Population Trends, Survival, and Sampling Methodologies for a Population of Rana draytonii
Estimating population trends provides valuable information for resource managers, but monitoring programs face trade-offs between the quality and quantity of information gained and the number of sites surveyed. We compared the effectiveness of monitoring techniques for estimating population trends of Rana draytonii (California Red-legged Frog) at Point Reyes National Seashore, California, USA, over a 13-yr period. Our primary goals were to: 1) estimate trends for a focal pond at Point Reyes National Seashore, and 2) evaluate whether egg mass counts could reliably estimate an index of abundance relative to more-intensive capture–mark–recapture methods. Capture–mark–recapture (CMR) surveys of males indicated a stable population from 2005 to 2009, despite low annual apparent survival (26.3%). Egg mass counts from 2000 to 2012 indicated that despite some large fluctuations, the breeding female population was generally stable or increasing, with annual abundance varying between 26 and 130 individuals. Minor modifications to egg mass counts, such as marking egg masses, can allow estimation of egg mass detection probabilities necessary to convert counts to abundance estimates, even when closure of egg mass abundance cannot be assumed within a breeding season. High egg mass detection probabilities (mean per-survey detection probability = 0.98 [0.89–0.99]) indicate that egg mass surveys can be an efficient and reliable method for monitoring population trends of federally threatened R. draytonii. Combining egg mass surveys to estimate trends at many sites with CMR methods to evaluate factors affecting adult survival at focal populations is likely a profitable path forward to enhance understanding and conservation of R. draytonii.Abstract
Location of Cemetery Pond (black star) at Point Reyes National Seashore, California, USA.
Annual abundance of Rana draytonii egg masses at Cemetery Pond, Point Reyes National Seashore, California, USA, 2000–2012. Abundance was estimated with a Jolly-Seber model (hollow red squares) and with a state-space model with a binomial detection component (red circles and line), raw egg mass counts (black asterisks), and adult male annual peak abundance as estimated with a robust-design Cormack-Jolly-Seber model (blue triangles and line). Points represent posterior modes (model-based egg mass abundance), maximum-likelihood estimates (male abundance), or raw data (egg mass counts); error bars represent 95% highest posterior density intervals (model-based egg mass abundance) or 95% confidence intervals (male abundance). Lines connect estimates that share abundance information across years. Points for model-based estimates of egg mass abundance are offset for clarity.
Weekly probability of Rana draytonii egg masses persisting at Cemetery Pond, Point Reyes National Seashore, California, USA, 2000–2012. Points represent posterior modes; error bars represent 95% highest posterior density intervals.
Detection probability of Rana draytonii egg masses during a single survey at Cemetery Pond, Point Reyes National Seashore, California, USA, 2000–2012. Points represent posterior modes; error bars represent 95% highest posterior density intervals.
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