Ecological meltdown in predator-free forest fragments

The manner in which terrestrial ecosystems are regulated is controversial. The "top-down" school holds that predators Limit herbivores and thereby pre vent them from overexploiting vegetation. "Bottom-up" proponents stress the role of plant chemical defenses in limiting plant depredation by herbivore s. A set of predator-free islands created by a hydroelectric impoundment in Venezuela allows a test of these competing world views. Limited area restr icts the fauna of small (0.25 to 0.9 hectare) islands to predators of inver tebrates (birds, lizards, anurans, and spiders), seed predators (rodents), and herbivores (howler monkeys, iguanas, and leaf-cutter ants). Predators o f vertebrates are absent, and densities of rodents, howler monkeys, iguanas , and leaf-cutter ants are 10 to 100 times greater than on the nearby mainl and, suggesting that predators normally Limit their populations. The densit ies of seedlings and saplings of canopy trees are severely reduced on herbi vore-affected islands, providing evidence of a trophic cascade unleashed in the absence of top-down regulation.