Predation on the eggs and larvae of invasive cane toads (Rhinella marina) by native aquatic invertebrates in tropical Australia

An invasive species is less likely to flourish if it is vulnerable to the predators it encounters in its newly-colonised range. The success of cane toads (Rhinella marina) in invading Australia has been attributed to the toads' powerful chemical defences, which render this toxic invader invulnerable to most vertebrate predators. However, invertebrate predators have been largely ignored. Our laboratory studies show that at least three invertebrate species (one coleopteran, two hemipterans) are unaffected by the toads' bufadienolide defences, and consume aquatic stages of the cane toad life history. The water beetle Cybister godeffroyi consumed eggs, hatchlings and tadpoles of R. marina, whereas the water bugs Diplonychus rusticus and Lethocerus insulanus consumed toad tadpoles but not eggs. In choice tests, C. godeffroyi and D. rusticus preferred cane toad tadpoles to simultaneously available native tadpoles or fish, possibly reflecting the smaller size and/or slower swimming speeds of larval cane toads. Rates of predation on cane toad tadpoles were high (e.g., a single water beetle often consumed five tadpoles per day in a complex environment). These abundant, voracious invertebrate predators could reduce cane toad recruitment success in some tropical waterbodies. Future research could usefully examine whether densities of water beetles and water bugs increase after toad invasion (potentially explaining why toad numbers often decline post-colonisation).