Mutualism – when and how important is it – a peek through a metacommunity lens

Background/Question/Methods

: Ecologists view negative density-dependent interactions among species as stabilizing their populations and coexistence. Competition and predation stand out here. We know that mutualism is also substantial, but we suggest its contribution is less clear because it involves destabilizing positive feedback. We also know that habitat heterogeneity, or patchiness, is a significant modifier of the interactions, and patchiness may have contrasting effects on negative vs positive interactions. Specifically, patchiness promotes coexistence by reducing predation and competition but may reduce species richness by disrupting mutualistic interactions. A question emerges: will mutualists be less successful in patchy than other species? If yes, then we will hypothesize that performance metrics will decline for mutualists. We use an agent-based, spatially explicit metacommunity model where species gain energy on suitable patches, with dispersal and reproduction incurring costs, and species interactions impart costs and rewards on the gradient from negative to positive (mutualistic) direct interactions. Ten species (generalists, specialists, mutualists) with random habitat specialization ranges dispersed, reproduced, and interacted over 250 steps under various levels of mutualism. We gauged the benefits of mutualism by several metrics. For example, how many habitat types mutualists use compared to non-mutualistic specialists indicates their ability to expand into otherwise unsuitable environments.

Results/Conclusions

: Surprisingly, mutualists performed much better than expected in the metacommunity occupying different suitability. Specifically, obligatory mutualists managed to occupy almost twice the habitat range as species designated as highly specialized controls (no benefits due to mutualism). While generalists performed best on the habitat matrix offered by the simulation, this was not surprising because generalists are, by definition, capable of using the broadest range of patch types. Facultative mutualism did not improve on the corresponding non-mutualistic specialists, underscoring that stronger mutualism improves species performance the most. Also, combined obligatory and facultative mutualists performed by about 33% better than combined specialist species (two combined controls). These results contradict our expectations that the difficulty of finding both a suitable habitat and a suitable partner should tax mutualist success. Instead, we see mutualism as a successful strategy to offset the costs of dispersal, survival, competition for resources, and other negative interactions that all species experienced in the model. Partial or facultative mutualists did not outperform their non-mutualist counterparts, possibly because a patchy environment requires robust benefits of obligatory mutualism to offset its costs. In conclusion, mutualistic interactions may be a prerequisite for the survival of specialists, the most significant component of biodiversity.