Finding desert tortoises is much harder during drought years, which means biologists may underestimate populations of the threatened reptiles. The finding, reported in the October issue of Conservation Biology, highlights the problems that scientists face in trying to estimate endangered populations that live in extreme environments.
Mojave Desert tortoises, which wander the most arid parts of North America's southwest, were declared "threatened" under the U.S. Endangered Species Act in 1990, after biologists estimated their numbers at 100,000 and dropping. But when the National Park Service asked ecologist Jerome Freilich to track tortoise numbers within California's Joshua Tree National Park, he didn't have enough money to run a large, labor-intensive census. Instead, he recruited volunteers for a more affordable annual survey of a one-kilometer-square plot from 1991 to 1996.
When researchers plugged the counts into a mathematical model that estimated populations, however, the numbers "went haywire," says Freilich, now with The Nature Conservancy in Lander, Wyoming. That's because the annual counts--which involved capturing, marking, and later recapturing tortoises--varied widely depending on rainfall. Animals seemed to disappear during drought years, when they apparently hid in underground burrows, only to reappear in wetter years. As a result, data from the four moister years indicated a population of between 62 and 83 mature animals in the study area. Data from two drought years produced less certain estimates, ranging from 43 to 97. The take-home message, Freilich says, is that tortoise-counting methods can be "plagued" by weather patterns, and that accurate estimates depend on sampling during nondrought years.
The study emphasizes how weather can skew population estimates, says Steven Corn, a zoologist with the U.S. Geological Survey (USGS) in Missoula, Montana. Still, he is "not sure that I would conclude from this paper that every single estimate in a drought year is an underestimate." And Kristin Berry, another USGS biologist in Riverside, California, says her own studies have found no significant differences in tortoise populations in dry and wetter years. "Experienced people," she says, "are able to find [tortoises] even in a drought."