Bullfrog (Lithobates catesbeianus) diet composition, life history, and time period for larval development in California’s north coast

Lithobates catesbeianus (American bullfrogs) are not native to states west of the Rocky Mountains. Since their introduction to California, they have been implicated in the decline of amphibian species. On California's north coast, bullfrogs may be causing local declines of native amphibians through predation, competition, or disease. Knowledge of the diet of introduced bullfrogs can help determine if bullfrogs are directly predating these amphibians and therefore directly causing decline. Moreover, Humboldt County's coastal areas have a climate that is cooler on average than the American bullfrogs' native climate. Cool, yet stable year-round temperatures may affect larval phenology, knowledge of which is important for timing the larval culling used to manage the species. Bullfrogs and their larvae were captured from five coastal sites in Humboldt County, CA. Sites varied from 2 m to 1400 m in elevation, and from 0.18 km to 27.25 km in straight-line distance from the coast. Frogs were measured for snout-vent length, gender was determined and stomach contents were categorized. For larvae, total length and stage of development (Gosner stage) were recorded. Results showed diet was greatly affected by body size and site location. Native herptiles made up a very small percentage (7%) of overall bullfrog diet by count, but a larger percentage (32% in adult females to 65% in juveniles) of diet by volume. Most larvae that were captured were large and young tadpoles (i.e., low Gosner stage) were rare. A late stage larva was captured in early spring, suggesting a multi-year development period. This study provides evidence that bullfrogs are directly predating on native herptiles, and suggests that larval management efforts would best be carried out in late summer, prior to tadpoles metamorphosing or overwintering.