Including intraspecific trait variability to avoid distortion of functional diversity and ecological inference: lessons from natural assemblages

1. Functional diversity assessments are crucial and increasingly used for
understanding ecological processes and managing ecosystems. The functional
diversity of a community is assessed by sampling traits at one or more
scales (individuals, populations, species) and calculating a summary index
of the variation in trait values. However, it remains unclear how the
scale at which traits are sampled and the indices used to estimate
functional diversity may alter the patterns observed and inferences about
ecological processes. 2. For 40 plant and 61 ant communities, we assess
functional diversity using six methods – encompassing various mean-based
and probabilistic methods – chosen to reflect common scenarios where
different levels of detail are available in trait data. We test whether
including trait variability at different scales (from individuals to
species) alter functional diversity values calculated using volume-based
and dissimilarity-based indices, Functional Richness (FRic) and Rao,
respectively. We further test whether such effects alter the functional
diversity patterns observed across communities and their relationships
with environmental drivers such as abiotic gradients and occurrences of
invasive species. 3. Intraspecific trait variability strongly determined
FRic and Rao. Methods using only species’ mean trait values to calculate
FRic (convex hulls) and Rao (Gower-based dissimilarity) distorted the
patterns observed when intraspecific trait variability was considered.
These distortions generated Type I and Type II errors for the effects of
environmental factors structuring the plant and ant communities. 4. The
high sensitivity of FRic to individuals with extreme trait values was
revealed in comparisons of different probabilistic methods including
among-individual and among-population trait variability in functional
diversity. By contrast, values and ecological patterns in Rao were
consistent among methods including different scales of intraspecific trait
variability. 5. Decisions about where traits are sampled and how trait
variability is included in functional diversity can drastically change the
patterns observed and conclusions about ecological processes. We recommend
sampling the traits of multiple individuals per species and capturing
their intraspecific trait variability using probabilistic methods. We
discuss how intraspecific trait variability can be reasonably estimated
and included in functional diversity in the common circumstance where only
limited trait data are available.