INVASIVE SPECIES REMOVAL INCREASES THREATENED CALIFORNIA RED-LEGGED FROG ABUNDANCE AND ALTERS THE DIET OF ENDANGERED SAN FRANCISCO GARTERSNAKES

Richard Kim; U.S. Geological Survey, Western Ecological Research Center; rkim@usgs.gov; Brian J. Halstead, Eric J. Routman, Michael L. Casazza, Julie Andersen

Non-native invasive species often have detrimental effects on native ecosystems and species. Eradicating introduced species, however, can disrupt established ecosystem processes. We examined the effects of eradicating introduced fish and American bullfrogs on threatened California red-legged frogs (Rana draytonii) and one of their important predators, endangered San Francisco Gartersnakes (Thamnophis sirtalis tetrataenia). We monitored an R. draytonii population between 2014 and 2015 during and after the invasive species were removed, and assessed if potential increase in R. draytonii abundance also increased their availability as prey for T. s. tetrataenia. We observed 0.18 (95% highest posterior density interval, 0-0.89) adult R. draytonii per survey in 2014, but 6.98 (4.82-8.82) per survey in 2015. Although no recruitment occurred in 2014, we estimated the peak mean density of R. draytonii metamorphs to be 1.98 (1.86-2.12) individuals/m2 in 2015. Twenty two percent of T. s. tetrataenia samples in 2015 had consumed R. draytonii, but none had in 2014. Reference ponds without exotics decreased in R. draytonii abundance over a similar period. Removal of bullfrogs and invasive fish directly facilitates the restoration of R. draytonii populations and may increase their availability as prey for the endangered T. s. tetrataenia.

Challenges and Conflict in Vertebrate Predator Management