Type: report

Article abstract: In 2003, we implemented a new monitoring program for the endangered arroyo toad (Bufo californicus) on Marine Corps Base Camp Pendleton (MCBCP). To address the problems associated with large variations in adult toad activity, we employed a spatial and temporal monitoring approach that tracks the presence of arroyo toad breeding populations by documenting the presence of eggs and larvae. Unlike adult toads, eggs and/or larvae remain visible in the water for months before metamorphosis and have a much higher probability of detection. This year, we began monitoring 89 km of potential toad breeding habitat within MCBCP. We divided the habitat into approximately 60 blocks, each divided into 6 survey site lengths. One site length within each block is surveyed yearly, while the other site lengths are surveyed on a 5 year rotation. We implemented the first year of this rotating panel design by comprehensively surveying 120 randomly stratified survey site lengths (30 km). We then used a loglinear modeling program to model the data and correct for varying detection probabilities. The program provides the framework for powerful statistical analysis of trends in metapopulation dynamics and breeding, as well as the effects of habitat, aquatic variables, and management actions on arroyo toad populations. In 2003, 78% of potential toad breeding habitat contained water during our survey efforts. Of these areas, 87.4% (se = 9.5) of the habitat was occupied by breeding arroyo toads. The greatest occupancy was recorded on the San Mateo watershed (97.9%), followed by the San Onofre (90.9%) and Santa Margarita (83.8%) watersheds. We evaluated over 14 habitat and survey specific variables in the models. These included landscape variables, environmental variables, and the presence of nonnative plant and aquatic vertebrate species. Results showed that the absence of crayfish was the single most significant predictor of the presence of arroyo toad larvae. Larvae were 20 times (95% CI: 2-249) more likely to be detected when crayfish were absent. Although data on the relationship between crayfish and arroyo toads are sparse, crayfish are known to prey upon amphibian eggs, larvae, and adults, and have been linked with declines in some amphibian populations. It is unknown whether this is a direct link or if crayfish are an indirect indicator of less than favorable habitat conditions. In order to provide continuity with previous monitoring efforts, monitor n