Monitoring Desert Tortoise Survivorship Through Use of Radio Telemetry [Proposal]

Douglas, Charles; Longshore, Kathleen

In recent years, desert tortoise (Gopherus agassizit) populations throughout the Mojave Desert have declined, mostly as a consequence of habitat loss and degradation, human-caused mortality (accidental or intentional) and URTD (Groombridge 1982, Luckenbach 1982, Berry 1984). As a result, they were officially listed in 1990 as "threatened" by the U.S. Fish and Wildlife Service (U.S. Fish and Wildlife Service 1993}. Effective management of this species requires knowledge of desert tortoise biology, including a thorough understanding of demographic parameters of existing populations. Desert tortoises are long-lived animals. Individuals do not reach sexual maturity until 12-25 years of age (Woodbury and Hardy 1948, Turner et al. 1987}, estimated life expectancy is greater than 40 years (Germano 1992, Berry and Woodbury 1984). Recruitment rates are low due to low survival of eggs (Turner et al. 1986, Lampkin 1966) and hatchlings (Vaughan 1984, Baxter 1987). Estimates of population growth are less than 0.5% per year (Turner et al. 1986). The life history strategy of the desert tortoise allows the species to persist in environments where resource availability is unpredictable and as a consequence, even moderate downward fluctuations in adult survival rates can result in rapid population declines (Stearns 1976). These characteristics make the long-term survival of reproductively active adults important for maintenance of viable populations. For development of demographic models and appropriate management techniques, it is particularly important to understand variations in survivorship and mortality rates of adults and how year-to-year differences are influenced by environmental variables. The following is a proposal designed to establish methodologies for long-term monitoring of desert tortoise populations and generate critical information on demographic parameters and dispersal.