PARENT SESSION
Oral Session #3: Conservation Ecology: Threatened and Endangered Species.
Presiding: J. Lawler
Monday, August 5. 8:00 AM to 11:30 AM. Graham Meeting Room, TCC.
Environmental factors impacting Yosemite toads in Yosemite National Park.
Sadinski, Walter*,1, Wilson, Michelle2, Cleaver, James3, 1 U. S. Geological Survey, La Crosse, WI2 Tetra Tech, Inc., Santa Barbara, CA3 University of California, San Francisco, San Francisco, CA
ABSTRACT- The Yosemite toad (Bufo canorus) inhabits only a small area of the Sierra Nevada in California. Reports suggest this species has declined in distribution and abundance. The U. S. Fish and Wildlife Service is considering listing it under the Federal Endangered Species Act. We studied Yosemite toads in Yosemite National Park from 1995 - 2001 to determine any evidence of declines or possible mechanisms for declines. We found over 60 breeding sites at elevations of 2600 - 3200 m between Tioga Pass and Tuolumne Meadows, typically in extremely shallow water. Only a few adults bred at most sites. Mortality among embryos was widespread and often dramatic. Larvae died often when hydroperiods were insufficient, but did not have detectable levels of pesticides. We did not observe widespread mortality among other life stages. Field observations and manipulative experiments showed mortality among embryos was due to freezing, water mold, and roundworms, but not ultraviolet radiation or pesticides. Freezing killed embryos, which allowed water mold to get established quickly on egg masses. The mold then spread over and killed otherwise healthy embryos before they could hatch. Roundworms often coincidentally killed large numbers of embryos. Reproductive success of Yosemite toads is particularly dependent upon freezing or shortened hydroperiods and should be monitored closely. Although mechanisms for decline did and do exist, it was not clear that the population of Yosemite toads in our study area was declining.
KEY WORDS: Yosemite toads, Yosemite National Park, freezing, water mold