Eighty years ago, an agricultural scientist named Reginald Mungomery brought cane toads to Australia, bred them, and released their offspring in sugar cane plantations near Cairns. Little did he know that he was setting in train one of the greatest ecological disasters to befall Australian wildlife. His decision has been universally condemned since.
But now, almost a century later, I am proposing that we can fix Mungomery’s historic blunder by doing almost exactly what he did. Ironically, we can buffer the devastating impact of the cane toad invasion by releasing juvenile toads at the invasion front.
This idea might sound ludicrous. The idea of releasing even more cane toads sounds like the height of academic folly. But it works, and my colleagues and I have even managed to convince initially sceptical management authorities and private landowners to adopt the method.
Our key discovery was that populations of most species of native predators aren’t affected by the toad invasion. A few individuals are fatally poisoned when they eat a toxic toad, but most predators aren’t killed by such a meal. If the toad it eats is fairly small (and thus not too poisonous), the predator becomes ill, and learns not to eat toads in future. After that learning experience, the predator can coexist indefinitely with cane toads – it doesn’t try to eat them, so it’s not at risk.
So why doesn’t this same kind of learning save the larger predators – such as quolls, goannas, bluetongue skinks and snakes – that die in droves as soon as cane toads arrive in an area? Our surveys show up to 95% mortality in these species.
The reason is that these large predators attack large toads, and these toads contain more than enough poison to kill even a massive goanna within minutes.
Sadly, the apex predators never get an opportunity to encounter the smaller toads that could have offered them a lifeline by teaching them to steer well clear.
On the frontline
The invasion vanguard is dominated by large adult toads, of a size that kills rather than educates any predator who eats it. Smaller toads (and reproductive females, heavy with eggs) aren’t at the frontline because they can’t keep up with the fastest invaders. As a result, cane toads don’t usually breed until a year or two after the first wave arrives – and by the time the area contains small as well as large toads, the predators have already been killed.
So native predators face a stark equation. If the first cane toad you meet is a large one, you die. If the first cane toad you meet is a small one, you learn not to eat toads, and you survive.
This suggests a straightforward way to buffer the ecological impact of the cane toad invasion: ensure that the predator’s first meeting is with a small toad rather than a large one. There’s a very easy way to do this: we can release small toads at the invasion front, or induce aversion by other methods (such as feeding native animals toad-flavoured sausages).
We have trialled this method, and it works. Three of the most vulnerable species – Northern Quolls, Bluetongue Skinks, and Yellow-spotted Goannas – all survive toad invasion if they are trained beforehand, but die if they are not.
For example, my research colleague Georgia Ward-Fear captured goannas on a Kimberley floodplain, radio-tracked them, and dangled small cane toads in front of some of the lizards. Many of them seized on the toad, became nauseous, and avoided toads afterwards. Of the goannas that Georgia trained in this way, half were still alive at the conclusion of her study 18 months later – whereas all of the untrained lizards died long before then.
But isn’t this just a stopgap solution, delaying the wave of predator deaths for just a single generation? Won’t the offspring of those trained predators need to be trained as well, and so on forever? No, they won’t.
Within a year or two of the toads’ arrival in an area, they begin to breed – and so the offspring of our trained predators grow up in a world with small as well as large cane toads. The young predators will encounter small toads, eat them, and learn to give them a wide berth. All we need is a single generation of education to provide a long-lasting benefit.
We are now working with Western Australia’s Department of Parks and Wildlife to fine-tune our methods, and then implement them on a landscape scale. We can’t stop the toad invasion from proceeding through the Kimberley, but we can substantially blunt the invaders’ impact. Ironically, the way we are doing it is almost exactly the same as the act that caused the problem in the first place, when Reginald Mungomery released those first young cane toads in cane fields on the other side of the continent.
Rick Shine is the recipient of the 2016 Prime Minister’s Prize for Science for his work on the cane toad problem.
Authors: Rick Shine, Professor in Evolutionary Biology, University of Sydney