Implications of Fire History for Conserving Bighorn Sheep

By Vernon C. Bleich

Steep, rocky terrain is widely recognized as an important component of bighorn sheep habitat. Visual openness, however, also is important because it enhances the probability that bighorn sheep can detect and evade predators. In southern California, some populations of bighorn sheep occupy chaparral habitat, which in the absence of fire can become an almost impenetrable barrier and impacts the ability of bighorn sheep to detect predators. As chaparral vegetation matures, it produces forage of poorer quality than that available in similar, but recently burned, areas.

My colleagues and I accumulated springtime aerial survey data over a period of 25 years, during which time the physical structure of chaparral vegetation occupied by bighorn sheep changed substantially. Those changes occurred as a function of time elapsed since fires, but also as a result of recent fires. We used those observations to examine the importance of fire (which reduces visual obstructions caused by dense vegetation) to habitat selection by bighorn sheep in the San Gabriel Mountains of eastern Los Angeles County and western San Bernardino County.

Our research indicated that bighorn sheep in the San Gabriel Mountains selected sites that had burned within 15 years. We also used our results to quantify potential changes in availability of bighorn sheep habitat based on fire history. In that exercise, we identified 150 square miles of suitable habitat that existed in 2002 (when only 63 bighorn sheep were tallied), 188 square miles in 1980 (when the bighorn sheep population was at its highest level), and 163 square miles in 2004, just after a series of large wildfires. We also estimated that 238 square miles of otherwise suitable habitat would be available in a hypothetical situation if the entire study area burned.

Results of this study indicate that:

• In chaparral ecosystems, fire history is an important parameter explaining habitat selection by bighorn sheep in areas where other features (e.g., slope, aspect, ruggedness, etc.) are suitable for sheep.

• Restoration of bighorn sheep to their historical distribution in chaparral-dominated ecosystems of the transverse ranges of southern California will depend upon more frequent fires.

• The presence of treasured big game animals, such as bighorn sheep, should not be taken for granted. Their presence is the result of ecological processes and natural resource management. This study has important implications for the conservation of bighorn sheep, and the results should be incorporated into land management plans.

Logistical and financial support for this project (Bleich, V. C., H. E. Johnson, S. A. Holl, L. Konde, S. G. Torres, and P. R. Krausman. 2008. Fire history in a chaparral ecosystem: implications for conservation of a native ungulate. Rangeland Ecology and Management 61:571-579) was provided by the California Department of Fish and Game, U.S. Forest Service, Society for the Conservation of Bighorn Sheep, and the Los Angeles County Fish and Game Commission. Readers can request a copy of the original paper upon which this summary is based from Vern Bleich.

Trophy Points: Big Game Research On Line is complied and edited by David G. Hewitt, a Professional Member of the Boone and Crockett Club and the Stuart W. Stedman Chair for White-tailed Deer Research at the Caesar Kleberg Wildlife Research Institute.