Functional diversity responses to changing species richness in reef fish communities

Functional diversity in biological communities is critical for providing the full suite of ecosystem functions and services, yet the relationship between functional diversity and species richness - how diversity is usually measured - remains largely unknown. Here we evaluate empirically how different measures of functional diversity change with increasing species richness across 36 Atlantic reef fish communities that vary greatly in species richness. We show that (1) the definition used for functional groups strongly influences the perceived relationship; (2) when using more comprehensive definitions of functional groups the relationship is asymptotic and highly unsaturated; and (3) increasing species richness primarily leads to the addition of species to a few key functional groups rather than the addition of novel functional groups. Consequently, non-random assembly of fish assemblages leaves many possible functional groups empty, and high species redundancy within functional groups is limited to relatively few groups. Consequences of future extirpation or extinction of species for the loss of ecosystem functions will be variable, in part depending on the proportion of species lost within a functional group, but potentially profound due to the large number of functional groups represented by only a single species and the targeted nature of most species extraction and extirpations.