Biologists have long thought that when large mammals, such as elephants and gazelles, are driven to extinction, small critters will inherit the earth. As those critters (think rodents) multiply, so will the number of disease-carrying fleas. Scientists have now experimentally confirmed this scenario, which is troubling because it could lead to a rise in human infection by diseases that can be transferred between animals and people.

The research started 20 years ago, when biologists working at the Mpala Research Center in Kenya embarked on a large-scale experiment to understand the importance of diversity. They divided swaths of the center's land into 10-acre chunks and changed the types of wildlife living on each one. In some, they removed all large animals, such as giraffes and zebras, leaving behind only mammals smaller than 33 pounds. As the years went on, they kept a record of the species that inhabited each plot of the savanna.

Using those records, Hillary Young, an ecologist at the University of California, Santa Barbara, compared rodent abundance in tracts free of large mammals with similar areas open to all wildlife. She and her colleagues discovered that the landscapes rid of these large mammals contained twice as many rodents as the uncontrolled areas. The findings were published in May in the journal Proceedings of the National Academy of Sciences USA.

Out of the 11 mammal species present on the land devoid of megafauna, Mearns's pouched mice were most populous, accounting for 75 percent of the total count. As expected, the chances of finding a flea carrying the bacterium Bartonella also doubled. Bartonella infects mammals, including humans, and can cause major organ damage.

“Changes in wildlife communities can and do cause significant impacts on disease risk,” says Young, who adds that the results should apply to all places, not just the African savanna. She hopes that if people understand that preserving wildlife also preserves their own health, perhaps they will be less likely to tolerate losses to biodiversity.