The Strategy...
...is simple
WHAT: Expose native predators to small doses of Cane toad toxin that make them sick but don't kill them ('teaching' them that toads are bad)
WHEN: Just before these animals meet the oncoming Cane toad invasion in the wild
HOW: Distribute small Cane toads or Cane toad baits (i.e. made of toad flesh) in areas of the landscape with high predator abundance
It's likely that all vertebrate species (even us) have the ability to develop taste aversion. This is because taste aversion enables animals to: i) detect that an ingested substance was toxic; ii) create a strong physiological association between taste and toxicity; iii) shift their foraging behaviour to avoid these dangerous meals in the future (substance aversion). Taste Aversion, therefore, is a powerful protective response that enables animals to survive alongside poisonous substances in the wild.
The only caveat - the initial dose can't be fatal!
VIDEO: Premise of CTA toad aversion and CTA deployment on the landscape scale
Teacher toads and CTA deployment
Putting it into practice
Our field studies have shown that we can harness an animal's natural ability to develop taste aversion to affect its own conservation when faced with the cane toad invasion. Animals can develop strong aversions after only one experience with a small cane toad.
Here is an example. In the video below 'Macca' a wild floodplain monitor has his first trial with a small toad. This is the first time he has seen one so he gobbles it down. The next clip shows Macca a month later, in a subsequent trial. He investigates the small toad extensively but recognises it by sight and smell, then leaves it alone. Macca survived in toad-infested areas until the end of the study (three years later). If the first toad he had met was a big one at the invasion front, he wouldn't be here today!
VIDEO: Macca's CTA trials in the wild
Macca's CTA trials
Where do we focus our effort?
Apex predators are 'bang for buck' when it comes to Cane toad impact mitigation strategies. This is because apex predators maintain ecosystem stability through predatory relationships from the top down. Maintaining healthy populations of predators in the system keep things running as they have been, in balance. It's when a new predator is added into the system (eg. cats) or a native one is taken out (eg. goannas) that chaos happens.
Take the Yellow-spotted Monitor for example. Below you will see some of the interactions it engages in within its ecosystem (Fig 1.). Through predation, these monitors keep meso(middle) -predators in check, such as medium sized snakes and even cats. But when goannas decline, these meso-predators breed up and exert enormous predation pressure on small birds, mammals and lizards (Fig 2.), causing declines in these species also. Furthermore, large pythons suddenly have substantially less prey in the environment.
Food webs exist in balance; small changes can have big impacts. Preserve the native predators, preserve the balance and the rest will follow.
Figure 1. A goanna's ecological web
Figure 2. One chain of predation within northern ecosystems. What happens when goannas decline?
Q&A
ISN'T RELEASING MORE CANE TOADS A CRAZY IDEA!?
No, this idea is supported by a decade of rigorous scientific research into invasion ecology, cane toad biology and impact mitigation. Importantly, we need to accept that not only are cane toads here to stay but they have greatly exceeded our expectations with respect to rate and range of spread. We have very little time left to act before Cane toads are through the Kimberley in very high densities, and populations of vulnerable predators are wiped out - as has happened elsewhere behind the invasion front. Once we researchers came to terms with that inevitability, we freed up the opportunity to focus on novel avenues of mitigation. In short - the toads are coming; we really have nothing to lose, but if we act now, we have lots to gain.
WON'T RELEASING SMALL TOADS SPREAD THE INVASION?
No, the Cane toad invasion is currently moving at 50 km a year in Northern Australia. It’s important that small toads are released IMMEDIATELY AHEAD of the invasion for learning to be still effective when the native animals encounter the big toads that dominate the vanguard of the invasion. Most of those small toads will not survive for long, and even if they do, the areas where they live will have been invaded before the young “teacher toads” are big enough to disperse or old enough to breed. So, releasing these small toads won’t make the toad invasion spread any faster or further than where it will already travel to.
WHAT IS THE EVIDENCE THAT TRAINING WILL HELP?
Field trials of the CTA method have been conducted in three species that are threatened by cane toad invasion: Northern Quolls, Yellow-spotted Goannas, and Bluetongue Lizards. We also have laboratory studies to show rapid aversion learning in other species like Freshwater Crocodiles. We have published all of these studies in peer-reviewed scientific journals. For all those species, taste-aversion learning is very effective. Untrained individuals die as soon as toads invade, but our trained individuals survive and breed.
WILL NATIVE ANIMALS PASS LEARNING ON TO THE NEXT GENERATION?
Probably not, although we know that young quolls stay with their mother for a long time and they may learn by watching her avoid cane toads. Our research has shown that these baby quolls do avoid toads (and so survive) but this may not be due to education from their mother. Instead, there is a simpler process. At the front of the toad invasion, all of the toads are very large. If a predator eats one it will die. But within a few years, the toads breed and so the landscape is full of tiny toads. The offspring of any surviving predators will encounter these small toads, eat them, become sick, and learn for themselves to leave toads alone. So, we only need to educate a single generation of predators – the one that first encounters toads - after that, the next generation can learn for themselves.
WON’T SOME PREDATORS BE KILLED BY EATING A “TEACHER TOAD”?
Yes, perhaps – but the numbers will be tiny compared to those that are killed by the main toad invasion – which will roll through the area within a few months anyway. The “teacher toads” we release will be so small that they are very unlikely to kill a predator, and if we release them in the right place at the right time, they will grow only very slowly so will remain a teacher not a killer for many months. We know that many, many animals can learn to avoid toads if they have a negative experience that doesn't kill them, and the toad invasion is coming their way anyway, so rather than adding to the problem, releasing toads may actually provide the only opportunity that we have to truly mitigate the impact of cane toads.
IF WE TRAIN PREDATORS, WON'T WE UNDERMINE THE NATURAL MECHANISMS THAT PRODUCE NATURALLY TOAD-SMART POPULATIONS?
Yes and no. These species do still exist in some long-invaded areas and we know that these animals are not genetically/physiologically immune to toad toxin, but are instead behaviourally averse to toads. If populations do persist, the animals are generally much less common, less genetically diverse and take a very long time to recover (imperilled predators are still very rare in areas of the NT that were invaded 15 years ago). Because toad toxin is so potent, interactions with toads at the invasion front are likely fatal in 100% of cases, so there is little room for variation (eg. nuances in attack style etc.) to be selected upon and create robust toad-smart populations; the declines are sharp and severe.
But, because predators play such important regulatory roles in the environment, we can't let their populations decline, even if they eventually recover. By the time predator populations start to recover naturally the damage is already done - the ecosystems have irrevocably changed. Thus it is important we ensure predator populations don't dip below the minimum viable population size in those first declines so that they recover much faster and so that the wider ecosystems remain as stable as possible. The only way to do this is to preserve as many animals as possible initially.
CAN WE SAVE PREDATORS RIGHT ACROSS THE KIMBERLEY?
Sadly, no. The area is just too big, and many places are inaccessible. But if we create “pockets” where predators survive, those animals and their offspring (which will learn to avoid toads without needing training from us – see above) can spread out and populate the nearby regions where toads have wiped out their species. So, the ecosystem will recover much faster if we can keep those “pockets” of surviving predators.
WHY NOT JUST REMOVE ALL THE TOADS?
We would love to do that, but it isn’t possible – at least, with any methods currently available. A female toad can lay more than 30,000 eggs in a clutch, so we simply can’t eradicate toads faster than they can replace the ones we remove. And it only takes a single large toad to kill a native predator, so just reducing toad abundance may not help very much.
WHY USE SAUSAGES AND TOAD BAITS AS WELL AS “TEACHER TOADS”?
For some imperilled predators, the nausea induced by eating a toad-flesh sausage may be enough to teach them. This method allows us to target specific sites with known amounts of toad toxin per “bait”, avoiding problems such as “teacher toads” dying, growing or dispersing. And some members of the Cane Toad Coalition have extensive experience in using sausage baits for control of other feral species. We are developing baits to suit a variety of circumstances.
HOW WILL YOU KNOW IF IT WORKS?
We survey the abundance of predators before and after toads arrive, in places where we deploy baits as well as places where we do not. We predict that predator populations will persist in the areas where we have provided an education not to eat toads. Our results have been positive with all the large reptilian predators that we have trialled in the wild so far.
WHAT QUESTIONS DO YOU HAVE?
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