Without Top Predators, Ecosystems Turn Topsy-Turvy

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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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