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

It seems intuitively obvious that species diversity promotes functional diversity: communities with more plant species imply more varied plant leaf chemistry, more species of crops provide more kinds of food, etc. Recent literature has nuanced this view, showing how the relationship between the two can be modulated along latitudinal or environmental gradients. Here we show that even without such effects, the evolution of functional trait variance can erase or even reverse the expected positive relationship between species- and functional diversity. We present theory showing that trait-based eco-evolutionary processes force species to evolve narrower trait breadths in more tightly packed, species-rich communities, in their effort to avoid competition with neighboring species. This effect is so strong that it leads to an overall reduction in trait space coverage whenever a new species establishes. Empirical data from land snail communities on the Galapagos Islands are consistent with this claim. The finding that the relationship between species- and functional diversity can be negative implies that trait data from species-poor communities may misjudge functional diversity in species-rich ones, and vice versa. The positive relationship between species diversity and functional diversity has been shown to vary. Here, the authors use theoretical models and data from Galapagos land snail communities to show how eco-evolutionary processes can force species to evolve narrower trait breadths in more species-rich communities to avoid competition, creating a negative relationship.Funding Agencies|Charles Darwin Foundation; Galapagos National Park Directorate [044-06, PC-45-14]; Swedish Research Council [VR 2017-05245]; National Institute of General Medical Sciences of the National Institutes of Health (IDeA) [P30 GM103324]; National Science Foundation [1523540, 1751157]; National Geographic Society; American Malacological Society; Western Society of Malacology; Conchologists of America; Systematics Research Fund; Research Foundation Flanders; University of Namur [FSR Impulsionnel 48454E1]; Fund for Scientific Research, FNRS [PDR T.0048.16]; ARC grant DIVERCE, a concerted research action from the special research fund [18/23-095]