Ecologists have unraveled an intricate skein of interactions among forest species that may govern upsurges of Lyme disease and tree-ravaging gypsy moths. In a 3-year study  described in today's Science, the researchers showed that bumper crops of acorns lead to an explosion of mice, and the mice in turn protect oak trees by eating gypsy moths. But the mice also host ticks that can spread Lyme disease, a sometimes disabling human infection.
To tease out these links, Clive Jones and Richard Ostfeld's team at the Institute of Ecosystem Studies in Millbrook, New York, and Jerry Wolff of Oregon State University in Corvallis first showed by trapping mice in experimental forest plots that fewer mice means much greater survival rates for gypsy moth pupae. Next they simulated a "masting"--the term for an abundant acorn season that occurs naturally every 2 to 5 years. In the fall of 1995, they spread 3500 kilograms of acorns on three experimental plots with few mice. Mouse populations skyrocketed. "Our data show that the key trigger [for moth outbreaks] is this relationship between the acorns and the mice," Jones says.
Along the way, the researchers also kept an eye on ticks, as white-footed mice are a reservoir of the Lyme disease spirochete, which they transmit to tick larvae. The group showed that the summer after the masting, there were far more tick larvae--an eightfold rise--in the acorn-rich plots as compared to other plots. The acorns had apparently attracted deer bearing adult ticks--which spawned the tick larvae--as well as boosted mouse numbers, Jones says.
More acorns, more mice, more deer, more ticks: It adds up to a larger Lyme disease risk, the researchers argue. But the study also highlights the challenge of managing ecosystems, because in this case trying to cut down on Lyme disease by, say, trapping mice could send gypsy moth numbers soaring. "Once we start tinkering with nature, we could get in a wonderful mess," says Princeton ecologist Andrew Dobson.
Although epidemiologists also praise the work, some say too many other factors determine Lyme disease outbreaks for the study to have much predictive value for now. "The question is still open," says Joseph Piesman, who heads the Lyme disease vector branch at the Centers for Disease Control and Prevention in Fort Collins, Colorado.