Assessing functional evenness with the FEve index: A word of warning

Functional diversity has recently become a key challenge in community ecology due to its major importance for the understanding of ecosystem processes such as ecosystem productivity, the resilience of ecosystems to disturbances or invasions and regulation of the flux of matter. Some authors have argued that functional diversity can be divided into three independent components: functional richness, functional evenness and functional divergence. Currently, the evenness component of functional diversity is mainly assessed on the basis of the FEve index of Villéger et al. (2008) that quantifies the regularity of interspecies distances and the homogeneity of species abundances. Here, we demonstrate that the FEve index could give values that are not consistent with its claimed properties, and we illustrate this inconsistency with various specific examples. We highlight the fact that the FEve index can give values close to 1, although all species are not regularly spaced and have not the same abundance. We also demonstrate that two communities could have the same FEve index value although they differ in terms of abundance distribution. We assume that these characteristics are due to the fact that FEve index combines in a single value two different components: functional distance and distributions of species abundances. Finally, we propose an alternative solution to overcome the problem of these drawbacks, and thus achieve a more complete and accurate view of the functional evenness of communities.