Halting cane toads in WA

A NERP workshop

(Broome, WA, May 2014)

#82 September 2014

Cane toads can reach remarkable densities around dams, like this one at Auvergne Station, NT. (Photo courtesy of Ruchira Somaweera).

Cane toads can reach remarkable densities around dams, like this one at Auvergne Station, NT. (Photo courtesy of Ruchira Somaweera).

Cane toads have reached the Kimberley and there is no sign that their conquest is nearing completion. Their remorseless advance across the Top End makes it seem like they are invincible, but we believe that by exploiting the toads’ inability to retain water, we might be able to control its spread. Few, if any toads can survive more than 10 days without water in the dry season. So, in very dry regions, we may be able to halt their spread by excluding them from permanent water sources. If we manage lots of water sources in the same area (eg, by fencing natural water bodies, or minimising leaks in tank and trough systems that provide water to cattle), we might be able to create a waterless barrier or ‘firebreak’ in the landscape that toads can’t penetrate.

Now this sounds ambitious – where could we manage all permanent water bodies to create a waterless firebreak? Well it just so happens that toads will need to march south towards the Pilbara through an arid corridor where permanent natural water is in short supply. Artificial water points and natural springs dot the corridor, forming a thin strip of suitable toad habitat along the coast. The combination of landscape and climate makes this corridor a potential bottleneck (or choke point) in which to create a barrier.

Many Australian frogs, such as this desert spadefoot that we found on our roadtrip, are capable of burrowing when times get tough. But cane toads lack such adaptation for an arid existence.

Many Australian frogs, such as this desert spadefoot that we found on our roadtrip, are capable of burrowing when times get tough. But cane toads lack such adaptation for an arid existence.

But how effective might a barrier be at halting the spread of toads? We’ve modelled the way toads spread through this corridor by combining information on the biology of toads, their dispersal behaviour in response to rainfall, and the location of water points in the corridor (Tingley et al. 2013). We ran the model under a ‘do nothing’ scenario and then tested how many water points would have to be managed to halt toad spread. Given the distribution of water points, our preliminary modelling suggests that we would only need to manage around 100 water points to stop their spread. A barrier of this size in the right location would stop the invasion dead in its tracks, and prevent toads from occupying more than 260,000 km2 of their potential distribution in Western Australia.

Natural and artificial water bodies in the study area. The black arrow in the lower left hand corner of the main figure shows the location of the De Grey River.

Natural and artificial water bodies in the study area. The black arrow in the lower left hand corner of the main figure shows the location of the De Grey River.

So our modelling suggests that creating a waterless barrier might be a sensible strategy, but we wanted to ask pastoralists and people who know this region firsthand what they thought of the idea. After all, it’s easy to ‘pretend’ to manage water points on a computer by simply deleting them! If this idea were ever to be implemented on the ground, we would want to be sure about the location of water points in the corridor, how easily we could exclude toads from those points, the willingness of stakeholders to participate, and of course, how much this all might cost. So, to get a handle on some of these questions, we travelled the entire length of the corridor and spoke to as many stakeholders as we could. Our trip was divided into two parts: a road trip and a workshop.

A road trip and a workshop

The first component of our trip involved talking to all of the pastoralists in the Kimberley-Pilbara corridor. We wanted to: 1) get feedback on the idea, 2) gain a better understanding of the landscape we’re trying to model, and 3) improve the accuracy of our water point data. The configuration of water bodies will have a strong influence on how easily toads can disperse, so it’s important to get this information right. As a result, we spent a considerable amount of time chatting with pastoralists about the locations of artificial water points (dams, tanks and wells), as well as natural water points such as perennial springs.

To help us work on the feasibility, practicalities and costs associated with stopping the spread of toads, we travelled to Broome to speak with people who know the country and the reality of getting things done. The goals of the workshop were to go back to square one and consider all possible actions we could employ to halt the spread of toads, to acquire further feedback on the feasibility of a waterless barrier in the Kimberley-Pilbara corridor, and to discuss trade-offs between different goals for landscape management, and between alternative management actions for stopping toads. In the end, a wide range of stakeholder groups attended, including academics, NGOs, indigenous groups, and employees from several state and federal government departments.

“Our preliminary modelling suggests that we would only need to manage around 100 water points to stop their spread. A barrier of this size in the right location would stop the invasion dead in its tracks.”

Outcomes

Reid Tingley discusses the feasibility of stopping the spread of toads at a workshop in Broome.

Reid Tingley discusses the feasibility of stopping the spread of toads at a workshop in Broome.

Overall, the road trip and workshop were a great success. We received positive feedback about the idea of a waterless firebreak to halt the spread of toads and now have a better understanding of the landscape we’re trying to model. However, perhaps the most promising aspect of the discussions was the realisation that a waterless barrier might create numerous opportunities for ‘win-win’ situations among environmentalists, pastoralists and indigenous communities. For example, this idea could present an opportunity to improve infrastructure and water usage on pastoral stations, while implementing and monitoring a barrier could provide potential employment opportunities for indigenous ranger groups in the area. We also managed to raise awareness of our work through several media interviews.

Now that the workshop is behind us, there is plenty of work to do. The first step is to refine our maps of water bodies and tighten up the modelling. Clearly, if we ever decided to spend large amounts of money implementing this project, we’d want to be confident in the models’ ability to make reliable predictions. Then over the next 6 months, we’ll use the model to estimate how much a project like this might cost, find the most cost-effective location for a barrier, and test how robust our decisions are to various sources of uncertainty.

So watch this space! There’s plenty of exciting work to come!


 

More info: Reid Tingly reid.tingley@unimelb.edu.au

Reference

Tingley R, BL Phillips, M Letnic, GP Brown, R Shine &SJE Baird (2013). Identifying optimal barriers to halt the invasion of cane toads Rhinella marina in arid Australia. Journal of Applied Ecology 50: 129-137.

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