MASS MORTALITY AND EXTINCTION IN A HIGH-ELEVATION POPULATION OF RANA-MUSCOSA

Rana muscosa is one of several high-elevation amphibians that have recently disappeared from seemingly pristine sites. The present study documents an event of mass mortality among larval and metamorphosed R. muscosa in a lake in Kings Canyon National Park, California, and the ultimate extinction of the population. In 1979 metamorphosed individuals declined from ca. 800 individuals in early summer to nearly zero in late summer. During this time many carcasses were collected, individuals showed symptoms of red-leg disease, and blood from an affected individual contained the bacterial pathogen characteristic of this disease, Aeromonas hydrophila. Also during the summer of 1979, nearly all of the approximately 1100 tadpoles began metamorphosis, but all metamorphosing individuals were consumed by Brewer's blackbirds (Euphagus cyanocephalus). This population of R. muscosa continued to exist until at least 1983, but was extinct by 1989. Recolonization of the site will probably never occur because streams connecting to extant populations of R. muscosa now contain introduced fishes.