Invasional meltdown-under? Toads facilitate cats by removing a naive top predator

Context. Australia has been a battleground of invasive versus native species for similar to 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 naive 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 generally been thought to be independent of one another. Aims. We aimed to determine the impacts of invasive cane toads on monitor lizards and feral cats by estimating changes in relative abundance before and after the toad invasion. Methods. We studied toad impacts at three sites in the Kimberley region of northern Australia. We used two methods for estimating relative abundance: camera traps and track station data. Data included greater than 4000 trap days and included 7 years over an 11-year period. Key results. As expected, invading cane toads rapidly decimated populations of two species of monitor lizards (97-99% declines), including the top-order predatory Varanus panoptes. Unexpectedly, this loss was associated with a >10-fold mean increase in detection rates of cats by 5 years after the loss of V. panoptes, reflecting relative increases of 3.3-8.7 individual cats per site. Conclusions. 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. This interaction likely reflects one type of invasional meltdown, whereby a non-native species (cane toad) facilitated any aspect of another's (feral cat) invasion (e.g. survival, reproduction, resource acquisition), but the latter has no detected influence on the former (+/0 interaction). Implications. Because both invaders cause declines in animal populations and are difficult to control, the potentially synergistic tandem of cane toads and feral cats could have chronic, irreversible effects on animal communities.