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The Lake Guri Experiment

What makes you most fearful for the future?
Terborgh: “I fear that complete nuclear disarmament will not become a fact until after the next nuclear war...”

See John Terborgh's full Q&A »

What do you like best about your profession?
Aponte: “The study of biology has been like an open window from which I [have] learned to appreciate the world from a different perspective...”

See César Aponte's full Q&A »

What makes you most hopeful for the future?
Balbás: “Mankind will be wise enough to choose the best road to be happy and self confident, living in a world where each living thing is important and [has] the right to stay...”

See Luis Balbás' full Q&A »

The importance of predators to a landscape is demonstrated by a great unplanned experiment deep in the heart of Venezuela.

Located in the eastern reaches of Venezuela, Lake Guri and hundreds of its islands are completely manmade. Their creation is the result of a large hydroelectric project which included the building of several dams to provide electricity for millions. Not long ago, most of this area was an unbroken expanse of green dominated by top predators like jaguars and harpy eagles. In the middle of the food chain were animals like howler monkeys which ruled the treetops protecting their territory with their signature calls. Things are quite different now. Life on these islands has responded dramatically to the landscape changes. In so doing, Lake Guri offers scientists an unprecedented opportunity to peer into the inner workings of a complex ecosystem.

Every time ecologist John Terborgh visits Lake Guri, he and his team members, including César Aponte and Luis Balbás, document significant changes to the life on these islands. They suspect that as the floodwaters created these islands, a key group of animals fled – the big predators. In their absence, their prey began wreaking havoc. According to Terborgh, the predators haunt the place by not eating their prey. On one island, iguanas are living at 10 times normal densities. On another, howler monkeys are living at 50 times higher density than on the mainland. And these normally vociferous primates are completely silent in such overcrowded conditions. On a different island, leaf-cutting ants are living at 100 times their normal numbers. Only the toughest plants now survive on these over-grazed quarters and these survivors are heavily defended with thorns or chemicals.

Lake Guri presents a classic example of a top-down mediated system wherein the removal of top predators initiates a cascade of effects on other populations – mainly the prey species and their food supplies. For example, predators keep prey populations at levels below the population size that would be observed in the absence of predators. On the other hand, if factors such as food and/or habitat availability are the main influences driving population fluctuations, that population is said to be regulated by bottom-up (resource) processes. In truth both processes often regulate populations, either through a seasonal or temporal shift from one process to the other, or when both processes act in concert.

» Terborgh, J., et al. (2001). Ecological Meltdown in Predator-Free Forest Fragments. Science, 294, 1923

Next: Wolves in Yellowstone Park »

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