Relative Abundance Differences of Two Invasive Toad Species on Minami-daito and Kita-daito Islands, Japan

Biological invasions are major threats to global biodiversity, and island ecosystems are especially vulnerable. Many previous studies have reported the presence of invasive species on islands, but relatively few studies have quantified the population status of such species. Furthermore, multiple invasive species are often introduced to the same region or island. In such case, quantifying the relative abundance of each species would provide us with significant insight into their population dynamics and potential species interactions. Rhinella marina and Bufo gargarizans miyakonis were introduced to both Minami-daito and Kita-daito Islands before 1945. Although some studies have reported their invasion and establishment, few studies have quantified their relative abundance. To reveal their relative abundance, we set 5 km line transects and counted the number of each species from 11 to 31 July 2019. We applied generalized linear models to examine the relationship between the number of individuals observed with species and environmental factors. We also applied these models to examine the relationship between the number of individuals observed with island and environmental factors. We found the relative abundance of R. marina was significantly higher than that of B. g. miyakonis on Minami-daito Island. This is the opposite pattern of 30 years ago, suggesting the population dynamics of these invasive toads is unstable on Minami-daito Island. Although B. g. miyakonis was found on both islands, no individual of R. marina was found on Kita-daito Island. The population dynamics may depend on environmental differences between islands and ecological differences between species. Future studies are needed to reveal the potential factors determining the differences in relative abundance between islands and between species.