Hierarchical effects of environmental filters on the functional structure of plant communities: a case study in the French Alps

Understanding the influence of the environment on the functional structure of ecological communities is essential to predict the response of biodiversity to global change drivers. Ecological theory suggests that multiple environmental factors shape local species assemblages by progressively filtering species from the regional species pool to local communities. These successive filters should influence the various components of community functional structure in different ways. In this paper, we tested the relative influence of multiple environmental filters on various metrics of plant functional trait structure (i.e. ‘community weighted mean trait’ and components of functional trait diversity, i.e. functional richness, evenness and divergence) in 82 vegetation plots in the Guisane Valley, French Alps. For the 211 sampled species we measured traits known to capture key aspects of ecological strategies amongst vascular plant species, i.e. leaf traits, plant height and seed mass (LHS). A comprehensive information theory framework, together with null model based resampling techniques, was used to test the various environmental effects. Particular community components of functional structure responded differently to various environmental gradients, especially concerning the spatial scale at which the environmental factors seem to operate. Environmental factors acting at a large spatial scale (e.g. temperature) were found to predominantly shape community weighted mean trait values, while fine‐scale factors (topography and soil characteristics) mostly influenced functional diversity and the distribution of trait values among the dominant species. Our results emphasize the hierarchical nature of ecological forces shaping local species assemblage: large‐scale environmental filters having a primary effect, i.e. selecting the pool of species adapted to a site, and then filters at finer scales determining species abundances and local species coexistence. This suggests that different components of functional community structure will respond differently to environmental change, so that predicting plant community responses will require a hierarchical multi‐facet approach.     

A multi-scale comparison of trait linkages to environmental and spatial variables in fish communities across a large freshwater lake

Species present in communities are affected by the prevailing environmental conditions, and the traits that these species display may be sensitive indicators of community responses to environmental change. However, interpretation of community responses may be confounded by environmental variation at different spatial scales. Using a hierarchical approach, we assessed the spatial and temporal variation of traits in coastal fish communities in Lake Huron over a 5-year time period (2001–2005) in response to biotic and abiotic environmental factors. The association of environmental and spatial variables with trophic, life-history, and thermal traits at two spatial scales (regional basin-scale, local site-scale) was quantified using multivariate statistics and variation partitioning. We defined these two scales (regional, local) on which to measure variation and then applied this measurement framework identically in all 5 study years. With this framework, we found that there was no change in the spatial scales of fish community traits over the course of the study, although there were small inter-annual shifts in the importance of regional basin- and local site-scale variables in determining community trait composition (e.g., life-history, trophic, and thermal). The overriding effects of regional-scale variables may be related to inter-annual variation in average summer temperature. Additionally, drivers of fish community traits were highly variable among study years, with some years dominated by environmental variation and others dominated by spatially structured variation. The influence of spatial factors on trait composition was dynamic, which suggests that spatial patterns in fish communities over large landscapes are transient. Air temperature and vegetation were significant variables in most years, underscoring the importance of future climate change and shoreline development as drivers of fish community structure. Overall, a trait-based hierarchical framework may be a useful conservation tool, as it highlights the multi-scaled interactive effect of variables over a large landscape.     

Scaling environmental change through the community-level: a trait-based response-and-effect framework for plants

Predicting ecosystem responses to global change is a major challenge in ecology. A critical step in that challenge is to understand how changing environmental conditions influence processes across levels of ecological organization. While direct scaling from individual to ecosystem dynamics can lead to robust and mechanistic predictions, new approaches are needed to appropriately translate questions through the community level. Species invasion, loss, and turnover all necessitate this scaling through community processes, but predicting how such changes may influence ecosystem function is notoriously difficult. We suggest that community‐level dynamics can be incorporated into scaling predictions using a trait‐based response–effect framework that differentiates the community response to environmental change (predicted by response traits) and the effect of that change on ecosystem processes (predicted by effect traits). We develop a response‐and‐effect functional framework, concentrating on how the relationships among species' response, effect, and abundance can lead to general predictions concerning the magnitude and direction of the influence of environmental change on function. We then detail several key research directions needed to better scale the effects of environmental change through the community level. These include (1) effect and response trait characterization, (2) linkages between response‐and‐effect traits, (3) the importance of species interactions on trait expression, and (4) incorporation of feedbacks across multiple temporal scales. Increasing rates of extinction and invasion that are modifying communities worldwide make such a research agenda imperative.     

Trait structure and redundancy determine sensitivity to disturbance in marine fish communities

Trait diversity is believed to influence ecosystem dynamics through links between organismal traits and ecosystem processes. Theory predicts that key traits and high trait redundancy—large species richness and abundance supporting the same traits—can buffer communities against environmental disturbances. While experiments and data from simple ecological systems lend support, large‐scale evidence from diverse, natural systems under major disturbance is lacking. Here, using long‐term data from both temperate (English Channel) and tropical (Seychelles Islands) fishes, we show that sensitivity to disturbance depends on communities’ initial trait structure and initial trait redundancy. In both ecosystems, we found that increasing dominance by climatically vulnerable traits (e.g., small, fast‐growing pelagics/corallivores) rendered fish communities more sensitive to environmental change, while communities with higher trait redundancy were more resistant. To our knowledge, this is the first study demonstrating the influence of trait structure and redundancy on community sensitivity over large temporal and spatial scales in natural systems. Our results exemplify a consistent link between biological structure and community sensitivity that may be transferable across ecosystems and taxa and could help anticipate future disturbance impacts on biodiversity and ecosystem functioning.     

Biological traits approaches in benthic marine ecology: Dead ends and new paths

Biological traits analysis (BTA) links community structure to both ecological functions and response to environmental drivers through species’ attributes. In consequence, it has become a popular approach in marine benthic studies. However, BTA will reach a dead end if the scientific community does not acknowledge its current shortcomings and limitations: (a) uncertainties related to data origins and a lack of standardized reporting of trait information; (b) knowledge gaps on the role of multiple interacting traits on driving the organisms’ responses to environmental variability; (c) knowledge gaps regarding the mechanistic links between traits and functions; (d) a weak focus on the spatial and temporal variability that is inherent to the trait expression of species; and, last but not least, (e) the large reliance on expert knowledge due to an enormous knowledge gap on the basic ecology of many benthic species. BTA will only reach its full potential if the scientific community is able to standardize and unify the reporting and storage of traits data and reconsider the importance of baseline observational and experimental studies to fill knowledge gaps on the mechanistic links between biological traits, functions, and environmental variability. This challenge could be assisted by embracing new technological advances in marine monitoring, such as underwater camera technology and artificial intelligence, and making use of advanced statistical approaches that consider the interactive nature and spatio‐temporal variability of biological systems. The scientific community has to abandon some dead ends and explore new paths that will improve our understanding of individual species, traits, and the functioning of benthic ecosystems.     

Fish communities diverge in species but converge in traits over three decades of warming

Describing the spatial and temporal dynamics of communities is essential for understanding the impacts of global environmental change on biodiversity and ecosystem functioning. Trait‐based approaches can provide better insight than species‐based (i.e. taxonomic) approaches into community assembly and ecosystem functioning, but comparing species and trait dynamics may reveal important patterns for understanding community responses to environmental change. Here, we used a 33‐year database of fish monitoring to compare the spatio‐temporal dynamics of taxonomic and trait structure in North Sea fish communities. We found that the majority of variation in both taxonomic and trait structure was explained by a pronounced spatial gradient, with distinct communities in the southern and northern North Sea related to depth, sea surface temperature, salinity and bed shear stress. Both taxonomic and trait structure changed significantly over time; however taxonomically, communities in the south and north diverged towards different species, becoming more dissimilar over time, yet they converged towards the same traits regardless of species differences. In particular, communities shifted towards smaller, faster growing species with higher thermal preferences and pelagic water column position. Although taxonomic structure changed over time, its spatial distribution remained relatively stable, whereas in trait structure, the southern zone of the North Sea shifted northward and expanded, leading to homogenization. Our findings suggest that global environmental change, notably climate warming, will lead to convergence towards traits more adapted for novel environments regardless of species composition.     

Both traits and phylogenetic history influence community structure in snakes over steep environmental gradients

Assemblages of closely related organisms are generated on axes of deep time diversification, biogeographic processes related to dispersal and habitat filtering, and competition. Using models that account for phylogeny, ecology, and traits, we examine how the interaction among biogeography, habitat filtering, and trait convergence influences community assemblage in Nearctic snakes. With 122 community surveys, environmental niche, trait data including size, diet, parity and habitat preference, and a nearly complete phylogeny of snakes from the United States, we ask 1) do phylogenetic species variability (PSV) and traits change in predictable and correlated ways given ecology and geographic distance, 2) are the measured traits variable within and across communities and how is this related to PSV at local scales, and 3) is there evidence of habitat filtering or trait divergence? Following a general trend of western to eastern North American origin and dispersal of major groups, we similarly show a significant decrease in PSV in this direction but unexpectedly with stable trait variance, showing that traits and phylogenetic variability are disconnected at the community level. We also demonstrate that trait variability and not PSV dominates local communities. Finally, regardless of phylogeny, we show that certain traits, such as reproductive mode (parity) frequency, change within communities in response to steep ecological gradients.     

Resilience and stability of a pelagic marine ecosystem

The accelerating loss of biodiversity and ecosystem services worldwide has accentuated a long-standing debate on the role of diversity in stabilizing ecological communities and has given rise to a field of research on biodiversity and ecosystem functioning (BEF). Although broad consensus has been reached regarding the positive BEF relationship, a number of important challenges remain unanswered. These primarily concern the underlying mechanisms by which diversity increases resilience and community stability, particularly the relative importance of statistical averaging and functional complementarity. Our understanding of these mechanisms relies heavily on theoretical and experimental studies, yet the degree to which theory adequately explains the dynamics and stability of natural ecosystems is largely unknown, especially in marine ecosystems. Using modelling and a unique 60-year dataset covering multiple trophic levels, we show that the pronounced multi-decadal variability of the Southern California Current System (SCCS) does not represent fundamental changes in ecosystem functioning, but a linear response to key environmental drivers channelled through bottom-up and physical control. Furthermore, we show strong temporal asynchrony between key species or functional groups within multiple trophic levels caused by opposite responses to these drivers. We argue that functional complementarity is the primary mechanism reducing community variability and promoting resilience and stability in the SCCS.     

Linking traits to species diversity and community structure in phytoplankton

In addition to answering Hutchinson’s question “Why are there so many species?”, we need to understand why certain species are found only under certain environmental conditions and not others. Trait-based approaches are being increasingly used in ecology to do just that: explain and predict species distributions along environmental gradients. These approaches can be successful in understanding the diversity and community structure of phytoplankton. Among major traits shaping phytoplankton distributions are resource utilization traits, morphological traits (with size being probably the most influential), grazer resistance traits, and temperature responses. We review these trait-based approaches and give examples of how trait data can explain species distributions in both freshwater and marine systems. We also outline new directions in trait-based approaches applied to phytoplankton such as looking simultaneously at trait and phylogenetic structure of phytoplankton communities and using adaptive dynamics models to predict trait evolution.     

Plant functional trait response to environmental drivers across European temperate forest understorey communities

• Functional traits respond to environmental drivers, hence evaluating trait‐environment relationships across spatial environmental gradients can help to understand how multiple drivers influence plant communities. Global‐change drivers such as changes in atmospheric nitrogen deposition occur worldwide, but affect community trait distributions at the local scale, where resources (e.g. light availability) and conditions (e.g. soil pH) also influence plant communities.
• We investigate how multiple environmental drivers affect community trait responses related to resource acquisition (plant height, specific leaf area (SLA), woodiness, and mycorrhizal status) and regeneration (seed mass, lateral spread) of European temperate deciduous forest understoreys. We sampled understorey communities and derived trait responses across spatial gradients of global‐change drivers (temperature, precipitation, nitrogen deposition, and past land use), while integrating in‐situ plot measurements on resources and conditions (soil type, Olsen phosphorus (P), Ellenberg soil moisture, light, litter mass, and litter quality).
• Among the global‐change drivers, mean annual temperature strongly influenced traits related to resource acquisition. Higher temperatures were associated with taller understoreys producing leaves with lower SLA, and a higher proportional cover of woody and obligate mycorrhizal (OM) species. Communities in plots with higher Ellenberg soil moisture content had smaller seeds and lower proportional cover of woody and OM species. Finally, plots with thicker litter layers hosted taller understoreys with larger seeds and a higher proportional cover of OM species.
• Our findings suggest potential community shifts in temperate forest understoreys with global warming, and highlight the importance of local resources and conditions as well as global‐change drivers for community trait variation.

     

The ecology of differences: assessing community assembly with trait and evolutionary distances

Species enter and persist in local communities because of their ecological fit to local conditions, and recently, ecologists have moved from measuring diversity as species richness and evenness, to using measures that reflect species ecological differences. There are two principal approaches for quantifying species ecological differences: functional (trait‐based) and phylogenetic pairwise distances between species. Both approaches have produced new ecological insights, yet at the same time methodological issues and assumptions limit them. Traits and phylogeny may provide different, and perhaps complementary, information about species' differences. To adequately test assembly hypotheses, a framework integrating the information provided by traits and phylogenies is required. We propose an intuitive measure for combining functional and phylogenetic pairwise distances, which provides a useful way to assess how functional and phylogenetic distances contribute to understanding patterns of community assembly. Here, we show that both traits and phylogeny inform community assembly patterns in alpine plant communities across an elevation gradient, because they represent complementary information. Differences in historical selection pressures have produced variation in the strength of the trait‐phylogeny correlation, and as such, integrating traits and phylogeny can enhance the ability to detect assembly patterns across habitats or environmental gradients.     

Integrating phylogenetic community structure with species distribution models: an example with plants of rock barrens

Approaches using phylogenetic pattern in ecological communities to deduce processes of community assembly have been criticised as disconnected from foundations in ecological mechanism, especially with respect to lack of data about abiotic and biotic niches. These criticisms can be addressed with analyses of organismal traits that underlie environmental filtering, competitive exclusion, and other candidate processes; however, the difficulty of assembling large trait databases means that such studies remain uncommon. We suggest a synthesis of phylogenetic community structure analysis and species distribution modeling that we believe can allow inference about community processes without prohibitive data requirements. We illustrate this method for angiosperm communities of rock barrens in eastern Canada. First, we analyzed phylogenetic community structure of four rock‐barren sites at three nested spatial scales (quadrat to region). For the nine most common species in our barrens, we used regional occurrence records to build species distribution models identifying environmental drivers of the nine species’ distributions. Coefficients of these models represent implicit trait data that summarize each species’ response to the environmental drivers in the model. We then tested for phylogenetic signal in these traits, to ask whether ecological forces acting on them could be generating phylogenetic community structure. We found strong phylogenetic clustering at the quadrat level, while patterns at larger scales were complex. Our distribution model suggested drought stress as the dominant driver for distributions of all the species, consistent with local correlations with soil depth, and the species’ responses to drought showed strong phylogenetic signal. The convergence of results from phylogenetic community structure and species distribution modeling suggests that barren communities are structured at the quadrat level by environmental filtering effects of moisture stress, to which species have phylogenetically patterned responses.     

Spatio-temporal dynamics of multi-trophic communities reveal ecosystem-wide functional reorganization

Large-scale alterations in marine ecosystems as a response to environmental and anthropogenic pressures have been documented worldwide. Yet, these are primarily investigated by assessing abundance fluctuations of a few dominant species, which inadequately reflect ecosystem-wide changes. In addition, it is increasingly recognized that it is not species identity per se, but their traits that determine environmental responses, biological interactions and ecosystem functioning. In this study, we investigated long-term, spatio-temporal variability in trait composition across multiple organism groups to assess whether functional changes occur in a similar way across trophic levels and whether shifts in trait composition link to environmental change. We combined extensive trait datasets with long-term surveys (30–40 yr) of four organism groups (phytoplankton, zooplankton, benthic invertebrates and fish) in three environmentally distinct areas of a large marine ecosystem. We found similar temporal trajectories in the community weighted mean trait time-series of the different trophic groups, revealing ecosystem-wide functional changes. The traits involved and their dynamics differed between areas, concurrent with climate-driven changes in temperature and salinity, as well as more local dynamics in nutrients and oxygen. This finding highlights the importance of considering both global climate, as well as local external drivers when studying ecosystem changes. Using a multi-trophic trait-based approach, our study demonstrates the importance of integrating community functional dynamics across multiple trophic levels to capture ecosystem-wide responses which could, ultimately, help moving towards a holistic understanding, assessment and management of marine ecosystems.     

Trait‐based community assembly of aquatic macrophytes along a water depth gradient in a freshwater lake

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Community mean traits as additional indicators to monitor effects of land-use intensity on grassland plant diversity

Semi-natural grasslands, biodiversity hotspots in Central-Europe, suffer from the cessation of traditional land-use. Amount and intensity of these changes challenge current monitoring frameworks typically based on classic indicators such as selected target species or diversity indices. Indicators based on plant functional traits provide an interesting extension since they reflect ecological strategies at individual and ecological processes at community levels. They typically show convergent responses to gradients of land-use intensity over scales and regions, are more directly related to environmental drivers than diversity components themselves and enable detecting directional changes in whole community dynamics. However, probably due to their labor- and cost intensive assessment in the field, they have been rarely applied as indicators so far.Here we suggest overcoming these limitations by calculating indicators with plant traits derived from online accessible databases. Aiming to provide a minimal trait set to monitor effects of land-use intensification on plant diversity we investigated relationships between 12 community mean traits, 2 diversity indices and 6 predictors of land-use intensity within grassland communities of 3 different regions in Germany (part of the German ‘Biodiversity Exploratory’ research network). By standardization of traits and diversity measures, use of null models and linear mixed models we confirmed (i) strong links between functional community composition and plant diversity, (ii) that traits are closely related to land-use intensity, and (iii) that functional indicators are equally, or even more sensitive to land-use intensity than traditional diversity indices. The deduced trait set consisted of 5 traits, i.e., specific leaf area (SLA), leaf dry matter content (LDMC), seed release height, leaf distribution, and onset of flowering. These database derived traits enable the early detection of changes in community structure indicative for future diversity loss. As an addition to current monitoring measures they allow to better link environmental drivers to processes controlling community dynamics.     

Evolution in plant populations as a driver of ecological changes in arthropod communities

Heritable variation in traits can have wide-ranging impacts on species interactions, but the effects that ongoing evolution has on the temporal ecological dynamics of communities are not well understood. Here, we identify three conditions that, if experimentally satisfied, support the hypothesis that evolution by natural selection can drive ecological changes in communities. These conditions are: (i) a focal population exhibits genetic variation in a trait(s), (ii) there is measurable directional selection on the trait(s), and (iii) the trait(s) under selection affects variation in a community variable(s). When these conditions are met, we expect evolution by natural selection to cause ecological changes in the community. We tested these conditions in a field experiment examining the interactions between a native plant (Oenothera biennis) and its associated arthropod community (more than 90 spp.). Oenothera biennis exhibited genetic variation in several plant traits and there was directional selection on plant biomass, life-history strategy (annual versus biennial reproduction) and herbivore resistance. Genetically based variation in biomass and life-history strategy consistently affected the abundance of common arthropod species, total arthropod abundance and arthropod species richness. Using two modelling approaches, we show that evolution by natural selection in large O. biennis populations is predicted to cause changes in the abundance of individual arthropod species, increases in the total abundance of arthropods and a decline in the number of arthropod species. In small O. biennis populations, genetic drift is predicted to swamp out the effects of selection, making the evolution of plant populations unpredictable. In short, evolution by natural selection can play an important role in affecting the dynamics of communities, but these effects depend on several ecological factors. The framework presented here is general and can be applied to other systems to examine the community-level effects of ongoing evolution.     

Biodiversity change under adaptive community dynamics

Compositional change is a ubiquitous response of ecological communities to environmental drivers of global change, but is often regarded as evidence of declining “biotic integrity” relative to historical baselines. Adaptive compositional change, however, is a foundational idea in evolutionary biology, whereby changes in gene frequencies within species boost population-level fitness, allowing populations to persist as the environment changes. Here, we present an analogous idea for ecological communities based on core concepts of fitness and selection. Changes in community composition (i.e., frequencies of genetic differences among species) in response to environmental change should normally increase the average fitnessof community members. We refer to compositional changes that improve the functional match, or “fit,” between organisms' traits and their environment as adaptive community dynamics. Environmental change (e.g., land-use change) commonly reduces the fit between antecedent communities and new environments. Subsequent change in community composition in response to environmental changes, however, should normally increase community-level fit, as the success of at least some constituent species increases. We argue that adaptive community dynamics are likely to improve or maintain ecosystem function (e.g., by maintaining productivity). Adaptive community responses may simultaneously produce some changes that are considered societally desirable (e.g., increased carbon storage) and others that are undesirable (e.g., declines of certain species), just as evolutionary responses within species may be deemed desirable (e.g., evolutionary rescue of an endangered species) or undesirable (e.g., enhanced virulence of an agricultural pest). When assessing possible management interventions, it is important to distinguish between drivers of environmental change (e.g., undesired climate warming) and adaptive community responses, which may generate some desirable outcomes. Efforts to facilitate, accept, or resist ecological change require separate consideration of drivers and responses, and may highlight the need to reconsider preferences for historical baseline communities over communities that are better adapted to the new conditions.     

A trait‐based framework for stream algal communities

The use of trait‐based approaches to detect effects of land use and climate change on terrestrial plant and aquatic phytoplankton communities is increasing, but such a framework is still needed for benthic stream algae. Here we present a conceptual framework of morphological, physiological, behavioural and life‐history traits relating to resource acquisition and resistance to disturbance. We tested this approach by assessing the relationships between multiple anthropogenic stressors and algal traits at 43 stream sites. Our “natural experiment” was conducted along gradients of agricultural land‐use intensity (0–95% of the catchment in high‐producing pasture) and hydrological alteration (0–92% streamflow reduction resulting from water abstraction for irrigation) as well as related physicochemical variables (total nitrogen concentration and deposited fine sediment). Strategic choice of study sites meant that agricultural intensity and hydrological alteration were uncorrelated. We studied the relationships of seven traits (with 23 trait categories) to our environmental predictor variables using general linear models and an information‐theoretic model‐selection approach. Life form, nitrogen fixation and spore formation were key traits that showed the strongest relationships with environmental stressors. Overall, FI (farming intensity) exerted stronger effects on algal communities than hydrological alteration. The large‐bodied, non‐attached, filamentous algae that dominated under high farming intensities have limited dispersal abilities but may cope with unfavourable conditions through the formation of spores. Antagonistic interactions between FI and flow reduction were observed for some trait variables, whereas no interactions occurred for nitrogen concentration and fine sediment. Our conceptual framework was well supported by tests of ten specific hypotheses predicting effects of resource supply and disturbance on algal traits. Our study also shows that investigating a fairly comprehensive set of traits can help shed light on the drivers of algal community composition in situations where multiple stressors are operating. Further, to understand non‐linear and non‐additive effects of such drivers, communities need to be studied along multiple gradients of natural variation or anthropogenic stressors.     

Trait response in communities to environmental change: effect of interspecific competition and trait covariance structure

The response of ecological communities to environmental disturbances depends not just on the number of species they contain but also on the functional diversity of the constituent species; greater variation in the tolerance of species to different environmental disturbances is generally thought to confer greater resistance to the community. Here, I investigate how the functional diversity of communities changes with environmental disturbances. Specifically, I assume that there is variation in traits among species that confer tolerance or sensitivity to environmental disturbances. When a disturbance occurs, variation in species tolerances causes changes in the relative abundances of species, which in turn changes the average tolerance of the community. For example, if tolerance to an environmental disturbance is conferred by large body size, then the environmental disturbance should be expected to increase the average body size of individuals in the community. Despite this expectation, ecological interactions among species can affect the average community response. For example, if larger species are also strong competitors with each other, then this might reduce the increase in average body size in the community, because interspecific competition limits the grow in population density of large bodied species. Similarly, when disturbances affect multiple traits, the covariance in the distribution of trait values among species may restrict the response of any one trait; if two traits provide tolerance to the same disturbance but negatively covary among species, then the response of one trait will limit the response of the other trait at the community level. Using a Lotka–Volterra model for competitive communities, I derive general formulae that generate explicit predictions about the changes in average trait values in a community subject to environmental disturbances. These formulae demonstrate that competition can impede the change in average community trait values. However, the impediment is not considerable in comparison to the predominant factors of trait variances and species selection effects when species with the most similar trait values also experience the greatest interspecific competition. Similarly, negative covariances among different traits that confer resistance to the same environmental disturbance will impede their responses. I illustrate these results using phytoplankton data from a whole-lake experiment in which manipulation to the zooplankton community created a disturbance to the phytoplankton that changed the selective consumption of large vs. small phytoplankton.