Island Study Suggests Predators Key To Healthy Ecosystem

DURHAM, N.C. - A study of animals and plants isolated since 1986 on small islands in Venezuela has yielded strong evidence that predators play a key role in perpetuating the diversity of plants and animals.

In a longstanding ecological controversy, this "top-down" view is countered by a "bottom-up" view, which holds that it is the mix of consumable plants in an area that determines the numbers and varieties of plant eaters, and of their predators as well.

The study by 11 scientists from seven nations, published in the Nov 30 issue of the journal Science, emphasized the importance of predators in maintaining a healthy ecosystem, said John Terborgh, James B. Duke Professor of Environmental Science at Duke University's Nicholas School of the Environment and Earth Sciences and the article's principal author.

Terborgh warned that the lack of such predators as wolves, bobcats and mountain lions in the eastern United States is already damaging forest ecosystems there. In a "parallel situation," the Science authors wrote, overgrazing by livestock can degrade semi-arid grasslands into thornier, scrubbier vegetation.

Their study was supported by the National Science Foundation.

The researchers studied 10 small- and medium-sized islands created by a hydroelectric impoundment at Lago Guri, a Venezuelan lake almost the size of Connecticut. A key ecological feature of the islands, which range in area from about 0.6 acres to 24 acres, is the absence of predators that prey on vertebrate animal species.

"A space of just a few acres simply doesn't support a self-sustaining population of any predator that feeds on vertebrates," added Terborgh, who is also co-director of the Duke Center for Tropical Conservation.

With predators excluded as a result of the flooding that isolated the islands, the numbers of plant-eating animals still living on these islands have become "hyperabundant," increasing by 10- to 100-fold, the Science article said. "This is very strong evidence for top-down regulation," Terborgh added. "If predators are gone, then the number of these consumers explodes.

"Some islands have monkeys living on them at densities well above combined densities in the most abundant primate communities ever discovered on Earth," he said. "It is something off the scale, never before seen. Similarly, we have other herbivores that are in extreme abundance - leaf cutter ants and common iguanas, and various kinds of small rodents."

According to data in the Science article, the capture rate for rodents was 35 times larger on small islands than on the mainland. Iguana numbers, based on counts of their dung, were 10 times larger. Leaf cutter ant populations were 100 times higher. And howler monkey densities reached the equivalent of 1,000 animals per square kilometer (about 0.38 square miles), compared with 20 to 40 per square kilometer on the mainland.

One finding - that in the absence of top-down regulation, bottom-up regulation takes over as consumers increase - was apparent in the suppressed reproductive rates among overpopulating howler monkeys, the Science article added. Two adult females observed on one 1-1/2-acre island produced the equivalent of 0.125 births per female per year, compared to 0.5 births per year among monitored females living on a 865-acre island.

But in general, support for the top-down view prevails because vegetation on the islands seems to be suffering from the dearth of meat-eaters. And facing a resulting food crunch, herbivores there are being forced to expand their diets.

For instance, howler monkeys normally "highly prefer certain foods, fruits and leaves of certain species of trees," Terborgh said. "When they're trapped on tiny islands at very high densities, their feeding becomes, in a statistical sense, almost completely non-selective."

And leaf cutter ants "are being forced by overpopulation to feed on a lot of things they'd never use otherwise. They harvest all kinds of things that are really surprising to us, the tough leaves, poisonous leaves, things you'd swear that they couldn't use."

The result is evidence for what the authors termed a "trophic cascade," in which the absence of predators leads to a "hyperabundance" of consumers, which overexploit adult trees. "Hyperabundant herbivores threaten to reduce species-rich forests to an odd collection of herbivore-resistant plants," the authors wrote in Science.

"The trees there before the flooding are now dying off at much higher than normal rates," Terborgh explained. Meanwhile, "the combination of rodents and leaf cutter ants and all these other herbivores are just eliminating all the young plants that appear," he added. "The vegetation is in a state of collapse. Plant species that can survive under this massive onslaught by herbivores are the ones that are incredibly tough and terribly toxic."

Terborgh compared the process at work on the islands to what ecologists have observed in dry regions of the western United States, where cattle, horses, sheep and goats are allowed to overgraze available grasses.

"It's absolutely predictable that the grassland will be turned into sagebrush, yucca, cactus, prickly pear or mesquite -- all the things that animals find a hard time eating," he said. "What has never been shown before is that this same thing can happen in a forest."

The Science article's authors write that "these observations are warnings, because large predators which impose top-down regulation have been extirpated from most of the continental U.S. and, indeed, much of the Earth's terrestrial realm."

Terborgh noted that woodlands in the eastern U.S. are already changing because they lack predators.

"We have too many white-tailed deer, too many beavers, too many racoons and possums," he said. "Evidence is just pouring in now that lots of forests are failing to reproduce themselves in a normal fashion.

"Oak trees, in particular, are being suppressed because the deer eat acorns in the fall and then eat the oak seedlings in the spring. So instead we're getting maples and tulip poplars and other species that were minor elements of the forest before."

Other authors of the Science report are affiliated with Florida International University in Miami; Universidad Nacional "San Antonio de Abad" de Cusco in Peru; the Wildlife Conservation Society in New York City and Cambridge University in England; the Chintan Environmental Research and Action Group in New Delhi, India; Universidad Ricardo Palma in Lima, Peru; Fundaciouseo de Ciencias in Caracas, Venezuela; Universidad Complutense de Madrid in Spain; the University of Toronto; the University of Wisconsin in Oshkosh; and EDELCA in Puerto Ordaz, Venezuela.

Note to editors: John Terborgh can be reached at (919) 490-9081 or manu@acpub.duke.edu.