Dispersal disruption alters plant spatial patterns and decreases plant survivorship: Analytical approximations to individual-based models

Background/Question/Methods

Extirpation of vertebrates by human activity results in "empty forests", with disrupted ecological processes, including seed dispersal of plants. Although seed dispersal is typically modeled as monotonically decreasing with distance from the tree, vertebrates disperse seeds in clumps to preferred areas. These seeds must survive the attack of insect seed predators in order to germinate, and clumped seed deposition can greatly alter the number and spatial distribution of germinating plants. We developed analytical approximations to examine how the interaction between seed dispersal by vertebrates and patterns of plant mortality due to insect seed predators shape the spatial pattern of seed survivorship. As these approximations do not capture the spatial structure created by frugivores, we used individual-based models to examine how clumping of seeds by frugivores affect seed survival. We compared the realizations from individual-based models to analytical approximations to examine the effect of clumping by frugivores on the spatial patterns of seed survivorship.

Results/Conclusions

Seedling recruitment patterns observed in the model reproduce the range of patterns observed empirically. Analytical solutions approximate seed dispersal by frugivores, but clumped seed deposition increased the variation in seed densities, resulting in higher survivorship of seeds. Survivorship of seeds decreased with tree abundance, the number of insect seed predators, and the detection area of insects. Survivorship of seeds increased when seeds escaped predation by dispersing further than their predators. Reductions in the number of seed dispersal events decreased survivorship of seeds. A basic understanding of how frugivores and seed predators affect plant survivorship will help us predict the future of plant communities faced by anthropogenic pressures that include the hunting of seed dispersers.