Population ecology of the desert tortoise at Goffs, California /

Abstract

In 1982 SCE announced changes in its energy policy, and the intention to promote development of solar, wind and geothermal energy sources. These land- intensive technologies raised issues associated with loss of habitats of various species of desert plants and animals. One of these species was the desert tortoise. Over the past 10 years this species has increased dramatically in social importance--not only in California, but also throughout its range in the western U.S. (including parts of Arizona, Nevada and Utah). State and federal agencies have supported studies of local distribution, abundance and habitats. These studies have shown that some habitats of the desert tortoise have been and are iintinuing to be seriously disturbed by human activities. Strong circumstantial evidence suggests that the abundance of the tortoise has declined in many parts of its range. Concern for the tortoise in southwestern Utah led to its listing as Threatened under the Endangered Species Act in 1980. In September 1984 the Defenders of Wildlife, Natural Resource Defense Council, and Environmental Defense Fund petitioned the U.S. Fish and Hildlife Service to list the tortoise as Endangered throughout its range in Arizona, California and Nevada. In December 1985 the Service determined that listng was warranted so the tortoise will remain a species of social and biological importance in California. Causes of tortoise habitat disruption are manifold. One source of impacts--both past and present--is construction and operation of energy facilities. Development and transmission of energy involves building and maintenance of gas and water pipelines, transmission and distribution lines, roads, railroad spurs, wells and power plants. The desert tortoise played a conspicuous role in mitigation measures and research prescribed by the State Energy Commission in its 1983 decision regarding certification of the Solar 100 facility in Johnson Valley. v The planning ofmlongetermistudies of the desert tortoisememphasized belief in the value of fundamental research relating to the population ecology of the species. Hhile it was understood that basic research would not necessarily enable the prediction of effects of mitigating actions, it was believed that such investigations would provide an improved basis for planning specific mitigation-related experiments. The Goffs project, dealing with tortoise reproduction and survival, and the integration of these processes into a life- table, was developed in keepng with these convictions. This report is the last in a series of four, and summarizes four years of work near Goffs, in eastern San Bernardino County, California. Rates of tortoise reproduction and survival were measured over this interval, and average rates of births and deaths used to construct a life-table. The table developed in this manner showed that the Goffs population is increasing at a rate of 22 per year. One result of simulation experiments with the life-table was the demonstration of how difficult it is to_predict changes in the well-being of populations whose members may live for 80 years or more. Destructive effects may not be clearly discernible for many years. On the other hand, our findings suggest that short-term negative influences--if reversible--will not be dangerous to the long-term perpetuation of the species. Irreversible deterioration of environments will, however, eventually lead to local extinctions.