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Global Warming in Yosemite

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Global warming is causing major shifts in the range of small mammals in Yosemite National Park, according to a new study that compared small mammal populations in the park today versus 90 years ago. Mammals like shrews, mice, and ground squirrels have moved to higher elevations or reduced their ranges in response to warmer temperatures.

“We didn’t set out to study the effects of climate change, but to see what has changed and why” since the last full-scale survey in Yosemite in 1918, says study leader Craig Moritz, director of the campus’s Museum of Vertebrate Zoology. “But the most dramatic finding…was the upward elevational shift of species. When we asked ourselves, ‘What changed?’ it hit us between the eyes: the climate.”

The population movements have not altered biodiversity in the park; however, Berkeley biologists say rapid changes in less than a century could be a problem. Although half the species shifted their ranges, the other half did not, which means communities have been altered and the species interacting with one another have changed.

The foundation for the current study is a landmark survey of Sierra Nevada birds, mammals, reptiles, and amphibians initiated early last century by zoology professor Joseph Grinnell. With a large number of colleagues and students, Grinnell trekked through the Sierra Nevada collecting specimens in Yosemite recording for posterity the variety of life then under threat from gold mining and overgrazing.

Steve Beissinger, professor of environmental science, policy and management, explains that thanks to these detailed field notes recording not only what the Grinnell team saw, but also what they failed to observe, today’s Berkeley biologists were able to perform a statistical analysis that validates the study results.

“One of the biggest problems we have when comparing the distribution of species now and in the past is false absences. If they didn’t see something back then, is it because it wasn’t there, or because it just wasn’t detected?” he said. Employing occupancy models developed in the past few years, he added, “the Grinnell group’s data allows us to go back and, night by night, reconstruct their trapping success for small mammals and develop a probability for detecting each species for Grinnell and for us.”

The UC Berkeley team, with the assistance of colleagues around the state, is continuing its resurvey of Grinnell’s transects and will eventually resurvey coastal mountains as well. This work will provide information about northward or southward movement of animals in addition to their elevational movement in response to climate and land use change.

-Robert Sanders


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