Snowmobiles raise the stress hormone levels of elk and wolves, a new study suggests. The findings come as the National Park Service considers whether to restrict snowmobile use for the sake of wildlife.
In April 2000 the Park Service announced plans to ban snowmobiles in national parks, but later reversed itself following a lawsuit by the snowmobile industry. Presently the ban is being debated again--and the machines' impact on wildlife is at the heart of the issue. Earlier studies demonstrated that mammals and birds move to avoid areas of snowmobile use and that their heart rates increase in the presence of the machines.
To look for more direct signs of stress, Scott Creel of Montana State University, Bozeman, and colleagues measured levels of glucocorticoid stress hormones in the feces of elk and gray wolves in the wild--a reliable indicator of levels in the bloodstream. In the June issue of Conservation Biology, Creel's group reveals that elk in Yellowstone National Park show higher levels of the stress hormone during the snowmobile season and that levels rise and fall with the amount of daily snowmobile traffic. Wolves in Voyageurs National Park in Minnesota, where snowmobile use is heavy, had more of the hormone than wolves in nearby Isle Royale National Park in Michigan, which is closed to snowmobiles. During the 2-year study, glucocorticoid levels at Voyageurs dropped 37%, paralleling a 37% drop in snowmobile activity.
Creel says the findings provide an "early warning" that the animals are stressed and the populations, which have been stable so far, could suffer in the future. Chronically elevated stress hormone levels in vertebrates interfere with the immune system, inhibit reproduction, and cause other health problems.
The study did not check the health of wolves or elk, however, and Joshua Millspaugh, a wildlife ecologist at the University of Missouri, Columbia, notes that it's not clear what glucocorticoid levels indicate potential harm to an animal. Even so, Samuel Wasser, a conservation biologist at the University of Washington, Seattle, says that policy-makers should err on the side of caution. "If we wait to show a fitness effect, it may already be too late to turn things around."