Does Mammal Community Composition Control Recruitment in Neotropical Forests? Evidence from Panama

Patterns of seed predation, germination, and seedling herbivory were investigated in Panamanian forests. We hypothesized that seed and seedling survival would vary with differences in mammal community composition. We tested this hypothesis at five sites in mainland forests adjacent to Gatun Lake, full terrestrial mammalian granivore/ herbivore communities with top predators; at five sites on Barro Colorado Island (BCI), also a full mammalian granivore/herbivore community but without the two largest cats; at one site each on five medium-sized islands, with rats, agouti, rabbit, and paca present; and on five small islands that support rats only. Experiments were replicated for Dipteryx panamensis, Gustavia superba, and Virola nobilis, all of which have large seeds. To assess seed removal, seeds were placed in wire exclosure cages and nearby outside the cages. There was no difference in removal rates between forest types, with almost all unprotected seeds removed at all sites. To assess post-removal seed fate, seeds of Gustavia and Virola were attached to threads and placed on the forest floor. All threaded seeds were victims of predation on small islands, whereas 34, 43, and 77% of threaded seeds were dispersed and buried on BCI, medium islands, and the mainland, respectively. To assess seedling herbivory, half of the wire exclosure cages were removed after germination, and seedling survival was assessed after 13–14 mo. Protection from mammals increased seedling survivorship by more than sixfold on the smallest islands, by threefold on the medium islands, by twofold on the mainland, and by less than twofold on BCI. The absence of the two largest cats and the exclusion of poachers from BCI was associated with lower seedling herbivory and higher seed predation than observed on the mainland. In contrast, extreme mammal defaunation on the small and medium islands had large and consistent effects on seedling recruitment, including increased seed predation and increased seedling herbivory relative to sites with more intact mammal communities.