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Notes:
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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. TRait OVerlAp) which performs computations to estimate species trait
dissimilaritywith different types of data.
Results: The trait overlap approach generally produces a less context-dependentmeasure
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 bemore 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,
especiallywhen 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
approachesmay 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.
This research was supported by the Grant Agency of the Czech Republic (GACR P505/12/1296 and P505/12/1390). CPC was supported by a FPI scholarship (BES-2008- 009821). |
