The gift that keeps on giving: Why does biological diversity accumulate around mutualisms?

It is increasingly apparent that mutualisms are central components of ecological communities, generating linkages among species in ways that lead to rich and persistent assemblages. I propose here an underlying rationale for why biological diversity tends to accumulate around mutualisms. I first highlight four key features of mutualism: most involve one-way or two-way exchange of resources; these resources are commonly used by many different mutualists; as they are difficult or impossible to modulate to mutualists' needs, there would appear to be an advantage to overproduction; and many species other than mutualists subsist on these resources. As a consequence, I argue, (a) interaction networks form around mutualisms that extend well beyond the mutualists themselves; (b) non-mutualists taking advantage of one species' resources may often be mutualists of other species in the community; and (c) different mutualistic networks are also linked, because successful mutualisms often generate resources that form the base of yet other mutualisms. I present three examples of plant/pollinator interactions that illustrate how mutualisms contribute to the accumulation of biological diversity in their local habitats. I conclude by offering a set of predictions to move these ideas further.