Evolutionary ecology of seed dispersal by frugivores: From exploitation to mutualism

Background/Question/Methods

Many plants exploit animal movement in order to disperse their seeds, thus reducing seed predation, parent-offspring competition and sibling competition, and promoting the colonization of favorable sites for germination and growth. Although seed dispersal has been extensively studied and quantified, its integration into the wider context of plant life-cycles has been rather incomplete. The present work aims to develop a better understanding of the evolutionary ecology of seed dispersal by means of frugivores, i.e. endozoochory. We developed a simple spatially implicit model that considers the positive as well as the negative effects of frugivory on plant recruitment. By taking into consideration that the adaptations for endozoochory (e.g. bigger fruits) involve costs for the plant, as well as the response of frugivores towards such adaptations, we studied the evolution of plant traits meant to induce animals to disperse seeds. Our modelling approach can be easily adjusted to consider other kinds of dispersers apart from frugivores (e.g. ants).

Results/Conclusions

Our principal finding is that natural selection can turn the plant-animal relationship from a purely exploitative one into a mutualistic one under a broader set of parameter conditions and without the explict consideration of animal population dynamics in time and space. Under density-independent selection settings our model predicts that endozoochory can be unfavorable when fruit traits (e.g. size, pigmentation) are costly to produce. Under density-dependent selection settings however, endozoochory tends to be favorable even if plant traits are costly, thus promoting the evolution of mutualism. Finally, under special scenarios of density-dependent and density-independent mortality factors acting on seeds and seedlings, and of sensory physiology of the frugivores, our model and individual based simulations predict the diversification of plant dispersal strategies by evolutionary branching. Future work must address how do animal population dynamics and coevolution will modify these predictions.