Using environmental niche models to elucidate drivers of the American bullfrog invasion in California

Abstract

Environmental niche models (ENMs) are commonly used to inform management of invasive species, but invasive species often violate two key assumptions of ENMs which assert that species’ ecological niches are stable in space and time, and that organisms are in equilibrium with their environment. The American bullfrog (Rana catesbeiana) invasion in California provides an excellent opportunity to use ENM approaches recommended for overcoming these assumption violations to inform management of a harmful invader. In the current study, we used dynamic ENM approaches and explicit comparisons between native and invaded niches to: (1) examine how environmental drivers of occurrence in California shifted during the invasion process, and (2) investigate the potential for continued bullfrog expansion by explicitly assessing their environmental equilibrium. Since their initial introduction to metropolitan areas of California in the late 1800s, bullfrogs have spread throughout the state with the most notable recent expansions in coastal regions, the Central Valley, and the mountains of northern California. Bullfrog occurrence in California was consistently associated with human activity and higher winter temperature. Small pockets of high human activity have sustained bullfrog populations in otherwise unsuitable areas. The areas in California with the highest predicted risk of increased bullfrog abundance and future bullfrog expansion include the southern Central Valley, the coast of northern California, and mid elevations in the mountains of northern California. Continued bullfrog invasion in these areas is likely to impact sensitive native amphibian populations.