Population dynamics and the ecological stability of obligate pollination mutualisms

Mutualistic interactions almost always produce both costs and benefits for each of the interacting species. It is the difference between gross benefit s and costs that determines the net benefit and the per-capita effect on ea ch of the interacting populations. For example, the net benefit of obligate pollinators. such as yucca and senita moths, to plants is determined by th e difference between the number of ovules fertilized from moth pollination and the number of ovules eaten by the pollinator's larvae. It is clear that if pollinator populations are large, then, because many eggs are laid, cos ts to plants are large, whereas, if pollinator populations are small, gross benefits are low due to lack of pollination. Even though the size and dyna mics of the pollinator population are likely to be crucial, their importanc e has been neglected in the investigation of mechanisms, such as selective fruit abortion, that can limit costs and increase net benefits. Here, we su ggest that both the population size and dynamics of pollinators are importa nt in determining the net benefits to plants, and that fruit abortion can s ignificantly affect these. We develop a model of mutualism between populati ons of plants and their pollinating seed-predators to explore the ecologica l consequences of fruit abortion on pollinator population dynamics and the net effect on plants. We demonstrate that the benefit to a plant population is unimodal as a function of pollinator abundance, relative to the abundan ce of flowers. Both selective abortion of fruit with eggs and random aborti on of fruit, without reference to whether they have eggs or not, can limit pollinator population size. This can increase the net benefits to the plant population by limiting the number of eggs laid, if the pollination rate re mains high. However, fruit abortion can possibly destabilize the pollinator population, with negative consequences for the plant population.