A macroecological perspective of trait patterns in stream communities

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Fine‐scale coexistence patterns along a productivity gradient in wet meadows: shifts from trait convergence to divergence

Semi‐natural meadows host a great number of species coexisting at fine spatial scales. Different assembly mechanisms, related to differences in functional traits between species, can influence such coexistence. Coexisting species could be either functionally dissimilar to occupy different niches (‘divergence’) or functionally similar due to exclusion of species with traits less adapted to the prevailing abiotic and biotic conditions (‘convergence’). Various theories differently predict how trait convergence and divergence should differ due to disturbance, along productivity gradients, and across different functional traits. We tested such theories in 21 wet meadows of different productivity in central Europe. In each meadow, four 1 × 1 m plots were established in which disturbance by mowing was combined with fertilization. Species presence was recorded in 100 quadrats 10 × 10 cm in size within the plots over five years. Convergence and divergence were assessed at very fine spatial scales (10 × 10 cm) to focus on the processes driven by the interactions for similar resources. Convergence emerged as the dominant pattern for all traits and across all years. It was particularly strong in the least productive conditions while divergence emerged in some of the most productive meadows. Mowing increased convergence in meadows with low productivity, but increased divergence in productive meadows. Fertilization generally increased convergence, with this increase being more pronounced in mown plots. Convergence in unproductive conditions could be caused by either higher fitness of stress‐tolerant species (more abundant in the species pool of these sites) or by functionally similar species sharing similar patches within fine‐scale heterogeneous plots. This outcome also suggests abiotic filters can have an important role at fine scales, where plant‐ecological theory usually predicts the prevalence of biotic processes.     

Disturbance regime changes the trait distribution,phylogenetic structure and community assembly of tropical rain forests

A rapidly increasing effort to merge functional community ecology and phylogenetic biology has increased our understanding of community assembly. However, studies using both phylogenetic‐ and trait‐based methods have been mainly conducted in old‐growth forests, with fewer studies in human‐disturbed communities, which play an increasingly important role in providing ecosystem services as primary forests are degraded. We used data from 18 1‐ha plots in tropical old‐growth forests and secondary forests with different disturbance histories (logging and shifting cultivation) and vegetation types (tropical lowland and montane forests) on Hainan Island, southern China. The distributions of 11 functional traits were compared among these six forest types. We used a null model approach to assess the effects of disturbance regimes on variation in response and effect traits and community phylogenetic structure across different stem sizes (saplings, treelets, and adult trees) and spatial scales (10–50 m). We found significant differences in the distribution of functional traits in highly disturbed lowland sites versus other forest types. Many individuals in highly disturbed lowland sites were deciduous, spiny, with non‐fleshy fruits and seeds dispersed passively or by wind, and low SLA. The response traits of coexisting species were clustered in all sites except for highly disturbed lowland sites where evenness was evident. There were different distributions of effect traits for saplings and treelets among different forest types but adult trees showed stronger clustering of trait values with increasing spatial scale among all forest types. Phylogenetic clustering predominated across all size classes and spatial scales in the highly disturbed lowland sites, and evenness in other forest types. High disturbance can lead to abiotic filtering, generating a community dominated by closely related species with disturbance‐adapted traits, where biotic interactions play a relatively minor role. In lightly disturbed and old growth forests, multiple processes simultaneously drive the community assembly, but biotic processes dominate at the fine scale.     

Global to community scale differences in the prevalence of convergent over divergent leaf trait distributions in plant assemblages

Aim The drivers of species assembly, by limiting the possible range of functional trait values, can lead to either convergent or divergent distributions of traits in realized assemblages. Here, to evaluate the strengths of these species assembly drivers, we partition trait variance across global, regional and community scales. We then test the hypothesis that, from global to community scales, the outcome of co‐occurring trait convergence and divergence is highly variable across biomes and communities. Location Global: nine biomes ranging from subarctic highland to tropical rain forest. Methods We analysed functional trait diversity at progressively finer spatial scales using a global, balanced, hierarchically structured dataset from 9 biomes, 58 communities and 652 species. Analyses were based on two key leaf traits (foliar nitrogen content and specific leaf area) that are known to drive biogeochemical cycling. Results While 35% of the global variance in these traits was between biomes, only 15% was between communities within biomes and as much as 50% occurred within communities. Despite this relatively high within‐community variance in trait values, we found that trait convergence dominated over divergence at both global and regional scales through comparisons of functional trait diversity in regional and community assemblages against random (null) models of species assembly. Main conclusions We demonstrate that the convergence of traits occurring from global to regional assemblages can be twice as strong as that from regional to community assemblages, and argue that large differences in the nature and strength of abiotic and biotic drivers of dominant species assembly can, at least partly, explain the variable outcome of simultaneous trait convergence and divergence across sites. Ultimately, these findings stress the urgent need to extend species assembly research to address those scales where trait variance is the highest, i.e. between biomes and within communities.     

Within‐species patterns challenge our understanding of the leaf economics spectrum

The utility of plant functional traits for predictive ecology relies on our ability to interpret trait variation across multiple taxonomic and ecological scales. Using extensive data sets of trait variation within species, across species and across communities, we analysed whether and at what scales leaf economics spectrum (LES) traits show predicted trait–trait covariation. We found that most variation in LES traits is often, but not universally, at high taxonomic levels (between families or genera in a family). However, we found that trait covariation shows distinct taxonomic scale dependence, with some trait correlations showing opposite signs within vs. across species. LES traits responded independently to environmental gradients within species, with few shared environmental responses across traits or across scales. We conclude that, at small taxonomic scales, plasticity may obscure or reverse the broad evolutionary linkages between leaf traits, meaning that variation in LES traits cannot always be interpreted as differences in resource use strategy.     

Landscape variables influence taxonomic and trait composition of insect assemblages in Neotropical savanna streams

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Quantifying the effects of ecological constraints on trait expression using novel trait‐gradient analysis parameters

Complex processes related to biotic and abiotic forces can impose limitations to assembly and composition of plant communities. Quantifying the effects of these constraints on plant functional traits across environmental gradients, and among communities, remains challenging. We define ecological constraint (C_i) as the combined, limiting effect of biotic interactions and environmental filtering on trait expression (i.e., the mean value and range of functional traits). Here, we propose a set of novel parameters to quantify this constraint by extending the trait‐gradient analysis (TGA) methodology. The key parameter is ecological constraint, which is dimensionless and can be measured at various scales, for example, on population and community levels. It facilitates comparing the effects of ecological constraints on trait expressions across environmental gradients, as well as within and among communities. We illustrate the implementation of the proposed parameters using the bark thickness of 14 woody species along an aridity gradient on granite outcrops in southwestern Australia. We found a positive correlation between increasing environmental stress and strength of ecological constraint on bark thickness expression. Also, plants from more stressful habitats (shrublands on shallow soils and in sun‐exposed locations) displayed higher ecological constraint for bark thickness than plants in more benign habitats (woodlands on deep soils and in sheltered locations). The relative ease of calculation and dimensionless nature of C_i allow it to be readily implemented at various scales and make it widely applicable. It therefore has the potential to advance the mechanistic understanding of the ecological processes shaping trait expression. Some future applications of the new parameters could be investigating the patterns of ecological constraints (1) among communities from different regions, (2) on different traits across similar environmental gradients, and (3) for the same trait across different gradient types.     

The importance of spatial scale for trait–abundance relations

The concept of community assembly through trait‐based environmental filtering has played a key role in our understanding of how communities change over space and time, however, the importance of spatial scale in the filtering process remains unclear. We propose that different environmental filters may operate at different spatial scales, and that filters at finer scales would be nested within those acting at coarser scales. We tested for the existence of spatially nested sets of trait‐based filters in a temperate native grassland by applying the recently proposed maximum entropy (MaxEnt) approach to trait‐based community assembly, which we extend through a trait selection procedure. We found that different traits were important in influencing the abundances of species at the three different spatial scales examined (micro‐habitat, habitat, landscape), supporting the idea that trait based filtering processes operating at coarse spatial scales can be quite distinct from those operating at fine scales. Despite this result, we identified several traits which were frequently related to abundance at all spatial scales. Taken together, our results support the proposition that trait‐based environmental filters at finer spatial scales are nested within those operating at coarser scales. We compared our results to those obtained using a simpler trait‐by‐trait analytical approach (correlation analysis and MaxEnt on individual traits). The capacity for MaxEnt to incorporate multiple traits simultaneously provided unique insights into the important traits at each spatial scale and presents significant advantages over existing univariate and multivariate approaches.     

Disparate relatives: Life histories vary more in genera occupying intermediate environments

Species within clades are commonly assumed to share similar life history traits, but within a given region some clades show much greater variability in traits than others. Are variable clades older, allowing more time for trait diversification? Or do they occupy particular environments, providing a wider range of abiotic or biotic opportunities for the establishment and maintenance of diverse trait attributes? Does environmental opportunity increase trait variability across all species, or is it specific to species belonging to the same clade, increasing only within-clade trait variability? We studied the variability of six life-history traits (initiation of flowering, duration of flowering, plant life span, seed mass, stress tolerance, type of reproduction) within 383 angiosperm genera from Central Europe distributed along six abiotic gradients. We compared patterns of within-genus variability to those present in the entire dataset, independent of genus membership. We found that trait variability differed strongly between genera, but did not depend on their age. Trait variability was higher within genera occupying intermediate positions along regional abiotic environmental gradients, compared with patterns across the entire dataset (and unbiased by geographical sampling, family membership or species richness). Increasing trait variability within genera reflected increasing independence of traits from the abiotic environment. We conclude that intermediate abiotic environments play an important role in maintaining and possibly generating the striking diversity of life history traits present within certain clades. They may do so by relaxing the abiotic constraints on the evolution and maintenance of species traits within clades.     

Conservation of taxonomic and biological trait diversity of European stream macroinvertebrate communities: a case for a collective public database

The use of databases for the conservation of biodiversity is increasing. During the last decade, such a database has been created for European stream macroinvertebrates. Today, it includes 527 sites that are the least human-impacted representatives of many stream types across many European regions. It includes data on the abundance of 312 invertebrate genera, several environmental site characteristics, collection methods, bibliographic data sources, and 11 biological traits of the genera (e.g. size, life cycle, food and feeding habits, described in 61 categories). The database will be useful in addressing many topics that are potentially relevant to biodiversity conservation. To illustrate this potential, we provide examples of how the data could be exploited. First, we describe the frequency of some taxonomic and biological characteristics (e.g. richness and diversity of genera and traits) of the macroinvertebrate communities and assess how these characteristics are related (e.g. how trait richness increases with genus richness). Second, we describe the frequency of some characteristics of the genera and traits (e.g. occurrence frequency, abundance, dispersion index) and again assess how these characteristics are related (e.g. how occurrence increases with abundance). Finally, we suggest how the database could be developed into a collective, publicly accessible database that covers stream types and regions of Europe more comprehensively.     

Fish assemblage convergence along stream environmental gradients: an intercontinental analysis

Species that pass through similar environmental filters, regardless of geographic proximity or evolutionary history, are expected to share many traits, resulting in similar assemblage trait distributions. Convergence of assemblage trait distributions among different biotic regions would indicate that consistent ecological processes produce repeated patterns of adaptive evolution. This study analyzes trait–environment relationships across multiple stream fish assemblages representing evolutionarily divergent faunas. We hypothesized that trait–environment patterns converge across regional faunas in response to a common set of environmental filters acting on functional traits. One hundred and ninety‐seven species and forty streams were sampled from five regions: Belize, Benin, Brazil, Cambodia and USA. By examining trait–environment plots, multiple congruent trait–environment patterns were found across all regions, indicative of a consistent set of environmental filters acting on local community assembly. The consistency of these patterns strongly suggests that water velocity and habitat structural complexity function as universal environmental filters, producing similar assemblage trait distributions in streams across all regions. Bivariate relationships were not universal, and only one of the associations between a single functional trait and single environmental variable was statistically significant across all five regions. Strong phylogenetic signal was found in traits and habitat use, which implies that niche conservatism also influenced assemblage trait distributions. Overall, results support the idea that habitat templates structure trait distributions of stream fish assemblages and do so in a consistent manner.     

Invertebrate community responses to land use at a broad spatial scale: trait and taxonomic measures compared in New Zealand rivers

1. Large‐scale comparative studies of ecological responses to anthropological stressors in rivers require measures that are consistent across a range of spatial scales. The biological trait profile of communities offers an alternative approach to traditional measures of macroinvertebrate taxonomic identity and is less constrained by biogeographic influences. 2. We compared the capacities of taxonomic composition and biological trait composition to discriminate the effects of land use (measured as percentage of the catchment in pasture) across a large geographic zone (the whole of New Zealand) in 30 sub‐catchments grouped into five ecoregions throughout the North and South Islands of New Zealand. In addition, we investigated trait consistency (i.e. whether similar traits had similar trait responses to land use at local (catchment) and broad scales). 3. The analysis of taxonomic composition showed that community structure was indeed influenced by land‐use intensity, but that relationships differed among ecoregions. In contrast, traditional assessment metrics (Macroinvertebrate Community Index, richness in Ephemeroptera, Plecoptera and Trichoptera taxa) and trait composition were uninfluenced by region. Trait responses were consistent at the broad and catchment scales, with similar traits responding to pastoral land use at both scales. 4. We used general linear modelling to investigate individual trait responses to land‐use intensity, catchment area and region, focussing on 15 trait categories known to be influenced by land‐use intensity at the catchment scale. Several trait categories varied with land‐use intensity and demonstrated consistency at both catchment and broad scales. Of these, the representation of shorter generation time, asexual reproduction and hermaphroditism, ability to lay eggs beneath the water surface, egg protection and respiration types tolerant of oxygen depletion generally increased in assemblages exposed to more intense pastoral land use. At the same time, the representation of short life duration of adults, prevalence of laying eggs at the water surface, sexual reproduction and low body flexibility decreased in assemblages exposed to land‐use intensification. 5. Our results highlight the value of developing predictive response measures that are relevant at multiple scales and provide the basis for new measures of river condition that are as effective as taxonomic identity in terms of response reliability.     

A conceptual framework for the spatial analysis of functional trait diversity

Functional trait diversity is a popular tool in modern ecology, mainly used to infer assembly processes and ecosystem functioning. Patterns of functional trait diversity are shaped by ecological processes such as environmental filtering, species interactions and dispersal that are inherently spatial, and different processes may operate at different spatial scales. Adding a spatial dimension to the analysis of functional trait diversity may thus increase our ability to infer community assembly processes and to predict change in assembly processes following disturbance or land‐use change. Richness, evenness and divergence of functional traits are commonly used indices of functional trait diversity that are known to respond differently to large‐scale filters related to environmental heterogeneity and dispersal and fine‐scale filters related to species interactions (competition). Recent developments in spatial statistics make it possible to separately quantify large‐scale patterns (variation in local means) and fine‐scale patterns (variation around local means) by decomposing overall spatial autocorrelation quantified by Moran's coefficient into its positive and negative components using Moran eigenvector maps (MEM). We thus propose to identify the spatial signature of multiple ecological processes that are potentially acting at different spatial scales by contrasting positive and negative components of spatial autocorrelation for each of the three indices of functional trait diversity. We illustrate this approach with a case study from riparian plant communities, where we test the effects of disturbance on spatial patterns of functional trait diversity. The fine‐scale pattern of all three indices was increased in the disturbed versus control habitat, suggesting an increase in local scale competition and an overall increase in unexplained variance in the post‐disturbance versus control community. Further research using simulation modeling should focus on establishing the proposed link between community assembly rules and spatial patterns of functional trait diversity to maximize our ability to infer multiple processes from spatial community structure.     

Integrating trait and phylogenetic distances to assess scale‐dependent community assembly processes

Biodiversity is structured by multiple mechanisms that are dependent, at least in part, on ecological similarities and differences among species. Integrating traits and phylogenies in diversity metrics may provide deeper insight into community assembly processes across spatial scales. However, different traits are influenced by processes at different spatial scales, and it is not clear how trait‐spatial scale mismatches skew our ability to detect assembly patterns. An additional complexity is how phylogenetic distances, which might capture unmeasured traits, reflect spatially dependent processes. Here we analyze a freshwater zooplankton dataset from 91 ponds and show that different traits are associated with processes at different spatial scales. We first assessed the response of individual traits to processes at both α‐ and β‐scales, and then quantified the power of different combinations of traits and phylogenetic distances to reveal environmental and spatial drivers of α‐ and β‐diversity. We found that explanatory power was maximised when we accounted for environmental and spatial drivers with single, but different traits for α‐ and β‐diversity. Using the most appropriate trait for each spatial scale outperformed phylogenetic information, but phylogenetic information outperformed the same traits when these were used at the wrong spatial scale, and all outperformed taxonomic analyses that ignore trait and phylogenetic information. We demonstrate that accounting for species’ similarities and differences provides important information about dominant assembly mechanisms at different spatial scales, and that phylogeny is especially useful when measured traits are uninformative at a given spatial scale or when there is lack of trait data. Our study also indicates, however, that trait‐scale mismatches among phylogenetically conserved traits may affect the performance of phylogenetic indices compared to indices that account only for the best single trait at each spatial scale.     

Intra-specific leaf trait responses to species richness at two different local scales

Plant functional traits, especially leaf traits, are accepted proxies for ecosystem properties. Typically, they are measured at the species level, neglecting within-species variation. While there is extensive knowledge about functional trait changes (both within and across species) along abiotic gradients, little is known about biotic influences, in particular at local scales. Here, we used a large biodiversity-ecosystem functioning experiment in subtropical China to investigate intra-specific trait changes of 16 tree species as a response to species richness of the local neighbourhood. We hypothesized that because of positive complementarity effects, species shift their leaf traits towards a more acquisitive growth strategy, when species richness of the local neighbourhood is higher. The trait shift should be most pronounced, when a focal tree's closest neighbour is from a different species, but should still be detectable as a response to species richness of the directly surrounding tree community. Consequently, we expected that trees with a con-specific closest neighbour have the strongest response to species richness of the surrounding tree community, i.e., the steepest increase of acquisitive traits. Our results indicate that species diversity promoted reduced competition and complementarity in resource use at both spatial scales considered. In addition, the closest neighbour had considerably stronger effects than the surrounding tree community. As expected, trees with a con-specific nearest neighbour showed the strongest trait shifts. However, the predicted positive effect of local hetero-specificity disappeared at the highest diversity levels of the surrounding tree community, potentially resulting from a higher probability to meet a strong competitor in a diverse environment. Our findings show that leaf traits within the same species vary not only in response to changing abiotic conditions, but also in response to local species richness. This highlights the benefit of including within-species trait variation when analysing relationships between plant functional traits and ecosystem functions.     

Environmental,spatial and phylogenetic determinants of fish life‐history traits and functional composition of Australian rivers

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Limitations of trait‐based approaches for stressor assessment: The case of freshwater invertebrates and climate drivers

The appeal of trait‐based approaches for assessing environmental vulnerabilities arises from the potential insight they provide into the mechanisms underlying the changes in populations and community structure. Traits can provide ecologically based explanations for observed responses to environmental changes, along with predictive power gained by developing relationships between traits and environmental variables. Despite these potential benefits, questions remain regarding the utility and limitations of these approaches, which we explore focusing on the following questions: (a) How reliable are predictions of biotic responses to changing conditions based on single trait–environment relationships? (b) What factors constrain detection of single trait–environment relationships, and how can they be addressed? (c) Can we use information on meta‐community processes to reveal conditions when assumptions underlying trait‐based studies are not met? We address these questions by reviewing published literature on aquatic invertebrate communities from stream ecosystems. Our findings help to define factors that influence the successful application of trait‐based approaches in addressing the complex, multifaceted effects of changing climate conditions on hydrologic and thermal regimes in stream ecosystems. Key conclusions are that observed relationships between traits and environmental stressors are often inconsistent with predefined hypotheses derived from current trait‐based thinking, particularly related to single trait–environment relationships. Factors that can influence findings of trait‐based assessments include intercorrelations of among traits and among environmental variables, spatial scale, strength of biotic interactions, intensity of habitat disturbance, degree of abiotic stress, and methods of trait characterization. Several recommendations are made for practice and further study to address these concerns, including using phylogenetic relatedness to address intercorrelation. With proper consideration of these issues, trait‐based assessment of organismal vulnerability to environmental changes can become a useful tool to conserve threatened populations into the future.     

Intraspecific functional trait variability across different spatial scales: a case study of two dominant trees in Korean pine broadleaved forest

Intraspecific functional trait variability plays an important role in the response of plants to environmental changes. However, it is still unclear how the variability differs across three nested spatial scales (individual, plot, and site) and which determinants (climatic, soil, and ontogenetic variables) shape the trait variability. Along a latitudinal gradient in Korean pine broadleaved forest of northeast China, we quantified the extent of intraspecific variability of four functional traits in two dominant trees Pinus koraiensis and Fraxinus mandshurica at eight sites, including specific leaf area, leaf dry matter content (morphological traits) and leaf nitrogen content, leaf phosphorus content (physiological traits). Results showed a large trait variation within and between species (coefficient variation: 6.07–23.3%). The leaf physiological traits of F. mandshurica and morphological traits of P. koraiensis were more responsive at site scale, while the morphological traits of F. mandshurica and physiological traits of P. koraiensis were more responsive at individual scale. In addition, abiotic and biotic factors explaining functional trait variation differ markedly between the two tree species, with physiological trait of F. mandshurica being more associated with climate and soil, while traits variability in P. koraiensis was not affected by climate, soil, and ontogeny, except for leaf phosphorus content. Overall, we can predict that the physiological traits of broadleaved species tend to be more sensitive to environmental changes, while pines are more sensitive to competition. It is critical to determine which spatial scale and trait type should be taken into account in predictive models of vegetation dynamics.     

Common ancestry or environmental trait filters: cross‐continental comparisons of trait–habitat relationships in tropical anuran amphibian assemblages

Aim To investigate whether trait–habitat relations in biological communities converge across three global regions. The goal is to assess the role of habitat templets in shaping trait assemblages when different assembly mechanisms are operating and to test whether trait–habitat relations reflect a common evolutionary history or environmental trait filters. Location Guiana Shield, South America; Upper Guinea Forest Block, West Africa; Borneo rain forests, Southeast Asia. Methods We compared large anuran amphibian data sets at both the regional and cross‐continental scale. We applied a combination of three‐table ordinations (RLQ) and permutation model‐based multivariate fourth‐corner statistics to test for trait–habitat relationships at both scales and used phylogenetic comparative methods to quantify phylogenetic signal in traits that enter these analyses. Results Despite the existence of significant trait–habitat links and congruent trait patterns, we did not find evidence for the existence of a universal trait–habitat relationship at the assemblage level and no clear sign for cross‐continental convergence of trait–habitat relations. Patterns rather varied between continents. Despite the fact that a number of traits were conserved across phylogenies, the phylogenetic signal varied between regions. Trait–habitat relations therefore not only reflect a common evolutionary history, but also more recently operating environmental trait filters that ultimately determine the trait composition in regional assemblages. Main conclusions Integrating trait–habitat links into analyses of biological assemblages can enhance the predictive power and general application of species assembly rules in community and macroecology, particularly when phylogenetic comparative methods are simultaneously applied. However, in order to predict trait composition based on habitat templets, trait–habitat links cannot be assumed to be universal but rather have to be individually established in different regions prior to model building. Only then can direct trait‐based approaches be useful tools for predicting fundamental community patterns.     

Environmental and spatial controls of taxonomic versus trait composition of stream biota

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