Article

Individuals and the Variation Needed for High Species Diversity in Forest Trees

February 2010
Science 327(5969):1129-32

February 2010
327(5969):1129-32

Source
PubMed

Authors:

Difference, Not Diversity In tropical forests, as in the ocean plankton, thousands of species may compete for the same resources. How they succeed in coexisting remains one of the central paradoxes in the study of biodiversity. Theory shows that coexisting species must partition the environment, but such partitioning is not obvious. Using data from coexisting forest trees in the southeastern United States, Clark (p. 1129 ) show that individual variation between members of the same species allows them to avoid direct competition: One plant may differ significantly from another in its requirements for light, nutrients, or moisture, yet remain within the general spectrum of features displayed by its conspecifics.

Major axes of variation in tree demography across global forests

Article

Full-text available

May 2024

The future trajectory of global forests is closely intertwined with tree demography, and a major fundamental goal in ecology is to understand the key mechanisms governing spatio‐temporal patterns in tree population dynamics. While previous research has made substantial progress in identifying the mechanisms individually, their relative importance among forests remains unclear mainly due to practical limitations. One approach to overcome these limitations is to group mechanisms according to their shared effects on the variability of tree vital rates and quantify patterns therein. We developed a conceptual and statistical framework (variance partitioning of Bayesian multilevel models) that attributes the variability in tree growth, mortality, and recruitment to variation in species, space, and time, and their interactions – categories we refer to as organising principles (OPs). We applied the framework to data from 21 forest plots covering more than 2.9 million trees of approximately 6500 species. We found that differences among species, the species OP, proved a major source of variability in tree vital rates, explaining 28–33% of demographic variance alone, and 14–17% in interaction with space , totalling 40–43%. Our results support the hypothesis that the range of vital rates is similar across global forests. However, the average variability among species declined with species richness, indicating that diverse forests featured smaller interspecific differences in vital rates. Moreover, decomposing the variance in vital rates into the proposed OPs showed the importance of unexplained variability, which includes individual variation, in tree demography. A focus on how demographic variance is organized in forests can facilitate the construction of more targeted models with clearer expectations of which covariates might drive a vital rate. This study therefore highlights the most promising avenues for future research, both in terms of understanding the relative contributions of groups of mechanisms to forest demography and diversity, and for improving projections of forest ecosystems.

Intraspecific alternative phenotypes contribute to variation in species’ strategies for growth

Article

Full-text available

Apr 2024
OECOLOGIA

Ecologists have historically sought to identify the mechanisms underlying the maintenance of local species diversity. High-dimensional trait-based relationships, such as alternative phenotypes, have been hypothesized as important for maintaining species diversity such that phenotypically dissimilar individuals compete less for resources but have similar performance in a given environment. The presence of alternative phenotypes has primarily been investigated at the community level, despite the importance of intraspecific variation to diversity maintenance. The aims of this research are to (1) determine the presence or absence of intraspecific alternative phenotypes in three species of tropical tree seedlings, (2) investigate if these different species use the same alternative phenotypes for growth success, and (3) evaluate how findings align with species co-occurrence patterns. We model species-specific relative growth rate with individual-level measurements of leaf mass per area (LMA) and root mass fraction (RMF), environmental data, and their interactions. We find that two of the three species have intraspecific alternative phenotypes, with individuals within species having different functional forms leading to similar growth. Interestingly, individuals within these species use the same trait combinations, high LMA \(\times\) low RMF and low LMA \(\times\) high RMF, in high soil nutrient environments to acquire resources for higher growth. This similarity among species in intraspecific alternative phenotypes and variables that contribute most to growth may lead to their negative spatial co-occurrence. Overall, we find that multiple traits or interactions between traits and the environment drive species-specific strategies for growth, but that individuals within species leverage this multi-dimensionality in different ways for growth success.

Rethinking the role of intraspecific variability in species coexistence

Preprint

Jan 2024

Intraspecific variability (IV) has been proposed as a new track to explain species coexistence. Previous studies generally assumed that IV results from intrinsic differences between conspecifics that widen species’ fundamental niches and blur differences among species, thus impeding stable coexistence, but also slowing down the rate of competitive exclusion. Based on a body of evidence, we here argue that IV does not necessarily imply differences among conspecifics, nor species niches overlap: conspecifics differ in their measured attributes mainly due to differences in the micro-environment they thrive in. Consequently, they respond more similarly to environmental variation than heterospecifics, thereby concentrating competition within species – a necessary condition for species coexistence. We call for new studies exploring observed IV as an outcome of species-specific responses to high-dimensional environmental variations that can lead to inversions of species hierarchy in space and time promoting stable coexistence.

Putting space into trait ecology: Trait, environment and biodiversity relationships at multiple spatial scales

Article

Full-text available

Jan 2024
J ECOL

Ecological processes such as environmental filtering and biotic interactions that shape species' traits and community diversity often vary with geographic distance, potentially generating spatial structures in trait variation, covariation and biodiversity data. Understanding spatial structures of trait, environment and biodiversity, or the spatial link between those factors, is fundamental to identifying spatially explicit assembly processes or biodiversity distributions in spatially heterogeneous landscapes but remains unclear. To address the issue, we gathered individual‐level leaf and diameter traits data paired with environmental data from a 4.8 ha subtropical Chinese forest and divided the forest into 25, 100, 400 and 1936 m² grids representing contrasting spatial grains. Using Moran's correlograms, we quantified the spatial structures of trait variation and covariation, environmental conditions and biodiversity. We assessed the links between those variables using path analyses. Most variables were spatially positively autocorrelated. However, trait mean was more autocorrelated than trait variation or covariation, and intraspecific trait variation was more autocorrelated than interspecific variation. Autocorrelations in those community properties were generally weak at the large grain. Path analyses indicated positive associations between interspecific trait variation and species diversity at a very small to medium scale and a positive association between intraspecific variation and small‐scale functional diversity. Trait covariation constrained biodiversity, and multi‐trait means were negatively linked to very small‐ to medium‐scale species diversity but positively to medium‐ and large‐scale functional diversity. Patterns regarding multi‐trait community structure–environment–biodiversity associations were generally held for individual traits. However, depending on the trait, spatial scale and plant ontogenic stage, the pattern's strength changed, or occasionally, their sign reversed. We attribute spatial patterns in multi‐trait mean and covariation to scale‐dependent variation in environmental heterogeneity and trait variation to scale‐dependent competition. Synthesis. Our study provides novel insights into spatial and scale‐dependent variability in functional community structure, environment and biodiversity, and their relationships. Our results demonstrate the usefulness of spatial trait analyses in identifying scale‐dependent assembly processes or finding the importance of processes to biodiversity distributions in spatially heterogeneous landscapes. A spatially explicit perspective is thus helpful for the progress of trait ecology.

High intraspecific growth variability despite strong evolutionary legacy in an Amazonian forest

Article

Full-text available

Oct 2023
ECOL LETT

Tree growth is key to species performance. However, individual growth variability within species remains underexplored for a whole community, and the role of species evolutionary legacy and local environments remains unquantified. Based on 36 years of diameter records for 7961 trees from 138 species, we assessed individual growth across an Amazonian forest. We related individual growth to taxonomy, topography and neighbourhood, before exploring species growth link to functional traits and distribution along the phylogeny. We found most variation in growth among individuals within species, even though taxonomy explained a third of the variation. Species growth was phylogenetically conserved up to the genus. Traits of roots, wood and leaves were good predictors of growth, suggesting their joint selection during convergent evolutions. Neighbourhood crowding significantly decreased individual growth, although much of inter‐individual variation remains unexplained. The high intraspecific variation observed could allow individuals to respond to the heterogeneous environments of Amazonian forests.

Rethinking the nature of intraspecific variability and its consequences on species coexistence

Article

Full-text available

Mar 2023

Intraspecific variability (IV) has been proposed to explain species coexistence in diverse communities. Assuming, sometimes implicitly, that conspecific individuals can perform differently in the same environment and that IV increases niche overlap, previous studies have found contrasting results regarding the effect of IV on species coexistence. We aim at showing that the large IV observed in data does not mean that conspecific individuals are necessarily different in their response to the environment and that the role of high-dimensional environmental variation in determining IV has largely remained unexplored in forest plant communities. We first used a simulation experiment where an individual attribute is derived from a high-dimensional model, representing “perfect knowledge” of individual response to the environment, to illustrate how large observed IV can result from “imperfect knowledge” of the environment. Second, using growth data from clonal Eucalyptus plantations in Brazil, we estimated a major contribution of the environment in determining individual growth. Third, using tree growth data from long-term tropical forest inventories in French Guiana, Panama and India, we showed that tree growth in tropical forests is structured spatially and that despite a large observed IV at the population level, conspecific individuals perform more similarly locally than compared with heterospecific individuals. As the number of environmental dimensions that are well quantified at fine scale is generally lower than the actual number of dimensions influencing individual at- tributes, a great part of observed IV might be represented as random variation across individuals when in fact it is environmentally driven. This mis-representation has important consequences for inference about community dynamics. We emphasize that observed IV does not necessarily impact species coexistence per se but can reveal species response to high-dimensional environment, which is consistent with niche theory and the observation of the many differences between species in nature.

Species α-diversity promotes but β-diversity restricts aboveground biomass in tropical forests, depending on stand structure and environmental factors

Article

Full-text available

Nov 2022

Forest plays a vital role in the global biogeochemical cycles through a high rate of carbon sequestration and harboring biodiversity. However, local species diversity is declining while also becoming increasingly homogenized across communities. Although effects of local biotic processes (e.g., species α -diversity and stand structural heterogeneity) and environmental factors on aboveground biomass (AGB) have been widely tested, there is a huge knowledge gap for the effect of regional biotic processes (i.e., taxonomic and functional β -diversity) in forests. Here, we hypothesized that regional and local environmental factors along with biotic processes jointly regulate AGB through species shifts in tropical forests. Using piecewise structural equation modeling (pSEM), we linked climatic water availability, soil fertility, stand structural heterogeneity (either tree DBH inequality, height inequality, or stand density), species α -diversity, taxonomic or functional β -diversity (and its two components; β -turnover and β -richness), and AGB across 189 inventory plots in tropical forests of Sri Lanka. Soil fertility and climatic water availability shaped local and regional biotic processes. Stand structural heterogeneity promoted species α -diversity but declined β -diversity (but increased β -taxonomic turnover). Species α -diversity and stand structural heterogeneity promoted AGB whereas taxonomic and functional β -diversity declined (but β -taxonomic turnover increased) AGB. The relationships of AGB with species α -diversity and β -diversity varied from significant to nonsignificant positive depending on the specific combinations of stand structural heterogeneity metrics used. This study shows that local biotic processes could increase AGB due to the local and regional niche complementarity effect whereas the regional biotic processes could restrict AGB due to the regional selection or functional redundancy effect under favorable environmental conditions. We argue that biotic homogenization, as well as drought conditions, may have strong divergent impacts on forest functions and that the impacts of tree diversity loss may greatly reduce carbon sequestration.

Interactions between leaf traits and environmental factors help explain the growth of evergreen and deciduous species in a subtropical forest

Article

Full-text available

Mar 2024
FOREST ECOL MANAG

It is well known that evergreen and deciduous species possess different functional traits and utilize different strategies in growth and adaptation to environments. However, little work has been done to elucidate whether leaf habits mediate the effect of trait-environment interactions on plant performance. In this study, our subjective was to illuminate whether relative growth rate of deciduous and evergreen species is influenced by multiple trait-environment interactions. We conducted measurements on eight leaf traits of 1230 individuals belonging to 25 species in a subtropical evergreen and deciduous broad-leaved mixed forest. Additionally, we collected data on topographic factors, edaphic variables and competition index. Subsequently, we employed generalized linear mixed model to analyze plant relative growth rate, considering high-order trait-environment interactions for both evergreen and deciduous species. We also visualized the effects of these interactions on growth patterns. Our results showed that leaf habits were divided by trait PC 1 (41.8%) which was related to leaf lifespan and resource acquisition. Evergreen species tended to have greater interspecific variation compared to deciduous species. Notably, the inclusion of trait-environment interactions significantly improved growth predictions for both leaf habits, although explanatory power of deciduous models was always higher than that of evergreen species. Furthermore, we observed variability in the effects of trait-environment interactions on plant performance varied between leaf habits, leading to different optimal models for each leaf habit, even when they shared similar trait-environment context. These results indicated that difference of life history strategies between leaf habits could be reflected by trait-environment interactions. We emphasized the importance of leaf habits in explaining forest productivity and functions, and future research should focus on the effects of leaf habits on other demographic metrics to understand species coexistence in mixed forests.

Beyond variance: simple random distributions are not a good proxy for intraspecific variability in systems with environmental structure

Article

Full-text available

Mar 2024

Aboveground carbon sequestration of Cunninghamia lanceolata forests: Magnitude and drivers

Article

Full-text available

Jan 2024

Diversity Patterns and Determinants of Soil Microorganisms and Nematodes along Elevation Gradients in a Temperate Forest in South Korea

Article

Full-text available

Dec 2023

The elevational patterns of soil microbial and nematodes diversity (SMND) and the determinants remain controversial. Moreover, how the SMND are modified simultaneously with an elevational gradient has not yet been established. In this study, we investigated the elevational patterns of the SMND and the relative importance among/within tree factors (i.e., tree diversity, identity, and quantity) and environmental factors (i.e., climate and soil) on the SMND. For this purpose, we analyzed datasets from 27 plots across nine elevation bands in the temperate forests of Mt. Gariwang, South Korea. We performed multimodel inference tests and subsequently conducted a variance partitioning to determine the most prominent factors controlling each SMND and compare the relative contribution of the trees and environmental effects. Our results revealed that bacterial and fungal diversity decreased along the elevation gradient. However, nematode diversity did not change significantly, indicating that site-specific environmental conditions may be more influential than the elevation per se. Moreover, this indicates that bacterial diversity was affected by the pH and functional dispersion of the leaf size, and that fungal diversity was governed only by the pH. However, nematode diversity was driven by aboveground biomass, ammonium-nitrogen, and tree size diversity. In summary, the soil microbial diversity was more strongly controlled by the environmental factors, whereas the tree factors were more important for nematodes. Our results show that the elevational patterns and determinants of SMND differed among the taxonomic groups in the common micro-food web. These findings provide new insights into the factors controlling the SMND in a temperate forest and expand the local knowledge of soil biodiversity which is necessary for promoting its mainstreaming. Thus, our results contribute to establishing a basis for more targeted and effective biodiversity conservation and management practices in forest ecosystems.

Interactions between Leaf Traits and Environmental Factors Enhance the Understanding of Leaf Habits in a Subtropical Forest

Preprint

Jan 2023

Enhancing the predictability of ecology in a changing world: A call for an organism-based approach

Article

Full-text available

Jan 2023

Ecology is usually very good in making descriptive explanations of what is observed, but is often unable to make predictions of the response of ecosystems to change. This has implications in a human-dominated world where a suite of anthropogenic stresses are threatening the resilience and functioning of ecosystems that sustain mankind through a range of critical regulating and supporting services. In ecosystems, cause-and-effect relationships are difficult to elucidate because of complex networks of negative and positive feedbacks. Therefore, being able to effectively predict when and where ecosystems could pass into different (and potentially unstable) new states is vitally important under rapid global change. Here, we argue that such better predictions may be reached if we focus on organisms instead of species, because organisms are the principal biotic agents in ecosystems that react directly on changes in their environment. Several studies show that changes in ecosystems may be accurately described as the result of changes in organisms and their interactions. Organism-based theories are available that are simple and derived from first principles, but allow many predictions. Of these we discuss Trait-based Ecology, Agent Based Models, and Maximum Entropy Theory of Ecology and show that together they form a logical sequence of approaches that allow organism-based studies of ecological communities. Combining and extending them makes it possible to predict the spatiotemporal distribution of groups of organisms in terms of how metabolic energy is distributed over areas, time, and resources. We expect that this “Organism-based Ecology” (OE) ultimately will improve our ability to predict ecosystem dynamics.

Major axes of variation in tree demography across global forests

Preprint

Full-text available

Jan 2023

Aim: Global forests and their structural and functional features are shaped by many mechanisms that impact tree vital rates. Although many studies have tried to quantify how specific mechanisms influence vital rates, their relative importance among forests remains unclear. We aimed to assess the patterns of variation in vital rates among species and in space and time across forests to understand and provide a baseline for expectations of the relative importance of the different mechanisms in different contexts. Location: 21 forest plots worldwide. Time period: 1981-2021 Major taxa studied: Woody plants Methods: We developed a conceptual and statistical framework (variance partitioning of multilevel models) that attributes the variability in growth, mortality, and recruitment to variation in species, space, and time, and their interactions, which we refer to as organising principles (OPs). We applied it to data from 21 forest plots covering more than 2.9 million trees of approximately 6,500 species. Results: Differences among species, the species OP, were a major source of variability in tree vital rates, explaining 28-33% of demographic variance alone, and in interaction with space 14-17%, totalling 40-43%. Models with small spatial grain sizes (quadrats at 5 x 5 m) retained most of the spatial OP, but a large proportion of variance remained unexplained (31-55%). The average variability among species declined with species richness across forests, indicating that diverse forests featured smaller interspecific differences in vital rates. Main conclusions: Decomposing variance in vital rates into the proposed OPs showed that taxonomy is crucial to predictions and understanding of tree demography. Our framework has a high potential for identifying the structuring mechanisms of global forest dynamics as it highlights the most promising avenues for future research both in terms of understanding the relative contributions of mechanisms to forest demography and diversity and for improving projections of forest ecosystems.

Dendrometry, production and nutritional value of Mimosa caesalpiniifolia Benth. under monocrop and silvopastoral system

Preprint

Full-text available

May 2024

Legumes have the potential to provide diverse ecosystem services, therefore, it is important to understand the quantitative and qualitative aspects of their development in different cropping systems. The objective of this study was to evaluate the dendrometric, productive characteristics and nutritional value of Mimosa caesalpiniifolia Benth. in monocrop and a silvopastoral system with signal grass, in Brazil. The treatments were distributed under a randomized block design and consisted of M. caesalpiniifolia monocrop system and silvopasture (signal grass + M. caesalpiniifolia ). Evaluations were carried out every 56 days for two years. The data were analyzed using repeated measures over time using SAS on demand (2021) and the treatment means were compared using PDIFF with Tukey's test (P < 0.05). There was no effect of cropping systems (P > 0.05) on plant height (~ 5.2 m). The greatest values for diameter at breast height (16.32 cm), basal diameter (20.54 cm), and dry matter production per plant (36 g DM plant − 1 ) were observed in the silvopasture system (P < 0.05). Forage mass was higher (P < 0.05) in the monocrop system compared to silvopasture (69 and 22 kg DM ha − 1 , respectively) in the first year of evaluation. Total forage accumulation was greater in the monocrop system compared to silvopasture (383 vs. 116 kg DM ha − 1 year − 1 ) in the first year of evaluation. The nutritional value was not influenced (p < 0.05) by the cultivation systems in the first year of evaluation, however, higher content of CP (221 g kg − 1 ), ADF (449 g kg − 1 ), and IVDMD (383 g kg − 1 ) were observed in the rainy season, while the highest DM content (426 g kg − 1 ) occurred in the dry season. M. caesalpiniifolia showed good dendrometric characteristics in the silvopasture system. The silvopasture provides greater forage production per plant of M. caesalpiniifolia and better nutritional value of the forage than the legume monocrop.

Functional trait patterns: investigating variation-covariation relationships and the importance of intraspecific variability along distinct vegetation types

Article

May 2024

Plants adjust to abiotic conditions by changing their anatomical, morphological, and physiological traits. Traits can vary independently or in an integrated manner, known as trait variation and covariation respectively. It has been hypothesized that a trade-off would emerge along a gradient of abiotic constraints in which trait variation would be favored under resource-rich conditions while covariation under resource-limited ones. Although many studies have provided empirical support for this trade-off, a consensus has not yet emerged, due to a lack of support in some cases. This study investigated variation and covariation in three leaf and four wood traits of 74 woody species from a rainforest, a semideciduous forest, and a Restinga heath vegetation in the Atlantic Forest, which are subjected to different water-related constraints. We asked: Is there a variation-covariation trade-off within and across vegetation types? How does incorporating intraspecific variability change the magnitude and pattern of trait covariation? We found a variation-covariation trade-off and a positive relationship both within and across vegetation types. Wood variation was higher and covariation was lower in the rainforest, likely due to the greater water availability. Conversely, wood trait covariation was higher and variation was lower in the Restinga and seasonal semideciduous forest. Differences between vegetation types are likely related to the species’ strategies to prevent hydraulic failure, particularly for Restinga species that adjust their wood density and xylem vessel density in a coordinated manner. Accounting for intraspecific trait variability increased covariation across all vegetation types, particularly in the Restinga. This highlights the loss of functional information when analyses are based solely on species’ mean trait values. Our results also contribute to this discussion by providing evidence that the trade-off or a positive relationship between trait variation and covariation may be context-dependent.

Oak genomics for nature-based solutions: shaping future forests and resilient landscapes in Canada

Article

Full-text available

May 2024
TREE GENET GENOMES

To preserve biodiversity and maintain ecosystem services provided by trees in the course of climate change, it is essential to consider challenging tree species, which are less studied primarily due to a lack of investment compared to commercial species. Species of the genus Quercus present an interesting case because of their economic and ecological importance, and their syngameon biology. As a model for exploring ecological diversification, and with recent advances in forest genomics, knowledge, data, and genomic resources for oak have accumulated and are summarized in this review to foster oaks as potential candidate species for future reforestation programs in Canadian natural, peri-urban, and urban ecosystems. We summarize the state of current genomic research in oak and the accompanying opportunities genomics can provide to achieve the potential of oak silviculture in Canada. Further, we highlight the socio-economic benefits of planting oaks and genomic tools for the development of a traceability system along the value chain. Finally, we discuss some of the remaining challenges to successfully integrate oaks into different forest management programs. In light of their increased drought resistance, oak species exhibit a strong potential as viable choices for future forests, resilient agricultural landscapes, and urban areas. By leveraging the progress made in oak genomics and the new applications that have been developed for commercial species, we can foster the successful management of oak genetic resources for the production of suitable seedlings, thereby aiding Canada in its ambitious pursuit of planting two billion trees to combat climate change.

Response of Individual-Tree Aboveground Biomass to Spatial Effects in Pinus kesiya var. langbianensis Forests by Stand Origin and Tree Size

Article

Full-text available

Feb 2024

To enhance forest carbon sequestration capacity, it is important to optimize forest structure by revealing the spatial effects of the aboveground biomass of individual trees, with particular emphasis on stand origin and tree size. Here, 0.3 ha clear-cut plots of Pinus kesiya var. langbianensis forest were selected in a typical plantation and natural stand. Then, the ordinary least squares model and spatial regression models were used to analyze the different responses between spatial position and individual tree biomass based on the stand origin and diameter at breast height (DBH) of the tree. Our study shows the following: (1) The spatial effect produced a stronger response in the natural stand than in the plantation. The amount of change in the adjusted R-squared (ΔRadj2) of tree component totaled 0.34 and 0.57 for Pinus kesiya var. langbianensis and other trees in the natural stand, compared to only 0.2 and 0.42 in the plantation; (2) Spatial effects had a stronger impact on the accuracy of the fit for the crown (ΔRadj2 = 0.52) compared to the wood and bark (ΔRadj2 = 0.03) in the plantation, and there were no significant differences in the natural stand (ΔRadj2 = 0.42, ΔRadj2 = 0.43); (3) When DBH reached a certain size, the impact of spatial effect for the crown showed a significant change from positive to negative. The sizes of DBH were 19.5 cm, 14 cm and 34.6 cm, 19 cm for branches of Pinus kesiya var. langbianensis and other tree species in the plantation and natural stand, and were 20.3 cm and 31.4 cm for the foliage of Pinus kesiya var. langbianensis. Differences in stand structure led to varied responses in the biomass of tree components to spatial effects.

Evaluating individual tree species classification performance across diverse environments

Article

Full-text available

Jan 2024

Vegetation species mapping using airborne imaging spectroscopy yields accurate results and is important for advancing conservation objectives and biogeographic studies. As these data become more readily available owing to the upcoming launch of spaceborne imaging spectrometers, it is necessary to understand how these data can be used to consistently classify species across large geographic scales. However, few studies have attempted to map species across multiple ecosystems; therefore, little is known regarding the effect of intra-specific variation on the mapping of a single species across a wide range of environments and among varying backgrounds of other non-target species. To explore this effect, we developed and tested species classification models for Metrosideros polymorpha, a highly polymorphic canopy species endemic to Hawai'i, which is found in a diverse array of ecosystems. We compared the accuracies of support vector machine (SVM) and random forest models trained on canopy reflectance data from each of eight distinct ecosystems (ecosystem-specific) and a universal model trained on data from all ecosystems. When applied to ecosystem-specific test datasets, the ecosystem-specific models outperformed the universal model; however, the universal model retained high (>81%) accuracies across all ecosystems. Additionally, we found that models from ecosystems with broad variation in M. polymorpha canopy traits, as estimated using chemometric equations applied to canopy spectra, accurately predicted M. polymorpha in other ecosystems. While species classifications across ecosystems can yield accurate results, these results will require sampling procedures that capture the intra-specific variation of the target species.

Wood density and leaf size jointly predict woody plant growth rates across (but not within) species along a steep precipitation gradient

Article

Full-text available

Dec 2023
J ECOL

Functional traits have been proposed to define key dimensions of plant ecological strategies, but we lack consensus on whether traits can accurately predict plant demography. Despite theoretical expectations, it has been challenging to find consistent relationships between functional traits and growth. In this study, we quantified inter‐ and intraspecific trait variation (ITV) and individual growth rates of woody plants across a steep moisture gradient that varies 10‐fold in annual precipitation (350–3700 mm) in southern Chile and used a hierarchical Bayesian model to predict growth as a function of trait values. We show that large‐leaved species with lower stem tissue density exhibited the fastest growth rates, and these two traits exhibited the highest proportion of interspecific variation. Predictions of growth improved considerably (R² of the best model increased from 0.28 to 0.49) when species‐level multiple traits and their interactions were considered. The inclusion of ITV, however, did not improve models of growth rate. We found that trait–growth rate relationships were not always consistent across levels of biological organization; relationships observed at the interspecific level did not necessarily hold at the intraspecific level. We found that the relationships between wood density or leaf size and growth were consistent in direction across the precipitation gradient, and the relationships between leaf economics traits and growth were weak and site‐specific. Synthesis. Although using more than one functional trait considerably improved growth predictions, wood density and leaf size successfully predicted growth rates across (not within) species, which is consistent with a whole‐plant carbon economy. We assert that these two traits are intimately linked and ultimately describe a continuum of plant architecture and carbon economy that covers multiple trait syndromes.

Tree diversity increases productivity through enhancing structural complexity across mycorrhizal types

Article

Full-text available

Oct 2023

Tree species diversity and mycorrhizal associations play a central role for forest productivity, but factors driving positive biodiversity-productivity relationships remain poorly understood. In a biodiversity experiment manipulating tree diversity and mycorrhizal associations, we examined the roles of above- and belowground processes in modulating wood productivity in young temperate tree communities and potential underlying mechanisms. We found that tree species richness, but not mycorrhizal associations, increased forest productivity by enhancing aboveground structural complexity within communities. Structurally complex communities were almost twice as productive as structurally simple stands, particularly when light interception was high. We further demonstrate that overyielding was largely explained by positive net biodiversity effects on structural complexity with functional variation in shade tolerance and taxonomic diversity being key drivers of structural complexity in mixtures. Consideration of stand structural complexity appears to be a crucial element in predicting carbon sequestration in the early successional stages of mixed-species forests.

Connecting higher-order interactions with ecological stability in experimental aquatic food webs

Article

Full-text available

Sep 2023

Community ecology is built on theories that represent the strength of interactions between species as pairwise links. Higher-order interactions (HOIs) occur when a species changes the pairwise interaction between a focal pair. Recent theoretical work has highlighted the stabilizing role of HOIs for large, simulated communities, yet it remains unclear how important higher-order effects are in real communities. Here, we used experimental communities of aquatic protists to examine the relationship between HOIs and stability (as measured by the persistence of a species in a community). We cultured a focal pair of consumers in the presence of additional competitors and a predator and collected time series data of their abundances. We then fitted competition models with and without HOIs to measure interaction strength between the focal pair across different community compositions. We used survival analysis to measure the persistence of individual species. We found evidence that additional species positively affected persistence of the focal species and that HOIs were present in most of our communities. However, persistence was only linked to HOIs for one of the focal species. Our results vindicate community ecology theory positing that species interactions may deviate from assumptions of pairwise interactions, opening avenues to consider possible consequences for coexistence and stability.

Seedling performance in a dioecious tree species is similar near female and male conspecific adults despite differences in colonization by arbuscular mycorrhizal fungi

Article

Aug 2023
OIKOS

Plant–soil feedbacks (PSFs) are a key driver of species diversity and composition in plant communities worldwide; however, the factors that may cause feedbacks to vary within species are rarely examined. In dioecious species, the strength of feedbacks may differ near female plants that produce seed versus near male plants (which do not) because repeated inputs of seeds and high seedling densities near females may cause accumulation of host-specific soil microbes that influence seedling performance. To test whether conspecific seedling performance is reduced near seed-producing female trees relative to male or heterospecific trees, we conducted shadehouse and field experiments with a dioecious tropical tree species, Virola surinamensis (Myristicaceae), on Barro Colorado Island, Panama. The shadehouse experiment isolated the effect of soil microbial communities on seedling growth and allowed us to quantify colonization by mutualistic arbuscular mycorrhizal (AM) fungi, while the field experiment allowed us to assess seedling survival and growth in the presence of nearby conspecific adults and seedlings. In both experiments, seedling performance was similar between seedlings grown in the soil microbial communities and field environments underneath female conspecific, male conspecific, and heterospecific trees. However, contrary to expectation, seedling colonization by AM fungi was higher in male conspecific soil microbial communities than in female or heterospecific soil microbial communities at the end of the shadehouse experiment. Together, our experiments show that while differences among female and male plants in dioecious species may influence the association of conspecific seedlings with AM fungi in their soils, this variation does not necessarily translate directly to differences in seedling performance, at least over the time frame of our experiments. Studies of additional dioecious species are needed to help determine differences in soil microbial communities beneath male and female plants and to assess the role of seed input versus adult root systems in driving PSFs.

Effects of plant diversity and big-sized trees on ecosystem function in a tropical montane evergreen broad-leaved forest

Article

Full-text available

Aug 2023

Introduction Scale dependencies play a vital role in defining the biodiversity-ecosystem functioning relationship in forest ecosystems, which varies based on the magnitude of multiple plant diversity attributes, soil properties, and aboveground biomass in forest ecosystems. However, the effects of plant diversity and big-sized trees on the relationship between plant diversity and aboveground biomass across different scales remain unclear in forest ecosystems. Methods Based on a 30-ha tropical montane evergreen broad-leaved forest dynamics plot in Yunnan province, China, we comparatively analyzed the importance of scale-dependent effects of multiple plant diversity attributes, soil properties, neighborhood competition intensity and aboveground biomass of big-sized trees, as well as stand structural complexity on aboveground biomass of all woody individuals. The aim is therefore to identify the main predictors for sustaining aboveground biomass of all woody individuals, considering multiple biotic and abiotic factors jointly, as well as underlying mechanisms. Results Our results suggest that indicators such as species richness and phylogenetic diversity did not strongly contribute to aboveground biomass of all woody individuals with increasing spatial scales, while aboveground biomass of big-sized trees exhibited the greatest contribution to aboveground biomass of all woody individuals. Stand structural complexity, characterized by variances in woody plant diameter at breast height, also contributed more to aboveground biomass of all woody individuals indirectly via neighborhood competition intensity and aboveground biomass of big-sized trees. Contributions of functional dispersion and community-weighted mean of leaf phosphorus concentration to aboveground biomass of all woody individuals became stronger with increasing spatial scales. Neighborhood competition intensity exhibited a negative linear relationship with aboveground biomass of all woody individuals at the smallest scale, but it affected positively aboveground biomass of all woody individuals across spatial scales, likely due to indirect effects via aboveground biomass of big-sized trees. Discussion Big-sized trees will likely become more important in biodiversity maintenance and ecosystem function management as deforestation and forest degradation.

Stand Diversity and Carbon Stock of a Tropical Forest in the Deng Deng National Park, Cameroon

Article

Full-text available

Jan 2023

The rate of species extinction in declining or fragmented ecological communities

Article

Full-text available

Jul 2023
PLOS ONE

Loss of habitat can take many forms, ranging from the fragmentation of once-continuous habitat to the slow erosion of populations across continents. Usually, the harm leading to biodiversity loss is not immediately obvious: there is an extinction debt. Most modelling research of extinction debt has focussed on relatively rapid losses of habitat with species loss happening in response afterwards. In this paper, using a niche-orientated community model we compare and contrast two different mechanisms and find contrasting patterns of extinction debt. From small fragments, we typically see the rapid initial loss of many species, followed by a slower loss of species on larger timescales. When we consider slow incremental declines of population sizes, we find initially a slow rate of extinction which subsequently increases exponentially. In such cases, the delayed extinctions may go undetected initially both because the extinctions may be small relative to background randomness and because rate itself is not constant and takes time to reach its maximum.

Functional diversity and identity influence the self‐thinning process in young forest communities

Article

Full-text available

Jun 2023
J ECOL

There is increasing evidence that the strength of tree diversity effects on productivity varies considerably over the course of forest development. Evidence points to canopy closure and the subsequent self‐thinning as key phases of forest development during which positive diversity effects emerge. A number of studies have shown that self‐thinning can differ among species, and also in mixtures compared with monocultures. Yet, how diversity influences the process of canopy closure and self‐thinning remains poorly understood. In this study, using 11 years of growth and mortality records from a large diversity experiment, we fitted self‐thinning trajectories for 37 tree communities with equal initial densities and explored whether and how functional diversity and identity may affect these trajectories. We then examined whether the diversity effects on self‐thinning were influenced by differences in growth or in mortality. We found that tree communities' functional diversity and identity strongly influence the self‐thinning process. First, we observed that tree communities dominated by early successional species, and slow‐growing evergreens begin self‐thinning at a larger mean tree size. Second, we found that mixing species with contrasting resource‐use strategies, and the dominance of deciduous, fast‐growing species, reduce tree mortality rate in relation to mean tree size during self‐thinning (i.e. shallower self‐thinning slope). The lower rates of self‐thinning in these functionally diverse communities seem to be explained by both an increase in tree growth and a reduction in density‐related mortality simultaneously over time. Synthesis. Overall, this study highlights that increasing tree diversity has the potential to enhance forest productivity in the long term through a better performance during the self‐thinning process when competition for resources is most intense.

Seed dispersal and tree legacies influence spatial patterns of plant invasion dynamics

Article

Full-text available

Mar 2023

Invasive plant species alter community dynamics and ecosystem properties, potentially leading to regime shifts. Here, the invasion of a non-native tree species into a stand of native tree species is simulated using an agent-based model. The model describes an invasive tree with fast growth and high seed production that produces litter with a suppressive effect on native seedlings, based loosely on Melaleuca quinquenervia , invasive to southern Florida. The effect of a biocontrol agent, which reduces the invasive tree's growth and reproductive rates, is included to study how effective biocontrol is in facilitating the recovery of native trees. Even under biocontrol, the invader has some advantages over native tree species, such as the ability to tolerate higher stem densities than the invaded species and its litter's seedling suppression effect. We also include a standing dead component of both species, where light interception from dead canopy trees influences neighboring tree demographics. The model is applied to two questions. The first is how the mean seedling dispersal rate affects the spread of the invading species into a pure stand of natives, assuming the same mean dispersal distance for both species. For assumed litter seedling suppression that roughly balances the fitness levels of the two species, which species dominates depends on the mean dispersal distance. The invader dominates at both very high and very low mean seedling dispersal distances, while the native tree dominates for dispersal distances in the intermediate range. The second question is how standing dead trees affect either the rate of spread of the invader or the rate of recovery of the native species. The legacy of standing dead invasive trees may delay the recovery of native vegetation. The results here are novel and show that agent-based modeling is essential in illustrating how the fine-scale modeling of local interactions of trees leads to effects at the population level.

Kin selection, kin recognition and kin discrimination in plants revisited: A claim for considering environmental and genetic variability

Article

Mar 2023
PLANT CELL ENVIRON

Considering kin selection in the study of interacting conspecific plants broadens our vision of plant behaviour and brings arguments to explain plant-plant positive interactions. These interactions are the subject of abundant research in community ecology and the role of relatedness in interactions between individuals has become a hot topic. Indeed, the past decade has seen a steady accumulation of exciting but also controversial results regarding the behaviour of plants in the presence of genetically related neighbours, i.e., kin discrimination. In addition to the existence of some methodological and semantic aspects that should be strengthened to accurately reveal the occurrence of kin recognition or selection, we claim here that some key aspects must be considered to increase our ability to detect kin recognition and selection in plants. These aspects include intraspecific variability, the fact that plant-plant interactions are context-dependent, and the lifespan of species (particularly long-lived ones). Additionally, we note that the study of the population spatial genetic structure could be a useful tool to identify candidate settings for the study of kin recognition and selection in plants.

Higher local intra- than interspecific variability in water- and carbon-related leaf traits among Neotropical tree species

Article

Full-text available

Mar 2023
ANN BOT-LONDON

Background and aims: Intraspecific variability in leaf water-related traits remains little explored despite its possible importance in the context of increasing drought frequency and severity. Studies comparing intra- and inter-specific variability of leaf traits often rely on unappropriate sampling designs that result in non-robust estimates, mainly due to an excess of the species/individual ratio in community ecology, or on the contrary to an excess of the individual/species ratio in population ecology. Methods: We virtually tested three strategies to compare intra- and interspecific trait variability. Guided by the results of our simulations, we carried out a field sampling. We measured 9 traits related to leaf water and carbon acquisition in 100 individuals from 10 neotropical tree species. We also assessed trait variation among leaves within individuals and among measurements within leaves to control for sources of intraspecific trait variability. Key results: The most robust sampling based on the same number of species and individuals per species revealed higher intraspecific variability than previously recognized, higher for carbon related traits (47-92% & 4-33% of relative and absolute variation resp.) than for water related traits (47-60% & 14-44% of relative and absolute variation resp.), which remained non negligible. Nevertheless, part of the intraspecific trait variability was explained by leaves variation within individuals (12-100% of relative variation) or measurements variations within leaf (0-19% of relative variation) and not only by individual ontogenetic stages and environmental conditions. Conclusions: We conclude that robust samplings based on the same number of species and individuals per species are needed to explore global or local variation in leaf water and carbon related traits within and among tree species, since our study revealed higher intraspecific variation than previously recognized.

The importance of timely metadata curation to the global surveillance of genetic diversity

Article

Full-text available

Jan 2023

Genetic diversity within species represents a fundamental yet underappreciated level of biodiversity. Because genetic diversity can indicate species resilience to changing climate, its measurement is relevant to many national and global conservation policy targets. Many studies produce large amounts of genome‐scale genetic diversity data for wild populations, but most (87%) do not include the associated spatial and temporal metadata necessary for them to be reused in monitoring programs or for acknowledging the sovereignty of nations or Indigenous Peoples. We undertook a “distributed datathon” to quantify the availability of these missing metadata and to test the hypothesis that their availability decays with time. We also worked to remediate missing metadata by extracting them from associated published papers, online repositories, and from direct communication with authors. Starting with 848 candidate genomic datasets (reduced representation and whole genome) from the International Nucleotide Sequence Database Collaboration, we determined that 561 contained mostly samples from wild populations. We successfully restored spatiotemporal metadata for 78% of these 561 datasets (N = 440 datasets comprising 45,105 individuals from 762 species in 17 phyla). Looking at papers and online repositories was much more fruitful than contacting authors, who only replied to our email requests 45% of the time. Overall, 23% of our email queries to authors unearthed useful metadata. Importantly, we found that the probability of retrieving spatiotemporal metadata declined significantly with the age of the dataset, with a 13.5% yearly decrease for metadata located in published papers or online repositories and up to a 22% yearly decrease for metadata that were only available from authors. This rapid decay in metadata availability, mirrored in studies of other types of biological data, should motivate swift updates to data sharing policies and researcher practices to ensure that the valuable context provided by metadata is not lost to conservation science forever. This article is protected by copyright. All rights reserved

Embracing variability: the use of mixed effects models in Ecology

Thesis

Full-text available

Dec 2022

Melina De Souza Leite

Ecology is a science that is constantly concerned with testing hypotheses or even understanding ecological processes from patterns in data, which are mostly described using statistical tools. Mixed-effects models are increasingly used in ecology given the hierarchical nature of most ecological data and are especially useful in partitioning sources of variability. Mixed-effects models can provide quantitative answers that describe what fraction of a particular pattern is explained by different mechanisms: the "how much" rather than the "which". This shift in perspective can prove very useful in ecology both to understand relative contribution of ecological processes, and to better describe global patterns of variability. In this thesis, I employ and advocate for the use of mixed-effects models in very different ecological questions, but with a common analytical framework: variability partitioning in mixed-effects models. In the first chapter, we seek to understand how matrix (non-habitat) modulates the relative contribution of community assembly processes (environmental filter and drift) of forest birds due to habitat loss in fragmented landscapes. To model bird occurrence, we used a mixed-effect model with association between species attributes and forest cover as fixed effects and several random components to measure relative strength of certain processes. We found that the environmental filter by habitat loss is the dominant process and can be relaxed or strengthened depending on matrix quality, evidencing that the matrix has a strong impact on modulating community assembly processes. In the second chapter, we develop and apply a conceptual and analytical framework through variance partitioning of a mixed-effect model to attribute variability in tree vital rates due to differences among species, space, and time, and their interactions. We apply the framework to growth, mortality, and recruitment rates of 21 globally distributed forests covering over 2.9 million trees of approximately 6,500 species. Our framework has a high potential for identifying the structuring mechanisms of forest dynamics, as it highlights the most promising avenues for future research by increasing understanding of the relative contributions of groups of mechanisms to forest demography and diversity. Along with variance partitioning tools, we highlight in this thesis the great potential of mixed-effects models in connecting statistical and ecological inference, incorporating variability as a diverse and good source of information both to understand relative contribution of ecological processes and to describe more detailed patterns in complex ecological systems.

A review on the potential effects of environmental and economic factors on sheep genetic diversity: Consequences of climate change

Article

Full-text available

Nov 2022

Climate change has a significant effect on the productivity of livestock including milk, meat, and reproduction. This could be attributed to the internal diversion of energy resources towards adaptive mechanisms. Among the climate change variables, thermal stress seems to be the major limiting factor in animal agriculture. A better understanding of the effects of climate change-influenced ecological factors on the genetic diversity of livestock species is warranted. Sheep is an ideal livestock species to be used in investigating environmental adaptation due to its wide range of agroecological habitats, genetic and phenotypic variability. There is a heavy reliance on sheep genetic diversity for future animal protein security, but the implications of climate change on their genetic diversity receive less attention. Here, the potential environmental factors influencing natural selection in sheep populations are presented. We argue that prolonged exposure to these factors plays a major role in influencing the development of adaptation traits in indigenous sheep breeds, consequently leading to the alteration of genetic diversity at specific loci. The factors discussed include hot temperatures (heat stress), insufficient water, low quantity and quality of forage, and prevalence of parasites, pests, and diseases. In addition, genetic diversity, some signatures of selection for adaptation and economic angles of selection are also briefly discussed. A better understanding of environmental factors influencing the genetic diversity of sheep populations will inform breeding and management programs and may offer an opportunity for greater production efficiency with low input costs.

Biological, structural and functional responses of tropical forests to environmental factors

Article

Dec 2022
BIOL CONSERV

As compared to temperate and boreal forests, the majority of tropical forests are biodiverse, structurally-complex and high-functioning ecosystems, yet their potential attributes are vulnerable to anthropogenic disturbances and environmental factors. Here, we hypothesize that the biological, structural and functional attributes of tropical forests respond to water-related climatic and soil nutrient-related factors. Thus, we quantified 27 forest attributes and 20 environmental factors across 189 plots of Sri Lankan tropical forests. Our results suggest that environmental conditions were characterized by both water-related and temperature-related factors, and as such, both coarse-textured and compacted-structured soils determined soil conditions. Forest conditions were characterized by high species-functional diversity, structural complexity and aboveground biomass-related functions. We found strong positive effects of water-related climatic factors (particularly mean annual precipitation) followed by temperature-related climatic factors (i.e., growing degree days) but negligible positive to negative effects of textured-related (i.e., gravel contents) and nutrient-related (i.e., bulk density) soil factors on most of the biological, structural and functional attributes. Overall the biodiversity and carbon stocks of Sri Lankan tropical forests are likely to increase with water-energy balance and improved soil conditions, and thus, studied forests could offset a substantial quantity of anthropogenic carbon emissions to achieve carbon neutrality which can have both regional and global significance if protected from anthropogenic disturbances. Broadly, this study suggests that conservation practices should consider multiple environmental and biotic attributes of the tropical forests during forest management which can have a win-win situation for biodiversity conservation and climate change mitigation.

Competition is critical to the growth of Larix gmelinii and Betula platyphylla in secondary forests in Northeast China under climate change

Article

Apr 2024

Inverse relationship between species competitiveness and intraspecific trait variability may enable species coexistence in experimental seedling communities

Article

Full-text available

Apr 2024

Theory suggests that intraspecific trait variability may promote species coexistence when competitively inferior species have higher intraspecific trait variability than their superior competitors. Here, we provide empirical evidence for this phenomenon in tree seedlings. We evaluated intraspecific variability and plastic response of ten traits in 6750 seedlings of ten species in a three-year greenhouse experiment. While we observed no relationship between intraspecific trait variability and species competitiveness in competition-free homogeneous environments, an inverse relationship emerged under interspecific competition and in spatially heterogeneous environments. We showed that this relationship is driven by the plastic response of the competitively inferior species: Compared to their competitively superior counterparts, they exhibited a greater increase in trait variability, particularly in fine-root traits, in response to competition, environmental heterogeneity and their combination. Our findings contribute to understanding how interspecific competition and intraspecific trait variability together structure plant communities.

Climate biogeography of Arabidopsis thaliana : Linking distribution models and individual variation

Article

Oct 2023

Aim Patterns of individual variation are key to testing hypotheses about the mechanisms underlying biogeographic patterns. If species distributions are determined by environmental constraints, then populations near range margins may have reduced performance and be adapted to harsher environments. Model organisms are potentially important systems for biogeographical studies, given the available range‐wide natural history collections, and the importance of providing biogeographical context to their genetic and phenotypic diversity. Location Global. Taxon Arabidopsis thaliana (‘Arabidopsis’). Methods We fit occurrence records to climate data, and then projected the distribution of Arabidopsis under last glacial maximum, current and future climates. We confronted model predictions with individual performance measured on 2194 herbarium specimens, and we asked whether predicted suitability was associated with life history and genomic variation measured on ~900 natural accessions. Results The most important climate variables constraining the Arabidopsis distribution were winter cold in northern and high elevation regions and summer heat in southern regions. Herbarium specimens from regions with lower habitat suitability in both northern and southern regions were smaller, supporting the hypothesis that the distribution of Arabidopsis is constrained by climate‐associated factors. Climate anomalies partly explained interannual variation in herbarium specimen size, but these did not closely correspond to local limiting factors identified in the distribution model. Late‐flowering genotypes were absent from the lowest suitability regions, suggesting slower life histories are only viable closer to the centre of the realized niche. We identified glacial refugia farther north than previously recognized, as well as refugia concordant with previous population genetic findings. Lower latitude populations, known to be genetically distinct, are most threatened by future climate change. The recently colonized range of Arabidopsis was well‐predicted by our native‐range model applied to certain regions but not others, suggesting it has colonized novel climates. Main Conclusions Integration of distribution models with performance data from vast natural history collections is a route forward for testing biogeographical hypotheses about species distributions and their relationship with evolutionary fitness across large scales.

Living at the edge: the functional niche occupation of woody plant communities in the submediterranean ecotone

Article

Sep 2023
ANN BOT-LONDON

Background and aims Submediterranean areas are rich ecotones, where slight modifications in environmental conditions can lead to substantial changes in the composition of plant communities. They thus offer an ideal scenario to examine plant community assembly. In this study, we followed a trait-based approach including intraspecific variability to elucidate (1) the relationship between niche occupancy components and species richness, (2) the processes governing the assembly of these communities, and (3) the contribution of intraspecific trait variability in shaping the functional trait space. Methods We measured eight morphological and chemical traits in 405 individuals across 60 plots located in different forest communities (Mediterranean, Eurosiberian and Mixed) coexisting within a submediterranean ecosystem in central Spain. We calculated three niche occupancy components related to Hutchinson’s n-dimensional hypervolumes: the total functional volume of the community, the functional overlap between species within the community and the average functional volume per species, and then used null models to explore the relative importance of habitat filtering, limiting similarity and intraspecific variability as assembly patterns. Key Results Both habitat filtering and niche differentiation drive the community assembly of Mediterranean communities, whereas limiting similarity and hierarchical competition shape Eurosiberian communities. Intraspecific responses were mostly explained by shifts in species’ niches across functional space (changes in the position of the centroids of hypervolumes). Conclusions Different assembly mechanisms govern the structure of Mediterranean, Eurosiberian and Mixed plant communities. Combining niche occupancy components with a null model approach at different spatial scales offers new insights into the mechanisms driving plant community assembly. Considering intraspecific variability is indispensable for understanding the mechanisms governing species coexistence in species-rich ecotones.

Effects of intraspecific variation in a native species' phenology on its coexistence with non‐native plants

Article

Sep 2023

Intraspecific trait variation is ubiquitous and is likely to influence species coexistence. Despite theoretical progress, empirical work on the effects of intraspecific variation on the dynamics of competing species is rare. This is because of the formidable empirical requirements necessary to link intraspecific variation in species' functional traits with intraspecific variation in the demographic and competitive rates that mediate coexistence. Here we partially overcome these challenges to determine how intraspecific variation in reproductive phenology in a native Californian annual plant species Lasthenia californica affects its ability to coexist with two non‐native species Bromus madritensis and Lactuca serriola that display contrasting phenological patterns. Using data from a field experiment, we empirically parameterize a model of competitive population dynamics, accounting for the effects of intraspecific phenological trait variation on the native species' response to both intra‐ and interspecific competition. We find that intraspecific variation in phenology drives differences in the native species' response to competition. Moreover, simulations of the parameterized model show that this variation improves the competitive performance of the native species. This occurs because of the effects of nonlinear averaging mediated by a nonlinear, concave‐up competition function that is a general feature of competition across a wide range of taxa. While intraspecific variation improves competitive performance, we also find that the magnitude of the benefit is predicted to be insufficient to prevent competitive exclusion against the non‐native species with early phenogy Bromus . Against the second non‐native species with later phenology Lactuca , intraspecific variation is predicted to result in coexistence where competitive exclusion would otherwise occur, but we could not rule out alternative qualitative outcomes for this interaction.

Masting is uncommon in trees that depend on mutualist dispersers in the context of global climate and fertility gradients

Article

Full-text available

Jun 2023

The benefits of masting (volatile, quasi-synchronous seed production at lagged intervals) include satiation of seed predators, but these benefits come with a cost to mutualist pollen and seed dispersers. If the evolution of masting represents a balance between these benefits and costs, we expect mast avoidance in species that are heavily reliant on mutualist dispersers. These effects play out in the context of variable climate and site fertility among species that vary widely in nutrient demand. Meta-analyses of published data have focused on variation at the population scale, thus omitting periodicity within trees and synchronicity between trees. From raw data on 12 million tree-years worldwide, we quantified three components of masting that have not previously been analysed together: (i) volatility, defined as the frequency-weighted year-to-year variation; (ii) periodicity, representing the lag between high-seed years; and (iii) synchronicity, indicating the tree-to-tree correlation. Results show that mast avoidance (low volatility and low synchronicity) by species dependent on mutualist dispersers explains more variation than any other effect. Nutrient-demanding species have low volatility, and species that are most common on nutrient-rich and warm/wet sites exhibit short periods. The prevalence of masting in cold/dry sites coincides with climatic conditions where dependence on vertebrate dispersers is less common than in the wet tropics. Mutualist dispersers neutralize the benefits of masting for predator satiation, further balancing the effects of climate, site fertility and nutrient demands.

Connecting higher order interactions with ecological stability in experimental aquatic food webs

Preprint

Full-text available

May 2023

Community ecology is built on theory that represents the strength of interactions between species as pairwise links. Higher order interactions occur when the presence of a third (or more) species changes the pairwise interaction between a focal pair. Recent theoretical work has highlighted the stabilizing role of higher order interactions for communities, yet it remains unclear how important higher order effects are in real communities. Here we used experimental communities of aquatic protists to examine the relationship between higher-order interactions and community stability (i.e., the persistence of species in a community). We cultured a focal pair of consumers in the presence of additional competitors and a predator and collected time series data of their abundances. We then fitted competition models with and without HOIs to measure interaction strength between the focal pair across different community compositions. We used survival analysis to measure the persistence of individual species. We found evidence that additional species positively affected persistence of the focal species and that HOIs were present in most of our communities. However, persistence was only linked to HOIs for one of the focal species. Our results vindicate community ecology theory positing that species interactions may deviate from assumptions of pairwise interactions, opening avenues to consider possible consequences for coexistence and community stability.

Age-Girth Stand Structure of Himalayan Fir and Growth-NDVI Relationship in the Treeline Transects of Western Himalaya: An Ecological Perspective

Chapter

Feb 2023

Subalpine forests are the important indicators of climate change and the future biomass stock under forest densification and treeline advancement. Analysing the stand structure and growth behaviour of trees is essential for the assessment of the functioning and sustainable management of forest resources. We provided the girth and age stand structure of Abies spp. (silver fir) from the treeline ecotone in the moist transects of Western Himalaya, and the relationship between fir growth and normalized difference vegetation index (NDVI) for assessing the forest health. The age and girth class analysis of fir trees from the four different treeline transects revealed the presence of mixed age and girth classes within the treeline ecotone. Fir trees were established around 200 years ago in the transects with subsequent densification, but the fir treeline showed static behaviour during the later part of the twentieth century. We also found variations in the girth size increment with age amongst the fir trees, which is statistically significant for the fir trees growing near treeline ecotone. The relationships between the tree-ring width chronologies and NDVI suggested the role of temperature in controlling forest health. Such studies could help in extending the existing vegetation cover records to more past for better evaluation of changes in forest health and interaction with climatic conditions.Keywords Abies BiomassHimachal PradeshKashmirTree-ring widthUttarakhandVegetation cover

Seedling performance in a dioecious tree species is similar near female and male conspecific adults despite differences in colonization by arbuscular mycorrhizal fungi

Preprint

Full-text available

Jan 2023

Patterns of seedling growth and survival near conspecific plants have important consequences for species diversity in plant communities, but the factors causing intraspecific variation in seedling performance are unclear. Greater seed production or size of adult plants could both drive the local accumulation of specialized antagonist species in the environment and affect conspecific seedling performance. Experiments with dioecious species, in which only female individuals produce seeds, decouple these factors in areas of high conspecific density. To assess whether conspecific seedling performance is reduced in the environments associated with seed-producing female trees relative to male or heterospecific trees, we conducted shadehouse and field experiments with a dioecious tropical tree species, Virola surinamensis (Myristicaceae), on Barro Colorado Island, Panama. The shadehouse experiment isolated the effect of soil microbial communities on seedling performance and allowed us to quantify colonization by mutualistic arbuscular mycorrhizal (AM) fungi, while the field experiment allowed us to assess seedling performance in a context that included the effects of nearby conspecific trees. Seedling biomass at the end of the 8-mo shadehouse experiment was similar between experimental plants exposed to soil microbial communities from underneath female conspecific, male conspecific, and heterospecific adult trees. However, seedling colonization by AM fungi was higher in male soil microbial communities. Similarly, at the end of the 7-mo field experiment, survival and biomass did not differ among experimental seedlings grown in female, male, and heterospecific environments (but biomass was reduced near larger conspecific adults). Together, our experiments did not support the hypothesis that conspecific seedling performance is reduced in female environments relative to male environments, despite reductions in colonization by mutualistic AM fungi in female soil microbial communities. Thus, intraspecific variation in the biotic interactions between conspecific seedlings, established adults, and mutualist species may not translate directly to patterns of tropical tree seedling survival.

Evolutionary effects of individual variation and dimensionality of higher-order interactions on the robustness of species coexistence

Preprint

Full-text available

Dec 2022

Although the eco-evolutionary effects of individual variation for species coexistence are still widely debated, theoretical evidence appears to support a negative impact on coexistence. Mechanistic models of eco-evolutionary effects of individual variation focus largely on pairwise interactions, while the dynamics of communities where both pairwise and higher-order interactions (HOIs) are pervasive are not known. In addition, most studies have focused on effects of high dimensional HOIs on species coexistence when in reality such HOIs could be highly structured and low-dimensional, as species interactions could primarily be mediated through phenotypic traits. Here, combining quantitative genetics and Lotka-Volterra equations, we explored the eco-evolutionary effects of individual variation on the patterns of species coexistence in a competitive community dictated by pairwise interactions and HOIs. Specifically, we compare six different models in which HOIs were modelled to be trait-mediated (low-dimensional) or random (high-dimensional) and evaluated its impact on robustness of species coexistence in the presence of different levels of individual variation. Across the six different models, we found that individual variation did not promote species coexistence, irrespective of whether interactions were pairwise or were of higher-order. However, individual trait variation could stabilize communities to external perturbation more so when interactions were of higher order. When compared across models, species coexistence is promoted when HOIs strengthen pairwise intraspecific competition more so than interspecific competition, and when HOIs act in a hierarchical manner. Additionally, across the models, we found that species traits tend to cluster together when individual variation in the community was low. We argue that, while individual variation can influence community patterns in many different ways, they more often lead to fewer species coexisting together.

Modeling Community Dynamics Through Environmental Effects, Species Interactions and Movement

Article

Nov 2022

Understanding how communities respond to environmental change is frustrated by the fact that both species interactions and movement affect biodiversity in unseen ways. To evaluate the contributions of species interactions on community growth, dynamic models that can capture nonlinear responses to the environment and the redistribution of species across a spatial range are required. We develop a time-series framework that models the effects of environment–species interactions as well as species–species interactions on population growth within a community. Novel aspects of our model include allowing for species redistribution across a spatial region, and addressing the issue of zero inflation. We adopt a hierarchical Bayesian approach, enabling probabilistic uncertainty quantification in the model parameters. To evaluate the impacts of interactions and movement on population growth, we apply our model using data from eBird, a global citizen science database. To do so, we also present a novel method of aggregating the spatially biased eBird data collected at point-level. Using an illustrative region in North Carolina, we model communities of six bird species. The results provide evidence of nonlinear responses to interactions with the environment and other species and demonstrate a pattern of strong intraspecific competition coupled with many weak interspecific species interactions. Supplementary materials accompanying this paper appear online.

The role of trait-mediated indirect interactions for multispecies plant–animal mutualisms

Chapter

Dec 2012

Rebecca E. Irwin

There is increasing evidence that the structure and functioning of ecological communities and ecosystems are strongly influenced by flexible traits of individuals within species. A deep understanding of how trait flexibility alters direct and indirect species interactions is crucial for addressing key issues in basic and applied ecology. This book provides an integrated perspective on the ecological and evolutionary consequences of interactions mediated by flexible species traits across a wide range of systems. It is the first volume synthesizing the rapidly expanding research field of trait-mediated indirect effects and highlights how the conceptual framework of these effects can aid the understanding of evolutionary processes, population dynamics, community structure and stability, and ecosystem function. It not only brings out the importance of this emerging field for basic ecological questions, but also explores the implications of trait-mediated interactions for the conservation of biodiversity and the response of ecosystems to anthropogenic environmental changes.

Evolutionary indirect effects: examples from introduced plant and herbivore interactions

Chapter

Dec 2012

Jennifer A. Lau

Natural enemy functional identity, trait-mediated interactions and biological control

Chapter

Dec 2012

Indirect evolutionary interactions in a multitrophic system

Chapter

Dec 2012

Trait-mediated indirect interactions in size-structured populations: causes and consequences for species interactions and community dynamics

Chapter

Dec 2012

Volker H W Rudolf

Janzen, D. H. Herbivores and number of tree species in tropical forests. American Naturalist

Article

Full-text available

Nov 1970

Daniel Janzen

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.

Long-Term Studies of Vegetation Dynamics

Article

Full-text available

Aug 2001

By integrating a wide range of experimental, comparative, and theoretical approaches, ecologists are starting to gain a detailed understanding of the long-term dynamics of vegetation. We explore how patterns of variation in demographic traits among species have provided insight into the processes that structure plant communities. We find a common set of mechanisms, derived from ecological and evolutionary principles, that underlie the main forces shaping systems as diverse as annual plant communities and tropical forests. Trait variation between species maintains diversity and has important implications for ecosystem processes. Hence, greater understanding of how Earth's vegetation functions will likely require integration of ecosystem science with ideas from plant evolutionary, population, and community ecology.

The Importance of Demographic Niches to Tree Diversity

Article

Full-text available

Aug 2006
SCIENCE

Most ecological hypotheses about species coexistence hinge on species differences, but quantifying trait differences across species in diverse communities is often unfeasible. We examined the variation of demographic traits using a global tropical forest data set covering 4500 species in 10 large-scale tree inventories. With a hierarchical Bayesian approach, we quantified the distribution of mortality and growth rates of all tree species at each site. This allowed us to test the prediction that demographic differences facilitate species richness, as suggested by the theory that a tradeoff between high growth and high survival allows species to coexist. Contrary to the prediction, the most diverse forests had the least demographic variation. Although demographic differences may foster coexistence, they do not explain any of the 16-fold variation in tree species richness observed across the tropics.

Climate variability has a stabilizing effect on coexistence of prairie grasses

Article

Full-text available

Sep 2006

How expected increases in climate variability will affect species diversity depends on the role of such variability in regulating the coexistence of competing species. Despite theory linking temporal environmental fluctuations with the maintenance of diversity, the importance of climate variability for stabilizing coexistence remains unknown because of a lack of appropriate long-term observations. Here, we analyze three decades of demographic data from a Kansas prairie to demonstrate that interannual climate variability promotes the coexistence of three common grass species. Specifically, we show that (i) the dynamics of the three species satisfy all requirements of “storage effect” theory based on recruitment variability with overlapping generations, (ii) climate variables are correlated with interannual variation in species performance, and (iii) temporal variability increases low-density growth rates, buffering these species against competitive exclusion. Given that environmental fluctuations are ubiquitous in natural systems, our results suggest that coexistence based on the storage effect may be underappreciated and could provide an important alternative to recent neutral theories of diversity. Field evidence for positive effects of variability on coexistence also emphasizes the need to consider changes in both climate means and variances when forecasting the effects of global change on species diversity. • climate change • competition • grassland • plant community • population dynamics

Mechanisms of plant survival and mortality during drought: Why do some plants survive while others succumb to drought?

Article

Full-text available

Feb 2008
NEW PHYTOL

Severe droughts have been associated with regional‐scale forest mortality worldwide. Climate change is expected to exacerbate regional mortality events; however, prediction remains difficult because the physiological mechanisms underlying drought survival and mortality are poorly understood. We developed a hydraulically based theory considering carbon balance and insect resistance that allowed development and examination of hypotheses regarding survival and mortality. Multiple mechanisms may cause mortality during drought. A common mechanism for plants with isohydric regulation of water status results from avoidance of drought‐induced hydraulic failure via stomatal closure, resulting in carbon starvation and a cascade of downstream effects such as reduced resistance to biotic agents. Mortality by hydraulic failure per se may occur for isohydric seedlings or trees near their maximum height. Although anisohydric plants are relatively drought‐tolerant, they are predisposed to hydraulic failure because they operate with narrower hydraulic safety margins during drought. Elevated temperatures should exacerbate carbon starvation and hydraulic failure. Biotic agents may amplify and be amplified by drought‐induced plant stress. Wet multidecadal climate oscillations may increase plant susceptibility to drought‐induced mortality by stimulating shifts in hydraulic architecture, effectively predisposing plants to water stress. Climate warming and increased frequency of extreme events will probably cause increased regional mortality episodes. Isohydric and anisohydric water potential regulation may partition species between survival and mortality, and, as such, incorporating this hydraulic framework may be effective for modeling plant survival and mortality under future climate conditions. Contents Summary 1 I. Introduction 2 II. Consequences of vegetation mortality 3 III. Global patterns of mortality 3 IV. Hypotheses on mechanisms of drought‐related mortality 4 V. Evidence for hypothesized mechanisms 5 VI. Implications of future climate on hypothesized mortality mechanisms 13 VII. Conclusions 15 Acknowledgements 15 References 15

Plant Strategies and the Dynamics and Structure of Plant Communities. (MPB-26), Volume 26

Book

Dec 1988

David Tilman

Stability of forest biodiversity

Article

Jul 2003

Two hypotheses to explain potentially high forest biodiversity have different implications for the number and kinds of species that can coexist and the potential loss of biodiversity in the absence of speciation. The first hypothesis involves stabilizing mechanisms, which include tradeoffs between species in terms of their capacities to disperse to sites where competition is weak, to exploit abundant resources effectively and to compete for scarce resources. Stabilization results because competitors thrive at different times and places. An alternative, 'neutral model' suggests that stabilizing mechanisms may be superfluous. This explanation emphasizes 'equalizing' mechanisms, because competitive exclusion of similar species is slow. Lack of ecologically relevant differences means that abundances experience random 'neutral drift', with slow extinction. The relative importance of these two mechanisms is unknown, because assumptions and predictions involve broad temporal and spatial scales. Here we demonstrate that predictions of neutral drift are testable using palaeodata. The results demonstrate strong stabilizing forces. By contrast with the neutral prediction of increasing variance among sites over time, we show that variances in post-Glacial tree abundances among sites stabilize rapidly, and abundances remain coherent over broad geographical scales.

Inaugural Article by a Recently Elected Academy Member:

Article

Jul 2004

David Tilman

Stochastic niche theory resolves many of the differences between neutral theory and classical tradeoff-based niche theories of resource competition and community structure. In stochastic niche theory, invading species become established only if propagules can survive stochastic mortality while growing to maturity on the resources left unconsumed by established species. The theory makes three predictions about community structure. First, stochastic niche assembly creates communities in which species dominate approximately equally wide "slices" of the habitat's spatial heterogeneity. These niche widths generate realistic distributions of species relative abundances for which, contrary to neutral theory but consistent with numerous observations, there are strong correlations among species traits, species abundances, and environmental conditions. Second, slight decreases in resource levels are predicted to cause large decreases in the probability that a propagule would survive to be an adult. These decreases cause local diversity to be limited by the inhibitory effects of resource use by established species on the establishment (recruitment) of potential invaders. If resource pulses or disturbance allowed invaders to overcome this recruitment limitation, many more species could indefinitely coexist. Third, the low invasibility of high diversity communities is predicted to result not from diversity per se, but from the uniformly low levels of resources that occur in high-diversity communities created by stochastic competitive assembly. This prediction provides a potential solution to the invasion paradox, which is the tendency for highly diverse regions to be more heavily invaded.

Plant strategies and the dynamics and structure of plant communities

Article

May 1989

Without Abstract

Community Equilibria and Stability, and an Extension of the Competitive Exclusion Principle

Article

Sep 1970

Simon Levin

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 Paradox of the Plankton

Article

May 1961

G.E. Hutchinson

Fecundity of trees and the colonization-competition hypothesis

Article

Aug 2004

Colonization–competition trade-offs represent a stabilizing mechanism that is thought to maintain diversity of forest trees. If so, then early-successional species should benefit from high capacity to colonize new sites, and late-successional species should be good competitors. Tests of this hypothesis in forests have been precluded by an inability to estimate the many factors that contribute to seed production and dispersal, particularly the many types of stochasticity that contribute to fecundity data. We develop a hierarchical Bayes modeling structure, and we use it to estimate fecundity schedules from the two types of data that ecologists typically collect, including seed-trap counts and observations of tree status. The posterior density is obtained using Markov-chain Monte Carlo techniques. The flexible structure yields estimates of size and covariate effects on seed production, variability associated with population heterogeneity, and interannual stochasticity (variability and se-rial autocorrelation), sex ratio, and dispersal. It admits the errors in data associated with the ability to accurately recognize tree status and process misspecification. We estimate year-by-year seed-production rates for all individuals in each of nine sample stands from two regions and up to 11 years. A rich characterization of differences among species and relationships among individuals allows evaluation of a number of hypotheses related to masting, effective population sizes, and location and covariate effects. It demonstrates large bias in previous methods. We focus on implications for colonization–competition and a related hypothesis, the successional niche—trade-offs in the capacity to exploit high re-source availability in early successional environments vs. the capacity to survive low-resource conditions late in succession. Contrary to predictions of trade-off hypotheses, we find no relationship between suc-cessional status and fecundity, dispersal, or expected arrivals at distant sites. Results suggest a mechanism for maintenance of diversity that may be more general than colonization– competition and successional niches. High variability and strong individual effects (vari-ability within populations) generate massive stochasticity in recruitment that, when com-bined with ''storage,'' may provide a stabilizing mechanism. The storage effect stabilizes diversity when species differences ensure that responses to stochasticity are not highly correlated among species. Process variability and individual effects mean that many species have the advantage at different times and places even in the absence of ''deterministic'' trade-offs. Not only does colonization vary among species, but also individual behavior is highly stochastic and weakly correlated among members of the same population. Although these factors are the dominant sources of variability in data sets (substantially larger than the deterministic relationships typically examined), they have not been not included in the models that ecologists have used to evaluate mechanisms of species coexistence (e.g., even individual-based models lack random individual effects). Recognition of the mechanisms of coexistence requires not only heuristic models that capture the principal sources of stochasticity, but also data-modeling techniques that allow for their estimation.

The impact of neutrality, niche differentiation and species input on diversity and abundance distributions

Article

Jun 2007
OIKOS

We present a spatially-explicit generalization of Hubbell's model of community dynamics in which the assumption of neutrality is relaxed by incorporating dispersal limitation and habitat preference. In simulations, diversity and species abundances were governed by the rate at which new species were introduced (usually called ‘speciation’) and nearly unaffected by dispersal limitation and habitat preference. Of course, in the absence of species input, diversity is maintained solely by niche differences. We conclude that the success of the neutral model in predicting the abundance distribution has nothing to do with neutrality, but rather with the species-introduction process: when new species enter a community regularly as singletons, the typical J-shaped abundance distribution, with a long tail of rare species, is always observed, whether species differ in habitat preferences or not. We suggest that many communities are indeed driven by the introduction process, accounting for high diversity and rarity, and that species differences may be largely irrelevant for either.

The Unified Neutral Theory of Biodiversity and Biogeography

Chapter

Jan 2001
Monogr Popul Biol

Stephen Hubbell

Silvertown J.. Plant coexistence and the niche. Trends Ecol Evol 19: 605-611

Article

Nov 2004
TRENDS ECOL EVOL

Jonathan Silvertown

How large numbers of competing plant species manage to coexist is a major unresolved question in community ecology. The classical explanation, that each species occupies its own niche, seems at first unlikely because most plants require the same set of resources and acquire these in a limited number of ways. However, recent studies, although few in number and incomplete in many ways, do suggest that plants segregate along various environmental niche axes, including gradients of light, soil moisture and root depth, and that partitioning of soil nutrients occurs, possibly through the mediation of microbial symbionts, some of which are more species specific than was previously thought. Although it is unlikely that niche separation along environmental axes is the only mechanism of coexistence in any large community, the evidence now suggests that it plays a more significant role than has been previously appreciated. More research into the consequences of various known tradeoffs is likely to uncover further cases of niche separation facilitating coexistence.

Plant Strategies and the Dynamic Structure of Plant Communities

Article

Jan 1988

D.G. Tilman

Coexistence in a Variable Environment

Article

Dec 1979

Richard Levins

A community which would not reach a stable equilibrium may nevertheless persist if there is temporal variation and nonlinear dynamics. A procedure is introduced for taking time averages of the rates of change. Since the average of a nonlinear function is not the function of the average, higher terms such as the variances of resources or covariances among species and environmental factors enter into the coexistence conditions. These measures behave as if they were resources. Therefore the number of consumer species cannot exceed the number of resources plus distinct nonlinearities. The nonlinearities arise from predator saturation, learning, group hunting, multiple nutritional requirements, or seasonally variable feeding rates. It is shown that there is no long term correlation between the abundance of a species and its rates of increase.

The Unified Neutral Theory of Biodiversity

Article

Jan 2000

S.P. Hubbell

A unified theory of biogeography and relative species abundance are of central importance in biogeography and community ecology, yet these two bodies of theory have heretofore have been largely unconnected. Incorporating speciation into into the theory of island biogeography unexpectedly results in unification of these two theories. The unified theory predicts the existence of fundamental biodiversity number q that controls not only species richness, but also realtive species abundance in the source area metacommunity at equilibrium between speciation and extinction. With additional parameters for island size and migration rate

Chesson P.. Mechanisms of maintenance of species diversity. Annu Rev Ecol Evol Syst 31: 343-366

Article

Nov 2000
Annu Rev Ecol Systemat

Peter Chesson

The focus of most ideas on diversity maintenance is species coexistence, which may be stable or unstable. Stable coexistence can be quantified by the long-term rates at which community members recover from low density. Quantification shows that coexistence mechanisms function in two major ways: they may be a) equalising because they tend to minimise average fitness differences between species b) stabilising because they tend to increase negative intraspecific interactions relative to negative interspecific interactions Stabilising mechanisms are essential for species coexistence and include traditional mechanisms such as resource partitioning and frequency-dependent predation, as well as mechanisms that depend on fluctuations of population densities and environemntal factors in space and time. Equalising mechanisms contribute to stable coexistence because they reduce average fitness inequalities which might negate the effects of stabilising mechanisms. Models of unstable coexistence, in which species diversity decays over time, have focused almost exclusively on equalising mechanisms. These models would be more robust if they included stabilising mechanisms, which arise in many and varied ways but need not be adequate for full stability of a system. Models of unstable coexistence invite a briader view of diversity maintenence incorporating species turnover.

Functional tradeoffs determine species coexistence via the storage effect

Article

Aug 2009
P NATL ACAD SCI USA

How biological diversity is generated and maintained is a fundamental question in ecology. Ecologists have delineated many mechanisms that can, in principle, favor species coexistence and hence maintain biodiversity. Most such coexistence mechanisms require or imply tradeoffs between different aspects of species performance. However, it remains unknown whether simple functional tradeoffs underlie coexistence mechanisms in diverse natural systems. We show that functional tradeoffs explain species differences in long-term population dynamics that are associated with recovery from low density (and hence coexistence) for a community of winter annual plants in the Sonoran Desert. We develop a new general framework for quantifying the magnitude of coexistence via the storage effect and use this framework to assess the strength of the storage effect in the winter annual community. We then combine a 25-year record of vital rates with morphological and physiological measurements to identify functional differences between species in the growth and reproductive phase of the life cycle that promote storage-effect coexistence. Separation of species along a tradeoff between growth capacity and low-resource tolerance corresponds to differences in demographic responses to environmental variation across years. Growing season precipitation is one critical environmental variable underlying the demographic decoupling of species. These results demonstrate how partially decoupled population dynamics that promote local biodiversity are associated with physiological differences in resource uptake and allocation between species. These results for a relatively simple system demonstrate how long-term community dynamics relate to functional biology, a linkage scientists have long sought for more complex systems.

Competition, Habitat Selection, and Character Displacement in a Patchy Environment

Article

Jul 1964

Niche tradeoffs, neutrality, and community structure: A stochastic theory of resource competition, invasion, and community assembly

Article

Aug 2004

David Tilman

LETTER Reconciling niche and neutrality: the continuum hypothesis

Article

May 2006
ECOL LETT

In this study, we ask if instead of being fundamentally opposed, niche and neutral theories could simply be located at the extremes of a continuum. First, we present a model of recruitment probabilities that combines both niche and neutral processes. From this model, we predict and test whether the relative importance of niche vs. neutral processes in controlling community dynamics will vary depending on community species richness, niche overlap and dispersal capabilities of species (both local and long distance). Results demonstrate that niche and neutrality form ends of a continuum from competitive to stochastic exclusion. In the absence of immigration, competitive exclusion tends to create a regular spacing of niches. However, immigration prevents the establishment of a limiting similarity. The equilibrium community consists of a set of complementary and redundant species, with their abundance determined, respectively, by the distribution of environmental conditions and the amount of immigration.

Neutral theory and the evolution of ecological equivalence

Article

Jul 2006

Stephen Hubbell

Since the publication of the unified neutral theory in 2001, there has been much discussion of the theory, pro and con. The hypothesis of ecological equivalence is the fundamental yet controversial idea behind neutral theory. Assuming trophically similar species are demographically alike (symmetric) on a per capita basis is only an approximation, but it is equivalent to asking: How many of the patterns of ecological communities are the result of species similarities, rather than of species differences? The strategy behind neutral theory is to see how far one can get with the simplification of assuming ecological equivalence before introducing more complexity. In another paper, I review the empirical evidence that led me to hypothesize ecological equivalence among many of the tree species in the species-rich tropical forest on Barro Colorado Island (BCI). In this paper, I develop a simple model for the evolution of ecological equivalence or niche convergence, using as an example evolution of the suite of life history traits characteristic of shade tolerant tropical tree species. Although the model is simple, the conclusions from it seem likely to be robust. I conclude that ecological equivalence for resource use are likely to evolve easily and often, especially in species-rich communities that are dispersal and recruitment limited. In the case of the BCI forest, tree species are strongly dispersal- and recruitment-limited, not only because of restricted seed dispersal, but also because of low recruitment success due to heavy losses of the seedling stages to predators and pathogens and other abiotic stresses such as drought. These factors and the high species richness of the community strongly reduce the potential for competitive exclusion of functionally equivalent or nearly equivalent species.

Long-Term CO₂ Enrichment of a Forest Ecosystem: Implications for Forest Regeneration and Succession

Article

Jul 2007

The composition and successional status of a forest affect carbon storage and net ecosystem productivity, yet it remains unclear whether elevated atmospheric carbon dioxide (CO2) will impact rates and trajectories of forest succession. We examined how CO2 enrichment (+200 microL CO2/L air differential) affects forest succession through growth and survivorship of tree seedlings, as part of the Duke Forest free-air CO2 enrichment (FACE) experiment in North Carolina, USA. We planted 2352 seedlings of 14 species in the low light forest understory and determined effects of elevated CO2 on individual plant growth, survival, and total sample biomass accumulation, an integrator of plant growth and survivorship over time, for six years. We used a hierarchical Bayes framework to accommodate the uncertainty associated with the availability of light and the variability in growth among individual plants. We found that most species did not exhibit strong responses to CO2. Ulmus alata (+21%), Quercus alba (+9.5%), and nitrogen-fixing Robinia pseudoacacia (+230%) exhibited greater mean annual relative growth rates under elevated CO2 than under ambient conditions. The effects of CO2 were small relative to variability within populations; however, some species grew better under low light conditions when exposed to elevated CO2 than they did under ambient conditions. These species include shade-intolerant Liriodendron tulipifera and Liquidambar styraciflua, intermediate-tolerant Quercus velutina, and shade-tolerant Acer barbatum, A. rubrum, Prunus serotina, Ulmus alata, and Cercis canadensis. Contrary to our expectation, shade-intolerant trees did not survive better with CO2 enrichment, and population-scale responses to CO2 were influenced by survival probabilities in low light. CO2 enrichment did not increase rates of sample biomass accumulation for most species, but it did stimulate biomass growth of shade-tolerant taxa, particularly Acer barbatum and Ulmus alata. Our data suggest a small CO2 fertilization effect on tree productivity, and the possibility of reduced carbon accumulation rates relative to today's forests due to changes in species composition.

Resolving the biodiversity debate

Article

Sep 2007
ECOL LETT

The paradox of biodiversity involves three elements, (i) mathematical models predict that species must differ in specific ways in order to coexist as stable ecological communities, (ii) such differences are difficult to identify, yet (iii) there is widespread evidence of stability in natural communities. Debate has centred on two views. The first explanation involves tradeoffs along a small number of axes, including 'colonization-competition', resource competition (light, water, nitrogen for plants, including the 'successional niche'), and life history (e.g. high-light growth vs. low-light survival and few large vs. many small seeds). The second view is neutrality, which assumes that species differences do not contribute to dynamics. Clark et al. (2004) presented a third explanation, that coexistence is inherently high dimensional, but still depends on species differences. We demonstrate that neither traditional low-dimensional tradeoffs nor neutrality can resolve the biodiversity paradox, in part by showing that they do not properly interpret stochasticity in statistical and in theoretical models. Unless sample sizes are small, traditional data modelling assures that species will appear different in a few dimensions, but those differences will rarely predict coexistence when parameter estimates are plugged into theoretical models. Contrary to standard interpretations, neutral models do not imply functional equivalence, but rather subsume species differences in stochastic terms. New hierarchical modelling techniques for inference reveal high-dimensional differences among species that can be quantified with random individual and temporal effects (RITES), i.e. process-level variation that results from many causes. We show that this variation is large, and that it stands in for species differences along unobserved dimensions that do contribute to diversity. High dimensional coexistence contrasts with the classical notions of tradeoffs along a few axes, which are often not found in data, and with 'neutral models', which mask, rather than eliminate, tradeoffs in stochastic terms. This mechanism can explain coexistence of species that would not occur with simple, low-dimensional tradeoff scenarios.

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Individuals and the Variation Needed for High Species Diversity in Forest Trees

Abstract

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