Carnivores play the key role in keeping the earth green
a recent study on the role of predators in keeping the earth green can well be called a fruit of development.
Scientists have long wondered how plants have survived despite the constant onslaught from herbivores. Opinion has been divided -- one camp believing the physical and chemical defence mechanism of plants was responsible for their survival with the other arguing that the predators played the key role by keeping a check on herbivore populations. Now, a team of researchers has found proof favouring the latter theory, called the green world hypothesis.
"Ecologists have been debating whether herbivores are limited by plant defences or by predators. The matter is trivially simple in principle, but in practice, the challenge of experimentally creating predator-free environments in which herbivores can increase without constraint has proven almost insurmountable," says John Terborgh of Duke University, usa, who is the lead author of the study published in the March 2006 issue of the Journal of Ecology (Vol 94, No 2).
So it was a rare stroke of luck for Terborgh and his team when the ideal conditions to test the green world hypothesis were created in the late 1980s. The chance came in the form of a vast hydroelectric scheme in the Caroni valley of Venezuela. Project developers had to flood 4,300 square kilometres in the valley. A large lake was created. But the parts at higher altitude stuck out of the water, creating hundreds of islands. What was interesting to Terborgh and his team was that some of these islands had no predators at all.
By 1997, the researchers found that sapling densities on small islands were only 37 per cent (about one-third) of those on large landmasses. Populations of herbivores such as iguanas, howler monkeys and leaf-cutter ants had exploded on the predator-free islands, devastating the forest understory and the trees. Even the chemical defences (for instance, producing toxins) of the plants were overwhelmed in the onslaught. By 2002, sapling densities on the small islands had plummeted to 25 per cent of those at the sites with predators. "When predators control the numbers of herbivores, plant-eaters select only the choicest plant material," Terborgh explains, adding: "But as herbivore populations escalate, food resources become scarce, and they feed indiscriminately."
In their research paper, the scientists note that a parallel situation exists in eastern us, where the absence of predators such as cougars, wolves and bobcats has allowed deer to proliferate, with a disastrous impact on forests.
Information reported in the Journal of Ecology is a five-year update of results published by Terborgh and 10 other scientists in the November 30, 2001, issue of Science (Vol 294, No 5548). "The Science article reported on the first plant census we did, involving 15,000 plants, but it only gave us a snapshot in time," says Terborgh, adding, "We could see that there were huge differences between the little islands that didn't have any predators on them, and larger islands that did have predators. But we couldn't say much about where that was going to go in the future." The re-census was conducted to see if the earlier findings stood the test of time. "Indeed, during that five-year interval the populations of small, sapling-level plants continued to decline quite radically. And there's no question that the vegetation on these islands is just in a state of collapse," Terborgh says.
The study has important implications for the debate raging in many countries over reintroduction of predators such as wolves. "The take-home message is clear: the presence of a viable carnivore guild is fundamental to maintaining biodiversity," the authors say. According to ecologist Egbert Leigh of the Smithsonian Tropical Research Institute, "Terborgh is very likely right about neotropical forest, and his conclusions merit further testing and close attention from researchers in other types of forest.
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