival of many is threatened, yet pitheciines have been little studied. This book is the first to review the biology of this fascinating and diverse group in full. It includes fossil history, reviews of the biology of each genus and, among others, specific treatments of vocalisations and foraging ecology. These studies are integrated into considerations of current status and future conservation requirements on a country-by-country basis for each species. A state-of-the-art summary of current knowledge, Evolutionary Biology and Conservation of Titis, Sakis and Uacaris is a collective effort from all the major researchers currently working on these remarkable animals.
The neotropical primate family Pitheciidae consists of four genera Cacajao (uacaris), Callicebus (titis), Chiropotes (bearded sakis) and Pithecia (sakis), whose 40+ species display a range of sizes, social organisations, ecologies and habitats. Few are well known and the future survival of many is threatened, yet pitheciines have been little studied. This book is the first to review the biology of this fascinating and diverse group in full. It includes fossil history, reviews of the biology of each genus and, among others, specific treatments of vocalisations and foraging ecology. These studies are integrated into considerations of current status and future conservation requirements on a country-by-country basis for each species. A state-of-the-art summary of current knowledge, Evolutionary Biology and Conservation of Titis, Sakis and Uacaris is a collective effort from all the major researchers currently working on these remarkable animals.
The neotropical primate family Pitheciidae consists of four genera Cacajao (uacaris), Callicebus (titis), Chiropotes (bearded sakis) and Pithecia (sakis), whose 40+ species display a range of sizes, social organisations, ecologies and habitats. Few are well known and the future survival of many is threatened, yet pitheciines have been little studied. This book is the first to review the biology of this fascinating and diverse group in full. It includes fossil history, reviews of the biology of each genus and, among others, specific treatments of vocalisations and foraging ecology. These studies are integrated into considerations of current status and future conservation requirements on a country-by-country basis for each species. A state-of-the-art summary of current knowledge, Evolutionary Biology and Conservation of Titis, Sakis and Uacaris is a collective effort from all the major researchers currently working on these remarkable animals.
The neotropical primate family Pitheciidae consists of four genera Cacajao (uacaris), Callicebus (titis), Chiropotes (bearded sakis) and Pithecia (sakis), whose 40+ species display a range of sizes, social organisations, ecologies and habitats. Few are well known and the future survival of many is threatened, yet pitheciines have been little studied. This book is the first to review the biology of this fascinating and diverse group in full. It includes fossil history, reviews of the biology of each genus and, among others, specific treatments of vocalisations and foraging ecology. These studies are integrated into considerations of current status and future conservation requirements on a country-by-country basis for each species. A state-of-the-art summary of current knowledge, Evolutionary Biology and Conservation of Titis, Sakis and Uacaris is a collective effort from all the major researchers currently working on these remarkable animals.
The neotropical primate family Pitheciidae consists of four genera Cacajao (uacaris), Callicebus (titis), Chiropotes (bearded sakis) and Pithecia (sakis), whose 40+ species display a range of sizes, social organisations, ecologies and habitats. Few are well known and the future survival of many is threatened, yet pitheciines have been little studied. This book is the first to review the biology of this fascinating and diverse group in full. It includes fossil history, reviews of the biology of each genus and, among others, specific treatments of vocalisations and foraging ecology. These studies are integrated into considerations of current status and future conservation requirements on a country-by-country basis for each species. A state-of-the-art summary of current knowledge, Evolutionary Biology and Conservation of Titis, Sakis and Uacaris is a collective effort from all the major researchers currently working on these remarkable animals.
This is a summary paper that identifies the assumptions and principles of the core theory of evolution based on the analysis of the explanatory structure of Darwin's core theory and the common features of adaptive dynamics, the extended theory of evolution, and the theory of robust coexistence. It is closely related to our former works in theory-based ecology.
p>In this research project, a variety of complimentary techniques have been used to investigate the potential mechanisms that may be maintaining the coexistence of five highly similar interdigitated species from the genus Bursera , within the seasonally dry tropical forest of Chamela, Jalisco, Mexico. The population dynamics, incidences of herbivore damage, plant chemistry, spatial patterns and microhabitat associations of the congeneric species Bursera instabilis, B. arborea, B. fagaroides, B. excelsa, and B. heteresthes were investigated.
Several coexistence theories were supported by evidence gathered during this research, where the predictions of those theories were explained by population processes determined by a seed size-seed number trade-off. The theories of patch dynamics, density- and distance-dependent mortality and dispersal limitation were suggested to interact within the larger framework of differential sensitivity storage dynamics. The differential sensitivity storage theory states that the coexistence of common and rare competing species is maintained by differential sensitivity to temporally varying environmental conditions, which results in fluctuations in seedling mortality and thus adult recruitment through time. Trade-offs between growth rate/performance and tolerance to herbivory/shade were suggested to explain the differential sensitivity of species within the genus Bursera . Temporal fluctuations in rainfall, which in turn influences canopy development and thus light conditions at the forest floor, in addition to herbivore pressure were then suggested to be potential environmental drivers of differential sensitivity storage dynamics.</p
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.
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.
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.
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.
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
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...
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
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.
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.
The Handbook of Vegetation Science is growing. After the first volumes und er my editorship have appeared the interest of the scientific community has been revived and many new volume editors have started their work. The present volume wasjointly designed by Drs. J. White and W. Beeftink. Due to unforseen developments Dr. White signs now as the sole editor. The development of this volume within the series had a special history as Dr. White pointsout in his preface. Adding to this I need only to state that I found it essential to include the topic of this volume into a Hand book of Vegetation Science. lt was included therefore in my first revised Iist of topics to be included in the Handbook when I took over from Dr. Tüxen. It is a great pleasure for me to see this volume appear. Having read through the many contributions to this volume I can certainly congratu late Drs. White and Beeftink for their success in generating so much interest in this volume among their colleagues. The cooperation on this volume is forme the first sign that the new concept of the Handbook has been understood by the generation of scientists which I have to address. The influence this volume will have on the field ofplant population studies only time can teil. It appears to me, however, that this volume will become a standard resource for some future. Dr. White asked me to have this volume dedicated to Dr. Rabotnov.
One of the largest unsolved problems in ecology is to identify factors controlling the richness of species with a similar life form living in the same habitat. The problem is illustrated most clearly by the latitudinal gradient of tree species diversity — tropical rain forests include by far more tree species than temperate forests of the same area.
This book had its origin when, about five years ago, an ecologist (MacArthur) and a taxonomist and zoogeographer (Wilson) began a dialogue about common interests in biogeography. The ideas and the language of the two specialties seemed initially so different as to cast doubt on the usefulness of the endeavor. But we had faith in the ultimate unity of population biology, and this book is the result. Now we both call ourselves biogeographers and are unable to see any real distinction between biogeography and ecology.
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Considers a number of logical and philosophical points concerning community properties, and describes case studies where factors other than interspecific competition are of great influence in the structure and functioning of communities.-P.J.Jarvis
Through use of the models Professor Horn has devised, plant ecologists, foresters, and botanists will be able to predict the growth and productivity of a forest, the invading and senile species in a forest, the effect of shade tolerance on forest succession, and similar questions.
@'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.
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.
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.
We postulate that in homogeneous, grazed, nutrient- and water-deficient environments many species can reach virtually all microsites within the plot, which we express through the idea of the ‘carousel model’. We also question the usefulness of the niche concept and re-interpret it by stating that all species of this plant community have the same habitat niche, while most of them are short-lived and have the same regeneration niche. The essential variation amongst the species is their individual ability to establish or re-establish by making use of favourable conditions appearing in microsites in an unknown, complex spatio-temporal pattern.
That niches of competitors in ecological communities are shaped by mutual coevolution, which thus allows many species to coexist, is a commonly-held view. Two species must live together consistently to coevolve, so since predators (or parasites) are dependent upon their prey, they will necessarily co-occur with them and so should coevolve. In contrast, competing species, which are not dependent on each other, need not consistently co-occur or coevolve. Increased diversity, by reducing the consistency of co-occurrence, also reduces the chance of coevolution. To demonstrate coevolutionary divergence of competitors one must show: 1) that divergence has actually occurred: this has been done for some fossil sequences but not for any extant competitors; 2) that competition, rather than some other mechanism, is responsible; and 3) that it has a genetic basis. To demonstrate 2) and 3) for natural populations requires appropriate field experiments, which are suggested in the paper. This has been done, in part, in only one case. Thus the notion of coevolutionary shaping of competitors' niches has little support at present. Theory and evidence suggest that it is probable only in low diversity communities. /// То, что ниши конкурентов в экологических сообщаствах оформились в процессе коэволюции, поэволяющей т.о. сосуществовать многим видам, общепринятая точка зрения. Два вида должны сосуществовать, последовательно коэволюционируя, так что, по-скольку хищники (или паразиты) зависят от своих жертв, они должны непременно встречаться вместе и т.о. могут коэволюционировать. В противоположность этому, конкурирующие виды, независимые друг от друга, наобязательно должны встречаться вместе и коэволюционировать. Повышение разнообрязия при снижении постоянства совместной встречаемости таюже снижает возможность коэволюции. Для демонстрации коэволюционной дивергенции конкурентных видов следует показать: 1. что дивергенция реально существует; это было сделано для некоторых ископаемых рядов, но не для современных конкурирующих форм. 2. что конкуренция более реактивна, чем любой другой механизм; 3. и что она имеет генетическую основу. Чтобы доказать второй и третий пункты для естественных популяций, необходимо проведение полевых экспериментов, которые предложены в статье. Это было частично проделано лишь в одном случае. Таким образом, представление о формах коэволюции ниш конкурирующих видов плохо обосновано. Теория и имеющиеся факты говорят о том, что это возможно лишь для сообществ с низким разнообразием.
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.
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...
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
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...
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.
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
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The species richness (diversity) of local plant and animal assemblages—biological communities—balances regional processes
of species formation and geographic dispersal, which add species to communities, against processes of predation, competitive
exclusion, adaptation, and stochastic variation, which may promote local extinction. During the past three decades, ecologists
have sought to explain differences in local diversity by the influence of the physical environment on local interactions among
species, interactions that are generally believed to limit the number of coexisting species. But diversity of the biological
community often fails to converge under similar physical conditions, and local diversity bears a demonstrable dependence upon
regional diversity. These observations suggest that regional and historical processes, as well as unique events and circumstances,
profoundly influence local community structure. Ecologists must broaden their concepts of community processes and incorporate
data from systematics, biogeography, and paleontology into analyses of ecological patterns and tests of community theory.