Type: report

Article abstract: In 2020, the U.S. Geological Survey (USGS) implemented an annual long-term monitoring program for the federally endangered arroyo toad (Anaxyrus californicus) across preserved lands in San Diego County, California. Surveys were conducted in 250-meter (m) segments with data recorded every 125m. Each 125m site was designated as either core, rotating, or discovery. Core sites were permanent sites sampled annually. Up to 50 rotating sites were surveyed each year, with the exact number depending on available resources. Discovery sites were potentially “high-quality” locations sampled during high rainfall years to increase the likelihood that they were wetted. Surveys targeted arroyo toad egg strings and tadpoles, which are substantially more detectable than adults, and are an indication of annual breeding. In addition to recording the presence or absence of arroyo toads, we collected data on several covariates, including the presence of other aquatic species (non-native and native), plus landscape, vegetation, and water attributes. A three-state integrated habitat occupancy model was used to analyze data that includes the probability a site is wet (or suitable for breeding), and if wet, the probability of arroyo toad breeding. Habitat, water, and the presence of non-native species were used to determine predictors of occupancy. Collecting annual metrics, such as rainfall, surface water availability, area occupied, and wet area occupied, will allow us to track trends in arroyo toad populations in San Diego County over time. In nearly all watersheds surveyed in 2022, we observed a continued decline in water availability compared to our 2021 surveys. Precipitation across the representative stations for San Diego County all recorded below normal rainfall in 2022, and only 36% of sites were wet during sampling. We estimated percent area occupied (PAO) to be 5.90% (95% Highest Density Interval = 3.00 – 10.8%) of the total available habitat in the monitoring area. We predicted 13.3% (5.64 – 27.1%) of wet sites were occupied, county-wide. The watershed with the greatest probability of occupancy was the Santa Margarita River. We also observed a significant decline in PAO and percent wet area occupied (PWAO) in the San Diego River in 2022. No other watersheds (San Dieguito, San Juan, San Luis Rey, Sweetwater, and Tijuana) had a significant positive or negative effect on occupancy probability. Sites within the San Juan watershed were completely dry for the second year in a row. Overall detection probability was lower than in previous years (0.37, 0.17 – 0.62), and cumulative detection probability after two surveys was 0.60 (0.31 – 0.86). Our conclusions regarding the effects of covariates were very limited, due to the lack of water and low observed occupancy. Below-average rainfall years often result in a lack of data for analysis and limit the conclusions that can be drawn because the presence of water is necessary for occupancy by arroyo toad tadpoles. The breeding season in 2022 occurred during a below-average rainfall year and followed a previous year of below-average rainfall with declining arroyo toad occupancy. Although fluctuations in annual rainfall and recruitment are natural, prolonged drought can lead to the deterioration of habitat and breeding conditions and impact arroyo toad populations.