An emerging disease and non-native fish predation illustrate complexity of restoration efforts for endangered amphibians

Amphibian declines are occurring worldwide and many potential causes have been identified, yet there is still little known about the relative importance of various factors. In the Sierra Nevada, California, the endangered mountain yellow-legged frog (Rana muscosa) is declining. This frog occurs in naturally fishless alpine lakes and streams; however, more than a century of misguided management has resulted in a landscape dominated by introduced fish. Previous research showed that non-native fish removal resulted in rapid recolonization and recovery of frog populations. Just as management agencies began restoration efforts on a larger-scale, mass die offs and local extinctions of R. muscosa were discovered. Frogs had succumbed to chytridiomycosis, a disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd). This study examined whether the persistence of Bd outside its host, R. muscosa, would preclude frog re-introductions to extinction sites. We introduced uninfected tadpoles into extinction sites associated with chytridiomycosis (n=5 sites; 60 tadpoles /site). We collected the tadpoles after 10 weeks of exposure and used real time PCR to determine if they were infected. Tadpoles from extinction sites (n=5) did not become infected, nor did our negative controls (n=2) yet in our positive control (n=1) all but one tadpole was infected, suggesting that Bd will become extinct in the absence of a host. However, because lab studies show that tadpoles can be harder to infect than frogs, we repeated the experiment in 2005-2006 by releasing 50 frogs and 100 tadpoles each into 3 extinction sites. We found no infection after the first year, but in the second year Bd positive animals were discovered at all three sites. Thus, the pathogen is either able to survive for long periods of time without an amphibian host, or it is able to quickly reinvade remote areas.