Indirect effects of a top predator on a rain forest understory plant community

Tropical rain forests, some of the most diverse natural communities, have been characterized either as inherently stable or as extremely fragile. Theoretically, complex food webs allow for alternative pathways and overlapping functions within and among trophic levels, such that cascading effects are buffered and perturbation is resisted. On the other hand, strong interdependence among species and trophic levels in these same systems can make them vulnerable to perturbation. To assess the ability of a tropical rain forest community to resist perturbation or to amplify perturbation through trophic cascades, we measured changes in overall vegetative cover and species richness of understory plants in response to the experimental addition of a top predator beetle. In August 2000, 30 randomly located patches of naturally occurring Piper cenocladum shrubs were found at sites with variable soil quality and light availability, and top predator addition treatments were randomly assigned to half the patches. Using an 8-m(2) subplot within each P. cenocladum patch, we estimated (nondestructively) vegetative cover of understory plants for nine months and measured herbivore damage, plant longevity and plant species richness for similar to1.5 years. For all understory species in our subplots, vegetative cover and total numbers of leaves declined significantly more in predator addition patches than in control patches. Understory cover also declined significantly more on poor soils than on rich soils. Mortality of small plants was high overall, but longevity was significantly greater in control patches than in patches with top predators. Herbivore damage was lowest, plant establishment highest, and the accumulation of new plant species highest in control plots on rich soils. The cumulative effect of top predators interacted with plant resources, such that plant species richness was suppressed when beetles were present in understory communities on rich soils. Patterns of herbivore damage suggest that trophic cascades from apex predators through ant predators to herbivores altered the structure of understory vegetation. The presence of indirect effects supports the characterization of rain forests as vulnerable ecosystems whose integrity may be threatened by the slow erosion of complex tight interactions among species.