Invasive Toads Shift Predator-prey Densities in Animal Communities by Removing Top Predators

Although invasive species can have substantial impacts on animal communities, cases of invasive species facilitating native species by removing their predators have rarely been demonstrated across vertebrate trophic linkages. The predictable spread of the invasive cane toad, however, offered a unique opportunity to quantify cascading effects. In northern Australia, three species of predatory monitor lizards suffer severe population declines due to toad-induced lethal toxic ingestion (yellow-spotted monitor, Merten's water monitor, Mitchell's water monitor). We thus predicted subsequent increases in the abundance and recruitment of prey species due to the reduction of those predators. Toad-induced population-level declines in the water monitor species approached 50% over a five-year period spanning the toad invasion, apparently causing fledging success of the crimson finch to increase from 55% to 81%. The consensus of our original and published long-term data is that invasive cane toads are causing monitor lizards to lose a foothold on top-down regulation of their prey, triggering shifts in the relative densities of predator and prey in the Australian tropical savannah ecosystem.