Mammals, edge effects, and the loss of tropical forest diversity

Relative to the surrounding mainland forests, a subset of tree species dominates wind-exposed, forested islands in Gatun Lake, Panama. We explored how tree diversity in these fragments has been affected by (1) impoverishment of the mammal community and (2) changes in abiotic conditions following island formation ca. 90 years ago. To test effects of changes in the mammal community, we assessed seed and seedling survival for nine tree species in five forests: small islands with no mammals; small islands with spiny rats but no larger mammals; medium islands (intermediate mammal community); Barro Colorado Island (intermittently present puma and jaguar); and mainland forests (intact mammal community). To test effects of abiotic stress, we chose experimental sites at wind-exposed, wind-protected, and interior forest sites. We predicted that fragments with less diverse mammal communities would be characterized by (1) fewer seeds dispersed and cached, (2) lower long-term seed survival, and (3) higher rates of seedling herbivory by mammals. Where alteration of the environment has caused greater exposure to dry-season winds, we predicted that (4) germination and seedling establishment and (5) dry-season seedling survival would be low. Further, we expected that (6) dry-season seedling survival would increase if soil moisture levels were raised, but that (7) wet-season seedling survival is independent of wind exposure. In the larger forests, seed and seedling survival were low, and differences in mammal community composition had little effect. Small islands that supported only rats had the same low seed survival as larger forests. Elimination of all mammals resulted in much higher seed survival, and slightly higher seedling survival. Germination and seedling survival were lower at wind-exposed than at protected sites, but irrigation had no effect on dry-season seedling survival. It appears that the dominant species on Gatun Lake fragments have passed through a series of filters: seeds must first avoid being eaten by spiny rats, survive the first dry season, and then avoid seedling herbivory by rats. Biotic and abiotic factors determine seedling survival in tropical forests; synergistic changes in these factors can thus lead to dramatic losses of diversity.