It seems intuitively obvious that species diversity promotes functional (trait) diversity. For example, the more plant species there are, the more varied their leaf chemistry will be; more species of crops provide more kinds of food; etc. We undertake a rigorous investigation of this intuition using a trait-based eco-evolutionary model. Built on quantitative genetics, the model keeps track of species' population densities, trait means, and trait variances (or covariance matrices, in multidimensional trait spaces). We ask whether model outcomes with higher species diversity also end up with higher functional diversity. We then look at two independent sources of empirical data, provided by natural evolution experiments whereby very similar communities with varying species richness have evolved independently for a long time. The first dataset is on soricid (shrew) communities in the Congo basin lowland rainforest; the other is on land snails across the Galapagos Islands. As in the model, we ask whether and how species- and functional diversity covary between the subcommunities of these two datasets.
Results/Conclusions
Model outcomes and empirical data both challenge the view that species diversity promotes functional diversity. In the model, when species richness is low, individual species evolve large trait variation, while in species-rich communities species evolve narrow trait breadths, in their effort to avoid competition with neighbouring species. This effect is so strong as to reduce overall trait space coverage, overhauling the expected positive relationship between species- and functional diversity. This outcome proved highly robust to changes in model setup and parameterization. Our finding was supported by both data sources: for both soricid and land snail communities, greater species diversity lead to diminished functional diversity. We conclude that functional diversity is not simply a consequence of species diversity; thus, research programs relying on species' trait means may overestimate ecosystem performance.