Rat removal converts shrublands to grass.

Rat removal converts shrublands to grass

James H. Brown wasn't looking for greener pastures, but he found them anyway. When he and his colleagues fenced off sections of the Chihuahuan Desert in 1977, excluding certain rat species from small plots of shrubland in southeastern Arizona, they had but a single goal: assessing the rats' ability to compete with native ants for the area's supply of large plant seeds.

But 13 years after initiating the desert study and four years after his original associates published their last report on the project, Brown has doggedly stayed on the job to record a remarkable transformation among eight of 24 small plots of land -- each surrounded by fine-mesh fences adjusted to exclude either all rodents or at least three species of kangaroo rats native to the sites. Each of these 2,500-square-meter study areas -- formerly patchworks of scraggly shrubs and parched earth -- has sprouted a dense blanket of knee-high grass. In contrast, none of the study areas that maintained their normal population of the hopping rodents underwent a similar transformation, notes Brown, an ecologist at the University of New Mexico in Albuquerque.

All three species of native kangaroo rats (genus Dipodomys) had to be excluded before the study sites -- bordering a transitional region that includes both desert shrubland and grassland -- began their startling botanical conversion, Brown says. This finding documents for the first time that the collective actions of several related types of animals -- not just the behavior of a single species -- can dramatically alter the fate of an ecosystem, he says.

Identifying a small group of animals, a "keystone guild," that can significantly change the ecology of a habitat may have profound implications for conservation efforts, Brown asserts. He and Edward J. Heske, also from the University of New Mexico, report their work in the Dec. 21 SCIENCE.

"People are only worried about the much more comprehensible and simple question of conservation -- whether a species is there or not," comments marine ecologist James A. Estes of the University of California, Santa Cruz. "But if [an ecosystem] is tied together by a guild rather than a single species, our emphasis ought to be on conservation of the guild rather than the species."

Brown suggests that if ecologists cannot prevent the extinction of species that appear to play a key role influencing their local environment, they should replace the lost animals with organisms able to serve a similar habitat-preserving function.

He adds that the ecological impact of eliminating a keystone guild, though initially subtle, can prove dramatic. For example, during the first five years of the desert experiment, only small changes occurred -- the lack of kangaroo rats, which eat large seeds, spurred rainy-season growth of annual plants that bear these seeds. After nine years, however, grasses began to replace those plants, restoring lasting greenery to the once-sparse landscape. A perennial known as Lehmann's lovegrass increased 20-fold and the annual grass Aristida adscensionis tripled in abundance.

Brown says he hasn't determined why banishing kangaroo rats should promote grassland, but he suggests two possibilities: Frequent burrowing by the rats may prevent grass seeds from taking root, and the accumulation of dead organic material that the rats normally help decompose may hold vital moisture needed for the grasses to thrive.