Identifying drivers of population dynamics for a stream breeding amphibian using time series of egg mass counts

Abstract

The decline in amphibian populations is one of the starkest examples of the biodiversity crisis. For stream breeding amphibians, alterations to natural flow regimes by dams, water diversions, and climate change have been implicated in declines and extirpations. Identifying drivers of amphibian declines requires long time series of abundance data because amphibian populations can exhibit high natural variability. Multiple population viability analysis (MPVA) models integrate abundance data and share information from different populations to estimate how environmental factors influence population growth. Flow alteration has been linked to declines and extirpations in the Foothill Yellow‐legged Frog (Rana boylii), a stream breeding amphibian native to California and Oregon. To date, no study has jointly analyzed abundance data from populations throughout the range of R. boylii in an MPVA model. We compiled time series of egg mass counts (an index of adult female abundance) from R. boylii populations in 36 focal streams and fit an MPVA model to quantify how streamflow metrics, stream temperature, and surrounding land cover affect population growth. We found population growth was positively related to stream temperature and was higher in the years following a wet year with high total annual streamflow. Density dependence was weakest (i.e., carrying capacity was highest) for streams with high seasonality of streamflow and intermediate rates of change in streamflow during spring. Our results highlight how altered streamflow can further increase the risk of decline for R. boylii populations. Managing stream conditions to better match natural flow and thermal regimes would benefit the conservation of R. boylii populations.

Chytrid infections exhibit historical spread and contemporary seasonality in a declining stream-breeding frog

“…The Feather River watershed is characterized by a unique combination of dammed and free-flowing streams with frogs present in an area that has experienced intense wildfires [ 96 ]. Rana boylii is designated as threatened under the US Endangered Species Act [ 36 ] in the Feather watershed, and frogs occur at low population densities relative to other regions [ 97 ]. Ongoing population monitoring in this region will provide important insights into the relationships between Bd, flow regulation and canopy cover.…”

“…On the population level, persistence of R. boylii may be attributed to demographic resilience. Population growth rates of R. boylii positively correlate with summer stream temperature and are greater following wet years with high total annual volume of stream-flow [ 97 ]. The longevity (greater than 10 years) and fecundity of some individuals [ 30 , 98 ], should allow populations to rebound from disease-related losses when instream conditions are favourable for recruitment.…”