Chasing Mr Toad

If there was a time in my life when I was absolutely dripping in sweat, then this was it.

There were beads of the stuff all over my face, and I was leaning into a blast of musty air con, only moving away to gulp down some electrolytes. I suppose that may sound like hell, but honestly, I was having the time of my life.

Welcome to fieldwork.

I was taking shelter in a dusty 4WD on a cattle station, sitting on the coast between Broome and Port Hedland in Western Australia. More importantly, I was chasing down a rather infamous Australian pest: the Cane Toad (Rhinella marina).

[caption id="attachment_82" align="alignnone" width="3888"] ...and here we have Mr Toad wearing a stunning ball chain belt complete with radio transmitter. Photo: R Hayes[/caption]

While us ecologists are usually keen on keeping our study species around, I was wishing this particular Mr Toad would go back to South America, and leave our poor native predators alone. Unfortunately, that ship sailed in 1935, and over the past 80 years the toad has thrived in our landscape.

With no natural predators, the toad is free to breed more efficiently than rabbits, and spread wherever the climate permits it. Any native animal attempting to take a bite of a toad at best end up reviling at the taste, and at worst are taken out by the toad’s toxin, resulting in species declines. The toad is now established in the north of Western Australia, and is expected to continue moving south-west towards the Pilbara, in the south of Western Australia.

This is what brought me to a cattle station in the middle of Western Australia.

The station sits within the so-called Kimberley-Pilbara corridor, an area which should be inhospitable to toads during the dry season, when all the natural water dries up. But this is cattle country, and so there are man-made water sources everywhere. And many of them leak.

[caption id="attachment_85" align="alignnone" width="3888"] Leaking pastoral tanks can result in massive pools of cane toad happy-places. Photo: R Hayes[/caption]

Previous computer simulations had predicted that by effectively removing these water sources (by upgrading infrastructure and preventing leaks), a barrier would be created, preventing toads from moving southwards towards the Pilbara.

But this idea needed on-ground testing. Enter me: a bright-eyed master’s student, keen for a challenging field project.

After half a year of writing and reviewing grant applications, ethics proposals and Park’s permits, I flew to Kununurra in Western Australia to collect 20 male toads and drive them down south to a cattle station within the barrier area.

Yes, I was moving a non-native animal into new territory; a horrible idea in most ecological circles. But these toads were all males (to prevent any breeding), and we were releasing them in a remote area far away from available water sources. More importantly, the toads were going to be radio-tracked intensively as part of my study, so the risk of establishment was ultimately minimal.

The waterless barrier strategy assumes that toads will be able to survive within the corridor during the wet season with no issues. During the dry, however, artificial water points (i.e. dams and tanks) should be the only water available to toads, meaning their removal will prevent their survival and create a barrier. The barrier simply needs to be wide enough to account for toad dispersal through the unrestrictive wet season.

But there were a few untested assumptions around previous work. Were we sure toads couldn’t survive here without those artificial water sources? What about shelter sites, such as goanna burrows? What about coastal fogs providing moisture through morning dew?

To answer these questions, I started by attaching a radio transmitter to each toad by measuring out little belts made from a ball chain and clipping them around the toads’ waists. They were released at night into the desert, far away from any artificial water sources.

[caption id="attachment_87" align="alignnone" width="3888"] A gorgeous landscape to be working in. Absolutely filled with ankle-twisting and toad-sheltering goanna burrows. Photo: R Hayes[/caption]

The following morning myself and a research volunteer rose early and went out to catch up with the toads. Waving an antenna around like madmen, we listened desperately for the tell-tale BEEP leading us to a sheltering toad.

One of the ways toads have adapted to the Australian landscape is through being active at night and sheltering during the day. In doing this they avoid the worst of the heat, reduce their water loss, and increase their survival ability.

We were therefore expecting the toads to shelter during the day, allowing us to determine which shelter sites they were selecting within the landscape. For the most part, they did (60% of the time, in fact), however, we found one headstrong individual not only not sheltering, but hopping along during the heat of the day!

Indeed, even sheltering toads moved substantial distances overnight, far above the distances we expected. One avid sprinter managed to move over 2.5 km in one night!

It's unsurprising then that occasionally a BEEP could not be found, and we had no choice but to trudge through the desert in 500 m blocks, heading up to 3 km away from our starting point, all in search of one singular spirited cane toad and the valuable data he represented. Never have I desired a dune buggy more.

Even once we tracked the BEEP to its source, finding the toads was not always easy. The toads were brilliant at finding shelter sites within the landscape, and within this landscape the majority of shelter sites were goanna burrows.

This meant that we often had to stick an inspection camera down the burrows to confirm exactly where the toads were. This also helped us to avoid sticking our hands down into potential snake territory...

Once we finally found our toad, I weighed them to approximate their water loss, and recorded their GPS coordinates and shelter site type, such as a goanna burrow or shrub.

As part of our ethics requirements, we planned to humanely euthanise toads once they had lost 40% of their body weight (toads can survive losing 50%). Unfortunately, due to their unexpected speediness (and the harshness of the corridor environment), we were unable to locate many toads before they reached critical dehydration.

While an undoubtedly grim experience for the toads (and an emotional and philosophical trial for their human followers), this short survival time is ultimately a positive outcome for the waterless barrier strategy.

[caption id="" align="aligncenter" width="2128"] Plots showing survival (i.e., proportion of individuals alive) over time. (a) Overall survival. Dashed lines indicate 95% confidence intervals. (b) Effect of habitat on survival. * indicates censored points where a lost toad was last seen alive[/caption]

Most toads only survived 1-2 days, and our analysis predicts a maximum survival time of 5 days in this environment, supporting the assumption that toads cannot survive in this area without access to artificial water sources.

In the end, this project indicates that the waterless barrier is a feasible means to halt cane toad movement down the west Australian coast. This relatively inexpensive strategy could therefore be an effective way to protect species in the Pilbara from the toad's impact.

For my part, this project was my first taste of real field work and my first experience within Australia's vast semi-arid and arid environments, which I've since fallen in love with. If you're someone who has never left the eastern coast I absolutely insist you go out and explore more of this exceptional country.

I myself can't wait to get back out there.

Simple study question: Can cane toads survive in a potential barrier area without access to artificial water bodies during the dry season?

Simple study answer: No. The model predicts a maximum survival of 5 days in this environment. (Gregg et al. 2019).

Importance: Supports the waterless barrier strategy, allaying fears that toads would be able to survive in the barrier despite the removal of artificial water bodies.

In a word: Barrier