A mixed model approach to modelling global habitat suitability and invasion risk of the American bullfrog

American bullfrogs (Lithobates catesbeianus), which are native to the eastern United States, have been widely introduced to other parts of the world through food and pet trade. These include parts of North, Central, and South America, Western Europe, and parts of Asia. In many of these regions, they have become invasive by predating or outcompeting native animals. Because of the potentially damaging impact on ecosystems where this species is not native, it is pertinent to delineate through habitat suitability models where it is most likely to find suitable habitat outside of its native range in order to effectively prevent future outbreaks. Here, we use presence points available through the Global Biodiversity Information Facility (GBIF) along with mixed method habitat suitability modelling to determine areas of highest suitability and areas most at risk of invasion. We first ran an ecological niche factor analysis (ENFA) on a global grid of presence and pseudo-absence points using five uncorrelated bioclimatic factors (Bio1: annual temperature; Bio2: mean diurnal range; Bio12: annual precipitation; Bio14: precipitation of driest month; Bio15: precipitation seasonality). The original and ENFA modelled presence-pseudoabsence points were then fed separately into random forest algorithms using the same five bioclimatic variables. The random forest result of original data was considerably more conservative than the result from the ENFA modelled points, which is more similar to previous MAXENT models of this species found in the literature; however, the result of the original data indicates areas where the species is already present and therefore presents more risk than what is predicted by ENFA modelling. The two results were subsequently averaged to indicate areas at greatest risk of invasion; points with a value > 0.5 were considered risk areas. Our final model showed that the highest risk areas are located in Western North American, South America, Western Europe, parts of Africa, Japan, Southeast Asia, Western Australia, and New Zealand. Within these areas, the American bullfrog already occurs at high levels in Western North America and moderate levels in South America, Western Europe, and Japan; the species also has limited occurrences in Southeast Asia.