Survey and Assessment of Amphibian Populations in Rocky Mountain National Park

Abstract

We conducted surveys in Rocky Mountain National Park, Colorado for amphibians in 1987-1994. Four species, Ambystoma tigrinum, Bufo boreas, Pseudacris maculata, and Rana sylvatica, were recorded. Pseudacris maculata was the most widely distributed and abundant species in the Park. Two populations of E maculata were estimated to contain 161 and 136 breeding males in 1988. There was no evidence of a decline of A. tigrinum or R. sylvatica, but these species were found at relatively few locations. We did not detect Rana pipiens, which had been known previously from 3 locations in the Park. We found 7 breeding populations of B. boreas, which has declined recently elsewhere in the southern Rocky Mountains, but all but 2 of these populations were small and may not reproduce annually. At least one of these small populations is thought to have been extirpated. Estimated numbers of males in the 2 large populations, which are 6.4 km apart in the same drainage, were stable or increasing slightly from 1992 to 1995, averaging 189 and 239 individuals. Current and known locations of amphibians did not differ in elevation, size, lake type, presence of shallow water or emergent vegetation on the north shore, or presence of trout. Water chemistry at amphibian breeding sites was variable, but pH decreased significantly with increasing elevation. Causes of declines of B. boreas and R. pipiens are not known. Populations of B. boreas in the North Fork of the Big Thompson River are critically important to the conservation of this species in the Rocky Mountains. Declines and extinctions of amphibian populations throughout the world are receiving considerable attention (Richards and others 1993; Blaustein 1994; Pechmann and Wilbur 1994; Pounds and Crump 1994; Pyke and Osborne 1996; Green 1997). In the western United States, recent declines in abundance and distribution of several pond-breeding amphibians have been identified (Corn and Fogleman 1984; Bradford 1989; Carey 1993; Corn 1994; Drost and Fellers 1996; Fisher and Shaffer 1996). Declining amphibian populations in the southern Rocky Mountains are particularly alarming because they occurred within the past 20 yr and in areas still considered pristine. However, except where habitat destruction has been observed, reasons for declines in amphibian populations are not well understood. The rapidity and magnitude of declines of some species emphasizes the need for baseline data on distribution and abundance of amphibians. Unfortunately, historical data on montane amphibians is often fragmentary or anecdotal, and life history information is also lacking or poorly understood. This lack of information confounds interpretation of data on trends in population size or distribution, and it makes it difficult to identify the factors causing declines. Corn and others (1989) determined that 2 1 Mailing address.