Predator suppression by a toxic invader does not cascade to prey due to predation by alternate predators

Abstract

Invasive species often have catastrophic direct effects on native species through increased competition and predation. Less well understood are indirect, cascading effects across trophic levels. To reveal trophic disruptions caused by invasive species, it is necessary to document interactions between species at different trophic levels and across guilds. Here, we take this approach to quantify the impact of the invasion of cane toads at a riparian site in the Kimberley, northwest Australia. These toads are toxic to many native vertebrate predators and following toad arrival we observed the expected severe population decline in monitor lizards. Contrary to expectations however, this did not facilitate species in the next trophic level down: the abundance of their reptilian prey, as well as nest success of a songbird whose nests were predominantly depredated by monitor lizards, remained unchanged. Instead, detailed observations suggest a change in the suite of nest predators, with monitor lizards being replaced by other—mainly avian—predators, possibly reflecting their release from competitors that are more efficient nest predators. Hence, our findings highlight complex indirect effects of an invasive species across trophic levels and indicate that trophic cascades can go undetected when failing to include direct observations on predator–prey interactions.