Density-Dependent Effects of Amphibian Prey on the Growth and Survival of an Endangered Giant Water Bug

Abstract: Amphibian predator–insect prey relationships are common in terrestrial habitats, but amphibian larvae are preyed upon by a variety of aquatic hemipterans in aquatic habitats. This paper suggests that the survival of the nymphs of the endangered aquatic hemipteran Kirkaldyia (=Lethocerus) deyrolli (Belostomatidae: Heteroptera) is directly and indirectly affected by the abundance of their amphibian larval prey (tadpoles). Young nymphs of K. deyrolli mainly feed on tadpoles, regardless of differences in prey availability. Nymphs provided with tadpoles grow faster than nymphs provided with invertebrate prey. Therefore, tadpole consumption seems to be required to allow the nymphs to complete their larval development. In addition, the survival of K. deyrolli nymphs was greater during the period of highest tadpole density (June) than during a period of low tadpole density (July). Higher tadpole density moderates predation pressure from the water scorpion Laccotrephes japonensis (Nepidae: Heteroptera) on K. deyrolli nymphs; i.e., it has a density-mediated indirect effect. These results suggest that an abundance of tadpoles in June provides food for K. deyrolli nymphs (a direct bottom-up effect) and moderates the predation pressure from L. japonensis (an indirect bottom-up effect). An abundance of amphibian prey is indispensable for the conservation of this endangered giant water bug species.