Current and future predicted distributions of invasive toads (Anura: Bufonidae) and bullfrogs (Anura: Ranidae) on Sado Island

Abstract

Island ecosystems are thought to be hotspots of biodiversity but are also susceptible to invasion by alien species. Sado Island in Japan is a habitat for endemic and rare species, such as the endemic Sado wrinkled frog. However, two alien frog species, Bufo japonicus formosus and Lithobates catesbeianus , which have been introduced to the island are threatening the biodiversity of the island and their distribution information is lacking. In this study, we conducted a distribution survey of B. japonicus and L. catesbeianus over Sado Island and predicted their current and future potential distributions using a species distribution model. We found that B. japonicus currently inhabits the southwestern part of the island near its introduction point and is in the process of expansion. Meanwhile, L. catesbeianus has a wider distribution range than B. japonicus and is almost expanding throughout most of its potential suitable habitat. We predict that, in the future, B. japonicus will expand their distribution to the southeastern part of the island. These findings show that it is necessary for B. japonicus to prevent their expansion into further area and L. catesbeianus to reduce the population, considering their distribution status.