Taste aversion training can educate free-ranging crocodiles against toxic invaders

Apex predators play critical ecological roles, making their conservation a
high priority. In tropical Australia, some populations of freshwater
crocodiles (Crocodylus johnstoni) have plummeted by >70% due to
lethal ingestion of toxic invasive cane toads (Rhinella marina).
Laboratory-based research has identified conditioned taste aversion (CTA)
as a way to discourage consumption of toads. To translate those ideas into
landscape-scale management, we deployed 2,395 baits (toad carcasses with
toxin removed and containing a nausea-inducing chemical) across four gorge
systems in north-western Australia and monitored bait uptake with remote
cameras. Crocodile abundance was quantified with surveys. Free-ranging
crocodiles rapidly learned to avoid toad baits but continued to consume
control (chicken) baits. Toad invasion at our sites was followed by high
rates of crocodile mortality (especially for small individuals) at a
control site but not at nearby treatment sites. In areas with high
connectivity to other waterbodies, repeated baiting over successive years
had continuing positive impacts on crocodile survival. In summary, we
succeeded in buffering the often-catastrophic impact of invasive cane
toads on apex predators.