Why islands are easier to invade: human influences on bullfrog invasion in the Zhoushan archipelago and neighboring mainland China

Islands are often considered easier to invade than mainland locations because of lower biotic resistance, but this hypothesis is difficult to test. We compared invasion success (the probability of establishing a wild reproducing population) for bullfrogs (Rana catesbeiana) introduced to enclosures on 26 farms on islands in the Zhoushan archipelago and 15 farms in neighboring mainland China. Bullfrogs were more likely to invade farms located on islands with lower native frog species richness than mainland farms, consistent with the biotic resistance hypothesis. However, human frog hunting pressure also differed between islands and the mainland and, along with the number of bullfrogs raised in enclosures, was a stronger predictor of invasion success than native frog richness in multiple regression. Variation in hunting pressure was also able to account for the difference in invasion success between islands and mainlands: islands had lower hunting pressure and thus higher invasion probability. We conclude that the ease with which bullfrogs have invaded islands of the Zhoushan archipelago relative to the mainland has little to do with biotic resistance but results from variation in factors under human control.