Indirect interactions in a system involving mutualism and antagonism: A model of pollinator-disperser antagonism

Background/Question/Methods

Indirect interactions, which occur when one species impacts another through its effect on a third species, play a critical role in the structuring of ecological communities and can overwhelm the impact of direct interactions in some instances.  Despite long-standing interest in the field of evolutionary ecology in the stability of mutualistic interactions, the effect of indirect interactions on the stability and dynamics of systems involving mutualisms remains largely unexplored. Many species simultaneously participate in multiple mutualisms with different partners. Although likely to be common in nature, antagonism between two species that share a mutualistic partner might be expected to destabilize the interaction network.  Here, we present a population dynamical model of such a situation in which seed dispersers interfere with pollinators of a shared plant species.  We use this model to understand how the dynamics and persistence of the mutualistic interactions are affected by a consumptive, i.e. predatory, and non-consumptive antagonism by seed dispersers on pollinators. Although such situations have received little attention, they are likely to be important for the ecological and evolutionary dynamics of many plant species.

Results/Conclusions

Our model predicts that pollinator-disperser antagonism can introduce oscillatory dynamics into an otherwise stable system, when indirect and net species interactions are of moderate strength. With increased levels of antagonism, net interactions are driven primarily by large indirect effects that can lead to the extinction of the plant, and, thus, the mutualistic interactions. Additionally, by boosting the population of the disperser, plants have an increasingly negative effect on the density of pollinators as the strength of the antagonism increases. Non-consumptive effects of dispersers on pollinators may increase the instability of the system by inducing more oscillations in pollinator density. However, the strength of the mutualisms interacts strongly with the effect of the antagonism on the system, indicating that plants may be able to mitigate these negative indirect effects through their investment of resources into benefits for mutualistic partners. We discuss the implications of these types of interactions for the evolution of specialized dispersal systems and plant reproductive phenology.