Rock climbing cuts mollusk diversity. (At a Snail's Place).

As rock climbing soars in popularity, some cliff-side snail populations may be crashing, according to new research. While focusing attention on cliff ecosystems, the finding is also instigating debates about tougher climbing regulations.

A cliff's cracks and crevices are home to many small species. On a single saucer-size cliff ledge in Wisconsin, for example, scientists have found evidence of one-quarter of the state's land-snail species, notes ecologist Jeffrey C. Nekola of the University of Wisconsin-Green Bay.

North America's 4 million rock climbers treat cliffs as natural jungle gyms, says cliff ecologist Douglas W. Larson of the Universityof Guelph in Ontario, Canada. Still, rock climbing is generally considered to have a low environmental impact. Most climbers, after all, leave behind little save chalk dust.

A closer look at the cliff's more humble tenants suggests that climbers are leaving a more lasting trace, Larson says. In earlier studies, he and his colleagues found that plants and lichens growing on the Niagara Escarpment--a 700-kilometer stretch of limestone cliffs in southern Ontario--decline in popular climbing spots compared with unclimbed locations. Now, a team led by Darson and Nekola reports that tiny land snails, some smaller than caraway seeds, also disappear from high-activity spots.

The researchers counted snail shells in soil samples from nooks of climbed and unclimbed faces of the limestone cliffs. Soil collected along established climbing routes harbored one-fifth the number of snail shells found at unclimbed spots, the researchers report in the April Conservation Biology. What's more, half of the 40 snail species identified in the unclimbed areas were absent from soil samples taken from climbing routes. The researchers say that removal and packing of soil by climbers likely spur the declines. The results should open up the possibility of restricting climbers to specific routes, they argue.

"I'm amazed by the size of the effect," says conservation biologist Menno Schilthuizen of the University of Malaysia in Sabah. Snails lost from an area can take a "very long time" to come back, Schilthuizen adds.

"It's another case of a restricted habitat type that doesn't appeal to big, charismatic animals and therefore tends to get missed [by conservationists]," says ecologist Robert R. Dunn of the University of Connecticut in Storrs.

People shouldn't put too much weight on a single study along a single escarpment, says Jason Keith, policy director of The Access Fund, a climbing organization in Boulder, Colo.

The crucial question for management strategy, says conservation ecologist Robert H. Cowie of the University of Hawaii in Honolulu, is to determine how much of the snails' habitat is being severely disturbed.