Evidence of decline for Bufo boreas and Rana luteiventris in and around the northern Great Basin, western USA.

Abstract/Summary

Revisiting historical amphibian sites can provide a relatively fast assessment of amphibian status in a region. Between 2000 and 2003 we repeatedly surveyed historically documented sites for two species of concern in the northern Great Basin: the western toad (Bufo boreas), and the Columbia spotted frog (Rana luteiventris). Both of these species have experienced population declines in other parts of their ranges but their status was relatively unknown in our study area. We estimate that B. boreas occupies 49.5% (34.1% - 65.0%) of 34 historical sites and that R. luteiventris occupies 52.9% (43.5% - 62.3%) of its 30 historical sites. B. boreas was more likely to be detected at human-altered sites while R. luteiventris was more likely to be detected at sites that were deeper than other sites in our study. B. boreas was detected at three of 41 (7.3%) potential sites (located near historic sites) while R. luteiventris was detected at six of 16 (37.5%). At 187 randomly selected sites, B. boreas was detected at one and R. luteiventris at three sites. Given that the number of historical sites available for resurveys was small, these results should be interpreted with caution. Moreover, a species can shift its distribution away from historical sites due to habitat succession or metapopulation dynamics without necessarily declining. We suggest that there is sufficient evidence to conclude that B. boreas and R. luteiventris have declined in and around the northern Great Basin. However, relatively high occupancy of Potential Sites by R. luteiventris suggests they may not have declined as much as B. boreas in the region we surveyed.