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Ecologists Question Grizzly Comeback
16 April 1999 8:00 pm
A new study suggests that a 24-year-long campaign to boost grizzly bear numbers in Yellowstone National Park has made little, if any, headway. The study throws cold water on the federal government's claims that the Yellowstone grizzly population has been recovering steadily and the animals could be removed from the threatened species list. The study, appearing in this month's issue of Ecology, also portends even harder times for the grizzlies, thanks to recent poor yields of whitebark pine seeds, one of the bear's key foods.
When the grizzly was placed on the list in 1975, an estimated 200 to 300 bears were left in Yellowstone. Park managers took steps to stop practices like bears feeding at open garbage pits to reduce maulings and allow the bears to lead a more natural life in the wild. To find out whether the population has indeed grown, ecological modeler Craig Pease of Vermont Law School and David Mattson, a U.S. Geological Survey (USGS) grizzly field biologist, looked at 20 years of data on grizzlies tracked with radio collars or spotted by federal scientists in Yellowstone.
The duo folded these data into a computer model of births and deaths, taking into account factors--such as age, sex, whitebark pine crop size, and whether a bear had become accustomed to humans--that influenced the odds of survival of any particular tracked bear. The two estimate that Yellowstone grizzly numbers have grown only about 1% a year between 1975 and 1995--much lower than the 5% annual increase claimed by the Interior Department, which runs the park.
Some experts applaud the work. "Pease and Mattson have the right answer. I'm absolutely convinced of that," says University of California, Santa Cruz, population biologist Dan Doak. Interior officials, not surprisingly, dispute the conclusions. "We directly count the bears in numerous ways," says Chris Servheen, U.S. Fish and Wildlife Service grizzly recovery coordinator. "The population's been going up for some time." Servheen adds that USGS biologists are conducting a marking and tracking study that by this summer might help resolve questions over the current population.