Birds and small mammals are found at different elevations in the Sierra Nevada Mountains today compared to a century ago.

Animals live within geographical areas or “ranges” where climatic and environmental conditions are suitable for their reproduction, growth and survival. One way species may respond to warmer temperatures and changes in precipitation is by moving to more favorable areas. Globally, these movements (called “range shifts”) have often occurred towards the poles and, on land, to higher elevations. A landmark study conducted over a century ago (known as the Grinnell Survey) provides a historical baseline for examining changes in habitat conditions as well as in the distribution and abundance of birds and small mammals in selected regions of California. For more information, download the Small Mammal and Avian Range Shifts chapter.

The alpine chipmunk (left) has disappeared from lower elevations in the Sierra Nevada while the fox sparrow (middle) has expanded to higher elevations. The bushy-tailed wood rat (right) has shrunk its range in the Northern and Central Sierra Nevada, but not in the Southern Sierra.

Credit: US National Park Service and US Fish and Wildlife

• Almost 75 percent of small mammal species and over 80 percent of bird species in three study regions of the Sierra Nevada live at different elevations today compared to a century ago.
• Some species have expanded their range by moving to higher or lower elevations. Other species have contracted their range by moving out of elevations they had historically occupied. Some did both.
• Among mammals, upward shifts were more than twice as likely as downward shifts. These shifts were equally likely among birds.

• Species that are not able to migrate fast enough to suitable environments may decline in population – or, in extreme cases, face local extinction.
• Although range shifts benefit the animals that find more suitable environmental conditions, these redistributions can disrupt the ecosystem. The loss of a species from one area and its introduction into another can alter predator-prey interactions and competition for food and other resources. These changes can ultimately lead to a decline in biodiversity.
• Monitoring species movement helps scientists better understand how small mammals and birds are affected by climate change, to guide conservation efforts.

• Changes in temperature or precipitation can cause species to respond by shifting the boundaries they occupy. Animals that can tolerate warmer and more variable climatic conditions are expected to adapt better.
• Species respond to changing conditions differently due to inherent biological factors and predator-prey interactions. Some species may also seek refuge in areas such as campgrounds where food and water are available, allowing them to live through a period of unfavorable conditions.

• The Mojave Desert lies south of the cooler and wetter Great Basin Desert. Over the past century, both deserts have warmed substantially, but the Mojave has become drier while the Great Basin has become wetter. Sixty percent of bird species shifted their ranges in both deserts, although birds in the Mojave but did not expand into the Great Basin.
• As the Mojave Desert became warmer and drier over the past century, bird communities collapsed while small mammal populations remained stable. Birds suffered from dehydration and were unable to cool themselves. Mammals, on the other hand, found shelter in cooler spaces such as underground burrows.
• In a resurvey of Grinnell’s study sites in California’s Central Valley, changes in the distribution of bird species since the early 20th century were found to be strongly affected by water availability, which is influenced by both precipitation and land use.

In the Mojave Desert, populations of (from left to right) turkey vultures, chipping sparrows, mourning doves declined in the past century.

Credit: US Fish and Wildlife

• UC Berkeley, Museum of Vertibrate Zooology, The Grinnell Resurvey Project