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Genetic “Backburning” Can Stop Cane Toads

Credit: Johan Larson/Adobe

While rabbits have tens of babies, cane toads have tens of thousands. Credit: Johan Larson/Adobe

By Ben Phillips

Could the cane toad’s march through the Kimberley be stopped in its tracks by introducing less-dispersive toads ahead of the invasion front?

It’s been a long day. The sun set an hour or so back, and since then the only thing we’ve managed to drag from the car is the cask wine and three tin cups. We are sitting there, in the dark, in the dust among the saltbush, going over the highs and lows of the past 3 days. We will camp here, next to one of the 600-odd bores between Broome and Port Hedland, and tomorrow is going to be more of the same: driving the long distances necessary to meet the pastoralists and indigenous folk that call this part of the world home.

We want to know if we can stop the cane toad invasion here, where the Great Sandy Desert meets the coast. Reid Tingley, Darren Southwell and myself are here to talk with the locals and see the country in detail. Stopping the toad invasion is a bold idea, but is it feasible? So far, our observations have yielded a big “maybe”. We are a little despondent, but we are about to stumble on a fascinating new idea.

Cane toads were introduced into Australia in the 1930s. The 101 toads originally introduced around Gordonvale in northern Queensland have, in the past 80 years, become billions of toads. From their initial release site, toads have spread to occupy more than 1.5 million km2 of the Australian mainland.

Because they are highly toxic, they have had major impacts on native predators such as goannas, quolls and freshwater crocodiles. The removal of these predators has, in turn, resulted in cascading changes in the ecological communities that the toads have invaded.

The introduction of toads, like the introduction of rabbits and foxes, is one of those actions that we wish we could take back. If we can’t take it back, we would at least like to stop the invasion from progressing any further.

Stopping any invasion is hard, but stopping the cane toad invasion is particularly tricky. Cane toads make rabbits – those paragons of fecundity – look chaste. While rabbits have tens of babies, cane toads have tens of thousands.

As well as this, cane toads – particularly those on the invasion front – can move astonishing distances: more than 50km in a wet season. The reason invasion front toads move so far is that they have, over the past 80 years, evolved to become hyper-dispersive.

Conditions on invasion fronts relentlessly select for the fastest dispersers in a population. As a consequence, toads at the invasion front move 5–10 times further each year than their counterparts in northern Queensland.

These astonishing capacities for reproduction and movement make the toad invasion very hard to stop. Cane toads are currently invading the Kimberley at a rate of 50 km/year, and massive community efforts at toad control over the past decade have done nothing to curb that spread.

Stopping the cane toad invasion is impossible, or at least that was my long-held view. But that view began to change when Mike Letnic showed me a map and sketched an idea. The map showed the La Grange region between Broome and Port Hedland, and it revealed that all of the permanent water in this region was confined to a narrow coastal strip: a 600 km bottleneck of toad habitat wedged between the Great Sandy Desert and the Indian Ocean.

Mike also pointed out that almost all of these permanent water sources are artificial water points: bores sunk by pastoralists to keep their cattle alive in this otherwise perishing landscape. If we could stop toads getting to these artificial water sources, said Mike, they would not survive the dry season. This corridor would therefore become a death trap, and the toad invasion would stop.

This was an intriguing idea, and the potential win was that we could keep cane toads out of the Pilbara: a 270,000 km2 region of the country chock-full of endemic species, and also perfectly suitable for cane toads. This was an idea worth looking at very seriously, so we did.

We drew on more than a decade of intensive ecological work on toads conducted by Rick Shine and Greg Brown in Sydney; we drew on international expertise in spread modelling; and we drew on new empirical work exploring the survival of toads cut off from these water sources. The result has been several published papers and nothing but support for the idea.

Based on everything we know about toads, the landscape and weather, we now believe it’s possible to stop the toad invasion somewhere between Broome and Port Hedland. However, it’s a long road from an idea to its implementation.

So we’d come to talk to the locals. Did they think it was feasible? Was there anything important that our modelling had missed? Did our maps of water availability concord with their experience?

We were 3 days into this process, and we had seen highs and lows. Most people felt that it could work, but there were issues raised: coastal fogs; plans for irrigated cropping; design of infrastructure; costs. We felt that most of these issues could ultimately be dealt with.

Our largest concern was finding a bulletproof stretch of country about 100 km wide where we were confident that access to all water in the dry season could be controlled. We were halfway through our tour, and so far we hadn’t found that perfect stretch of country.

The idea behind this barrier is that, during the wet season, toads are free to disperse wherever they choose. There is abundant water during the wet season, particularly when cyclones are around. During these times of high water availability, toads will move freely and breed in numerous places in the landscape; there is nothing that can be done to prevent this.

Our strategy focuses on the dry season. By September, all the surface water is gone except for a few permanent natural water bodies and the artificial water points of the pastoralists. If we create a 100 km stretch of country where these water sources are not available to toads, the toads can disperse into it during the wet season but none can survive through the ensuing dry. The invasion stops.

The real frustration was that we had found stretches of country that were almost wide enough. For example, there was a nice 70 km stretch just down the road. Our modelling work told us that a 70 km stretch had a very good chance of working, but we wanted to be completely sure: 100km would give us a high level of certainty. If only our waterless barrier didn’t need to be so wide we would already have found a perfect place for it.

And that’s when we stumbled across a fascinating possibility. What if we weren’t trying to stop the invasion front toads with their insane evolved tendency to move huge distances? What if, instead, we were trying to stop the relatively sedentary toads that are found in northern Queensland where toads were first introduced?

And from there the idea of the genetic backburn sprang from the dust, fully-formed and surprising. Who would have thought that we might be able to stop an invasive species by introducing more of them ahead of the invasion front?

It’s a counter-intuitive idea, but it can work. The key to it is that, on invasion fronts of any invading population, including toads, there is intense evolutionary pressure operating to increase rates of dispersal. Only the individuals that move the furthest can be on the invasion front each generation, so dispersal rates on the invasion front can rapidly increase relative to the initial population.

This is not just a theoretical idea: it has been demonstrated in the Petri dish, in species shifting their range due to climate change and, perhaps most convincingly, in cane toads as they spread across northern Australia. Dispersal rates on invasion fronts will often evolve to high levels despite the costs. Thus, when placed in competition with individuals well back from the invasion front, the highly dispersive invaders will likely be less fit.

There is very clear evidence that this is the case in toads. After the invasion front has passed, dispersal rates steadily evolve downwards over time as fitter, less dispersive genotypes win out.

If we were to establish a waterless barrier and then, as the toad invasion approaches, introduce toads from Queensland on the nearside of the barrier, we would set off a new invasion, a backburn of less-dispersive genes heading back towards the oncoming invasion front. Because our less-dispersive toads can outcompete their highly dispersive brethren, the highly-dispersive toads (and their genes) never make it to the barrier. For the modest cost of moving some animals across the country, we can, in theory, make the barrier substantially more effective.

But it’s a long road from idea to implementation. Based on all the information and feedback gathered during our tour of the barrier region, Darren has improved our models, stress-tested them, and completed an economic analysis. His analysis suggests that, even without the genetic backburn, the barrier can work and the Pilbara kept toad-free.

Moreover, the cost of the entire exercise works out to be astonishingly cheap relative to many other conservation actions: about $90,000 per year. This money would be spent on ensuring that pastoral infrastructure was well maintained, leak-free and therefore inaccessible to toads.

Thus the idea offers potential wins to pastoralists, indigenous communities and the environment. Executing a genetic backburn would add almost no additional cost but would make the barrier substantially more difficult to breach. Whether we used the backburn to reduce the cost of the exercise (by implementing a narrower barrier) or simply as a cost-neutral mechanism to really make sure that our barrier holds is a decision for the future. Indeed, whether we should use the genetic backburn at all is a decision that requires careful consideration.

For now, however, it is simply exciting to have a completely new strategy up our sleeves for controlling the spread of invasive species. The toad is the most obvious application, but now that the idea is out it will be fascinating to see where it goes.

Rapid evolution happens during invasions of many kinds, from tumours to damselflies and toads, and even during the spread of disease. Can we exploit this rapid evolution to effect novel management outcomes? Time will tell.

Ben Phillips is Head of the Spatial Ecology and Evolution Lab at The University of Melbourne.