Birds, chipmunks and other small vertebrates in Yosemite National Park have shifted their habitats over the last century, researchers have found, giving possible new insight into how climate change may be affecting the wild ecosystem.
Biologists from Yosemite and the University of California, Berkeley, said drawing firm conclusions about the role of climate change in Yosemite's habitat structure will take more research. Even so, their findings offer a rare glimpse at a century's worth of environmental change in one of the nation's most prized landscapes.
Observing the range shifts was made possible by the foresight of biologists 100 years ago. Between 1911 and 1920, Joseph Grinnell and his colleagues at the Museum of Vertebrate Zoology at Berkeley conducted an exhaustive inventory of the mammals, birds, reptiles and amphibians across 1,500 square miles of the central Sierra Nevada, including Yosemite Valley.
Grinnell made sure the data and observations were recorded in such a way that current researchers at the museum and park were able to closely replicate the Yosemite portion of the survey between 2003 and 2005.
"Grinnell was brilliant in his initial assessment that the original data would be useful not necessarily right away but in 100 years," said Leslie Chow, the data manager for Yosemite's inventory and monitoring program. "It provides us with a baseline that very few other locations have for knowing what was here, and knowing what changes have occurred."
Researchers were able to relocate 21 original survey sites inside Yosemite by drawing on 2,000 pages of archived field notes and 817 photographs. They trapped small animals and observed birds to determine which species are still in those spots, which ones are now absent and any new ones that are present. Measurements and DNA samples collected from the animals during the recent survey were compared against the 2,795 specimens collected during the original inventory.
The resulting report, "A Re-survey of the Historic Grinnell-Storer Vertebrate Transect through Yosemite National Park, California," was published earlier this year on the centennial of the original survey.
Some survey methods and tools have changed since the early 20th century. For example, biologists no longer shoot mice, shrews and chipmunks; they capture them in live traps. The differences make it difficult to directly compare some data sets, such as species abundance.
However, researchers can confidently compare species distribution -- where animals are present or absence in the park -- to reveal how species' ranges have shifted since the original survey.
Of 30 vertebrate species with sufficient data for comparison, 14 exhibited no change, six expanded their ranges and 10 species experienced contractions. Typically, lower-elevation species expanded their ranges upward and high-elevation species' ranges retracted upward.
"Of the small species of mammals that have moved up, the average shift is 500 meters in elevation, which is a lot," said Craig Moritz, the director of the Museum of Vertebrate Zoology who helped coordinate the resurvey.
The shift is consistent with what researchers projected, given minimum temperatures in the central Sierra Nevada have increased about 3 degrees Celsius over the last century, Moritz said.
"We didn't set out to show the effects of global warming," Moritz said. "It was really surprising the effects were so strong."
Some animals, such as alpine chipmunks, had more dramatic range contractions than others. During Grinnell's survey, the chipmunks were observed at 8,600 feet. But in the most recent survey, it was found only at elevations above 10,500 feet. Experts hypothesize the shift could be due to a direct physical response to warmer temperatures, or from competition from other species that have expanded their range.
"We suspect climate change plays a role, but it can't explain all of them," Chow said. "Some species expanded their range downwards."
While the inventories are extensive, researchers do not know when the shifts began or exactly what is causing them. The surveys provide a foundation to conduct future research to answer those questions and fill in knowledge gaps.
For example, DNA analysis -- something Grinnell could only dream about -- has shown alpine chipmunks have lost half of their genetic diversity. The populations living at lower elevations have not migrated to higher elevations, they've "winked out," Chow said.
"The assumption in national parks has been if you don't do anything to grossly manipulate the habitat, whatever was originally here will continue to persist," Chow said. "With Grinnell's work, we're seeing that is not necessarily true."
Some park managers are at a loss of how to apply these insights to their mission of conserving biological diversity.
"We're seeing changes in areas that have the highest level of protection," said Steve Thompson, a Yosemite branch chief wildlife biologist. "I'm not sure what we can do to improve the chances of survival of species that are vulnerable to climate change."
While some species appear to be in trouble, Moritz said the surveys underline the importance of national parks. Yosemite has not lost any species yet because the park has provided the large habitats spanning the environmental gradient for animals to move as the climate has changed.
"Yosemite is doing its job protecting diversity," Moritz said. "These protected landscapes do matter, and they give the best chance for survival."
Click here to read the report.
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