On higher ground, chipmunk gene pool shrinks

Global warming has forced alpine chipmunks to higher ground, prompting a decline in their genetic diversity. The study, published in the journal Nature Climate Change, is one of the first to show a hit to the genetic diversity of a species because of a recent climate-induced change in the animals’ geographic range. What’s more, the genetic erosion occurred in the relatively short span of 90 years, highlighting the rapid threat that changing climate can pose to a species. With low genetic diversity a species can be more vulnerable to the effects of inbreeding, disease, and other problems that threaten species survival, the University of California, Berkeley, researchers say. “Climate change is implicated as the cause of geographic shifts observed among birds, small mammals and plants, but this new work shows that, particularly for mountain species like the alpine chipmunk, such shifts can result in increasingly fragmented and genetically impoverished populations,” says study lead author Emily Rubidge, who conducted the research while a doctoral student at UC Berkeley. “Under continued warming, the alpine chipmunk could be on the trajectory towards becoming threatened or even extinct.” The new findings build upon previous research that found major shifts in the range of small mammals in Yosemite National Park since the early 1900s. In 2003, biologists at UC Berkeley began an ambitious resurvey of Yosemite’s birds, mammals, reptiles, and amphibians, retracing the steps originally taken between 1914 and 1920 by Joseph Grinnell, founder and former director of the Museum of Vertebrate Zoology. Craig Moritz, an integrative biologist who contributed to the study, led the Grinnell Resurvey Project with museum colleagues. They found that many small mammals in Yosemite moved or retracted their ranges to higher, cooler elevations over the past century, a period when the average temperature in the park increased by 3 degrees Celsius, or about 5.4 degrees Fahrenheit. It is no surprise that the alpine chipmunk (Tamias alpinus) would be more sensitive to the temperature change, since it is a high-elevation species endemic to California’s Sierra Nevada, the researchers say. In the early 1900s, Grinnell and colleagues sighted alpine chipmunks at elevations of 7,800 feet. Now, the alpine chipmunk appears to be sticking to even higher elevations, retracting its range by about 1,640 feet upslope. To test the genetic impact from that loss of geographic range, researchers compared genetic markers from 146 modern-day alpine chipmunks with those from 88 of their historical counterparts. Samples were collected from seven paired sites throughout Yosemite. As a control, the researchers also looked at the genetics—both historic and modern—of lodgepole chipmunks (Tamias speciosus), a lower elevation species that had not changed its range over the past century. The analysis of genetic markers revealed a significant decline in “allele richness” among the recently sampled alpine chipmunk populations compared with their historic counterparts. Moreover, the researchers noted that the modern chipmunks were more genetically differentiated across sites than in the past, a sign of increased fragmentation in the alpine chipmunk population. In comparison, there were no significant changes in genetic diversity detected among the lodgepole chipmunks, a species found at elevations from 4,900 to 9,800 feet. “Much of what we read and hear about the effects of climate change on biodiversity is based on model projections and simulations, and these models typically involve many moving parts and lots of uncertainty,” says Justin Brashares, associate professor in the department of environmental science, policy, and management, who also worked on the study. “Thanks to the baseline provided by Joseph Grinnell’s pioneering efforts in the early 20th century, we are able to go beyond projections to document how climate is altering life in California. The research led by Emily is novel and important because it shows empirically that climate change has led to the loss of genetic diversity in a wild mammal over the last several decades.” Moritz adds that this study exemplifies how patterns of change in California’s ecosystems can be uncovered through analyses of fossil, historic, and modern records. “At the heart of this whole enterprise is the incredibly dense historic record and specimens we have at UC Berkeley from 100 years ago,” says Moritz. “These collections allow us to conduct sophisticated analyses to better understand how ecosystems are reacting to environmental changes, and to create more detailed models of future changes.” The Natural Sciences and Engineering Research Council of Canada, UC Berkeley’s Museum of Vertebrate Zoology, the Yosemite Fund, the National Geographic Society, and the National Science Foundation provided funding for the study, which included additional co-authors from UC Berkeley.