Small Mammal Populations in Yosemite

Feeling the Squeeze

The earth is changing. Every day we are told as we sit in our cars on our way to work, pumping collective tones of carbon dioxide and myriad other greenhouse gases and pollutants into the atmosphere. The climate is shifting and our industrial lives are certainly not putting the brakes on the process. By now anyone who can read a newspaper or turn on a television probably knows we are undergoing a period of global warming trend, and while this might bring to mind the cataclysmic melting polar icecaps and plunging into another ice age there are very different icecaps at risk as well.

In the mountainous environment temperatures generally drop as elevation increasing which is why we can have snow capped mountains even in a warm place like California. The Sierra Nevada are one such mountain environment and in Yosemite National Park the warming trend is becoming apparent. When one thinks of a natural park, often the forests of imagination are populated by a multitude of squirrels, chip-munks, and other small animals and Yosemite is home to a wide variety of such small mammal species. These species live in a wide variety of habitats, not just at the foot of the mountains but all the way from sea level to 3300 meters, high up on the flanks of the mountains.

While global warming has caused the range of the lower level species to expand, the reduction in habitat range of those species residing in higher elevations is much less studied and with this in mind a group of scientists from the University of California, Berkeley and Colorado State University set out to investigate. Led by Craig Moritz of the Department of Integrative Biology at U of C the team took to the hills.

Their aim was to investigate a century’s climate change and the resulting impact on the small mammal populations of Yosemite National Park. To do this they made use of the survey data compiled between 1914 and 1920 by Joseph Grinnell and colleagues which sampled the region’s biodiversity. With this extensive body of historical data as a record of the region’s native animal life, the team could reevaluate the survey using modern techniques and compare their results with those from 1920. In this way they were able to quantify a century’s worth of change on the range of those species in Yosemite.

The team captured specimens at one hundred and thirty three sites and looked at over forty species of small mammals on both the east and west slopes of a transect which covered and elevation range from almost sea level up to more than three thousand meters above sea level. By using the same area as Grinnell and trapping in the same regions they increased the accuracy of their findings. They also made use of trapping records in the area to aid in occupancy modeling to estimate the probability of a species being detected at any one site. The best model of a set of thirty six was then used to calculate the probability that a species should have been detected where it may not have been due to chance. Using these statistics in combination with hypothesized relationships between occupation, time, and elevation the team put together occupancy – elevation profiles using the averages of their extensive and thorough statistical analysis and modeling. These profiles were used to identify changes in habitat range of the species being observed.

The results were consistent with the estimates warming of approximately three degrees Celsius over the past hundred years. When taking into account the average temperature change of six degrees per kilometer rise in elevation the overall upward trend of about five hundred meters is right where it would be predicted. Some forty percent of the west slope species exhibited large shifts of lower habitat limits. ON average the range of those species shrunk upwards by about four hundred and seventy five meters in elevation. But in contrast only fourteen percent of the species’ upper habitat ranges expanded. The higher elevation animals generally saw a range decrease, fifty percent of the total mid - high range species lower range contracted. And fifty percent of the lower elevation species had their habitat range shift upwards.

The survey also found that closely related species did not respond the same to the change in temperatures. The scientists observed that apart from the original elevation inhabited, life history and ecological traits were not very good predictors of which species would have their lower ranges contracted. This was especially true of the higher elevation species whose lower ranges shrunk.

While the warming trend is not harming the biodiversity of Yosemite, it is a serious problem for some of the species of small mammals which are losing ground up in the mountains. As their habitable ranges continue to shrink these small animals are being confined to ever smaller areas. Some are endemic to the high Sierra like the Alpine chip-munk and need to be carefully observed. With the protection of large-scale elevation gradients some of the species can migrate in response to climate shifts. As the paper points out, the importance of protected landscapes has never been greater.

References:

Moritz, C., Patton, J.L., Conroy, C.J., Parra, J.L., White, G.C. and Beissinger, S.R. 2008. Impact of a century of climate change on the small-mammal communities in Yosemite National Park, USA. Science, 322: 261-264.