ISOLATION OF REMAINING POPULATIONS OF THE NATIVE FROG, RANA-MUSCOSA, BY INTRODUCED FISHES IN SEQUOIA AND KINGS CANYON NATIONAL-PARKS, CALIFORNIA

Rana muscosa (mountain yellow-legged frog) was eliminated by introduced fishes early in this century in many of the lakes and streams in Sequoia and Kings Canyon National Parks, California. In waters not inhabited by fish, however, R. muscosa has disappeared from many sites within the parks during the past 30 years, and it appears to have gone extinct in some drainage systems. Fragmentation of populations may have caused or contributed to these recent extinctions, because R. muscosa populations are significantly more isolated from one another by fish at present than in prestocking conditions. A total of 312 lake-sites in 95 drainage basins were surveyed for amphibians and fish in 1989-1990. For the 109 sites containing R- muscosa, we delineated networks of sites connected to one another via fishless streams, and we compared these present fishless networks (''present networks'') to those expected for the same sites assuming that fish bad not been introduced to the parks (''former networks''). Most present networks consist of only one site (mean = 1. 4), whereas the former networks, average 5.2 sites. This difference represents approximately a 10-fold difference in connectivity of populations, which is defined is the mean number of potential dispersal links (fishless streams) per network. Connectivity averages only 0.43 in present networks, in contrast to 4.15 in former ones.