Review
Traits Without Borders: Integrating Functional Diversity Across Scales

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Trends

Functional trait diversity, in other words the variation of traits between organisms, can be used to address a great number of pressing ecological questions. Consequently, trait-based approaches are increasingly being used by ecologists.

However, functional diversity comprises several components that can be evaluated at different spatial scales. Because of this conceptual complexity, there is an overabundance of disparate approaches for estimating it, which leads to confusion among users and hampers the comparability of different studies.

A single mathematical framework encompassing different approaches while providing a seamless continuity between spatial scales is needed.

Reconciling the approaches based on the concept of the niche as a hypervolume and those that consider traits in probabilistic terms is the first step towards the foundation of a unified framework.

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.

Section snippets

A Multi-Faceted FD

The responses of species to environmental conditions, disturbance, and biotic interactions, as well as their effects on ecosystem processes, are determined by their functional traits (see Glossary) 1, 2, 3, 4, 5. Consequently, functional trait-based approaches have great potential to address a variety of ecological questions [6], including the impact of global change on biodiversity and ecosystem service delivery 3, 7, 8, 9, ecological restoration [10], or the assembly of biological communities

TPD: Towards a Scale-Independent FD

If ecologists want to ultimately simplify the labyrinth of methods for quantifying FD, and make results of different studies more readily comparable, they will need to define a single framework that encompasses different approaches while providing the flexibility to move between multiple scales. Below we show that the foundations of such a framework can be found in studies using the concept of the niche as a hypervolume 47, 48 and those considering the probabilistic nature of functional traits

Incorporating Existing Methods into the TPD Framework

The TPD framework has the potential to unify existing FD approaches into a single and consistent structure, effectively incorporating ITV and the multidimensional nature of functional trait space across scales. In this section we first present corresponding adaptations of weighted mean trait values (TWM) [3], FD components [13], and of any method based on the trait dissimilarity between ecological units 61, 62, 63, 64, 65, highlighting when and how these adaptations solve several problems

Diversity Between and Across Ecological Units

As anticipated above, the promise of the TPD framework goes well beyond calculations within units. TPD functions can be used to evaluate mechanisms driving the spatial or temporal differences in the functional structure between populations, communities, or regions, that is, β-FD (see 76, 77 for reviews on the questions that can be tackled by studying β-diversity). We envisage that the TPD framework will lead to new techniques for estimating diversity across scales, for example quantifying the

Concluding Remarks

Because FD encompasses a variety of concepts and components, a combination of conceptual and mathematical approaches has, to date, been necessary to quantify it comprehensively 1, 19, 72. In this paper we have shown how the TPD concept can be applied to individuals, populations, communities, and regions, and we have provided an assortment of methods to estimate several aspects of FD using one or multiple traits within a single framework. The TPD framework implements, for the first time, the

Acknowledgments

We thank P. Craze and two anonymous reviewers for their helpful comments that improved this review. We also thank Marc Cadotte, Cristina Rota, Javier Seoane, and Carol Pedrero for their suggestions and feedback on these ideas. C.P.C. was supported by a Marie Curie Intra-European Fellowship within the European Commission 7th Framework Programme (TANDEM; project 626392) and by the Spanish MINECO (Project CGL2014-53789-R). F.d.B. and J.L. were supported by Czech Science Foundation grants GACR

Glossary

Convex hull
smallest convex volume that contains a set of points. In trait-based ecology, it is used to quantify the functional volume occupied by a species or community, as well as β-FD and its decomposition into nestedness and turnover components. Convex hulls are sensitive to outliers and do not detect gaps in the occupation of functional space.
Ecological unit
any scale at which it is meaningful to estimate FD. Examples are individual organisms, populations, species, communities,

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