The Effects of Above- and Belowground Mutualisms on Orchid Speciation and Coexistence

Both pollination by animals and mycorrhizal symbioses with fungi are believed to have been important for the diversification of flowering plants. However, the mechanisms by which these above- and belowground mutualisms affect plant speciation and coexistence remain obscure. We provide evidence that shifts in pollination traits are important for both speciation and coexistence in a diverse group of orchids, whereas shifts in fungal partner are important for coexistence but not for speciation. Phylogenetic analyses show that recently diverged orchid species tend either to use different pollinator species or to place pollen on different body parts of the same species, consistent with the role of pollination-mode shifts in speciation. Field experiments provide support for the hypothesis that colonization of new geographical areas requires adaptation to new pollinator species, whereas co-occurring orchid species share pollinator species by placing pollen on different body parts. In contrast to pollinators, fungal partners are conserved between closely related orchid species, and orchids recruit the same fungal species even when transplanted to different areas. However, co-occurring orchid species tend to use different fungal partners, consistent with their expected role in reducing competition for nutrients. Our results demonstrate that the two dominant mutualisms in terrestrial ecosystems can play major but contrasting roles in plant community assembly and speciation.