Predators Are Essential for the Ecology

For almost 50 years ecologists have wondered what keeps herbivores from eating up all the plants: is it the predators or is it some sort of plant defense mechanism? The dilemma seemed impossible to test empirically because of the impossibility of finding an entire ecological system without predators.

However, a team of scientists from Peru and United States have found just that. When 20 years ago Venezuela flooded the Caroni Valley, an area of 4300 square kilometers in order to create a lake (Lago Guri) as part of a huge hydroelectrical project, hundreds of islands, which were formerly fragments of a continuous landscape, were left behind. On some of these islands no predators were left. This offers the opportunity of an unexpected natural experiment.

John Terborgh, a professor of environmental science at Duke University's Nicholas School of the Environment and Earth Sciences, and his team monitored the vegetation at 14 sites of various sizes. Among these, nine of them have no predators left, while the rest have a complete or nearly complete suite of predators.

So, the question was: Are the predator-free islands on the verge of loosing their vegetation due to herbivores multiplying out of control? Or is it that the predators are dispensable and play no large-scale ecological role?

The team found that predators undoubtedly play a vital role in "keeping the world green". The flooding of the Caroni Valley happened in 1986. Since then the plant densities on the predator-free islands have continuously dropped. By 1997, they were only 37 percent of the other islands and of the large land masses. By 2002 the percentage had fallen to just 25 percent.

The herbivores species themselves started disappearing. Animals that eat fruits have already disappeared and only "generalist" herbivores such as iguanas, howler monkeys and leaf-cutter ants have survived. Insofar, they are hyper-abundant. But it may be that they will too start disappearing. Unless something changes... (And it did.)

"Mere numbers do not do justice to the bizarre condition of herbivore-impacted islets," the authors wrote. "The understory is almost free of foliage, so that a person standing in the interior sees light streaming in from the edge around the entire perimeter. There is almost no leaf litter, and the ground is bright red from the subsoil brought to the surface by leaf-cutter ants. Dead twigs, branches and vine stems from canopy dieback litter the ground, and in places lie in heaps. But in striking contrast with this scenario of destruction, the medium islands presented a relatively normal appearance."

Besides proving correct the "green world hypothesis" that claims the importance of the predators, Terborgh's team's results are significant in the debate over reintroduction of top predators such as wolves. The autors write: "The take-home message is clear: the presence of a viable carnivore guild is fundamental to maintaining biodiversity."

The team intended to see what happened in the meantime on the predator-free islands but have discovered that the Venezuelan natural experiment was stopped in 2003 by nature itself: a drought that began in 2001 and reached extreme levels in 2003 has caused a significant drop in the lake level; this left only a few isolated islands and predators managed to populate once again virtually all the islands.

"The fundamental premise of the whole project was our ability to study predator-free space, and that condition was no longer met. In one period in 2003 we found six different predators on islands that previously had no predators."

Photo Credits: Sea Studios Foundation; Duke University