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Functional traits are expected to modulate plant competitive dynamics. However, how traits and their plasticity in response to contrasting environments connect with the mechanisms determining species coexistence remains poorly understood. Here, we couple field experiments under two contrasting climatic conditions to a plant population model describing competitive dynamics between 10 annual plant species in order to evaluate how 19 functional traits, covering physiological, morphological and reproductive characteristics, are associated with species’ niche and fitness differences. We find a rich diversity of univariate and multidimensional associations, which highlight the primary role of traits related to water- and lightuse- efficiency for modulating the determinants of competitive outcomes. Importantly, such traits and their plasticity promote species coexistence across climatic conditions by enhancing stabilizing niche differences and by generating competitive trade-offs between species. Our study represents a significant advance showing how leading dimensions of plant function connect to the mechanisms determining the maintenance of biodiversity.

Ecologists have identified a growing number of functional traits that promote invasion. However, whether trait differences between exotic and native species promote invasion success by enhancing niche differences or giving invaders competitive advantages is poorly understood. We explored the mechanisms by which phenology determines invasion success in a California annual plant community by quantifying how the seasonal timing of growth relates to niche differences that stabilize coexistence, and the competitive ability differences that drive dominance and exclusion. We parameterized models of community dynamics from experimentally assembled annual communities in which exotic plants displayed earlier, coincident, or later phenology than native residents. Using recent theoretical advances from the coexistence literature, we found that differences in phenology promote stabilizing niche differences between exotic and native species. However, phenology was more strongly related to competitive ability differences, allowing later invaders to outcompete earlier native competitors and native residents to outcompete earlier invaders in field experiments. Few of these insights could be inferred by comparing the competitive outcomes across invaders, highlighting the need to quantify niche and competitive ability differences when disentangling how species differences drive invasion success.

Questions related to understanding which characteristics of the recipient communities make them vulnerable to invasion (i.e. invasibility) are commonly formulated in such a way that they consider ecological communities as whole entities. Yet, species within recipient communities have specific responses to the introduction of exotic species. As a consequence, little is known about the role of species interaction networks within and between trophic levels in determining the invasion likelihood of exotics introduced in previously uninvaded communities and the temporal dynamics of invaded communities. To obtain a mature understanding of invasion processes and their impacts, I propose here to apply a structural stability approach of species coexistence to the field of biological invasions. The main aim here is to distinguish how direct and indirect species’ responses to invasion in competitive and trophic networks translate to the mechanisms determining the maintenance and stability of species diversity in multispecies and multitrophic assemblages. This approach brings the further opportunity to integrate the field of biological invasions within a broader perspective about the study of novel ecosystems, that is, ecosystems that contain exotic species and face new introductions sometimes under abrupt environmental changes. Moreover, it can help to target specific mechanisms separating winners from losers in novel ecosystems. Synthesis. A solid framework to predict biological invasions and understand community assembly along temporal scales under novel ecosystems can be obtained by framing species’ responses to invasion within recent theoretical and methodological advances stemming from coexistence theory, which seeks to determine how diversity is maintained. A plain language summary is available for this article. Plain Language Summary

Understanding the processes maintaining species diversity is a central problem in ecology, with implications for the conservation and management of ecosystems. Although biologists often assume that trait differences between competitors promote diversity, empirical evidence connecting functional traits to the niche differences that stabilize species coexistence is rare. Obtaining such evidence is critical because traits also underlie the average fitness differences driving competitive exclusion, and this complicates efforts to infer community dynamics from phenotypic patterns. We coupled field-parameterized mathematical models of competition between 102 pairs of annual plants with detailed sampling of leaf, seed, root, and whole-plant functional traits to relate phenotypic differences to stabilizing niche and average fitness differences. Single functional traits were often well correlated with average fitness differences between species, indicating that competitive dominance was associated with late phenology, deep rooting, and several other traits. In contrast, single functional traits were poorly correlated with the stabilizing niche differences that promote coexistence. Niche differences could only be described by combinations of traits, corresponding to differentiation between species in multiple ecological dimensions. In addition, several traits were associated with both fitness differences and stabilizing niche differences. These complex relationships between phenotypic differences and the dynamics of competing species argue against the simple use of single functional traits to infer community assembly processes but lay the groundwork for a theoretically justified trait-based community ecology. Significance Biologists have long understood that differences between species in traits such as bill shape or rooting depth can maintain diversity in communities by promoting specialization and reducing competition. Here we test the assumption that phenotypic differences drive the stabilizing niche differences that promote coexistence. Using advances in ecological theory and detailed experiments we quantify average fitness and stabilizing niche differences between 102 plant species pairs and relate these differences to 11 functional traits. Individual traits were correlated with fitness differences that drive competitive exclusion but not stabilizing niche differences that promote coexistence. Stabilizing niche differences could only be described by combinations of traits, representing differentiation in multiple dimensions. This challenges the simplistic use of trait patterns to infer community assembly.

Summary One of the most pervasive concepts in the study of community assembly is the metaphor of the environmental filter, which refers to abiotic factors that prevent the establishment or persistence of species in a particular location. The metaphor has its origins in the study of community change during succession and in plant community dynamics, although it has gained considerable attention recently as part of a surge of interest in functional trait and phylogenetic‐based approaches to the study of communities. While the filtering metaphor has clear utility in some circumstances, it has been challenging to reconcile the environmental filtering concept with recent developments in ecological theory related to species coexistence. These advances suggest that the evidence used in many studies to assess environmental filtering is insufficient to distinguish filtering from the outcome of biotic interactions. We re‐examine the environmental filtering metaphor from the perspective of coexistence theory. In an effort to move the discussion forward, we present a simple framework for considering the role of the environment in shaping community membership, review the literature to document the evidence typically used in environmental filtering studies and highlight research challenges to address in coming years. The current usage of the environmental filtering term in empirical studies likely overstates the role abiotic tolerances play in shaping community structure. We recommend that the term ‘environmental filtering’ only be used to refer to cases where the abiotic environment prevents establishment or persistence in the absence of biotic interactions, although only 15% of the studies in our review presented such evidence. Finally, we urge community ecologists to consider additional mechanisms aside from environmental filtering by which the abiotic environment can shape community pattern. Lay Summary

Although observations of species-rich communities have long served as a primary motivation for research on the coexistence of competitors, the majority of our empirical and theoretical understanding comes from two-species systems. How much of the coexistence observed in species-rich communities results from indirect effects among competitors that only emerge in diverse systems remains poorly understood. Resolving this issue requires simple, scalable, and intuitive metrics for quantifying the conditions for coexistence in multispecies systems, and how these conditions differ from those expected based solely on pairwise interactions. To achieve these aims, we develop a structural approach for studying the set of parameter values compatible with n-species coexistence given the geometric constraints imposed by the matrix of competition coefficients. We derive novel mathematical metrics analogous to stabilizing niche differences and fitness differences that measure the range of conditions compatible with multispecies coexistence, incorporating the effects of indirect interactions emerging in diverse systems. We show how our measures can be used to quantify the extent to which the conditions for coexistence in multispecies systems differ from those that allow pairwise coexistence, and apply the method to a field system of annual plants. We conclude by presenting new challenges and empirical opportunities emerging from our structural metrics of multispecies coexistence.

Intransitive competition is often projected to be a widespread mechanism of species coexistence in ecological communities. However, it is unknown how much of the coexistence we observe in nature results from this mechanism when species interactions are also stabilized by pairwise niche differences. We combined field-parameterized models of competition among 18 annual plant species with tools from network theory to quantify the prevalence of intransitive competitive relationships. We then analyzed the predicted outcome of competitive interactions with and without pairwise niche differences. Intransitive competition was found for just 15–19% of the 816 possible triplets, and this mechanism was never sufficient to stabilize the coexistence of the triplet when the pair-wise niche differences between competitors were removed. Of the transitive and intransitive triplets, only four were predicted to coexist and these were more similar in multidimensional trait space defined by 11 functional traits than non-coexisting triplets. Our results argue that intransitive competition may be less frequent than recently posed, and that even when it does operate, pairwise niche differences may be key to possible coexistence.

Ecologists have long argued that higher functioning in diverse communities arises from the niche differences stabilizing species coexistence and from the fitness differences driving competitive dominance. However, rigorous tests are lacking. We couple field-parameterized models of competition between 10 annual plant species with a biodiversity-functioning experiment under two contrasting environmental conditions, to study how coexistence determinants link to biodiversity effects (selection and complementarity). We find that complementarity effects positively correlate with niche differences and selection effects differences correlate with fitness differences. However, niche differences also contribute to selection effects and fitness differences to complementarity effects. Despite this complexity, communities with an excess of niche differences (where niche differences exceeded those needed for coexistence) produce more biomass and have faster decomposition rates under drought, but do not take up nutrients more rapidly. We provide empirical evidence that the mechanisms determining coexistence correlate with those maximizing ecosystem functioning. It is unclear how biodiversity-ecosystem functioning and species coexistence mechanisms are linked. Here, Godoy and colleagues combine field-parameterised competition models with a BEF experiment to show that mechanisms leading to more stable species coexistence lead to greater productivity, but not necessarily to enhanced functions other than primary production.

The increase of species richness with area is a universal phenomenon on Earth. However, this observation contrasts with our poor understanding of how these species-area relationships (SARs) emerge from the collective effects of area, spatial heterogeneity, and local interactions. By combining a structuralist approach with five years of empirical observations in a highly-diverse Mediterranean grassland, we show that spatial heterogeneity plays a little role in the accumulation of species richness with area in our system. Instead, as we increase the sampled area more species combinations are realized, and they coexist mainly due to direct pairwise interactions rather than by changes in single-species dominance or by indirect interactions. We also identify a small set of transient species with small population sizes that are consistently found across spatial scales. These findings empirically support the importance of the architecture of species interactions together with stochastic events for driving coexistence- and species-area relationships. Local patterns of species coexistence across scales could determine the shape of species-area relationships. Here the authors apply a structuralist approach to empirical data on annual plant communities to assess how species interactions shape coexistence- and species-area relationships.

1. Plastic responses to spatiotemporal environmental variation strongly influence species distribution, with widespread species expected to have high phenotypic plasticity. Theoretically, high phenotypic plasticity has been linked to plant invasiveness because it facilitates colonization and rapid spreading over large and environmentally heterogeneous new areas. 2. To determine the importance of phenotypic plasticity for plant invasiveness, we compare well-known exotic invasive species with widespread native congeners. First, we characterized the phenotype of 20 invasive-native ecologically and phylogenetically related pairs from the Mediterranean region by measuring 20 different traits involved in resource acquisition, plant competition ability and stress tolerance. Second, we estimated their plasticity across nutrient and light gradients. 3. On average, invasive species had greater capacity for carbon gain and enhanced performance over a range of limiting to saturating resource availabilities than natives. However, both groups responded to environmental variations with high albeit similar levels of trait plasticity. Therefore, contrary to the theory, the extent of phenotypic plasticity was not significantly higher for invasive plants. 4. We argue that the combination of studying mean values of a trait with its plasticity can render insightful conclusions on functional comparisons of species such as those exploring the performance of species coexisting in heterogeneous and changing environments.