With a potential devourer around every corner--and no way to flee--it's a wonder plants have stuck around for so long. Perhaps they're just really good at defending themselves with spines and poison. Or maybe predators keep things in check by killing off herbivores. New research shows that this latter scenario is more likely and argues that reintroducing predators could help rebalance highly disturbed ecosystems.
The idea that predators play the primary role in keeping the world green debuted in 1960, but it's been difficult to prove experimentally. In 1986, ecologist John Terborgh of Duke University in Durham, North Carolina, recognized a golden opportunity when, as part of a vast hydroelectric scheme, developers flooded 4300 square kilometers in Venezuela's Caroni Valley. The flood created a lake containing hundreds of islands. Over 12 years, Terborgh and his team monitored the vegetation on nine small, predator-free islands and at five sites on larger islands and the mainland where predators such as pumas, harpy eagles, and ant-eating armadillos roamed.
When predators disappeared, plants paid the price. Populations of herbivores such as iguanas, howler monkeys, and leaf-cutter ants had exploded on the small islands, devastating the forest understory and the trees. By 2002, densities of saplings on the predator-free islands had plummeted to 25% of those at the sites with predators. Even the chemical defenses of the plants were overwhelmed in the onslaught, Terborgh says. "When predators control the numbers of herbivores, plant-eaters select only the choicest plant material," he says, "but as [herbivore] populations escalate, food resources become scarce, and they feed indiscriminately."
In striking contrast, the islands with predators look relatively normal, the researchers report in the March issue of the British Ecological Society's Journal of Ecology. Terborgh notes that a parallel situation exists in the eastern United States, where the absence of predators such as cougars, wolves, and bobcats has allowed deer to proliferate, with disastrous impacts on forests there.
Ecologist Egbert Leigh of the Smithsonian Tropical Research Institute, Panama, says the surprising revelation that predators protect tropical forests from leaf-cutter ants points to the current sketchy understanding of tropical ecology. Leigh says that while Terborgh has not completely proved his case, "he is very likely right about neotropical forest, and his conclusions merit further testing and close attention from researchers in other types of forest."