Post-Dispersal Seed Predation, Herbivory, and Trampling

Question:
Do mammals significantly affect the density and diversity of seedlings?

Why do we care?
Mammals may play a major role in limiting the recruitment of subsets of the woody plant community. If their effects are negatively density-dependent, falling disproportionately upon common species, they will positively affect seedling species diversity. If their effects fall upon other subsets, for example, large-seeded species, they may limit the abundance of those species. Mammal effects are particularly interesting since in so many otherwise-pristine tropical forests, hunting has decimated mammal communities. The effects of defaunation on plant community structure and diversity are just now becoming known.

Approach:
Together with Harald Beck, I added seeds of 15 species of trees to five types of mammal-excluding cages. In combination, these mammal exclosures prevent the access of small, medium and large mammals from the added seeds. We monitored the survival of the added seeds over 18 months. As seeds germinate, they are individually tagged and measured. Our seed response variables are the time until the first seed disappears and the fifth seed disappears. Our seedling responses are the number of seedlings alive at the end of the 18 months, and the species richness (per exclosure). This design allows an estimation of the effects of each mammal size class, as well as investigation of interactions with seed size and other covariates.

Results:
Mammal access has powerful effects on seed survival and seedling establishment. Results are still coming in, but small mammals seem to consume more seeds than do larger mammals.