Ecologists explain why the world is green

Hydroelectric schemes usually generate a barrage of criticism from conservationists. But the flooding of a Venezuelan valley 20 years ago has provided ecologists with the ideal outdoor laboratory to answer one of ecology’s oldest and thorniest questions: why is the world green?

Reporting their results in the March issue of the British Ecological Society’s Journal of Ecology, a team lead by Professor John Terborgh of Duke University says that the role of predators is the key to keeping the world green, because they keep the numbers of plant-eating herbivores under control. Their results support the so-called “green world hypothesis” first proposed in 1960 by Hairston, Smith and Slobodkin and seem to lay to rest the competing theory that plants protect themselves from being eaten through the physical and chemical defences they have developed.

Despite being nearly 50 years old, the green world hypothesis has been almost impossible to test until now. According to Terborgh: “Since the landmark paper by Hairston et al, ecologists have been debating whether herbivores are limited by plant defences or by predators. The matter is trivially simple in principle, but in practice the challenge of experimentally creating predator-free environments in which herbivores can increase without constraint has proven almost insurmountable.”

Along with colleagues from Harvard and Wake Forest University, Terborgh realised that the hypothesis could be tested on a vast hydroelectric scheme in Venezuela’s Caroni Valley, where in 1986 an area of 4,300 square kilometres was flooded to create a lake (Lago Guri) containing hundreds of land-bridge islands that were formerly fragments of a continuous landscape.

Terborgh and his team monitored the vegetation at 14 sites of differing size. Nine of the sites were on predator-free islands, while the others were on the mainland or on islands with a complete or nearly complete suite of predators. They found that by 1997, small sapling densities on small islands were only 37% of large land masses and by 2002 this had fallen to just 25%. Most of the vertebrates present in regional the dry forest ecosystem had disappeared from small islands, including fruit eaters and predators of vertebrates, leaving a hyperabundance of generalist herbivores such as iguanas, howler monkeys and leaf-cutter ants.

“Mere numbers do not do justice to the bizarre condition of herbivore-impacted islets. 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,” Terborgh says.

As well as proving that the green world hypothesis is correct, Terborgh’s results have important implications for the debate raging in many countries over reintroduction of top predators such as wolves. “The take-home message is clear: the presence of a viable carnivore guild is fundamental to maintaining biodiversity,” he says.