Convergence of form and function under similar environmental conditions has intrigued biologists for centuries. At the same time, phylogenetic conservatism has emerged as a general principal, suggesting that evolutionary constraints may limit the extent of convergence. These observations lead to the question “How has the balance of evolutionary history and environmental selection shaped species’ niches”? We addressed this question with respect to biotic interactions in alpine plant communities across 5 continents. Specifically, we studied the effects of “cushion” plants – a convergent growth form that has evolved independently many times during angiosperm evolution – on the structure and diversity of the entire community. Cushion plants generally facilitated co-occurring species, however this effect varied substantially across communities and environmental gradients. In an attempt to further explain this variation in community structure, we tested the hypotheses that the outcome of biotic interactions is influenced by (1) phylogenetic conservatism in cushion effects (Effect Conservatism), (2) phylogenetic conservatism in non-cushion species responses (Response Conservatism), and (3) relatedness between the cushion species and the rest of the community (Phylogenetic Repulsion).
There was weak support for effect conservatism of cushion plants, with a phylogenetic signature of cushion effects (i.e. mean relative interaction intensity, or RII, at the community level) less than but not significantly different from a Brownian motion model of trait evolution. Furthermore, RII was not at all conserved across the non-cushion phylogeny, providing no support for the response conservatism hypothesis. However, the phylogenetic repulsion hypothesis was strongly supported by a positive relationship between mean RII and the mean phylogenetic distance from the cushion species to the non-cushion species within a community. In other words, cushions that were closely related to the rest of the community tended to have competitive effects, whereas cushion species that were distantly related to the rest of the community had facilitative effects. Our results suggest that niche overlap between dominant species and the rest of the community strongly influences community structure, and the degree of niche overlap is in turn influenced by the biogeographic and evolutionary history of the regional species pool. We further explore the biogeographic patterns of cushion plant evolution and the role of adaptive radiations in determining community structure in alpine plant communities.