Invasional Meltdown-Under? Toads Facilitate Cats By Removing A Naïve Top Predator

Abstract

Australia has been a battleground of invasive vs. native species for ~200 years. Two of the most impactful invasive species to Australian animal communities are the toxic cane toad (Rhinella marina) and the predatory feral cat (Felis catus). Australia’s native fauna is evolutionarily naïve to both invaders because neither’s taxonomic group is native to the continent. Both invaders have had severe effects on Australian native animal communities including species extinctions, extirpations, and severe population declines, but until now their effects have been thought to be independent of one another. Here we present evidence for invasional meltdown: Invasive cane toads apparently facilitated feral cats by removing a naïve, native top predator from the ecosystem. Camera trap and track station data (>4,000 trap nights) spanning 11 years demonstrated, as expected, that invading cane toads rapidly decimated populations of two species of monitor lizards (97-99% declines), including the top-order predatory Varanus panoptes, at three sites in northwestern Australia. Unexpectedly, this loss was associated with a >10-fold mean increase in detection rates of cats by five years after the loss of V. panoptes, reflecting relative increases of 3.3-8.7 individual cats per site. Although some unknown factor may have caused an increase in cats, their similar trophic position and niche to V. panoptes suggests that toads facilitated cats by effectively removing the lizards from the animal community. The potentially synergistic tandem of cane toads and feral cats could have chronic, irreversible effects on animal communities due to the extreme difficulty of controlling these invaders.