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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.

To Terborgh's surprise, the carrying capacity—the upper limit of a sustainable population—of these plant-eaters turned out to be vastly greater than he had imagined, with populations surging ten to a hundred times higher than the animals' density on the nearby mainland.

The booming herbivore populations devoured the islands' vegetation.

"The impact of massive herbivory [plant eating] was to increase the mortality of trees, especially small saplings," Terborgh said. "Our model showed very dramatically that the vegetation [on the islands in Lake Guri] is in a state of collapse."

Return of the Wolves

The loss of top predators could also explain the disappearance of aspens and willows in the oldest national park in the United States: Yellowstone.

Scientists determined that aspens stopped regenerating in Yellowstone in the 1930s, around the time that wolves went extinct in the area.

Research suggests that the elimination of Yellowstone's wolves allowed one of their prey animals, elk, to browse aspens and willows undisturbed. This led to the disappearance of trees and streamside vegetation—and the loss of beaver habitat.

Since wolves were reintroduced into Yellowstone in 1995, aspen and willow have begun growing again, and a few beavers have returned to the park.

Algae Invasion

Similar phenomena have been observed in the oceans.

Overfishing may have caused the populations of hammerhead sharks in the Atlantic Ocean to drop by as much as 90 percent in some places.

"Sharks are top predators … they keep their prey in check. And that, in turn, helps their prey's prey," said Mike Heithaus, a marine biology professor at Florida International University in Miami. "These effects cascade through the whole ecosystem."

Overfishing may also at least partly explain why a suffocating layer of algae now blankets the once vibrant Discovery Bay coral reef in Jamaica.

Fisheries in the area first decimated top predatory fish, such as sharks, groupers, and jacks. Smaller fish became the next commercial target, including plant-eating species that kept fast-growing algae in check.

But the story is more complicated, says Rich Aronson, a senior marine scientist at the Dauphin Island Sea Lab in Alabama. Sampling the fossil record of the reef, Aronson found that the corals stopped growing in the early 1980s, after a hurricane hit Jamaica and destroyed much of the reef.

A few years later a lethal disease swept across the Caribbean Sea, killing the main remaining grazers, sea urchins, and paving the way for the algae to take over.

"The combination of killing the corals and the loss of herbivores resulted in this vast goo of seaweed," Aronson said. "It has to be some kind of outside disturbance that kills the coral and provides the entrée for the algae."

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