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by PBS.org
John Terborgh
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.
Howler monkey populations are
exploding due to a lack of big predators.
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.
References
» Terborgh, J., et al. (2001). Ecological Meltdown in Predator-Free
Forest Fragments. Science, 294, 1923
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