Challenges in linking trait patterns to niche differentiation

When Do Trait-Based Higher Order Interactions and Individual Variation Promote Robust Species Coexistence?

Models on the effects of individual variation often focus on pairwise interactions, but communities could harbor both pairwise and higher order interactions (HOIs). Theoretical studies on HOIs, where a third species modulates pairwise species competition, tend to assign them at random, even though they could be mediated and structured by one-dimensional traits. Here, we consider two different classes of models of both pairwise and higher order trait-mediated interactions: competition alleviated by increasing trait distance, and hierarchical competition where species higher in the hierarchy exert more competition on those lower and vice versa. Combining these models with evolutionary dynamics based on quantitative genetics, we compare their impact on species diversity, community pattern, and robustness of coexistence. Regardless of individual variation, trait-mediated HOIs generally do not promote and often hinder species coexistence, but there are some notable exceptions to this. We present an analytical argument to make sense of these results and argue that while the effects of trait-based HOIs on diversity may appear confusing on the surface, we can understand what outcome to expect in any given scenario by looking at the shape of the effective interaction kernel that arises from the joint action of pairwise and HOI terms. In addition, we find that (i) communities structured by competitive trait hierarchies are highly vulnerable to external perturbations, regardless of HOIs, and (ii) trait-based HOIs with distance-dependent competition create the most robust communities, with minimal impact from individual variation, and (iii) both individual variation and HOIs consistently lead to a more even distribution of species traits than would occur by chance. These findings suggest that trait-mediated HOIs foster coexistence only under special conditions, raising the question of whether HOIs must involve multiple traits to positively affect coexistence in competitive communities.

Why ecologists struggle to predict coexistence from functional traits

The rationale behind trait-based ecology is that shifting focus from species' taxonomic names to their measurable characteristics ('functional traits') leads to greater generality and predictive power. This idea has been applied to one of ecology's most intractable problems: the coexistence of competing species. But after 20 years, we lack clear evidence that functional traits effectively predict coexistence. Here, we present a theory-based argument for why this might be the case. Specifically, we argue that coexistence often depends on special quantities called 'process-informed metrics' (PIMs), which combine multiple traits and demographic characteristics in non-intuitive ways, obscuring any direct ties between individual traits and coexistence. We then lay a path forward for trait-based coexistence research that builds on mechanistic models of competition.

Ecomorphological Convergence Following Niche Shifts in Montane Ground Beetles (Carabidae: Nebria)

A critical step toward uncovering generalizable patterns of phenotype-niche relationships is understanding how functional traits have evolved as species occupy new habitats. Ecomorphological traits impact how organisms function in their environment and are predictive of habitat use and niche. Studying ecomorphological variation in the context of strong environmental filtering can provide opportunities to understand the role of convergent evolution in forming trait-habitat use patterns. By integrating a molecular phylogeny, habitat use, and morphometrics, this study aimed to understand the role of ancestry and convergent evolution in ecomorphological trait evolution. This study examined these processes using an assemblage of 79 species in the ground beetle genus Nebria (Carabidae: Nebriini). Species in this genus are habitat specialists who occupy montane and alpine streambeds, talus, and snowfields in Holarctic mountain ranges (0-5000 m. above sea level). Morphological measurements, including pronotal ratio (widest point divided by the base), elytral length, elytral ratio (length divided by width), antennal scape length, and pronotal and elytral shape (Fourier decomposition), were used in this study, in conjunction with measurements of habitat use habitat where specimens were collected (1970-2021). Morphological variation was examined in relation to habitat use and phylogenetic relatedness, and morphological trait evolution was tested for convergence. Ecomorphological traits are evolving slower than expected under a null model of Brownian motion evolution. Nebria species cluster in multivariate morphospace according to relatedness, but habitat use and relatedness together are the best predictors of morphological variation. There is evidence for convergence in riparian species based on morphologicy alone, and additional evidence for morphological convergence in riparian and alpine species when phylogenetic distance is considered. In species assemblages of Nebria, we found evidence of rapid diversification followed by a slow rate of ecomorphological evolution, with convergent evolution playing a significant role in shaping trait-habitat use patterns and niche acquisition.

The critical need for long-term biomonitoring: The case study of a major river system in Anatolia

The long-term monitoring of freshwater ecosystems has been globally recognized for its critical role in preserving natural biodiversity and human well-being over the past century, yet monitoring efforts remain heterogeneously distributed. The Anatolia region in Turkey is known to host three out of the world's 34 biodiversity hotspots, boasting high biodiversity, but is increasingly threatened by anthropogenic effects and invasive freshwater fish species. Although the introduction of non-native fish species and pollution has detrimental effects on different river catchments in this area, long-term biomonitoring studies in Anatolia remain scarce and inconsistent. Collecting information of the Gediz River basin, a major Anatolian river catchment as a model that has been significantly impacted by human activities, published between 1971 and 2023, a total of 21 fish species from 12 families were identified. The spatial and temporal occurrences of these species fluctuated significantly due to heterogeneous sampling efforts that did not follow a clear pattern, indicating a complex ecological and conservation scenario characterized by both spatial and temporal variability in species prevalence, endemism and abundance. The presented results emphasize the need for comprehensive long-term biodiversity monitoring for anthropogenically impacted catchments like the Gediz River basin. Only with a coherent basis of high-resolution monitoring data can the multiple stressors threatening Anatolia's freshwater biodiversity be disentangled and, subsequently, be mitigated. Using the Gediz River basin as an example, this study emphasizes the urgent need for comprehensive, long-term biodiversity monitoring in understudied regions worldwide to protect regional biodiversity and ensure ecosystem health.

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

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 × low RMF and low LMA × 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.

A Spatial Signal of Niche Differentiation in Tropical Forests

AbstractExplaining diversity in tropical forests remains a challenge in community ecology. Theory tells us that species differences can stabilize communities by reducing competition, while species similarities can promote diversity by reducing fitness differences and thus prolonging the time to competitive exclusion. Combined, these processes may lead to clustering of species such that species are niche differentiated across clusters and share a niche within each cluster. Here, we characterize this partial niche differentiation in a tropical forest in Panama by measuring spatial clustering of woody plants and relating these clusters to local soil conditions. We find that species were spatially clustered and the clusters were associated with specific concentrations of soil nutrients, reflecting the existence of nutrient niches. Species were almost twice as likely to recruit in their own nutrient niche. A decision tree algorithm showed that local soil conditions correctly predicted the niche of the trees with up to 85% accuracy. Iron, zinc, phosphorus, manganese, and soil pH were among the best predictors of species clusters.

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

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 the 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. Consideration of intraspecific variability is key for understanding the mechanisms governing species coexistence in species-rich ecotones.

Alien species and climate change drive shifts in a riverine fish community and trait compositions over 35 years

Alien fish substantially impact aquatic communities. However, their effects on trait composition remain poorly understood, especially at large spatiotemporal scales. Here, we used long-term biomonitoring data (1984-2018) from 31 fish communities of the Rhine river in Germany to investigate compositional and functional changes over time. Average total community richness increased by 49 %: it was stable until 2004, then declined until 2010, before increasing until 2018. Average abundance decreased by 9 %. Starting from 198 individuals/m² in 1984 abundance largely declined to 23 individuals/m² in 2010 (-88 %), and then consequently increased by 678 % up to 180 individuals/m² until 2018. Increases in abundance and richness starting around 2010 were mainly driven by the establishment of alien species: while alien species represented 5 % of all species and 0.1 % of total individuals in 1993, it increased to 30 % (7 species) and 32 % of individuals in 2018. Concomitant to the increase in alien species, average native species richness and abundance declined by 26 % and 50 % respectively. We identified increases in temperature, precipitation, abundance and richness of alien fish driving compositional changes after 2010. To get more insights on the impacts of alien species on fish communities, we used 12 biological and 13 ecological traits to compute four trait metrics each. Ecological trait dispersion increased before 2010, probably due to diminishing ecologically similar native species. No changes in trait metrics were measured after 2010, albeit relative shares of expressed trait modalities significantly changing. The observed shift in trait modalities suggested the introduction of new species carrying similar and novel trait modalities. Our results revealed significant changes in taxonomic and trait compositions following alien fish introductions and climatic change. To conclude, our analyses show taxonomic and functional changes in the Rhine river over 35 years, likely indicative of future changes in ecosystem services.

Trait-mediated competition drives an ant invasion and alters functional diversity

The assumption that differences in species' traits reflect their different niches has long influenced how ecologists infer processes from assemblage patterns. For instance, many assess the importance of environmental filtering versus classical limiting-similarity competition in driving biological invasions by examining whether invaders' traits are similar or dissimilar to those of residents, respectively. However, mounting evidence suggests that hierarchical differences between species' trait values can distinguish their competitive abilities (e.g. for the same resource) instead of their niches. Whether such trait-mediated hierarchical competition explains invasions and structures assemblages is less explored. We integrate morphological, dietary, physiological and behavioural trait analyses to test whether environmental filtering, limiting-similarity competition or hierarchical competition explain invasions by fire ants on ant assemblages. We detect both competition mechanisms; invasion success is not only explained by limiting similarity in body size and thermal tolerance (presumably allowing the invader to exploit different niches from residents), but also by the invader's superior position in trait hierarchies reflecting competition for common trophic resources. We find that the two mechanisms generate complex assemblage-level functional diversity patterns-overdispersion in some traits, clustering in others-suggesting their effects are likely missed by analyses restricted to a few traits and composite trait diversity measures.

Seasonal modifications of longitudinal distribution patterns within a stream: Interspecific interactions in the niche overlap zones of two Ephemera mayflies

Niche differentiation between closely related species leads to differentiation of their habitats. Segregation based on slight differences in environmental factors, that is niche differentiation on the microhabitat scale, allows more species to inhabit a certain geographic space. Therefore, such fine scale niche differentiation is an important factor in the support of species diversity. In addition, niche differentiation on the microhabitat scale and/or the differentiation of breeding seasons can be considered typical mechanisms that facilitate multispecies' co-existence. In this study, sister species (Commonly, Ephemera japonica inhabit at upstream region and Ephemera strigata inhabit at middle stream region), which often coexist in the upper to middle reaches of river systems of the Japanese Islands, were targeted and the following aspects were investigated. First, differences in habitat preference and interspecific differences in flow distribution patterns on a geographically fine scale were tracked in detail. Subsequently, the temporal transitions of their distribution patterns were investigated in detail and seasonal changes were investigated. Finally, we thoroughly investigated the disappearance of nymphs of each species from the river due to emergence affected the distribution of each species (by conducting daily emergence surveys). Combining results of these multiple studies also suggested that there may be spatiotemporal interspecial interaction between these two species within/around their overlapping regions. Traditionally, the longitudinal distribution pattern of these two Ephemera mayflies has been thought to be established based on a difference in habitat preferences, but this study revealed that the interspecific interaction between the two species also plays an important role. This study provides new insights into species diversity and distribution pattern formation in river-dwelling species.

Emergent Shapes of Trait-Based Competition Functions from Resource-Based Models: A Gaussian Is Not Normal in Plant Communities

AbstractIn community ecology, it is widely assumed that organisms with similar traits compete more intensely with one another for resources. This assumption is often encoded into theory and empirical tests via a unimodal competition function, which predicts that per capita competitive effect declines with separation in traits. Yet it remains unknown how well this function represents the true effect of traits on competitive outcomes, especially for long-lived plant communities, where lifetime fitness is difficult to estimate. Here, we evaluate the shape of competition functions embedded in two resource-based (RB) models, wherein plants compete for shared, essential resources. In the first RB model individuals compete for two essential nutrients, and in the second they compete for light in a size-based successional setting. We compared the shapes of the competition functions that emerged from interactions within these RB models to the unimodal function and others shapes commonly applied. In few instances did the trait-based competition function emerging from the RB model even vaguely resemble any of the shapes previously used. The mismatch between these two approaches suggests that theory derived using fixed competition functions based on trait separation may not apply well to plant systems, where individuals compete for shared resources. The more promising path will be to model depletion of resources by populations in relation to their traits, with its consequences for fitness landscapes and competitive exclusion.

The evolution of niche overlap and competitive differences

Competition can result in evolutionary changes to coexistence between competitors but there are no theoretical models that predict how the components of coexistence change during this eco-evolutionary process. Here we study the evolution of the coexistence components, niche overlap and competitive differences, in a two-species eco-evolutionary model based on consumer-resource interactions and quantitative genetic inheritance. Species evolve along a one-dimensional trait axis that allows for changes in both niche position and species intrinsic growth rates. There are three main results. First, the breadth of the environment has a strong effect on the dynamics, with broader environments leading to reduced niche overlap and enhanced coexistence. Second, coexistence often involves a reduction in niche overlap while competitive differences stay relatively constant or vice versa; in general changes in competitive differences maintain coexistence only when niche overlap remains constant. Large simultaneous changes in niche overlap and competitive difference often result in one of the species being excluded. Third, provided that the species evolve to a state where they coexist, the final niche overlap and competitive difference values are independent of the system's initial state, although they do depend on the model's parameters. The model suggests that evolution is often a destructive force for coexistence due to evolutionary changes in competitive differences, a finding that expands the paradox of diversity maintenance.

Two centuries for an almost complete community turnover from native to non-native species in a riverine ecosystem

Non-native species introductions affect freshwater communities by changing community compositions, functional roles, trait occurrences and ecological niche spaces. Reconstructing such changes over long periods is difficult due to limited data availability. We collected information spanning 215 years on fish and selected macroinvertebrate groups (Mollusca and Crustacea) in the inner-Florentine stretch of the Arno River (Italy) and associated water grid, to investigate temporal changes. We identified an almost complete turnover from native to non-native fish (1800: 92% native; 2015: 94% non-native species) and macroinvertebrate species (1800: 100% native; 2015: 70% non-native species). Non-native fish species were observed ~50 years earlier compared to macroinvertebrate species, indicating phased invasion processes. In contrast, α-diversity of both communities increased significantly following a linear pattern. Separate analyses of changes in α-diversities for native and non-native species of both fish and macroinvertebrates were nonlinear. Functional richness and divergence of fish and macroinvertebrate communities decreased non-significantly, as the loss of native species was compensated by non-native species. Introductions of non-native fish and macroinvertebrate species occurred outside the niche space of native species. Native and non-native fish species exhibited greater overlap in niche space over time (62%-68%) and non-native species eventually replaced native species. Native and non-native macroinvertebrate niches overlapped to a lesser extent (15%-30%), with non-natives occupying mostly unoccupied niche space. These temporal changes in niche spaces of both biotic groups are a direct response to the observed changes in α-diversity and species turnover. These changes are potentially driven by deteriorations in hydromorphology as indicated by alterations in trait modalities. Additionally, we identified that angling played a considerable role for fish introductions. Our results support previous findings that the community turnover from native to non-native species can be facilitated by, for example, deteriorating environmental conditions and that variations in communities are multifaceted requiring more indicators than single metrics.

Compartmentalized organization of ecological niche occupation in insular invertebrate communities

Understanding the mechanisms of species distribution within ecosystems is a fundamental question of ecological research. The current worldwide changes and loss of habitats associated with a decline in species richness render this topic a key element for developing mitigation strategies. Ecological niche theory is a widely accepted concept to describe species distribution along environmental gradients where each taxon occupies its own distinct set of environmental parameters, that is, its niche. Niche occupation has been described in empirical studies for different closely related taxa, like ant, ungulate, or skink species, just to name a few. However, how species assemblages of whole ecosystems across multiple taxa are structured and organized has not been investigated thoroughly, although considering all taxa of a community would be essential when analyzing realized niches. Here, we investigated the organization of niche occupation and species distribution for the whole ground-associated invertebrate community of small tropical insular ecosystems. By correlating environmental conditions with species occurrences using partial canonical correspondence analysis (pCCA), we demonstrated that the ground-associated invertebrate community does not spread evenly across the overall niche space, but instead is compartmentalized in four distinct clusters: crustacean and gastropod taxa occurred in one cluster, attributable to the beach habitat, whereas hexapods and spider taxa occurred in three distinct inland clusters, attributable to distinct inland habitats, that is, grassland, open forest, and dense forest. Within the clusters, co-occurrence pattern analysis suggested only a few negative interactions between the different taxa. By studying ground-associated insular invertebrate communities, we have shown that species distribution and niche occupation can be, similar to food webs, organized in a compartmentalized way. The compartmentalization of the niche space might thereby be a mechanism to increase ecosystem resilience, as disturbances cascade more slowly throughout the ecosystem.

Mapping the bacterial metabolic niche space

The rise in the availability of bacterial genomes defines a need for synthesis: abstracting from individual taxa, to see larger patterns of bacterial lifestyles across systems. A key concept for such synthesis in ecology is the niche, the set of capabilities that enables a population's persistence and defines its impact on the environment. The set of possible niches forms the niche space, a conceptual space delineating ways in which persistence in a system is possible. Here we use manifold learning to map the space of metabolic networks representing thousands of bacterial genera. The results suggest a metabolic niche space comprising a collection of discrete clusters and branching manifolds, which constitute strategies spanning life in different habitats and hosts. We further demonstrate that communities from similar ecosystem types map to characteristic regions of this functional coordinate system, permitting coarse-graining of microbiomes in terms of ecological niches that may be filled.

Coexistence of plant species under harsh environmental conditions: an evaluation of niche differentiation and stochasticity along salt marsh creeks

Background

Ecologists have achieved much progress in the study of mechanisms that maintain species coexistence and diversity. In this paper, we reviewed a wide range of past research related to these topics, focusing on five theoretical bodies: (1) coexistence by niche differentiation, (2) coexistence without niche differentiation, (3) coexistence along environmental stress gradients, (4) coexistence under non-equilibrium versus equilibrium conditions, and (5) modern perspectives.

Results

From the review, we identified that there are few models that can be generally and confidently applicable to different ecological systems. This problem arises mainly because most theories have not been substantiated by enough empirical research based on field data to test various coexistence hypotheses at different spatial scales. We also found that little is still known about the mechanisms of species coexistence under harsh environmental conditions. This is because most previous models treat disturbance as a key factor shaping community structure, but they do not explicitly deal with stressful systems with non-lethal conditions. We evaluated the mainstream ideas of niche differentiation and stochasticity for the coexistence of plant species across salt marsh creeks in southwestern Denmark. The results showed that diversity indices, such as Shannon–Wiener diversity, richness, and evenness, decreased with increasing surface elevation and increased with increasing niche overlap and niche breadth. The two niche parameters linearly decreased with increasing elevation. These findings imply a substantial influence of an equalizing mechanism that reduces differences in relative fitness among species in the highly stressful environments of the marsh. We propose that species evenness increases under very harsh conditions if the associated stress is not lethal. Finally, we present a conceptual model of patterns related to the level of environmental stress and niche characteristics along a microhabitat gradient (i.e., surface elevation).

Conclusions

The ecology of stressful systems with non-lethal conditions will be increasingly important as ongoing global-scale climate change extends the period of chronic stresses that are not necessarily fatal to inhabiting plants. We recommend that more ecologists continue this line of research.

Both selection and plasticity drive niche differentiation in experimental grasslands

The way species avoid each other in a community by using resources differently across space and time is one of the main drivers of species coexistence in nature^1,2. This mechanism, known as niche differentiation, has been widely examined theoretically but still lacks thorough experimental validation in plants. To shape niche differences over time, species within communities can reduce the overlap between their niches or find unexploited environmental space³. Selection and phenotypic plasticity have been advanced as two candidate processes driving niche differentiation^4,5, but their respective role remains to be quantified⁶. Here, we tracked changes in plant height, as a candidate trait for light capture⁷, in 5-year multispecies sown grasslands. We found increasing among-species height differences over time. Phenotypic plasticity promotes this change, which explains the rapid setting of differentiation in our system. Through the inspection of changes in genetic structure, we also highlighted the contribution of selection. Altogether, we experimentally demonstrated the occurrence of species niche differentiation within artificial grassland communities over a short time scale through the joined action of both plasticity and selection.

Dome patterns in pelagic size spectra reveal strong trophic cascades

In ecological communities, especially the pelagic zones of aquatic ecosystems, certain body-size ranges are often over-represented compared to others. Community size spectra, the distributions of community biomass over the logarithmic body-mass axis, tend to exhibit regularly spaced local maxima, called “domes”, separated by steep troughs. Contrasting established theory, we explain these dome patterns as manifestations of top-down trophic cascades along aquatic food chains. Compiling high quality size-spectrum data and comparing these with a size-spectrum model introduced in this study, we test this theory and develop a detailed picture of the mechanisms by which bottom-up and top-down effects interact to generate dome patterns. Results imply that strong top-down trophic cascades are common in freshwater communities, much more than hitherto demonstrated, and may arise in nutrient rich marine systems as well. Transferring insights from the general theory of non-linear pattern formation to domes patterns, we provide new interpretations of past lake-manipulation experiments.

An important question in ecology is how much species at higher trophic levels affect lower levels through top-down cascades. Here the authors show through analyses of pelagic size spectra that such cascades are strong in freshwater systems and can also arise in nutrient rich marine systems.

Generalizing clusters of similar species as a signature of coexistence under competition

Patterns of trait distribution among competing species can potentially reveal the processes that allow them to coexist. It has been recently proposed that competition may drive the spontaneous emergence of niches comprising clusters of similar species, in contrast with the dominant paradigm of greater-than-chance species differences. However, current clustering theory relies largely on heuristic rather than mechanistic models. Furthermore, studies of models incorporating demographic stochasticity and immigration, two key players in community assembly, did not observe clusters. Here we demonstrate clustering under partitioning of resources, partitioning of environmental gradients, and a competition-colonization tradeoff. We show that clusters are robust to demographic stochasticity, and can persist under immigration. While immigration may sustain clusters that are otherwise transient, too much dilutes the pattern. In order to detect and quantify clusters in nature, we introduce and validate metrics which have no free parameters nor require arbitrary trait binning, and weigh species by their abundances rather than relying on a presence-absence count. By generalizing beyond the circumstances where clusters have been observed, our study contributes to establishing them as an update to classical trait patterning theory.

Evolutionarily stable communities: a framework for understanding the role of trait evolution in the maintenance of diversity

Biological diversity depends on the interplay between evolutionary diversification and ecological mechanisms allowing species to coexist. Current research increasingly integrates ecology and evolution over a range of timescales, but our common conceptual framework for understanding species coexistence requires better incorporation of evolutionary processes. Here, we focus on the idea of evolutionarily stable communities (ESCs), which are theoretical endpoints of evolution in a community context. We use ESCs as a unifying framework to highlight some important but under-appreciated theoretical results, and we review empirical research relevant to these theoretical predictions. We explain how, in addition to generating diversity, evolution can also limit diversity by reducing the effectiveness of coexistence mechanisms. The coevolving traits of competing species may either diverge or converge, depending on whether the number of species in the community is low (undersaturated) or high (oversaturated) relative to the ESC. Competition in oversaturated communities can lead to extinction or neutrally coexisting, ecologically equivalent species. It is critical to consider trait evolution when investigating fundamental ecological questions like the strength of different coexistence mechanisms, the feasibility of ecologically equivalent species, and the interpretation of different patterns of trait dispersion.

Emergent niche structuring leads to increased differences from neutrality in species abundance distributions

Species abundance distributions must reflect the dynamic processes involved in community assembly, but whether and when specific processes lead to distinguishable signals is not well understood. Biodiversity and species abundances may be shaped by a variety of influences, but particular attention has been paid to competition, which can involve neutral dynamics, where competitor abundances are governed only by demographic stochasticity and immigration, and dynamics driven by trait differences that enable stable coexistence through the formation of niches. Key recent studies of the species abundance patterns of communities with niches employ simple models with pre-imposed niche structure. These studies suggest that species abundance distributions are insensitive to the relative contributions of niche and neutral processes, especially when diversity is much higher than the number of niches. Here we analyze results from a stochastic population model with competition driven by trait differences. With this model, niche structure emerges as clumps of species that persist along the trait axis, and leads to more substantial differences from neutral species abundance distributions than have been previously shown. We show that heterogeneity in "between-niche" interaction strength (i.e., in the strength of competition between species in different niches) plays the dominant role in shaping the species abundances along the trait axis, acting as a biotic filter favoring species at the centers of niches. Furthermore, we show that heterogeneity in "within-niche" interactions (i.e., in the competition between species in the same niche) counteracts the influence of heterogeneity in "between-niche" interactions on the SAD to some degree. Our results suggest that competitive interactions that produce niches can also influence the shapes of SADs.

Intra- and interspecific trait variations reveal functional relationships between specific leaf area and soil niche within a subtropical forest

Background and Aims

How functional traits vary with environmental conditions is of fundamental importance in trait-based community ecology. However, how intraspecific variability in functional traits is connected to species distribution is not well understood. This study investigated inter- and intraspecific variation of a key functional trait, i.e. specific leaf area (leaf area per unit dry mass; SLA), in relation to soil factors and tested if trait variation is more closely associated with specific environmental regimes for low-variability species than for high-variability species.

Methods

In a subtropical evergreen forest plot (50 ha, southern China), 106 700 leaves from 5335 individuals of 207 woody species were intensively collected, with 30 individuals sampled for most species to ensure a sufficient sample size representative of intraspecific variability. Soil conditions for each plant were estimated by kriging from more than 1700 observational soil locations across the plot. Intra- and interspecific variation in SLA were separately related to environmental factors. Based on the species-specific variation of SLA, species were categorized into three groups: low-, intermediate- and high-intraspecific variability. Intraspecific habitat ranges and the strength of SLA-habitat relationships were compared among these three groups.

Key Results

Interspecific variation in SLA overrides the intraspecific variation (77 % vs. 8 %). Total soil nitrogen (TN, positively) and total organic carbon (TOC, negatively) are the most important explanatory factors for SLA variation at both intra- and interspecific levels. SLA, both within and between species, decreases with decreasing soil nitrogen availability. As predicted, species with low intraspecific variability in SLA have narrower habitat ranges with respect to soil TOC and TN and show a stronger SLA-habitat association than high-variability species.

Conclusions

For woody plants low SLA is a phenotypic and probably adaptive response to nitrogen stress, which drives the predominance of species with ever-decreasing SLA towards less fertile habitats. Intraspecific variability in SLA is positively connected to species' niche breadth, suggesting that low-variability species may play a more deterministic role in structuring plant assemblages than high-variability species. This study highlights the importance of quantifying intraspecific trait variation to improve our understanding of species distributions across a vegetated landscape.

Competition-driven niche segregation on a landscape scale: Evidence for escaping from syntopy towards allotopy in two coexisting sibling passerine species

The role of interspecific competition for generating patterns in species' distribution is hotly debated and studies taking into account processes occurring at both large and small spatial scales are almost missing. Theoretically, competition between species with overlapping niches should result in divergence of their niches in sympatry to reduce the costs of competition. Many species show a mosaic distribution within sympatric zones, with the syntopic sites occupied by both species, and allotopic sites where only one species occurs. It is unclear whether such mosaics arise as a consequence of competition-driven niche segregation or due to the decline of their abundances towards range edges driven by environmental gradients. If the interspecific competition matters, we should observe (1) a shift in habitat preferences of one or both species between syntopy and allotopy, and (2) between allopatry and allotopy. Moreover, (3) species should show greater divergence in their habitat preferences in allotopy than in allopatry where (4) no differences in habitat preferences may occur. Finally, (5) shifts should be generally greater in the competitively subordinate species than in the dominant species. We used a unique dataset on abundance of two closely related passerine species, the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (Luscinia luscinia), collected across their syntopy, allotopy and allopatry. The predictions were tested within a generalized mixed-effects modelling framework. After accounting for environmental gradients perpendicular to the species' contact zone, we found a strong support for all but one prediction. Habitat preferences of both species shifted markedly between syntopy and allotopy, as well as between allopatry and allotopy. Whereas the species preferred the same habitats in allopatry, their preferences became strikingly different in allotopy where the abundance of the Common Nightingale increased towards dry and warm sites with low coverage of pastures, while the abundance of the Thrush Nightingale showed exactly opposite trends. Fifth prediction was not supported. Our results indicate that the competition between closely related species can result in considerable changes in habitat use across their geographic ranges accompanied with divergence in their habitat preferences in sympatry. Here, the species "escape" from competition to allotopic sites covered by habitats avoided by the competitor. Therefore, we argue that the interspecific competition is an important driver of species' distribution at both large and small spatial scales.

Biodiversity maintenance may be lower under partial niche differentiation than under neutrality

Niche differentiation is normally regarded as a key promoter of species coexistence in competitive systems. One might therefore expect that relative to neutral assemblages, niche-differentiated communities should support more species with longer persistence and lower probability of extinction. Here we compare stochastic niche and neutral dynamics in simulated assemblages, and find that when local dynamics combine with immigration from a regional pool, the effect of niches can be more complex. Trait variation that lessens competition between species will not necessarily give all immigrating species their own niche to occupy. Such partial niche differentiation protects certain species from local extinction, but precipitates exclusion of others. Differences in regional abundances and intrinsic growth rates have similar impacts on persistence times as niche differentiation, and therefore blur the distinction between niche and neutral dynamical patterns-although niche dynamics will influence which species persist longer. Ultimately, unless the number of niches available to species is sufficiently high, niches may actually heighten extinction rates and lower species richness and local persistence times. Our results help make sense of recent observations of community dynamics, and point to the dynamical observations needed to discern the influence of niche differentiation.

Higher-order interactions stabilize dynamics in competitive network models

Ecologists have long sought a way to explain how the remarkable biodiversity observed in nature is maintained. On the one hand, simple models of interacting competitors cannot produce the stable persistence of very large ecological communities. On the other hand, neutral models, in which species do not interact and diversity is maintained by immigration and speciation, yield unrealistically small fluctuations in population abundance, and a strong positive correlation between a species' abundance and its age, contrary to empirical evidence. Models allowing for the robust persistence of large communities of interacting competitors are lacking. Here we show that very diverse communities could persist thanks to the stabilizing role of higher-order interactions, in which the presence of a species influences the interaction between other species. Although higher-order interactions have been studied for decades, their role in shaping ecological communities is still unclear. The inclusion of higher-order interactions in competitive network models stabilizes dynamics, making species coexistence robust to the perturbation of both population abundance and parameter values. We show that higher-order interactions have strong effects in models of closed ecological communities, as well as of open communities in which new species are constantly introduced. In our framework, higher-order interactions are completely defined by pairwise interactions, facilitating empirical parameterization and validation of our models.

Persistent bill and corolla matching despite shifting temporal resources in tropical hummingbird-plant interactions

By specialising on specific resources, species evolve advantageous morphologies to increase the efficiency of nutrient acquisition. However, many specialists face variation in resource availability and composition. Whether specialists respond to these changes depends on the composition of the resource pulses, the cost of foraging on poorly matched resources, and the strength of interspecific competition. We studied hummingbird bill and plant corolla matching during seasonal variation in flower availability and morphology. Using a hierarchical Bayesian model, we accounted for the detectability and spatial overlap of hummingbird-plant interactions. We found that despite seasonal pulses of flowers with short-corollas, hummingbirds consistently foraged on well-matched flowers, leading to low niche overlap. This behaviour suggests that the costs of searching for rare and more specialised resources are lower than the benefit of switching to super-abundant resources. Our results highlight the trade-off between foraging efficiency and interspecific competition, and underline niche partitioning in maintaining tropical diversity.

The effect of intraspecific variation and heritability on community pattern and robustness

Intraspecific trait variation is widespread in nature, yet its effects on community dynamics are not well understood. Here we explore the consequences of intraspecific trait variation for coexistence in two- and multispecies competitive communities. For two species, the likelihood of coexistence is in general reduced by intraspecific variation, except when the species have almost equal trait means but different trait variances, such that one is a generalist and the other a specialist consumer. In multispecies communities, the only strong effect of non-heritable intraspecific variation is to reduce expected species richness. However, when intraspecific variation is heritable, allowing for the possibility of trait evolution, communities are much more resilient against environmental disturbance and exhibit far more predictable trait patterns. Our results are robust to varying model parameters and relaxing model assumptions.