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Climate change threatens California bighorn sheep

| 11 February 2004

 






A study led by Berkeley researchers has linked population declines among California’s desert bighorn sheep with the effects of climate change. What’s more, many of the remaining bighorn-sheep populations in the state could face extinction if certain global-warming forecasts for the next 60 years come true.

In the study, which is published in the current issue of Conservation Biology, the authors found that of 80 groups of desert bighorn sheep (Ovis canadensis nelsoni) known to have roamed California’s mountains over the past century, 30 are now extinct.

In their investigation of the population decline, the researchers evaluated impacts ranging from contact with domestic livestock (which can lead to the spread of disease and competition for food) to poaching, mining and other human-caused habitat disturbances, and additional factors. They also analyzed climatic variables, such as temperature and precipitation, that affect the availability of vegetation and dependable sources of spring water for the sheep.

“Climate was consistently correlated with extinction in a way the other factors weren’t,” said Clinton Epps, a doctoral student in environmental science, policy and management at the College of Natural Resources (CNR) and lead author of the paper. “The harsh environment inhabited by desert bighorn sheep already has them walking on a knife’s edge. It doesn’t take too much to push them off. The bottom line is that more than one-third of the populations that were once known are now gone.”

Between 1901 and 1987, the mean annual temperature in the deserts of the southwestern United States increased by about 1.8 degrees Fahrenheit, which is considered significant by climatologists. In addition, annual precipitation dropped about 20 percent in southeastern California over the last century. According to the study, groups of bighorn sheep were more likely to be lost in lower-elevation mountains, where there were higher average temperatures and less precipitation.

The authors examined population data on the state’s desert bighorn sheep that had been collected since 1940 by biologists and California Department of Fish and Game researchers. They also used historical records from local areas where the sheep were known to have lived, but had since disappeared.

Desert bighorn sheep live primarily in small, isolated groups throughout the mountain ranges of the Sonoran, Mojave and Great Basin deserts of the southwestern United States. They are also found in the Transverse and Peninsular mountain ranges of southwestern California. The species has been the focus of concern among conservationists since they were first protected by California legislation in 1873. Statewide, the population of desert bighorn is estimated at 3,500. Desert bighorn in the Peninsular Ranges (as well as a separate bighorn subspecies in the Sierra Nevada) are listed as endangered by state and federal agencies.

“When you start losing bighorn sheep from some mountain ranges, it affects the collective population,” said Dale McCullough, professor of ecosystem sciences in CNR and co-author of the paper. “Any decline in vegetation makes it more difficult for sheep to move among the different habitat clusters, which have become fewer and more spread out. Where they used to be able to move more readily to other mountain ranges to help repopulate or recolonize a habitat area, they must now cross an intervening desert.”

McCullough noted that effects on populations from global warming are easier to detect in bighorn sheep than in species of other large mammals that are more uniformly distributed over the landscape. “Other large mammals may shift north more gradually as a result of global warming, but that trend is harder to detect in the short term,” he said.

The authors also looked at predictions for how climate may change in the next 60 years. For scenarios that predict a minimum temperature increase of 1.3 degrees Fahrenheit, there is no significant increase in the average probability of extinction.

However, in scenarios that predict a more serious temperature increase of 3.6 degrees Fahrenheit and a 12-percent decrease in precipitation, the probability of extinction increases by half, from a baseline of 20 percent to 30 percent in the next 60 years. Epps points out that this increase is an average of the extinction probability calculated for each group remaining in southern California. The risks for some individual groups, particularly those that live at lower elevations, are significantly higher.

“Our study illustrates how sensitive certain populations can be to changes in climate, whether man-made or not,” said Epps. “Cases like this give conservationists some ammunition when talking about the importance of controlling global warming.”