Distribution Characteristics of the Introduced Non-Native Toad Bufo formosus 20 Years after a Volcanic Eruption on Miyake-Jima Island

Amphibians are poorly dispersed owing to their low osmotic-pressure and salt tolerance, and are rarely found on islands. However, amphibians that invade islands through anthropogenic introduction can have high population densities and enormous impact on native ecosystems. Bufo formosus, a large Japanese amphibian, was artificially introduced as a nonnative species, mainly on islands, and has had a significant impact on the native ecosystems of Hokkaido and Sado Island. However, knowledge of this introduced species on the Izu Islands is limited. Therefore, in this study, we conducted distribution and habitat-density surveys and created a species distribution model (SDM) to clarify the distribution status of introduced nonnative B. formosus on Miyake-jima Island, where approximately 20 years have passed since a volcanic eruption and species invasion. Occurrence record surveys revealed that B. formosus was found in a wide area of Miyake-jima Island. While SDM predicted that this species inhabits not only in forest but also grasslands formed after volcanic eruption, it was predicted that the potential distribution probability of this species inhabiting coastline and summit areas were low. A population density survey revealed the highest density around Tairo Pond on Miyake-jima Island, where 538 toads were found in 10 minutes. The lack of predators and competitors for this species on Miyake-jima Island may explain its widespread and high-density population. In the future, it will be necessary to clarify the impact of such a high-density population on the native ecosystem, especially on insect fauna.