Severe limitations of the FEve metric of functional evenness and some
alternative metrics
Abstract
1. The metric of functional evenness FEve is an example of how
approaches to conceptualizing and measuring functional variability may
go astray. 2. The index of functional evenness FEve has critical
conceptual and practical drawbacks: a) 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. b) Very minor differences in even one pairwise distance
can result in very different values of FEve. c) 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. d) 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. 3. We recommend not using 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 are in question. 4. Instead, we suggest three
alternative metrics that combines variability in species distances in
trait space with abundance in various ways, and more broadly 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.