LETTER Mutualism can mediate competition and promote coexistence Russell J. Schmitt* and Sally J. Holbrook Coastal Research Center, Marine Science Institute and Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA 93101, USA *Correspondence: E-mail: schmitt@lifesci.ucsb.edu Abstract Mutualistic interactions are not believed to promote coexistence of competitors because mutualisms produce positive feedbacks on abundances whereas coexistence requires negative feedbacks. Here we show that a mutualism between an anemonefish (Amphiprion) and its sea anemone host mediates the effect of asymmetrical competition for space between the anemonefish and another damselfish (Dascyllus) in a manner that fosters their coexistence. Amphiprion stimulates increases in host area, the shared resource, but social interactions cap the number of anemonefish to two adults per host. Space generated by the mutualism becomes differentially available to Dascyllus because the effectiveness of an anemonefish in excluding its competitor declines with increases in the area it defends. This alters AmphiprionÕs ratio of per capita intra- to interspecific effects and thus facilitates coexistence of the fishes. This mechanism may be prevalent in nature, adding another major pathway by which mutualism can enhance diversity. Keywords Mutualism, competition, coexistence, diversity, indirect effects, damselfish, anemonefish, sea anemones. Ecology Letters (2003) 6: 898–902 INTRODUCTION A cardinal focus in ecology centres on processes that shape patterns of biodiversity. Because interspecific competition has the inherent potential to reduce diversity via exclusion of inferior competitors, attention has focused on the processes and mechanisms that permit coexistence of competitors (Paine 1966; Tilman 1994; Chesson 2000). A central tenet of Lokta–Volterra competition theory is that coexistence is possible when the per capita effects of intraspecific competition on per capita rates of population growth are greater than those of interspecific competition (Chesson 2000). It has long been known that, under certain circumstances, predation can mediate competition between prey species to reduce the probability of competitive exclusion (Paine 1966; Sih et al. 1985; Chase et al. 2002), particularly when predators reduce the interspecific effects of competition relative to intraspecific effects (Chesson 2000; Chase et al. 2002). We present evidence here that mutualism also can mediate effects of interspecific competition, thereby increasing the probability that competing species will coexist. Mutualisms are pervasive in nature and their ecological importance is well acknowledged, despite the fact that dynamical aspects of the interaction have been studied far less commonly than for competition and predation (Boucher et al. 1982; Klausmeier 2001; Bruno et al. 2003). Most explorations of mutualism have focused on features of the interaction itself, such as the nature of the benefits gained by each participant and the conditions that give rise to positive effects (Boucher et al. 1982; Cushman & Beattie 1991; Bronstein 1994; Connor 1995). Mutualisms can, of course, enhance diversity because such mutualistic species as reef forming corals can function as Ôecosystem engineersÕ, providing habitat for species that otherwise would not be present (Stachowicz & Hay 1999; Bruno et al. 2003). Indirect or apparent mutualism, as well as facilitation, can arise in food webs and have positive influences on diversity (Vandermeer 1980; Hacker & Gaines 1997; Callaway & Pennings 2000; Bruno et al. 2003). However, because mutualisms typically create positive feedbacks on abundances and coexistence of competitors rests on negative feedbacks, their role in enhancing diversity by promoting coexistence of competitors has not been explored extensively (May 1982; Bever 1999, 2002). It is known that mutualistic arbuscular mycorrhizal fungi can increase the competitive ability of plants that host them (see Urcelay & Diaz 2003). Bever (1999, 2002) showed that among species of plants with mycorrhizal fungi, changes in the fungal Ecology Letters, (2003) 6: 898–902 doi: 10.1046/j.1461-0248.2003.00514.x Ó2003 Blackwell Publishing Ltd/CNRS