Diet Composition of the Invasive American Bullfrog (Lithobates catesbeianus) in Onuma Quasi-National Park, Hokkaido, Japan

An invasive alien species American bullfrog (hereafter, 'bullfrog') is found in freshwater lakes in Onuma Quasi-National Park, Hokkaido, Japan. Bullfrog commonly feeds on red swamp crayfish in many areas. However, red swamp crayfish has not been confirmed in Komuna Lake, Onuma Quasi-National Park. The purpose of this study is to examine the trend of predation on native biomes in areas without crayfish presence. We detected the dietary composition of bullfrogs. The stomach contents of 469 individuals were analyzed and classified. The stomach contents of adult frogs accounted for 67.4% of volume were vertebrates including Actinopterygii and Amphibia such as Japanese crucian carp, topmouth gudgeon, bullfrog juveniles, bullfrog tadpoles and Japanese common toad. Further, aquatic animals were preyed more than terrestrial animals in volume (60.0%) and frequency (90.0%). From these results, bullfrogs in Onuma Quasi-National Park used most conspicuous alien aquatic species such as alien fish and frogs of the same species as food resources and further revealed that these alien aquatic species function as a substitute food for the crayfish. While the majority of bullfrog food resources are alien aquatic species, several rare aquatic animals were also preyed on. In the future, it will be necessary to investigate the predation pressure of bullfrogs on local biodiversity.