Where Elephants Tread, Geckos Thrive, Study Finds

John Roach, National Geographic News 1 Feb 08;

As elephants bash and browse through the trees of the African savanna, they create nooks and crannies for little geckos to hide from predators and the hot sun, according to a new study.

The research shows that the population of Kenya dwarf geckos increases proportionally with the number of trees with limbs snapped, trunks split, bark stripped, and branches fallen in the wake of an elephant run-in.

African bush elephants, which weigh more than 15,000 pounds (7,000 kilograms), are known to modify habitats dramatically, noted Robert Pringle, an ecologist and conservation biologist at Stanford University in California.

Usually, he added, the modification is considered a destructive force.

"But here I seem to have evidence that they can also be a constructive force, with positive effects on the abundance of other species and perhaps on biodiversity more generally," Pringle wrote in an email from Kenya.

The damage caused by elephants creates crevices in broken trees, as well as a cover of strewn branches, providing geckos with secure spaces to rest or lay eggs, he explained.

Pringle conducts his research at the Mpala Research Center in central Kenya. He explains the positive effect of the elephant browsing on gecko population in the January issue of the journal Ecology.

Ecosystem Engineering

The finding is a "very nice demonstration" of ecosystem engineering, said Clive Jones, an ecologist at the Cary Institute of Ecosystem Studies in Millbrook, New York.

Ecosystem engineering is the process by which the activities of one kind of animal can structurally modify habitats of another, potentially benefitting other species. Jones and his colleagues first introduced the concept in 1994.

Jones said that many examples of ecosystem engineering have been based on inferences.

For example, if a beaver pond is full of salamanders not found anywhere else, the inference is that the beavers' modification of the habitat benefits the salamanders.

"Pringle's study," Jones said, "very clearly establishes the connection between the engineering activities of the elephants and the effects on lizards as being largely due to habitat creation."

Conservation Implications

The connection between geckos and elephants also has implications for the conservation and management of elephants, Pringle said.

"At the most basic level, these findings really underscore the importance of conserving elephants in particular," he said.

"They provide this crucial form of disturbance, and if you lose them, then you might well lose populations of other species in a cascade of extinction."

But determining how many elephants must be conserved is a complex calculation, Pringle noted.

His earlier work has shown that a heavy concentration of large browsers like elephants, gazelles, and giraffes tends to negatively effect the abundance of trees, beetles, and geckos.

When all large herbivores are excluded from a patch of land, geckos thrive, perhaps because the vegetation grows so thick and dense that it is full of hiding spots—similar to elephant-damaged trees.

A worrisome aspect of conservation management, Pringle noted, is that elephants are often excluded from small reserves like Kenya's Nairobi and Nakuru National Parks.

The new study underscores the need for some elephants, he said. The goal is to find the right balance—enough elephants to provide the benefits but not too many to outcompete other critters.

"There is probably a middle range of elephant density that is optimal from the perspective of overall biodiversity," Pringle said.