Biologists at Yellowstone National Park in Wyoming are worried that climate change might set off a series of changes that could threaten the park’s grizzly bear population. As temperatures rise, the atmosphere can hold more moisture, and in Yellowstone this could spur more cases of blister rust, a disease that attacks whitebark pines. These trees, which thrive in harsh high altitudes just below the timberline, produce pine nuts, which some experts believe may be the source of as much as 40 percent of a bear’s layer of winter fat. Fewer nuts could mean fewer bears, and because there are only an estimated 200 to 400 bears in the park, a modest increase in deaths could have serious effects. Scientists are considering various steps to take if blister rust becomes more prevalent.