Scientific Colloquium
January 16, 2004
The manner in which
terrestrial ecosystems are regulated is controversial.
The "top-down" school
holds that predators limit herbivores and thereby
prevent them from
overexploiting vegetation, whereas "bottom-up" proponents
stress the role of
plant chemical defenses in limiting the depredations of
herbivores. A set of
predator-free islands created by a hydroelectric
impoundment in
Venezuela allows a test of these competing world views.
Limited area restricts
the fauna of small (0.25-0.9 hectare) islands to
predators of arthropods
(birds, lizards, anurans, spiders) granivores
(rodents), and
herbivores (howler monkeys, iguanas, leaf-cutter
ants). Predators of
vertebrates are absent, and densities of rodents, howler
monkeys, iguanas, and
leaf-cutter ants are 10 to 100 times greater than on
the nearby mainland,
implying that predators normally limit their
populations. The
densities of seedlings and saplings of canopy trees are
severely reduced on
herbivore impacted islands, providing evidence of a
trophic cascade
unleashed by the absence of top-down regulation.