Without Top Predators, Ecosystems Turn Topsy-Turvy

Stefan Lovgren
for National Geographic News
April 26, 2005

When the construction of a hydroelectric dam on Venezuela's Caroni River was finally completed in 1986, it flooded an area twice the size of Rhode Island, creating one of South America's largest human-made lakes: Lake Guri.

As floodwaters turned hilltops into islands, a key group of animals—predators such as jaguars, harpy eagles, and armadillos—disappeared from the islands. Some swam or flew away. Others drowned or starved to death.

In the predator's absence, their prey—howler monkeys, iguanas, leaf-cutting ants—began multiplying. Soon these plant-eaters had devoured most of the once pristine forest.

It is a classic cautionary tale of the dangers of removing top predators from an ecosystem.

"Taking out predators has a cascade of effects on other populations, down to the plant life," said John Terborgh, a professor of environmental science at Duke University in Durham, North Carolina.

The Lake Guri story is recounted in a National Geographic four-part TV series, Strange Days on Planet Earth, which airs this Wednesday on PBS.

Mass Exodus

The creation of Lake Guri may have been an ecological disaster. But it offered biologists like Terborgh an unprecedented opportunity to study the effects of removing top predators from an ecosystem.

The first phase of the two-part Guri dam project was completed in 1968. It raised the water level over 390 feet (120 meters) above that of the original Caroni River.

When the dam's second phase was finished in 1986, the water level rose steadily over a year by another 164 feet (50 meters), and about a thousand hilltops became islands in a human-made lake.

Terborgh found that predators such as pumas, jaguars, anacondas, eagles, armadillos, and some weasels were not able to persist on islands smaller than 37 acres (15 hectares). Some of the animals swam or flew from the islands. Others starved to death.

Not surprisingly, the mass exodus of predators had a huge impact on their prey. Populations of howler monkeys, iguanas, and leaf-cutting ants exploded.

Continued on Next Page >>




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