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

This paper documents changes in the size of a breeding population of Y osemite 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 19 83 to 1991. Six other populations in the eastern Sierra Nevada were su rveyed occasionally from 1973 to 1990. The total number of marked male s entering the largest breeding pools at Tioga Pass declined nine-fold from 1974 to 1982. The mean number of toads found in daily searches d uring breeding also dropped over the 20 year study. Similar declines o ccurred at smaller breeding sites throughout Tioga Pass Meadow and at the six other populations. In 1990 we found evidence for continued rep roduction by B. canorus at all but one location (Sylvester Meadow), al though reproduction was minimal at Tioga Pass. No single source of mor tality 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 th e 62 year average, resulting in low tadpole survival. The death of adu lts from disease at Tioga Pass, and from predation at another site (Mi ldred Lake) also contributed to the declines. Although habitat destruc tion and localized human activities were not major factors, some natur al variables responsible for the declines may have had anthropogenic c omponents.