Wild Sheep of North America
ENDANGERED SPECIES PROFILE
WHO? Peninsular bighorn sheep (Ovis canadensis nelsoni)
WHAT? Listed as a federally endangered species
WHEN? March 18, 1998
WHERE? In the United States (from Palm Springs, CA south to the U.S./Mexico international border)
WHY? Substantial population declines due to disease, predation, habitat loss, and human disturbance
Summary of endangered listing
Six years after they were originally proposed for endangered listing, on March 18, 1998, the United States Peninsular Ranges population of desert bighorn sheep was federally listed as an endangered species pursuant to the Endangered Species Act (ESA) of 1973, as amended. The U.S. Fish & Wildlife Service (overseers of the Federal ESA) determined that Peninsular bighorn sheep are in danger of extinction throughout a significant portion of their range due to:
insufficient lamb recruitment
habitat loss, degradation, and fragmentation by urban and commercial development
predation coinciding with low population numbers
Peninsular bighorn habitat and numbers
Peninsular bighorn sheep inhabit dry, rocky, low-elevation desert slopes, canyons, and washes from the San Jacinto and Santa Rosa mountains near Palm Springs, California south into Baja California, Mexico. These sheep are known as low elevation bighorn because they use habitat from 400' - 4,000' elevation. Within the U.S., Peninsular bighorn are distributed in a metapopulation structure (a group of subpopulations linked by the movement of a limited number of animals) comprised of at least 8 demes or subpopulations. Helicopter surveys conducted in the fall of 2016 indicated that approximately 780 Peninsular bighorn inhabit the U.S. and the most recent surveys of Mexico estimate the Baja California Peninsular bighorn population at 2,000-2,500 bighorn. In the 1970s, Peninsular bighorn sheep were estimated to number nearly 1,200 in the U.S. and 4,500-7,800 in Baja California. Because there are currently considerably more animals in Mexico than in the U.S., and the Mexican government has established a new conservation program for managing Peninsular bighorn in Mexico, only the U.S. population was included in the endangered listing.
Reasons for the listing
Peninsular bighorn sheep have been listed under the California State Endangered Species Act (ESA) since 1971, but they continued to decline despite management plans, research, habitat acquisition, and the establishment of ecological reserves. A disease epizootic followed by several years of low recruitment (at least in most portions of the range) resulted in a vast population decline within the U.S. In the 1990's, heavy mountain lion predation accompanied by variable lamb recruitment rates continued to suppress bighorn populations. While predation is not typically a concern for healthy bighorn populations, it can severely impact those already debilitated by disease, habitat loss, or low numbers. Furthermore, habitat loss for Peninsular bighorn sheep has occurred at an alarming rate.
In a portion of their range, Peninsular bighorn sheep are also exposed to dangers at the urban-mountain interface including automobile collisions, poisonous plants, high predator densities, and parasites. In the northern Santa Rosa Mountains between 1991 and 1996, Bighorn Institute documented five bighorn struck and killed by cars, five bighorn killed by ingesting poisonous ornamental plants, and one bighorn strangled in a wire fence. During this six-year study, urbanization accounted for 34% of the adult bighorn mortalities, which made it the leading known cause of death for bighorn in this area. Urbanization in bighorn habitat also indirectly affects bighorn sheep by altering their habitat use, diet and behavior. These changes make bighorn more prone to parasites and disease, and potentially more vulnerable to predators.
What the endangered listing should accomplish
Listing of the species requires recovery actions and it prohibits activities that harm the species. Federal, State, and private biologists wrote a detailed recovery plan which describes actions needed for recovery, and establishes interim and long-term goals and objectives for the restoration of Peninsular bighorn. Ultimately, the goal of the recovery plan is to "delist" or remove Peninsular bighorn from the endangered list. The recovery plan was released in November 2000.
Most importantly, endangered species status provides greater habitat protection. For example, under the ESA, the U.S. Fish & Wildlife Service can modify a development project if it is determined that the project threatens the viability of Peninsular bighorn. Additional funding for habitat acquisition may also become available.
By recognizing the imperiled status of the species, the listing also encourages conservation actions by Federal, State, and private agencies, as well as individuals. Peninsular bighorn sheep are a natural part of southern California's heritage and are important culturally and economically. Recognition as a federally listed endangered species is a significant step toward the recovery of Peninsular bighorn sheep, but the struggle is not yet over. Research, habitat acquisition, population augmentation, and of course, public support must continue in order for the goals of the recovery plan to be met.