COS 12-4
The role of competition for benefits in mutualist coexistence: An ‘M*’ theory of competitive exclusion in mutualism?
Mutualism involves reciprocal exploitation in which the benefits species obtain from partners outweigh the costs they incur by engaging in the mutualism. Benefits can also represent limiting resources for which mutualists compete. Competition for the benefits provided by partners likely plays a fundamental role in mutualist coexistence, and therefore, the evolution of mutualism. Here, we develop a consumer-resource framework for incorporating competition for benefits within mutualism. The novel feature of our model that distinguishes it from previous theory is that benefits are explicitly modeled, and thus, may be depleted or even exhausted by mutualists. We investigate two key issues: (i) how competition for benefits affects mutualist coexistence and (ii) the conditions underlying coexistence versus competitive exclusion.
Results/Conclusions
Most existing models fail to capture the effects of competition for benefits because mutualists effectively produce inexhaustible benefits; hence, competition is impossible. Species only go extinct in the trivial case when they acquire benefits at an insufficient rate to overcome the inherent deficit between births and deaths (in obligate mutualisms); otherwise, species always coexist. Our model predicts that if mutualists compete exclusively for the same partner, species that furthest deplete shared benefits will competitively displace other mutualists. We derive an ‘M*’ rule for competitive exclusion, thereby extending R* theory to mutualism. Finally, we collapse our model by incorporating competition for benefits within species’ functional responses to capture the depletion of shared benefits. We find that trade-offs between foraging efficiency (finding partners) and benefit acquisition (gaining benefits once an interaction ensues) promote the coexistence of mutualists sharing common partners (analogous to the competition-colonization trade-off). Our findings illuminate the important role of competition for benefits on mutualist coexistence.