The evolution of trait variance creates a tension between species diversity and functional diversity

It seems intuitive that species diversity promotes functional diversity. For example, more plant species imply more diverse leaf chemistry and thus more kinds of food for herbivores. Here we argue that the evolution of functional trait variance challenges this view. We show that trait-based eco-evolutionary processes force species to evolve narrower trait breadths in tightly packed communities, in their effort to avoid competition with neighboring species. This effect is so strong as to reduce overall trait space coverage, overhauling the expected positive relationship between species- and functional diversity. Empirical data from Galapagos land snail communities proved consistent with this claim. As a consequence, trait data from species-poor communities may misjudge functional diversity in species-rich ones, and vice versa.