Trends in site occupancy by California red-legged frog (Rana draytonii) and American bullfrog (Rana catesbiana)

Background/Question/Methods

Aquatic habitats across the American west have been altered by urban development, unprecedented drought, introduced species, and wildlife disease.  These sources of habitat modification occur across the 8180 acres owned by Stanford  University, California, USA.  Creeks, reservoirs, and constructed wetlands span a rural to urban gradient from the Santa Cruz Mountains to Silicon Valley.  California red-legged frog (Rana draytonii) is a federally threatened species native to these aquatic habitats and hypothesized to be threatened by the invasive American bullfrog (Rana catesbiana).  To determine the status and population trend of these two species, aquatic habitats across Stanford University lands were surveyed twice annually from 1997 to 2014 for the presence of California red-legged frog (Rana draytonii) and American bullfrog (Rana catesbiana).  

Results/Conclusions

Occupancy models indicate that site occupancy by R. draytonii declined over time while R. catesbiana occupancy has fluctuated widely. Habitat type and availability, disease, or invasive species predation may have caused the disappearance of R. draytonii from some aquatic sites within the historic range of R. draytonii on Stanford University lands.