PMID: 12858272;Abstract:
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 (bE) slightly exceeds that of the pollinators. At higher bE, 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 bE 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.
Abstract:
Mutualisms (interspecific cooperative interactions) are ubiquitously exploited by organisms that obtain the benefits mutualists offer, while delivering no benefits in return. The natural history of these exploiters is well-described, but relatively little effort has yet been devoted to analysing their ecological or evolutionary significance for mutualism. Exploitation is not a unitary phenomenon, but a set of loosely related phenomena: exploiters may follow mixed strategies or pure strategies at either the species or individual level, may or may not be derived from mutualists, and may or may not inflict significant costs on mutualisms. The evolutionary implications of these different forms of exploitation, especially the threats they pose to the stability of mutualism, have as yet been minimally explored. Studies of this issue are usually framed in terms of a "temptation to defect" that generates a destabilizing conflict of interest between partners. I argue that this idea is in fact rather inappropriate for interpreting most observed forms of exploitation in mutualisms. I suggest several alternative and testable ideas for how mutualism can persist in the face of exploitation.
Abstract:
1. While the relationship between facilitation and competition has been explored extensively in recent years, there is also a natural link between facilitation and mutualism, as both are interspecific interactions that confer benefits. Yet, the relationship between these two interactions has been minimally explored. 2. Here, I explore parallels and differences between mutualism and facilitation. Five focal areas organize current research on mutualism evolution: trait evolution; the continuum from specialization to generalization; the evolutionary origins and maintenance of the interaction; co-evolution of partners; and the prevalence and implications of cheating. These foci are also helpful for investigating how facilitation evolves, a much less explored issue. 3. Testable hypotheses regarding the evolution of facilitation include the following: selection should be stronger on traits of facilitated species than on traits of facilitators; facilitative interactions with mutualistic (++) and commensal (+0) outcomes should exhibit greater evolutionary stability than those with antagonistic (+-) outcomes; co-evolution should be possible in mutualistic and antagonistic facilitation only; when co-evolution occurs, it should produce a geographic mosaic of interaction outcomes; and antagonistic facilitation could lead to selection on facilitators to either escape or to tolerate the neighbours that benefit from them. 4.Synthesis. Three gaps in our knowledge currently impede progress on evolutionary questions surrounding facilitation. First, reciprocal effects are rarely investigated; facilitation might evolve like mutualism, commensalism or antagonism, depending on effects on the facilitator species. Secondly, the genetics of relevant traits are not yet well explored; the traits themselves are better known for facilitator species than for the facilitated, which are more likely to evolve in the context of the interaction. Finally, the fitness costs and benefits associated with facilitation have rarely been measured. Filling these gaps should permit rapid progress in understanding how facilitation arises, persists and evolves. © 2009 British Ecological Society.
