Functional‐diversity indices can be driven by methodological choices and species richness

Abstract

Abstract. Functional diversity is an important concept in community ecology because it captures information on functional traits absent in measures of species diversity. One popular method of measuring functional diversity is the dendrogram-based method, FD. To calculate FD, a variety of methodological choices are required, and it has been debated about whether biological conclusions are sensitive to such choices. We studied the probability that conclusions regarding FD were sensitive, and that patterns in sensitivity were related to alpha and beta components of species richness. We developed a randomization procedure that iteratively calculated FD by assigning species into two assemblages and calculating the probability that the community with higher FD varied across methods. We found evidence of sensitivity in all five communities we examined, ranging from a probability of sensitivity of 0 (no sensitivity) to 0.976 (almost completely sensitive). Variations in these probabilities were driven by differences in alpha diversity between assemblages and not by beta diversity. Importantly, FD was most sensitive when it was most useful (i.e., when differences in alpha diversity were low). We demonstrate that trends in functional-diversity analyses can be largely driven by methodological choices or species richness, rather than functional trait information alone.

What North America's skeleton crew of megafauna tells us about community disassembly

“…This non-random trait loss better explains the changes in ecosystem function seen after the Pleistocene extinction [22] than gross functional richness and contradicts suggestions that long-term disassembly may produce smaller communities with relatively similar functional group ratios to the initial pre-extinction state [16]. Functional richness, no matter how it is measured, is an abstract value that is heavily dependent on methodological choices and difficult to compare between studies [31,48].…”

Understanding global patterns of mammalian functional and phylogenetic diversity

“…We also used a partial regression approach to partition the variance among the 'pure' environmental components, the 'pure' geographical patterns and their overlap, for each diversity metric. Finally, PD and FD are known to show a monotonic relationship with species richness [13,55] since the addition of each species will invariably lead to an increase in PD/FD by adding branches to the minimum spanning tree/dendrogram. Therefore, to evaluate the environmental determinants of PD and FD independent of the inherent relationship with species richness we ran the same analysis using a three-way partial regression [51] with squared richness as a third set, and reported the partial R 2 of SEVM and environmental variables independent of the effect of richness.…”

Response of herbaceous vegetation functional diversity to land use change across five sites in Europe and Israel

“…the convergence to Simpson diversity of FDvg calculated for increasing numbers of traits may reflect the fact that ultimately individual species present unique trait combinations. De Bello et al (2006) did not find robust proofs of this effect, even with seven traits, but this could be due to their using species frequencies rather than biomass as species abundance weights, and thereby overrepresenting the effects of less abundant species (see Lavorel et al, 2008, andPoos et al, 2009, for discussion). nevertheless, our results suggest that, consistent with the Leaf-Height-Seed scheme (Westoby, 1998), species tend to represent unique combinations of SLA, height, and seed weight.…”