POPULATION DECLINES OF YOSEMITE TOADS IN THE EASTERN SIERRA-NEVADA OF CALIFORNIA

This paper documents changes in the size of a breeding population of Yosemite toads, Bufo canorus, over 20 years at Tioga Pass, California. Surveys of & canorus numbers in breeding aggregations were made yearly from 1971 to 1982, and less systematic observations were made from 1983 to 1991. Six other populations in the eastern Sierra Nevada were surveyed occasionally from 1973 to 1990. The total number of marked males entering the largest breeding pools at Tioga Pass declined nine-fold from 1974 to 1982. The mean number of toads found in daily searches during breeding also dropped over the 20 year study. Similar declines occurred at smaller breeding sites throughout Tioga Pass Meadow and at the six other populations. In 1990 we found evidence for continued reproduction by B. canorus at all but one location (Sylvester Meadow), although reproduction was minimal at Tioga Pass. No single source of mortality was responsible for all the declines. We interpret the decline in our study populations as caused primarily by drought, disease, and predation. Spring snow depths in 1976,1977 and 1987-1991 were below the 62 year average, resulting in low tadpole survival. The death of adults from disease at Tioga Pass, and from predation at another site (Mildred Lake) also contributed to the declines. Although habitat destruction and localized human activities were not major factors, some natural variables responsible for the declines may have had anthropogenic components.