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Linking multidimensional functional diversity to quantitative methods: A graphical hypothesis-evaluation framework
Ecology
Abstract and Figures
Functional trait analysis is an appealing approach to study differences among biological communities because traits determine species' responses to the environment and their impacts on ecosystem functioning. Despite a rapidly expanding quantitative literature, it remains challenging to conceptualize concurrent changes in multiple trait dimensions ("trait space") and select quantitative functional diversity methods to test hypotheses prior to analysis. To address this need, we present a widely applicable framework for visualizing ecological phenomena in trait space to guide the selection, application, and interpretation of quantitative functional diversity methods. We describe five hypotheses that represent general patterns of responses to disturbance in functional community ecology and then apply a formal decision process to determine appropriate quantitative methods to test ecological hypotheses. As a part of this process, we devise a new statistical approach to test for functional turnover among communities. Our combination of hypotheses and metrics can be applied broadly to address ecological questions across a range of systems and study designs. We illustrate the framework with a case study of disturbance in freshwater communities. This hypothesis-driven approach will increase the rigor and transparency of applied functional trait studies.
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Supplementary resources (5)
... Conversely, similar changes in species richness and functional diversity across a gradient might indicate low redundancy. Abrupt changes in species and/or traits (functional) along elevational gradients also indicate regions where the environment acts as a strong selective agent in shaping community structure 28 . Therefore, by addressing how both species and their traits are distributed in mountain ecosystems, we can assess the relative role of habitat heterogeneity and elevation on community identity and function. ...
... Although species occupying over a 1000 m gradient below treeline would experience a very wide gradient of climatic conditions, especially early in the breeding season, harsher climatic conditions above treeline (more persistent snow and colder temperatures with more storms) may further promote turnover, selecting for species that are well adapted to surviving and breeding in extreme and exposed environments 6,13 . We also demonstrated support for functional turnover, above treeline only, as the alpine bird community showed both significantly less functional dispersion compared to bird communities at or below treeline, and the most distant centroid in multi-dimensional trait space 28 . The latter indicates specialization as alpine birds are functionally closer to each other than bird communities in other mountain habitat types. ...
... Functional dispersion is the mean distance of individual species from the centroid of all species in a community and was calculated by combining species traits with the estimated species density per point-count survey 69 . Functional distance, the Euclidean distance between the non-density weighted centroids of two communities in a trait space, was calculated using a presence/absence matrix and a Gower distance trait matrix 28 . From the resulting 2.3 million pairwise comparisons, we assessed the effect of mountain habitat types on the functional distances between the centroids using the following mixed-effect model structure: ...
Mountains produce distinct environmental gradients that may constrain or facilitate both the presence of avian species and/or specific combinations of functional traits. We addressed species richness and functional diversity to understand the relative importance of habitat structure and elevation in shaping avian diversity patterns in the south temperate Andes, Chile. During 2010-2018, we conducted 2202 point-counts in four mountain habitats (successional montane forest, old-growth montane forest, subalpine, and alpine) from 211 to 1,768 m in elevation and assembled trait data associated with resource use for each species to estimate species richness and functional diversity and turnover. We detected 74 species. Alpine specialists included 16 species (22%) occurring only above treeline with a mean elevational range of 298 m, while bird communities below treeline (78%) occupied a mean elevational range of 1,081 m. Treeline was an inflection line, above which species composition changed by 91% and there was a greater turnover in functional traits (2-3 times greater than communities below treeline). Alpine birds were almost exclusively migratory, inhabiting a restricted elevational range, and breeding in rock cavities. We conclude that elevation and habitat heterogeneity structure avian trait distributions and community composition, with a diverse ecotonal sub-alpine and a distinct alpine community.
... Just as taxonomic beta diversity can be partitioned into two components, so functional beta diversity (the dissimilarity in functional strategies between two communities) can be disaggregated into turnover and NRD components (Table 1, Villéger et al. 2013). High functional turnover along a disturbance gradient would reflect the introduction of novel trait combinations at disturbed sites (Boersma et al. 2016). By contrast, the progressive removal of extreme trait values by environmental filtering would create high true functional nestedness, as disturbed sites would contain only a subset of the functional strategies of undisturbed communities (Villéger et al. 2013). ...
... We expected a weaker trend in functional beta diversity, given that invertebrate community responses to drying are often buffered by high functional redundancy (Bêche and Statzner 2009, Boersma et al. 2014, Schriever et al. 2015. Finally, we predicted NRD to be a more important contributor to functional beta diversity than turnover, as changes in functional richness following stream drying have previously been reported even where species turnover has been prevalent (Boersma et al. 2016). ...
... Contrary to our expectations, NRD was not always the dominant driver of functional dissimilarity between treatments, as would be expected in cases where disturbed communities are functionally homogenised (i.e. lower functional richness), which has previously been reported following supraseasonal drought (Boersma et al. 2016). NRD was important at distinguishing between treatments with small differences in drought intensity, implying that some of the trait combinations displayed by slightly and moderately disturbed communities represented generalist strategies that were also viable under moderate and high intensity drought, respectively. ...
Climatic extremes are becoming more frequent and intense across much of the globe, potentially transforming the biodiversity and functioning of affected ecosystems. In freshwaters, hydrological extremes such as drought can regulate beta diversity, acting as powerful environmental filters to dictate the complement of species and functional traits found at local and landscape scales. New methods that enable beta diversity and its functional equivalent to be partitioned into turnover (replacement of species/functions) and nestedness-resultant (gain/loss of species/functions) components may offer novel insights into the parallel impacts of drought on ecosystem structure and function. Using a series of artificial channels (mesocosms) designed to mimic perennial headwater streams, we experimentally manipulated streamflows to simulate a gradient of drought intensity. We then modelled taxonomic and functional turnover and nestedness of macroinvertebrate communities along this gradient, validating direct gradient approaches (bootstrapping, Mantel tests) against null models of nestedness. Drought intensification produced significant environmental distance decay trends (i.e. communities became increasingly taxonomically and functionally dissimilar the more differentially disturbed by drought they were). Taxonomic distance decay was primarily driven by turnover, while the functional trend reflected a combination of richness differences and turnover at different points along the gradient. Taxonomic and functional distance decay slopes were not significantly different, implying that communities were functionally vulnerable to drying. The increased frequency and intensity of droughts predicted under most climate change scenarios could thus profoundly modify not only the structure of running water invertebrate communities, but also the ecosystem functions they underpin.
... Temporal variation of assembly rules in response to a disturbance. We used published dataset of Bogan & Lytle 55 , also used in Boersma et al. 56 . An aquatic invertebrate community in a small and isolated stream (French Joe Canyon) in southeast Arizona was sampled before and after a severe drought and resultant stream drying event (8 years separating the two sampling periods). ...
... An aquatic invertebrate community in a small and isolated stream (French Joe Canyon) in southeast Arizona was sampled before and after a severe drought and resultant stream drying event (8 years separating the two sampling periods). Seven categorical functional traits were selected that are associated with biological responses to drought in arid-land streams: body size, functional feeding group, dispersal ability, locomotion, voltinism, respiration and diapause [56][57][58] . We assessed the impact of this drying event on the community structure and composition in applying our framework on the invertebrate communities before and after the disturbance. ...
Describing how communities change over space and time is crucial to better understand and predict the functioning of ecosystems. We propose a new methodological framework, based on network theory and modularity concept, to determine which type of mechanisms (i.e. deterministic versus stochastic processes) has the strongest influence on structuring communities. This framework is based on the computation and comparison of two networks: the co-occurrence (based on species abundances) and the functional networks (based on the species traits values). In this way we can assess whether the species belonging to a given functional group also belong to the same co-occurrence group. We adapted the Dg index of Gauzens et al. (2015) to analyze congruence between both networks. This offers the opportunity to identify which assembly rule(s) play(s) the major role in structuring the community. We illustrate our framework with two datasets corresponding to different faunal groups and ecosystems, and characterized by different scales (spatial and temporal scales). By considering both species abundance and multiple functional traits, our framework improves significantly the ability to discriminate the main assembly rules structuring the communities. This point is critical not only to understand community structuring but also its response to global changes and other disturbances.
... Historically these relationships have been analyzed by directly comparing taxa abundance to habitat characteristics (Chase and Myers 2011) because abundances should respond as environmental conditions change. By using traits in addition to taxonomic abundance information, connections between abundance and habitat are seen that are not always apparent using only one of these measures alone (Schulze and Mooney 1993;Flynn and Palmer 2011;Parravicini et al 2014) and may better explain distribution patterns (Boersma et al 2016). Naturally occurring gradients that encompass a diversity of habitat characteristics present opportunities to observe how communities respond to changes in biotic and abiotic conditions. ...
The world’s most extensive freshwater sand dunes lie along the eastern shore of Lake Michigan, USA. These dunes follow a succession gradient from open canopy, grass-covered dunes to forested dunes further inland with wetlands interspersed in the dune landscape. We asked if macroinvertebrate assemblages in interdunal wetlands showed predictable change along the dune succession gradient. In April through October 2017, we collected physical–chemical data, characterized wetland habitat, and macroinvertebrate assemblages at 11 interdunal wetlands distributed along an open-forested dune gradient. We evaluated patterns of taxonomic diversity and abundance and functional richness, community composition, and community dissimilarity along the gradient. The dune gradient represented changes in water chemistry variables associated with terrestrial and aquatic vegetation. Overall, interdunal wetlands in open dune habitat showed lower taxonomic diversity and were dominated by communities with functional traits tailored to variable habitats (active dispersal, bi/multi-voltine). Variation in assemblage composition along the gradient is correlated with differences in water temperature, pH, dissolved oxygen, and amount of surrounding terrestrial vegetation. Community dissimilarity is driven primarily by terrestrial vegetation surrounding wetlands and secondarily by spatial location. This is the first study to document aquatic diversity across a dune succession gradient illustrating that terrestrial dune vegetation has a large impact on patterns of aquatic community and functional structure. To maintain high species diversity in Great Lakes sand dunes ecosystems we promote protection for both forest and dune habitat to safeguard unique species and biological traits that use interdunal wetlands.
... After 2000, bibliographic methods, status qua description, and analysis have become the main methods to study sports science and technology cooperation. Based on the SCI sport co-authored papers from 2004-2008, the authors of [6] analyzed the five universities with the highest participation in sport cooperation institutions and carried out the analysis of cooperation networks of key institutions [7]. e authors of [8] used the SCI papers from 1978-2005 as a sample and analyzed the cooperation trends, key cooperation institutions, and the main participating third countries. ...
Sport trade frictions have continued to evolve and escalate, which has a great impact on sport academic cooperation. In order to objectively assess the impact of sports scholarship on China and provide evidence to support future changes in sports academic cooperation, this study takes 269,647 academic papers produced by sports alone from 2010–2018 as the research object and integrates explicit, implicit, and performance information contained in the paper output to construct a multidimensional matrix assessment framework. The horizontal dimension splits the collaborative research output into three mutually exclusive subsets: China-led collaborative research, sport-led collaborative research, and bisectional collaborative research; the vertical dimension systematically analyzes the characteristics of collaborative sport academic research in terms of participants, research content, and research level. The purpose of this study is to characterize the role and status of academic cooperation between the two countries through a long period, large sample, and multidimensional perspective, to make an objective assessment of the impact of academic cooperation between the two countries, and to provide evidence to support a reasonable response to the impact of changes in the relationship between the two countries on academic cooperation.
... Two mechanisms likely underpin positive FD-area relationships. First, FD was correlated with species richness (Supporting information; Petchey and Gaston 2002), suggesting that additional species add novel trait combinations to communities (Boersma et al. 2016). Therefore, an increase in FD with area is likely a consequence of increasing species richness with area, which could occur under random assembly processes on islands (Fig. 4e-f; Si et al. 2017). ...
Community assembly processes on islands are often non‐random. The mechanisms behind non‐random assembly, however, are generally difficult to disentangle. Functional diversity in combination with a null model approach that accounts for differences in species richness among islands can be used to test for non‐random assembly processes, but has been applied rarely to island communities. By linking functional diversity of trees on islands with a null model approach, we bridge this gap and test for the role of stochastic versus non‐random trait‐mediated assembly processes in shaping communities by studying functional diversity–area relationships. We measured 11 plant functional traits linked to species dispersal and resource acquisition strategies of 57 tree species on 40 tropical islands. We grouped traits into four life‐history dimensions representing 1) dispersal ability, 2) growth strategy, 3) light acquisition and 4) nutrient acquisition. To test for non‐random assembly processes, we used null models that account for differences in species richness among the islands. Our results reveal contrasting responses of the four life‐history dimensions to island area. The dispersal and the growth strategy dimensions were underdispersed on smaller islands, whereas the light acquisition dimension was overdispersed. The nutrient acquisition dimension did not deviate from null expectations. With increasing island area, shifts in the strength of non‐random assembly processes increased the diversity of dispersal and acquisition strategies in island communities. Our results suggest that smaller islands may be more difficult to colonize and provide more limited niche space compared to larger islands, whose tree communities are likely determined by stochastic processes and higher niche diversity. Our null model approach highlights that analyzing the functional diversity of different life‐history dimensions provides a powerful framework to unravel community assembly processes on islands. These complex, non‐random assembly processes are masked by measures of functional diversity that do not account for differences in species richness between islands.
griculture has been recognized as an important sector in Indonesia. In addition to providing food and industrial raw materials, it contributes significantly to the growth of gross domestic product (GDP), foreign exchange, employment, and rural household income. One of the important triggers for the decline in harvested area and rice production in Musi Rawas Regency is the decline in rice field area due to the conversion of rice fields to non-rice fields. Conflicts over water resources vary from one area to another because water is not evenly distributed both in space and time. Therefore, this paper describes changes in the household population of rice farmers and changes in the population of aquaculture farmers in Musi Rawas Regency. This research was conducted using quantitative methods through a descriptive observational approach to secondary data obtained from the Government of Musi Rawas Regency, South Sumatra Province, Indonesia. The distribution of descriptive data was analyzed univariately using the SPSS Statistic 25.0 application. The area of rice fields increased from 2019 to 2021, followed by the number of farmers which also increased. The area of aquaculture is relatively stable in 2020 and 2021 and has increased compared to 2019. Meanwhile, aquaculture households from 2019 to 2021 continue to increase. he findings indicate the expansion of new rice fields (extensification) due to the need for new rice fields due to the increase in the population of farmers. The population of fish farmers is increasing even though the fishery area is constant. This condition indicates the fragmentation of fishery land.
Aims
Quantify changes in taxonomic and functional diversity (FD) and identity to determine if responses to a disturbance severity gradient follow a “colonization/competition” hypothesis: diversity will (a) increase with disturbance severity as more open conditions favor species with traits linked to colonization; and (b) become more similar between regeneration and overstorey layers as environmental filtering favors species with traits linked to increasing competition for light.
Location
North Carolina, USA.
Methods
Taxonomic (richness [S], evenness [E], and Shannon diversity [H′]) and functional diversity (richness [FRic], evenness [FEve], and dispersion [FDis]) and identities were calculated for regeneration and overstorey before and after restoration treatments: control (CONT); “undesirable” subcanopy stems removed via herbicide (HERB); repeated burning (RRXF); and timber harvest followed by burning (HARV).
Results
In the overstorey, HARV affected taxonomic and FD. In HARV, S and H′ were lower than in other treatments. At high levels of species richness, FEve was lower in HARV than in HERB and RRXF. Similarly, at high levels of species richness, FDis was greater in HARV than other treatments. In the regeneration layer, taxonomic and FD did not differ across treatments during any of the post‐treatment years. In the regeneration stratum, HARV increased the means of traits associated with rapid post‐disturbance establishment, carbon capture, and maximum height. Greater dissimilarity in composition between the overstorey and regeneration suggests stronger treatment effects on regeneration in HARV than less severe treatments.
Conclusion
Patterns do not support the hypothesis that taxonomic diversity increases with disturbance severity or decreases with time after disturbance in the regeneration layer. Of the restoration treatments tested in this study, only HARV affected aspects of functional identity of the regeneration stratum; even so, FD remained unchanged. In mixed Quercus forests, functional identity rather than taxonomic or FD may provide insight into nuanced effects of restoration treatments on ecosystem function.
Temperature variation within a year can impact biological processes driving population abundances. The implications for the ecosystem services these populations provide, including food production from marine fisheries, are poorly understood. Whether and how temperature variability impacts fishery yields may depend on the number of harvested species and differences in their responses to varying temperatures. Drawing from previous theoretical and empirical studies, we predict that greater temperature variability within years will reduce yields, but harvesting a larger number of species, especially a more functionally diverse set, will decrease this impact. Using a global marine fisheries dataset, we find that within-year temperature variability reduces yields, but current levels of functional diversity (FD) of targeted species, measured using traits related to species' responses to temperature, largely offset this effect. Globally, high FD of catch could avoid annual losses in yield of 6.8% relative to projections if FD were degraded to the lowest level observed in the data. By contrast, species richness in the catch and in the ecosystem did not provide a similar mitigating effect. This work provides novel empirical evidence that short-term temperature variability can negatively impact the provisioning of ecosystem services, but that FD can buffer these negative impacts.
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Owing to the conceptual complexity of functional diversity (FD), a multitude of different methods are available for measuring it, with most being operational at only a small range of spatial scales. This causes uncertainty in ecological interpretations and limits the potential to generalize findings across studies or compare patterns across scales. We solve this problem by providing a unified framework expanding on and integrating existing approaches. The framework, based on trait probability density (TPD), is the first to fully implement the Hutchinsonian concept of the niche as a probabilistic hypervolume in estimating FD. This novel approach could revolutionize FD-based research by allowing quantification of the various FD components from organismal to macroecological scales, and allowing seamless transitions between scales.
Functional diversity is an important component of biodiversity, yet in comparison to taxonomic diversity, methods of quantifying functional diversity are less well developed. Here, we propose a means for quantifying functional diversity that may be particularly useful for determining how functional diversity is related to ecosystem functioning. This measure of functional diversity ''FD'' is defined as the total branch length of a functional dendrogram. Various characteristics of FD make it preferable to other measures of functional diversity, such as the number of functional groups in a community. Simulating species' trait values illustrates how the relative importance of richness and composition for FD depends on the effective dimensionality of the trait space in which species separate. Fewer dimensions increase the importance of community composition and functional redundancy. More dimensions increase the importance of species richness and decreases functional redundancy. Clumping of species in trait space increases the relative importance of community composition. Five natural communities show remarkably similar relationships between FD and species richness.
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.
Multiple studies have shown that biodiversity loss can impair ecosystem processes, providing a sound basis for the general application of a precautionary approach to managing biodiversity. However, mechanistic details of species loss effects and the generality of impacts across ecosystem types are poorly understood. The functional niche is a useful conceptual tool for understanding redundancy, where the functional niche is defined as the area occupied by a species in an n-dimensional functional space. Experiments to assess redundancy based on a single functional attribute are biased towards finding redundancy, because species are more likely to have non-overlapping functional niches in a multi-dimensional functional space. The effect of species loss in any particular ecosystem will depend on i) the range of function and diversity of species within a functional group, ii) the relative partitioning of variance in functional space between and within functional groups, and iii) the potential for functional compensation (degree of functional niche overlap) of the species within a functional group. Future research on functional impairment with species loss should focus on identifying which species, functional groups, and ecosystems are most vulnerable to functional impairment from species loss, so that these can be prioritized for management activities directed at maintaining ecosystem function. This will require a better understanding of how the organization of diversity into discrete functional groups differs between different communities and ecosystems.
Functional trait diversity provides a powerful means of addressing ecology's persistent questions, through its dual role as an indicator of mechanisms driving differences in species composition between communities and as a predictor of ecosystem-level processes. Functional traits provide a means of testing mechanisms behind species turnover between communities because environmental heterogeneity, competition and disturbance influence species fitness via their traits. Functional traits also provide a link between species and multiple ecosystem-level processes, such as primary productivity, nutrient fluxes and resilience, since species influence these processes via their traits. This special issue demonstrates that functional diversity offers a practical means of investigating ecology's persistent questions.
There is a recent proposal to apply convex hulls to the measurement of habitat filtering, trophic diversity and functional richness. Although this approach has successful applications, some conceptual difficulties with the interpretation of results should not be overlooked. The basic assumption that trait convergence and the associated deflation of the convex hull is a result of environmental (habitat) filtering does not always hold, because 1) some traits may converge as a result of competition as well, and 2) environmental factors, such as disturbance, may lead to divergence, rather than convergence for certain characters. There is neither evidence nor theoretical proof that increasing correlations between traits and reduction in trait combinations are always caused by habitat filtering, especially when individual trait tranges are unchanged. Diversity measurements in terms of convex hull volumes may be misleading because zero or near zero values may results no matter how wide the individual trait ranges are. For these reasons, applications of convex hulls cannot be viewed uncritically, and considerable care must be taken even if the method is used in combination with other techniques.
Which functional diversity indices have the power to reveal changes in community assembly processes along abiotic stress gradients? Is their power affected by stochastic processes and variations in species richness along stress gradients?
We used a simple community assembly model to explore the power of functional diversity indices across a wide range of ecological contexts. The model assumes that with declining stress the influence of niche complementarity on species fitness increases while that of environmental filtering decreases. We separately incorporated two trait-independent stochastic processes – mass and priority effects – in simulating species occurrences and abundances along a hypothetical stress gradient. We ran simulations where species richness was constant along the gradient, or increased, decreased or varied randomly with declining stress. We compared observed values for two indices of functional richness – total functional dendrogram length (FD) and convex hull volume (FRic) – with a matrix-swap null model (yielding indices SESFD and SESFRic) to remove any trivial effects of species richness. We also compared two indices that measure both functional richness and functional divergence – Rao quadratic entropy (Rao) and functional dispersion (FDis) – with a null model that randomizes abundances across species but within communities. This converts them to pure measures of functional divergence (SESRao and SESFDis).
When mass effects operated, only SESRao and SESFDis gave reasonable power, irrespective of how species richness varied along the stress gradient. FD, FRic, Rao and FDis had low power when species richness was constant, and variation in species richness greatly influenced their power. SESFRic and SESFD were unaffected by variation in species richness. When priority effects operated, FRic, SESFRic, Rao and FDis had good power and were unaffected by variation in species richness. Variation in species richness greatly affected FD and SESFD. SESRao and SESFDis had low power in the priority effects model but were unaffected by variation in species richness.
Our results demonstrate that a reliable test for changes in assembly processes along stress gradients requires functional diversity indices measuring either functional richness or functional divergence. We recommend using SESFRic as a measure of functional richness and either SESRao or SESFDis (which are very closely related mathematically) as a measure of functional divergence. Used together, these indices of functional richness and functional divergence provide good power to test for increasing niche complementarity with declining stress across a broad range of ecological contexts.
Functional diversity is a component of biodiversity that generally concerns the range of things that organisms do in communities and ecosystems. Here, we review how functional diversity can explain and predict the impact of organisms on ecosystems and thereby provide a mechanistic link between the two. Critical points in developing predictive measures of functional diversity are the choice of functional traits with which organisms are distinguished, how the diversity of that trait information is summarized into a measure of functional diversity, and that the measures of functional diversity are validated through quantitative analyses and experimental tests. There is a vast amount of trait information available for plant species and a substantial amount for animals. Choosing which traits to include in a particular measure of functional diversity will depend on the specific aims of a particular study. Quantitative methods for choosing traits and for assigning weighting to traits are being developed, but need much more work before we can be confident about trait choice. The number of ways of measuring functional diversity is growing rapidly. We divide them into four main groups. The first, the number of functional groups or types, has significant problems and researchers are more frequently using measures that do not require species to be grouped. Of these, some measure diversity by summarizing distances between species in trait space, some by estimating the size of the dendrogram required to describe the difference, and some include information about species abundances. We show some new and important differences between these, as well as what they indicate about the responses of assemblages to loss of individuals. There is good experimental and analytical evidence that functional diversity can provide a link between organisms and ecosystems but greater validation of measures is required. We suggest that non-significant results have a range of alternate explanations that do not necessarily contradict positive effects of functional diversity. Finally, we suggest areas for development of techniques used to measure functional diversity, highlight some exciting questions that are being addressed using ideas about functional diversity, and suggest some directions for novel research.
Around the world, the human-induced collapses of populations and species have triggered a sixth mass extinction crisis, with rare species often being the first to disappear. Although the role of species diversity in the maintenance of ecosystem processes has been widely investigated, the role of rare species remains controversial. A critical issue is whether common species insure against the loss of functions supported by rare species. This issue is even more critical in species-rich ecosystems where high functional redundancy among species is likely and where it is thus often assumed that ecosystem functioning is buffered against species loss. Here, using extensive datasets of species occurrences and functional traits from three highly diverse ecosystems (846 coral reef fishes, 2,979 alpine plants, and 662 tropical trees), we demonstrate that the most distinct combinations of traits are supported predominantly by rare species both in terms of local abundance and regional occupancy. Moreover, species that have low functional redundancy and are likely to support the most vulnerable functions, with no other species carrying similar combinations of traits, are rarer than expected by chance in all three ecosystems. For instance, 63% and 98% of fish species that are likely to support highly vulnerable functions in coral reef ecosystems are locally and regionally rare, respectively. For alpine plants, 32% and 89% of such species are locally and regionally rare, respectively. Remarkably, 47% of fish species and 55% of tropical tree species that are likely to support highly vulnerable functions have only one individual per sample on average. Our results emphasize the importance of rare species conservation, even in highly diverse ecosystems, which are thought to exhibit high functional redundancy. Rare species offer more than aesthetic, cultural, or taxonomic diversity value; they disproportionately increase the potential breadth of functions provided by ecosystems across spatial scales. As such, they are likely to insure against future uncertainty arising from climate change and the ever-increasing anthropogenic pressures on ecosystems. Our results call for a more detailed understanding of the role of rarity and functional vulnerability in ecosystem functioning.
Question
Many functional diversity indices require the calculation of functional trait dissimilarities between species. However, very little is known about how the dissimilarity measure used might affect conclusions about ecological processes drawn from functional diversity.
Methods
We simulated real applications of functional diversity, to illustrate the key properties of the two most common families of dissimilarity measures: (1) ‘Gower’ distance, using only ‘mean trait’ value per species and then standardizing each trait, e.g. relative to its range; (2) ‘trait overlap’ between species, which takes into account within‐species trait variability. We then examine how these approaches could affect conclusions about ecological processes commonly assessed with functional diversity. We also propose a new R function (‘trova’, i.e. TR ait OV erlAp) which performs computations to estimate species trait dissimilarity with different types of data.
Results
The trait overlap approach generally produces a less context‐dependent measure of functional dissimilarity. For example, the results are less dependent on the transformation of trait data (often required in empirical datasets) and on the particular pool of species considered (i.e. trait range, regularity and presence of outliers). The results therefore could be more easily compared across studies and biomes. Further, trait overlap more reliably reproduces patterns expected when niche differentiation structures communities. The Gower approach, on the contrary, more reliably detects environmental filtering effects.
Conclusion
The two approaches imply different conceptions of how species dissimilarities relate to niche differentiation. Trait overlap is suitable for testing the effect of species interactions on functional diversity within local communities, especially when relatively small differences in species traits are linked to different resource acquisition. Gower is better suited to detecting changes in functional diversity along environmental gradients, as greater differences in trait values reflect increased niche differentiation. Combining trait overlap and Gower approaches may provide a novel way to assess the joint effects of environmental filtering and niche complementarity on community assembly. We suggest that attention should be given not only to the index of functional diversity considered but also whether the dissimilarity used is appropriate for the study context.
Functional diversity is the diversity of species traits in ecosystems. This concept is increasingly used in ecological research, yet its formal definition and measurements are currently under discussion. As the overall behavior and consistency of functional diversity indices have not been described so far, the novice user risks choosing an inaccurate index or a set of redundant indices to represent functional diversity.
In our study we closely examine functional diversity indices to clarify their accuracy, consistency, and independence. Following current theory, we categorize them into functional richness, evenness, or divergence indices. We considered existing indices as well as new indices developed in this study. The new indices aimed at remedying the weaknesses of currently used indices (e.g., by taking into account intraspecific variability). Using virtual data sets, we test (1) whether indices respond to community changes as expected from their category and (2) whether the indices within each category are consistent and independent of indices from other categories. We also test the accuracy of methods proposed for the use of categorical traits.
Most classical functional richness indices either failed to describe functional richness or were correlated with functional divergence indices. We therefore recommend using the new functional richness indices that consider intraspecific variability and thus empty space in the functional niche space. In contrast, most functional evenness and divergence indices performed well with respect to all proposed tests. For categorical variables, we do not recommend blending discrete and real‐valued traits (except for indices based on distance measures) since functional evenness and divergence have no transposable meaning for discrete traits. Nonetheless, species diversity indices can be applied to categorical traits (using trait levels instead of species) in order to describe functional richness and equitability.
Understanding the processes shaping biological communities under multiple disturbances is a core challenge in ecology and conservation science. Traditionally, ecologists have explored linkages between the severity and type of disturbance and the taxonomic structure of communities. Recent advances in the application of species traits, to assess the functional structure of communities, have provided an alternative approach that responds rapidly and consistently across taxa and ecosystems to multiple disturbances. Importantly, trait-based metrics may provide advanced warning of disturbance to ecosystems because they do not need species loss to be reactive. Here, we synthesize empirical evidence and present a theoretical framework, based on species positions in a functional space, as a tool to reveal the complex nature of change in disturbed ecosystems.
Current successional models, primarily those based on floral succession, propose several distinct trajectories based on the integration of two key hypotheses from succession theory: convergence versus divergence in species composition among successional sites, and progression towards versus deviation from a desired reference state. We applied this framework to faunal succession, including differential colonization between active and passive dispersers, and the nested patterns generated as a consequence of this peculiarity. Nine man-made wetlands located in three different areas, from 0-3 years from wetland creation, were assessed. In addition, 91 wetlands distributed throughout the region were used as references for natural macroinvertebrate communities. We predicted the following: (1) highly nested structures in pioneering assemblages will decrease to lower mid-term values due to a shift from active pioneering taxa to passive disperser ones; (2) passive idiosyncratic taxa will elicit divergent successional trajectories among areas; (3) the divergent trajectories will provoke lower local and higher regional diversity values in the mid-term assemblages than in pioneer assemblages. Our results were largely congruent with hypotheses (1) and (2), diverging from the anticipated patterns only in the case of the temporary wetlands area. However, overall diversity trends based on hypothesis (3) did not follow the expected pattern. The divergent successional trajectories did not compensate for regional biodiversity losses that occurred as a consequence of pioneering colonizer decline over time. Consequently, we suggest reconsidering wetland construction for mitigation purposes within mid-term time frames (≤3 years). Wetlands may not offset, within this temporal scenario, regional biodiversity loss because the ecosystem may not support idiosyncratic taxa from natural wetlands.
Landscape genetics aims to assess the effect of the landscape on intraspecific genetic structure. To quantify interdeme landscape structure, landscape genetics primarily uses landscape resistance surfaces (RSs) and least-cost paths or straight-line transects. However, both approaches have drawbacks. Parameterization of RSs is a subjective process, and least-cost paths represent a single migration route. A transect-based approach might oversimplify migration patterns by assuming rectilinear migration. To overcome these limitations, we combined these two methods in a new landscape genetic approach: least-cost transect analysis (LCTA). Habitat-matrix RSs were used to create least-cost paths, which were subsequently buffered to form transects in which the abundance of several landscape elements was quantified. To maintain objectivity, this analysis was repeated so that each landscape element was in turn regarded as migration habitat. The relationship between explanatory variables and genetic distances was then assessed following a mixed modelling approach to account for the nonindependence of values in distance matrices. Subsequently, the best fitting model was selected using the statistic. We applied LCTA and the mixed modelling approach to an empirical genetic dataset on the endangered damselfly, Coenagrion mercuriale. We compared the results to those obtained from traditional least-cost, effective and resistance distance analysis. We showed that LCTA is an objective approach that identifies both the most probable migration habitat and landscape elements that either inhibit or facilitate gene flow. Although we believe the statistical approach to be an improvement for the analysis of distance matrices in landscape genetics, more stringent testing is needed.
Functional diversity is hypothesised as being beneficial for ecosystem functions, such as productivity and resistance to invasion. However, a precise definition of functional diversity, and hence a framework for its quantification, have proved elusive. We present a definition based on the analogy of the components of species diversity – richness, evenness and divergence. These concepts are applied to functional characters to give three components of functional diversity – functional richness, functional evenness and functional divergence. We demonstrate how each of these components may be calculated. It is hoped that our definition of functional diversity and its components will aid in elucidation of the mechanisms behind diversity/ecosystem-function relationships.
1. Indices quantifying the functional aspect of biodiversity are essential in understanding relationships between biodiversity, ecosystem functioning and environmental constraints. Many indices of functional diversity have been published but we lack consensus about what indices quantify, how redundant they are and which ones are recommended.
2. This study aims to build a typology of functional diversity indices from artificial data sets encompassing various community structures (different assembly rules, various species richness levels) and to identify a set of independent indices able to discriminate community assembly rules.
3. Our results confirm that indices can be divided into three main categories, each of these corresponding to one aspect of functional diversity: functional richness, functional evenness and functional divergence. Most published indices are highly correlated and quantify functional richness while quadratic entropy (Q) represents a mix between functional richness and functional divergence. Conversely, two indices (FEve and FDiv respectively quantifying functional evenness and functional divergence) are rather independent to all the others. The power analysis revealed that some indices efficiently detect assembly rules while others performed poorly.
4. To accurately assess functional diversity and establish its relationships with ecosystem functioning and environmental constraints, we recommend investigating each functional component separately with the appropriate index. Guidelines are provided to help choosing appropriate indices given the issue being investigated.
5. This study demonstrates that functional diversity indices have the potential to reveal the processes that structure biological communities. Combined with complementary methods (phylogenetic and taxonomic diversity), the multifaceted framework of functional diversity will help improve our understanding of how biodiversity interacts with ecosystem processes and environmental constraints.
The metacommunity concept is an important way to think about linkages between different spatial scales in ecology. Here we review current understanding about this concept. We first investigate issues related to its definition as a set of local communities that are linked by dispersal of multiple potentially interacting species. We then identify four paradigms for metacommunities: the patch-dynamic view, the species-sorting view, the mass effects view and the neutral view, that each emphasizes different processes of potential importance in metacommunities. These have somewhat distinct intellectual histories and we discuss elements related to their potential future synthesis. We then use this framework to discuss why the concept is useful in modifying existing ecological thinking and illustrate this with a number of both theoretical and empirical examples. As ecologists strive to understand increasingly complex mechanisms and strive to work across multiple scales of spatio-temporal organization, concepts like the metacommunity can provide important insights that frequently contrast with those that would be obtained with more conventional approaches based on local communities alone.
Three commonly used measures of diversity, Simpson's index, Shannon's entropy, and the total number of species, are related to Renyi's definition of a generalized entropy. A unified concept of diversity is presented, according to which there is a continuum of possible diversity measures. In a sense which becomes apparent, these measures provide estimates of the effective number of species present, and differ only in their tendency to include or to ignore the relatively rarer species. The notion of the diversity of a community as opposed to that of a sample is examined, and is related to the asymptotic form of the species-abundance curve. A new and plausible definition of evenness is derived.
Natural populations consist of phenotypically diverse individuals that exhibit variation in their demographic parameters and intra- and inter-specific interactions. Recent experimental work indicates that such variation can have significant ecological effects. However, ecological models typically disregard this variation and focus instead on trait means and total population density. Under what situations is this simplification appropriate? Why might intraspecific variation alter ecological dynamics? In this review we synthesize recent theory and identify six general mechanisms by which trait variation changes the outcome of ecological interactions. These mechanisms include several direct effects of trait variation per se and indirect effects arising from the role of genetic variation in trait evolution.
The impact of anthropogenic activity on ecosystems has highlighted the need to move beyond the biogeographical delineation of species richness patterns to understanding the vulnerability of species assemblages, including the functional components that are linked to the processes they support. We developed a decision theory framework to quantitatively assess the global taxonomic and functional vulnerability of fish assemblages on tropical reefs using a combination of sensitivity to species loss, exposure to threats and extent of protection. Fish assemblages with high taxonomic and functional sensitivity are often exposed to threats but are largely missed by the global network of marine protected areas. We found that areas of high species richness spatially mismatch areas of high taxonomic and functional vulnerability. Nevertheless, there is strong spatial match between taxonomic and functional vulnerabilities suggesting a potential win–win conservation-ecosystem service strategy if more protection is set in these locations.
Manipulating community assemblages to achieve functional targets is a key component of restoring degraded ecosystems. The response-and-effect trait framework provides a conceptual foundation for translating restoration goals into functional trait targets, but a quantitative framework has been lacking for translating trait targets into assemblages of species that practitioners can actually manipulate. This study describes new trait-based models that can be used to generate ranges of species abundances to test theories about which traits, which trait values and which species assemblages are most effective for achieving functional outcomes. These models are generalisable, flexible tools that can be widely applied across many terrestrial ecosystems. Examples illustrate how the framework generates assemblages of indigenous species to (1) achieve desired community responses by applying the theories of environmental filtering, limiting similarity and competitive hierarchies, or (2) achieve desired effects on ecosystem functions by applying the theories of mass ratios and niche complementarity. Experimental applications of this framework will advance our understanding of how to set functional trait targets to achieve the desired restoration goals. A trait-based framework provides restoration ecology with a robust scaffold on which to apply fundamental ecological theory to maintain resilient and functioning ecosystems in a rapidly changing world.
The metacommunity concept, describing how local and regional scale processes interact to structure communities, has been successfully applied to patterns of taxonomic diversity. Functional diversity has proved useful for understanding local scale processes, but has less often been applied to understanding regional scale processes. Here, we explore functional diversity patterns within a metacommunity context to help elucidate how local and regional scale processes influence community assembly. We detail how each of the four metacommunity perspectives (species sorting, mass effects, patch dynamics, neutral) predict different patterns of functional beta- and alpha-diversity and spatial structure along two key gradients: dispersal limitation and environmental conditions. We then apply this conceptual model to a case study from alpine tundra plant communities. We sampled species composition in 17 ‘sky islands’ of alpine tundra in the Colorado Rocky Mountains, USA that differed in geographic isolation and area (key factors related to dispersal limitation) and temperature and elevation (key environmental factors). We quantified functional diversity in each site based on specific leaf area, leaf area, stomatal conductance, plant height and chlorophyll content. We found that colder high elevation sites were functionally more similar to each other (decreased functional beta-diversity) and had lower functional alpha-diversity. Geographic isolation and area did not influence functional beta- or alpha-diversity. These results suggest a strong role for environmental conditions structuring alpine plant communities, patterns consistent with the species sorting metacommunity perspective. Incorporating functional diversity into metacommunity theory can help elucidate how local and regional factors structure communities and provide a framework for observationally examining the role of metacommunity dynamics in systems where experimental approaches are less tractable.
How is the loss of plant species richness, associated with invasions, related to changes in functional diversity? What is the relationship between the traits of invasive species and those of invaded communities?
Different Central European vegetation types within the Czech Republic.
Functional diversity was calculated for 260-paired relevés, half non-invaded and half invaded by one of 13 widespread invasive species in Central Europe. Four traits (height, SLA, seed mass and clonal index) were considered as a way to understand the functional space occupied by native and alien species in the data set (410 species altogether).
Some of the functional diversity (FD) indices used (mean trait dissimilarity, mean nearest neighbour dissimilarity and SD of the mean nearest neighbour dissimilarity) revealed higher trait diversity for the invaded vegetation and negative relationship with species richness, while functional richness and evenness gave higher values for the uninvaded vegetation and positive relationship with species richness. Adding hypothetically the invader into the FD calculations for the uninvaded vegetation was found to increase most of the FD indices used, while excluding it from the FD calculations of the invaded vegetation decreased functional richness and also mean trait dissimilarity.
Results suggest that invading aliens tend to be functionally different from native species and are therefore likely to occupy an empty niche in the invaded vegetation. Similarly, the resident species in the non-invaded communities are not likely to occupy the whole potential niche space, which could remain available for the invasive species with different traits. This study suggests that the probability of a successful invasion is related to functional dissimilarities between the alien invader and native species of the resident communities.
Questions
Is trait convergence more intense when soil resource availability and disturbance constrain productivity and limit above‐ground competition? Do the effects of productivity and disturbance on functional diversity differ between the local and metacommunity scales?
Location
Semi‐arid grasslands in N ew Z ealand (43°59′ S, 170°27′ E).
Methods
We measured trait convergence and divergence in grasslands along gradients of primary productivity and disturbance at local (i.e. 1 m × 1 m) and metacommunity (8 m × 50 m) scales, using long‐term (27‐yr) manipulations of soil resource availability and grazing intensity. We compared trait dispersion metrics to those expected under different null models.
Results
At the metacommunity scale, we found stronger trait convergence with increasing productivity and grazing intensity, where all short, slow‐growing species were excluded from the potential species pool. However, once this broad‐scale filter on species pool was taken into account, we found that at the local scale, abundance‐weighted functional dispersion of co‐occurring species was stronger than expected under our null model, thereby suggesting limiting similarity. Moreover, trait divergence became stronger at higher productivity and lower grazing intensity, where size‐asymmetric competition for light is likely to have been more intense.
Conclusions
At the metacommunity scale, environmental filtering led to species with particular traits being excluded from the species pool. In contrast, at the local community scale where individuals interact, there was evidence of limiting similarity. Our results suggest that environmental filtering and limiting similarity are not mutually exclusive and jointly determine community structure, but can operate at different spatial scales.
Question: How do multivariate methods perform in relating species- and community-level trait responses to the environment? Location: (1) Field data from grazed semi-natural grasslands, NE Germany; (2) artificial data. Methods: Research questions associated with trait—environment relationships were briefly reviewed and seven available methods evaluated. The main distinction between research questions is whether trait—environment relationships should be addressed at community or species level. A redundancy analysis (RDA) of mean trait values of species in a plot weighted by their abundances (CWM-RDA) is exclusively suitable for the community level. The other six methods address the species level. A double inertia analysis of two arrays (RLQ) and double canonical correspondence analysis (double CCA) use combinations of ordinations to simultaneously analyse species and trait responses to the environment. A combination of the outlying mean index with generalized additive models (OMI-GAM) predicts the response of species to environmental variables on trait gradients. RDA-RegTree first analyses species responses to the environment with RDA and then uses a regression tree to classify trait expressions according to scores of species responses on the ordination axes. Cluster regression uses cluster analyses and logistic regression to search for trait combinations with the best response to the environmental variables. This method models the distribution of functional groups on environmental gradients. All methods and data are available as R scripts. Results: All methods consistently revealed the main trait responses to environment in the field data set, namely that life history was associated with available phosphorus while grazing intensity was related to leaf C:N ratio and canopy height. At community level, CWM-RDA gave a good overview of trait— environment relationships, as also provided by the species-based methods RLQ and double CCA. OMI-GAM revealed non-linear relationships in the field data set. Field and artificial data gave that the number and stability of functional groups produced by Cluster regression and RDA-RegTree varied more strongly than RLQ, double CCA and OMI-GAM. Conclusions: Each method addresses particular ecological concepts and research questions. If a user asks for the response of average trait expressions of communities to environmental gradients, CWM-RDA may be the first choice. However, species-based methods should be applied to address questions regarding co-existence of different life histories or to assess how groups of species respond to environmental changes. The artificial data set revealed that the methods differed in sensitivity to gradient lengths and random data.
Seasonal droughts are predictable components of arid‐land stream hydrology, and many arid‐land aquatic taxa have adapted to their extreme environment. However, climate change is altering this predictable hydrology, producing longer and more severe droughts and creating novel disturbance regimes for resident organisms.
The hydrologic transitions from flowing stream to fragmented pools to dry stream bed are frequently associated with steep decreases in taxonomic and functional diversity, referred to as thresholds of biodiversity loss. Less is known about how taxa respond between these thresholds, as fragmented pools gradually dry and abiotic conditions intensify. While an increasingly extreme environment may be expected to reduce taxonomic and trait richness, species adapted to predictable seasonal fragmentation may be resistant to declining water levels until all surface water is lost. We used aquatic mesocosms to test two competing hypotheses of the relationship between richness and pool drying for arid‐land stream invertebrates: (i) the drought vulnerability hypothesis (richness gradually decreases with drying) and (ii) the drought resistance hypothesis (richness remains constant until complete drying occurs).
We inoculated replicate mesocosms with aquatic invertebrates from arid‐land streams in Arizona, U.S.A., and applied three drying treatments representing a continuum of drying stress commonly observed in local streams during the summer dry season (water depths: 10, 7 and 1 cm). Mesocosms were covered to restrict dispersal and colonisation processes and to isolate resistance ( in situ survival of species) from resilience (community recovery following disturbance). After 45 days, we destructively sampled all invertebrates in the mesocosms and calculated various taxonomic and functional trait metrics.
Taxonomic richness and composition did not differ between drying treatments, providing strong support for the drought resistance hypothesis. Severe drying was associated with lower invertebrate abundances and higher densities than the moderate and control treatments. This finding suggests that density‐dependent processes generated by decreased available habitat may be more important determinants of community composition during droughts than abiotic stress in this system.
We observed a near‐complete overlap of trophic traits (body size and functional feeding group) and resistance traits (respiration mode and diapause) among the three treatments. This high functional redundancy may provide a buffer against changes to ecosystem functioning, even in cases of severe‐drying‐induced habitat contraction and fragmentation.
1. Ecological communities can be relatively stable for long periods of time, and then, often as a result of disturbance, transition rapidly to a novel state. When communities fail to recover to pre-disturbance configurations, they are said to have experienced a regime shift or to be in an alternative stable state.
2. In this 8-year study, we quantified the effects of complete water loss and subsequent altered disturbance regime on aquatic insect communities inhabiting a formerly perennial desert stream. We monitored two study pools seasonally for 4 years before and 4 years after the transition from perennial to intermittent flow to evaluate pre-drying community dynamics and post-drying recovery trajectories.
3. Mean species richness was not affected by the transition to intermittent flow, though seasonal patterns of richness did change. Sample densities were much higher in post-drying samples.
4. The stream pool communities underwent a catastrophic regime shift after transition to intermittent flow, moving to an alternative stable state with novel seasonal trajectories, and did not recover to pre-drying configurations after 4 years. Six invertebrate species were extirpated by the initial drying event, while other species were as much as 40 times more abundant in post-drying samples. In general, large-bodied top predators were extirpated from the system and replaced with high abundances of smaller-bodied mesopredators.
5. Our results suggest that the loss of perennial flow caused by intensified droughts and water withdrawals could lead to significant changes in community structure and species composition at local and regional scales.
Intensified exploitation of natural populations and habitats has led to increased mortality rates and decreased abundances of many species. There is a growing concern that this might cause critical abundance thresholds of species to be crossed, with extinction cascades and state shifts in ecosystems as a consequence. When increased mortality rate and decreased abundance of a given species lead to extinction of other species, this species can be characterized as functionally extinct even though it still exists. Although such functional extinctions have been observed in some ecosystems, their frequency is largely unknown. Here we use a new modelling approach to explore the frequency and pattern of functional extinctions in ecological networks. Specifically, we analytically derive critical abundance thresholds of species by increasing their mortality rates until an extinction occurs in the network. Applying this approach on natural and theoretical food webs, we show that the species most likely to go extinct first is not the one whose mortality rate is increased but instead another species. Indeed, up to 80% of all first extinctions are of another species, suggesting that a species' ecological functionality is often lost before its own existence is threatened. Furthermore, we find that large-bodied species at the top of the food chains can only be exposed to small increases in mortality rate and small decreases in abundance before going functionally extinct compared to small-bodied species lower in the food chains. These results illustrate the potential importance of functional extinctions in ecological networks and lend strong support to arguments advocating a more community-oriented approach in conservation biology, with target levels for populations based on ecological functionality rather than on mere persistence.
A novel conceptual framework is presented that proposes to apply trait-based approaches to predicting the impact of environmental change on ecosystem service delivery by multi-trophic systems. Development of the framework was based on an extension of the response–effect trait approach to capture functional relationships that drive trophic interactions. The framework was populated with worked examples to demonstrate its flexibility and value for linking disparate data sources, identifying knowledge gaps and generating hypotheses for quantitative models.
Given a resource gradient (e.g. light intensity, prey size) in a community, species evolve to use different parts of this gradient; competition between them is thereby reduced. Species relationships in the community may be conceived in terms of a multidimensional coordinate system, the axes of which are the various resource gradients (and other aspects of species relationships to space, time, and one another in the community). This coordinate system defines a hyperspace, and the range of the space that a given species occupies is its niche hypervolume, as an abstract characterization of its intra‐community position, or niche. Species evolve toward difference in niche, and consequently toward difference in location of their hypervolumes in the niche hyperspace. Through evolutionary time additional species can fit into the community in niche hypervolumes different from those of other species, and the niche hyperspace can become increasingly complex. Its complexity relates to the community's richness in species, its alpha diversity.
Species differ in the proportions of the niche hyperspace they are able to occupy and the share of the community's resources they utilize. The share of resources utilized is expressed in species' productivities, and when species are ranked by relative productivity (or some other measurement) from most to least important, importance‐value or dominance‐diversity curves are formed. Three types of curves may represent manners in which resources are divided among species: (a) niche pre‐emption with strong dominance, expressed in a geometric series, (b) random boundaries between niches, expressed in the MacArthur distribution, and (c) determination of relative importance by many factors, so that species form a frequency distribution on a logarithmic base of importance values, a lognormal distribution. The forms of importance‐value curves do not permit strong inference about resource division, but are of interest for their expression of species relationships and bearing on measurement of diversity.
1. Species diversity is most simply measured by counting species. More complicated measures, which take into account the relative abundance of the species, have been derived from information theory or from parameters of statistical distributions fitted to the census data. The information theory formulae can also be used to measure habitat diversity and differences between communities or habitats. In this way, changes in the pattern of species diversity can be compared with changes in the environment.
2. Small or remote islands and islands with uniform topography have fewer species than large or complex islands or islands nearer the source of colonization. For birds and some orders of insects it appears that the rate of colonization of new species is virtually balanced by the rate of extinction, so that the number of species has reached equilibrium. For other organisms, such as mammals, and for all organisms on the most remote islands, this equilibrium has probably not been reached and further increases in the fauna may be expected. The comparison of impoverished island faunas with the mainland faunas whence they were derived shows the effect of relaxed competition.
3. Local variations in the species diversity of small uniform habitats can usually be predicted in terms of the structure and productivity of the habitat. Habitats of similar structure on islands and mainland often have similar species diversities; the impoverishment of the island is reflected in the fact that different habitats on the island have nearly the same species, while different habitats on the mainland have more different species. This is interpreted as evidence that uniform habitats are nearly saturated with species and that new species usually colonize by occupying different habitats from present species.
4. The theory of competition and the facts of character displacement indicate that there is a limiting similarity to species which co‐exist within a habitat. Species more similar than this limiting value must occupy different habitats. According to the theory, this limiting value should be less where productivity is high, where family size is low and where the seasons are relatively uniform. It should also be less for pursuing hunters than for species which search for stationary prey.
5. Total species diversities, from areas composed of many types of habitat, are usually, but not always, much greater in the tropics than in temperate regions. This is accomplished by a finer subdivision of habitats (habitat selection) more than by a marked increase in diversity within habitats. This total diversity may still be increasing and may have not reached saturation.
A general coefficient measuring the similarity between two sampling units is defined. The matrix of similarities between all pairs of sample units is shown to be positive semidefinite (except possibly when there are missing values). This is important for the multidimensional Euclidean representation of the sample and also establishes some inequalities amongst the similarities relating three individuals. The definition is extended to cope with a hierarchy of characters.
Biodiversity can influence ecosystem functioning through changes in the amount of resource use complementary among species. Functional diversity is a measure of biodiversity that aims to quantify resource use complementarity and thereby explain and predict ecosystem functioning. The primary goal of this article is to compare the explanatory power of four measures of functional diversity: species richness, functional group richness, functional attribute diversity, and FD. The secondary goal is to showcase the novel methods required for calculating functional attribute diversity and FD. We find that species richness and functional group richness explain the least variation in above-ground biomass production within and across grassland biodiversity manipulations at six European locations; functional attribute diversity and FD explain greater variation. Rea-sons for differences in explanatory power are discussed, such as the relatively greater amount of information and fewer assumptions included in functional attribute diversity and FD. We explore the opportunities and limitations of the particular methods we used to calculate functional attribute diversity and FD. These mainly concern how best to select the information used to calculate them.
In the face of large‐scale environmental changes, predicting the consequences for species composition from species’ traits can be a major step towards generalizing ecological patterns and management. Few studies, however, have explored the applicability of this tool in relation to different climatic conditions. Here, the changes in species composition along a gradient of sheep‐grazing pressure (high, low, abandonment) were used to test whether a common set of plant functional traits (PFT) would provide consistent predictions of species’ responses to grazing in different biogeographical regions.
Data were collected across an altitudinal and climatic gradient from Mediterranean rangelands to subalpine grasslands in north‐eastern Spain. Species’ responses were calculated using partial constrained ordination to account only for the effect of grazing intensity. Regression trees and general linear models were applied to identify traits that could predict species’ responses.
Results were mostly consistent with the ruderal vs. competitive strategy ( sensu Grime ), in terms of life cycle, life form and plant height, and their expected responses to repeated disturbance.
However, the predictive capacity of the investigated traits changed with climatic conditions. Traits generally related to grazing did not show a strong repeatability across the climatic gradient. Convergent selection of climatic conditions and grazing indicated that plant features might reflect an adaptation to multiple selective forces. The climatic conditions acted as filters on the pool of PFT available and shifted the relevance of plant traits as potential predictors. Results were not substantially different after applying phylogenetically independent contrasts (PIC).
Synthesis and applications . At a local scale, plant functional traits are useful tools in predicting species’ responses to grazing and, for conservation purposes, identifying species vulnerable to land‐use changes. However, predictions cannot be extrapolated from one climatic region to another. The methodology proposed in this study to detect predicting traits can be applied more generally. Regression trees, in particular, appear to be a useful tool because they account for non‐additive effects and allow visualizations of trait combinations.
In landscapes subject to intensive agriculture, both soil fertility and vegetation disturbance are capable of impacting strongly, evenly and simultaneously on the herbaceous plant cover and each tends to impose uniformity on the traits of constituent species. In more natural and ancient grasslands greater spatial and temporal variation in both productivity and disturbance occurs and both factors have been implicated in the maintenance of species‐richness in herbaceous communities. However, empirical data suggest that disturbance is the more potent driver of trait differentiation and species co‐existence at a local scale. This may arise from the great diversity in opportunities for establishment, growth or reproduction that arise when the intensity of competition is reduced by damage to the vegetation.
In contrast to the diversifying effects of local disturbances, productivity‐related plant traits (growth rate, leaf longevity, leaf chemistry, leaf toughness, decomposition rate) appear to be less variable on a local scale. This difference in the effects of the productivity and disturbance filters arises from the relative constancy of productivity within the community and the diversity in agency and in spatial and temporal scales exhibited by disturbance events. Also, evolutionary responses to disturbances involve minor adaptive shifts in phenological and regenerative traits and are more likely to occur as micro‐evolutionary steps than the shifts in linked traits in the core physiology associated with the capacity to exploit productive and unproductive habitats.
During the assembly of a community and over its subsequent lifespan filters with diversifying and convergent effects may operate simultaneously on recruitment from the local species pool and impose contrasted effects on the similarity of the trait values exhibited by co‐existing species. Moreover, as a consequence of the frequent association of productivity with the convergence filter, an additional difference is predicted in terms of the effects of the two filters on ecosystem functioning. Convergence in traits selected by the productivity filter will exert effects on both the plant community and the ecosystem while divergent effects of the disturbance filter will be restricted to the plant community.
The influence of environmental stochasticity and dispersal in producing patterns in population synchrony was examined for 53 woods censused annually from 1990 to 1999 for nine resident bird species (wren Troglodytes troglodytes, dunnock Prunella modularis, robin Erithacus rubecula, blackbird Turdus merula, song thrush Turdus philomelos, long-tailed tit Aegithalos caudatus, blue tit Parus caeruleus, great tit Parus major, and chaffinch Fringilla coelebs) and four migrant bird species (garden warbler Sylvia borin, blackcap Sylvia atricapilla, chiffchaff Phylloscopus collybita and willow warbler Phylloscopus trochilus). Twelve species showed global synchrony of population counts due to regional population trends and widespread annual population fluctuations. There was a clear link between population fluctuations and winter weather for wren, and three other species showed their largest population declines after the coldest winters. Eight species showed a decline in synchrony with distance between woods, and there was evidence for dispersal causing this pattern in three species. Landscape structure affected patterns of synchrony in several species, with lower synchrony in landscapes with less woodland. For three species, this difference in synchrony across a landscape gradient of decreasing woodland cover accounted for the decline in synchrony over distance. Three species showed greater synchrony between woods with similar amounts of hedgerow in the surrounding landscape, suggesting that the surroundings of a wood influence the population dynamics of some species breeding in the wood. Habitat fragmentation can alter the processes contributing to population synchrony. Loss of woodland reduces the relative abundance of woodland bird species. The remaining patches of habitat are smaller, more isolated and are set in a more hostile landscape, all of which may disrupt dispersal between patches and alter the population dynamics within woods.
1. The goal of conservation and restoration activities is to maintain biological diversity and the ecosystem services that this diversity provides. These activities traditionally focus on the measures of species diversity that include only information on the presence and abundance of species. Yet how diversity influences ecosystem function depends on the traits and niches filled by species.
2. Biological diversity can be quantified in ways that account for functional and phenotypic differences. A number of such measures of functional diversity (FD) have been created, quantifying the distribution of traits in a community or the relative magnitude of species similarities and differences. We review FD measures and why they are intuitively useful for understanding ecological patterns and are important for management.
3. In order for FD to be meaningful and worth measuring, it must be correlated with ecosystem function, and it should provide information above and beyond what species richness or diversity can explain. We review these two propositions, examining whether the strength of the correlation between FD and species richness varies across differing environmental gradients and whether FD offers greater explanatory power of ecosystem function than species richness.
4. Previous research shows that the relationship between FD and richness is complex and context dependent. Different functional traits can show individual responses to different gradients, meaning that important changes in diversity can occur with minimal change in richness. Further, FD can explain variation in ecosystem function even when richness does not.
5. Synthesis and applications . FD measures those aspects of diversity that potentially affect community assembly and function. Given this explanatory power, FD should be incorporated into conservation and restoration decision‐making, especially for those efforts attempting to reconstruct or preserve healthy, functioning ecosystems.
1. Environmental policies that use ecological restoration to offset the destruction of natural ecosystems assume that restorations readily compensate for the losses because they progress reliably and predictably over time, following deterministic successional trajectories. However, succession and restoration are spatially and historically contingent processes, often characterized by divergent trajectories that deviate from expectations.
2. We develop a framework for monitoring restorations that integrates two ideas from succession theory: convergence vs. divergence in species composition among successional sites, and progression towards vs. deviation from an expected community state. We apply this framework to short- and long-term monitoring data from 11 restored wetlands in Illinois, USA, by comparing plant species composition among restored wetlands over time and between restored wetlands and two sets of reference wetlands (high integrity, ‘target’ wetlands and low integrity, degraded wetlands).
3. Over the first 4 years, restored wetlands that were initially similar in species composition diverged, progressing towards different high integrity target states. Planting a large number of native species in restorations increased their similarity to reference wetlands.
4. Over longer time scales (5–11 years post-restoration), however, restored wetlands deviated from the ideal trajectory and converged upon the species composition of degraded wetlands, mainly because of non-native species invasion.
5. Synthesis and applications. Framing restoration trajectories in terms of compositional convergence/divergence and progression towards/deviation from an acceptable range of reference sites is useful for monitoring restoration progress, identifying constraints to success and predicting restoration outcomes. Barriers to restoration, including non-native species and a lack of native propagules, can limit long-term progression towards target communities and constrain restoration to undesirable outcomes. Furthermore, convergence of restored wetlands on an undesirable community state limits the effectiveness of wetland mitigation policies.
There is growing interest in assessing relation ships between two or more distance matrices, where distances are based on
genetic, geographical, and/or environmental measures of dissimilarity for all pairwise combinations of n populations. Methods are developed and assessed for estimating confidence limits for the regression relationship between
dependent matrix Y and matrix X and for estimating the value of x given critical y. Methods include a regression mixed model that incorporates an additional population effects variance and a jackknife-by-population
regression method that omits the (n −1) distance observations for each population in turn. The approaches are illustrated using data to quantify rates of gene
flow with distance between wild plant populations of sea beet and are assessed using simulations.
Recent experimental and theoretical studies of community development, as well as more general studies of complex systems, indicate that historical differences and intrinsic indeterminism can lead to long-term community divergence. This observation has sparked a series of new investivations of community structure in systems that developed under controlled or natural conditions.
Recent experimental and theoretical studies of community development , as well as more general studies of complex systems, indicate that historical differences and intrinsic determinism can lead to long-term community divergence. This observation sparked a series of new investigations of community structure in systems that developed under controlled or natural conditions
It is suggested that local animal species diversity is related to the number of predators in the system and their efficiency in preventing single species from monopolizing some important, limiting, requisite. In the marine rocky intertidal this requisite usually is space. Where predators capable of preventing monopolies are missing, or are experimentally removed, the systems become less diverse. On a local scale, no relationship between latitude (10⚬ to 49⚬ N.) and diversity was found. On a geographic scale, an increased stability of annual production may lead to an increased capacity for systems to support higher-level carnivores. Hence tropical, or other, ecosystems are more diverse, and are characterized by disproportionately more carnivores.
Experimental investigations of the relationship between biodiversity and ecosystem functioning (BEF) directly manipulate diversity then monitor ecosystem response to the manipulation. While these studies have generally confirmed the importance of biodiversity to the functioning of ecosystems, their broader significance has been difficult to interpret. The main reasons for this difficulty concern the small scales of the experiment, a bias towards plants and grasslands, and most importantly a general lack of clarity in terms of what attributes of functional diversity (FD) were actually manipulated. We review how functional traits, functional groups, and the relationship between functional and taxonomic
diversity have been used in current BEF research. Several points emerged from our review.
First, it is critical to distinguish between response and effect functional traits when quantifying or manipulating FD. Second, although it is widely done, using trophic position as a functional group designator does not fit the effect-response trait division needed in BEF research. Third, determining a general relationship between taxonomic and FD is neither necessary nor desirable in BEF research. Fourth, fundamental principles in community and biogeographical ecology that have been largely ignored in BEF research could serve to dramatically improve the scope and predictive capabilities of BEF research. We suggest that distinguishing between functional response traits and functional effect traits both in combinatorial manipulations of biodiversity and in descriptive studies of BEF could markedly improve the power of such studies. We construct a possible framework for predictive, broad-scale BEF research that requires integrating functional, community, biogeographical, and ecosystem ecology with taxonomy.
Generalized linear mixed models provide a flexible framework for modeling a range of data, although with non-Gaussian response variables the likelihood cannot be obtained in closed form. Markov chain Monte Carlo methods solve this problem by sampling from a series of simpler conditional distributions that can be evaluated. The R package MCMCglmm implements such an algorithm for a range of model fitting problems. More than one response variable can be analyzed simultaneously, and these variables are allowed to follow Gaussian, Poisson, multi(bi)nominal, exponential, zero-inflated and censored distributions. A range of variance structures are permitted for the random effects, including interactions with categorical or continuous variables (i.e., random regression), and more complicated variance structures that arise through shared ancestry, either through a pedigree or through a phylogeny. Missing values are permitted in the response variable(s) and data can be known up to some level of measurement error as in meta-analysis. All simu- lation is done in C/ C++ using the CSparse library for sparse linear systems.
Ecology Letters (2010) 13: 267–283
Predicting changes in community composition and ecosystem function in a rapidly changing world is a major research challenge in ecology. Traits-based approaches have elicited much recent interest, yet individual studies are not advancing a more general, predictive ecology. Significant progress will be facilitated by adopting a coherent theoretical framework comprised of three elements: an underlying trait distribution, a performance filter defining the fitness of traits in different environments, and a dynamic projection of the performance filter along some environmental gradient. This framework allows changes in the trait distribution and associated modifications to community composition or ecosystem function to be predicted across time or space. The structure and dynamics of the performance filter specify two key criteria by which we judge appropriate quantitative methods for testing traits-based hypotheses. Bayesian multilevel models, dynamical systems models and hybrid approaches meet both these criteria and have the potential to meaningfully advance traits-based ecology.