Concord Naval Weapons Station (CNWS) has served as a military installation since 1942 and has been known to support special-status amphibian species for many years. The California red-legged frog (Rana aurora draytonii), federally listed as threatened, has a long history of occurrence at CNWS. In the early 198O?s, this species was thought to have little chance for survival into the future at CNWS. At the time, non-native bullfrogs (Rana catesbeiana) were considered abundant at CNWS. Area-restricted visual encounter surveys at CNWS from August 1998 to August 1999 showed California red-legged frogs were abundant and have essentially replaced the bullfrog at all ponds in which ranid frogs are present. The California tiger salamander (Ambystoma californiense), a California species of special concern and a federal candidate for listing, is also present at CNWS, but prior to our study, its distribution across the facility was not known We found this species in five ponds and at many upland areas of CNWS. Our surveys showed the California red-legged frog and California tiger salamander do not inhabit the tidal portion of CNWS. The sustained presence of California red-legged frogs and California tiger salamanders at CNWS is likely due to a combination of factors including absence of aquatic predators, low occurrence of migration barriers, presence of upland refugia, connectivity between water resources, and low levels of human disturbance relative to urban areas that border CNWS.
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