28th Annual Meeting and Symposium of the
Desert Tortoise Council, February 21-23, 2003
Abstracts

Comparison of Sonoran and Mojave Tortoises: Is the Sonoran Desert Tortoise Likely to be Listed, and if so, What Can We Learn from the Mojave Experience

Roy C. Averill-Murray
Arizona Game and Fish Department, 2221 W. Greenway Rd. Phoenix, AZ 85023

The U.S. Fish and Wildlife Service (Service) listed the Mojave population of the desert tortoise as Threatened under the Endangered Species Act in 1990, but in 1991 found that listing of the Sonoran population was not warranted. Nevertheless, the Service and the Arizona Game and Fish Department (Department) have considered the species to be of special concern since that time. The Arizona Interagency Desert Tortoise Team (AIDTT), co-chaired by the Service and Department, published a management plan for the Sonoran population in 1996 and issued an updated status report in 2000. Population monitoring between 1987 and 2000 indicated that Sonoran tortoise populations in Arizona were generally stable. However, several populations showed dramatic declines in 2001 and 2002 surveys.

While some apparent declines or their degrees may be a result of changes in population estimation methodology, high proportions of carcasses have been recovered relative to live tortoises found at some populations. Reasons for declines vary between sites, including natural mortality from mountain lions and potentially disease- and human-related causes, but are not apparent in all cases. Comparison of factors that resulted in listing of the Mojave population indicate that most threats are still less severe in the Sonoran population. Current threats to Sonoran tortoises include disease - upper respiratory tract disease (although no populations with high incidence of signs have been found) or cutaneous dyskeratosis (present in some proportion in nearly all populations); invasion of exotic plants and increased fire hazards; predation by feral dogs in local areas; fragmentation of populations by development, road, canals, etc.; and natural causes, such as drought.

Agencies have implemented the AIDTT's management plan to varying extents, but the lack of specific goals, objectives, or commitments has prevented the plan from achieving maximal success. Therefore, the AIDTT is leading an effort to develop a State Conservation Agreement to provide a more coordinated commitment to Sonoran tortoise management. Such an agreement will provide cost benefits through pro-active conservation and will help insure that the Sonoran population does not reach the dire straights for which the more intensive regulation of the Endangered Species Act becomes necessary.