Ecological impacts of invasive cane toads

Cane toads (Rhinella marina) were brought to Australia in 1935, and released in northeastern Queensland. They have since spread westwards (through Queensland, the Northern Territory, and into Western Australia) and southwards (into NSW). Concern about the toads’ impacts on wildlife spawned predictions that most native species encountering toads would be severely affected, and thus decline in abundance. Recent research is revealing a more complex picture. Toads do indeed cause precipitous population declines (of >80%) in some large-bodied predator species, due to lethal poisoning when predators attempt to eat toads. However, populations of smaller predator species are unaffected; some individuals are fatally poisoned, but most survive (because small toads contain far less toxin than large adult toads) and learn to avoid toads thereafter. Most Australian birds and rodents have inherited a tolerance of toad poison from Asian ancestors, and thus are not affected. Even for heavily impacted predators, populations might eventually recover. Simplistic predictions on the ecological impact of cane toads in Australia have not been supported by field studies – indirect impacts of toads often outweigh direct effects. For example, the abundance of native frogs appears to be unaffected by cane toads, because negative impacts (e.g. frogs being eaten by toads, poisoned by toads, competing with toads) are balanced by positive impacts (e.g. toads reducing abundance of frog-predators). Some native snake species predicted to be vulnerable to toads, have become more common since toad invasion – presumably because of the disappearance of predatory goannas. In summary, the impacts of invaders may be complex, and difficult to predict. Any disruption to one native species is likely to have indirect effects on others. Some native species are winners not losers as the toad invasion rolls across the landscape.