Severe limitations of the FEve metric of functional evenness and some alternative metrics

Abstract

The metric of functional evenness FEve is an example of how approaches to conceptualizing and measuring functional variability may go astray. This index has several critical conceptual and practical drawbacks: Different values of the FEve index for the same community can be obtained if the species have unequal species abundances; this result is highly likely if most of the traits are categorical.Very minor differences in even one pairwise distance can result in very different values of FEve.FEve uses only a fraction of the information contained in the matrix of species distances. Counterintuitively, this can cause very similar FEve scores for communities with substantially different patterns of species dispersal in trait space.FEve is a valid metric only if all species have exactly the same abundances. However, the meaning of FEve in such an instance is unclear as the purpose of the metric is to measure the variability of abundances in trait space. We recommend not using the FEve metric in studies of functional variability. Given the wide usage of FEve index over the last decade, the validity of the conclusions based on those estimates is in question. Instead, we suggest three alternative metrics that combine variability in species distances in trait space with abundance in various ways. More broadly, we recommend that researchers think about which community properties (e.g., trait distances of a focus species to the nearest neighbor or all other species, variability of pairwise interactions between species) they want to measure and pick from among the appropriate metrics.

Keywords: biodiversity; functional diversity; functional evenness; nearest‐neighbor distance.

Figure 1

A community network in which the distances between all three species are identical, which results in three possible minimum spanning trees (MSTs) and multiple FEve estimates for the same community

Figure 2

In these communities, two of the three species are equally distant in both communities (d ₁₂ = d ₂₃ = d) with a distance that is smaller than the third distance (d ₁₃). If the abundances of the three species are $w_1=1$, $w_2=2,$ and $w_3=3$, then FEve = 0.75, even though community B seems much more functionally irregular than community A

Figure 3

This community consists of three species in which d ₂₃ is larger than d ₁₃ and d ₁₂. The abundances (w) and distances result in values of ${\mathit{EW}}_{12}={\mathit{EW}}_{13}=1/6$, ${\mathit{PEW}}_{12}={\mathit{PEW}}_{13}=0.5$, and FEve = 1

Figure 4

Variability of FEve estimates for the actual abundances of eleven virulence phenotypes of Puccinia graminis, and the Y‐modification and Z‐modification of abundances (see Table 3 for details)