Population dynamics and methodological assessments from a 15-year period of Amphibian monitoring in British Columbia's Southern Gulf Islands

With amphibian populations facing a multitude of threats, including habitat loss, climate change, invasive species and infectious diseases, it is important to identify valuable amphibian habitat and the imminent pressures these environments face. Between 2004 and 2019, 6 years of amphibian surveys were conducted at Greenburn, Roe and McLean lakes in the Southern Gulf Islands of British Columbia, Canada. We assessed (1) species composition and trends of native amphibians, including at-risk northern red-legged frog (Rana aurora); (2) observations of invasive American bullfrog (Lithobates catesbeianus); and (3) the efficacy of visual encounter and trapping survey methods in determining multi-species amphibian occupancy. The shallow, semi-ephemeral McLean Lake hosted more amphibian species and more breeding activity than the larger, deeper waters of Greenburn and Roe lakes. Despite multiple observations, bullfrogs have thus far not established a detectable population within these lakes, with the presence of native and introduced predators as potential contributing factors. Declining trends in occupancy of native populations of R. aurora, Pacific chorus frog (Pseudacris regilla) and rough-skinned newt (Taricha granulosa) were observed at all three lakes. Results varied within years by species and survey method, highlighting the importance of effective replication and employing complementary survey methods to optimize studies of amphibian occupancy. These observations also emphasize the value of shallow, small- to medium-sized waterbodies to native amphibian populations in the Southern Gulf Islands. As these waterbodies become increasingly threatened by global climate change and habitat degradation, the potential impacts of declining freshwater ecosystem health on amphibian populations should be considered.