Three-way coexistence in obligate mutualist-exploiter interactions: the potential role of competition

Many mutualisms host "exploiter" species that consume the benefits provided by one or both mutualists without reciprocating. Exploiters have been widely assumed to destabilize mutualisms, yet they are common. We develop models to explore conditions for local coexistence of obligate plant/pollinating seed parasite mutualisms and nonpollinating exploiters. As the larvae of both pollinators and (at a later time) exploiters consume seeds, we examine the importance of intraspecific and (asymmetric) interspecific competition among and between pollinators and exploiters for achieving three-way coexistence. With weak intra- and interspecific competition, exploiters can invade the stable mutualism and coexist with the mutualists (either stably or with oscillations), provided the exploiters' intrinsic birthrate (b(E)) slightly exceeds that of the pollinators. At higher b(E), all three species go locally extinct. When facing strong interspecific competition, exploiters cannot invade and coexist with the mutualists if intraspecific competition in pollinators and exploiters is weak. However, strong intraspecific competition in pollinators and exploiters facilitates exploiter invasion and coexistence and greatly expands the range of b(E) over which stable coexistence occurs. Our results suggest that mutualist/exploiter coexistence may be more easily achieved than previously thought, thus highlighting the need for a better understanding of competition among and between mutualists and exploiters.