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The toad we love to hate

For some unfathomable reason, cane toads stir the popular imagination. Most invasive species are simply not noticed by most of us, or, if they are, they are quickly assimilated into our mental landscape…

Australia’s best known pest. AAP Image/Dave Hunt

For some unfathomable reason, cane toads stir the popular imagination. Most invasive species are simply not noticed by most of us, or, if they are, they are quickly assimilated into our mental landscape and forgotten. Who watches a honey bee flitting between dandelions and thinks “invasive species, both of them”?

Very few invasive species generate even a flicker of concern, and of those that do the cane toad is surely the one that attracts the most visceral of reactions: people either love them or hate them.

Why are toads here? What is their real impact on Australia’s native species? And what, if anything, can be done to stop them? Research over the last decade has revealed substantial new insights into these questions.

Toads were brought to Australia in 1935 by scientists working for the sugarcane industry. Australian toads had a long journey. They were originally native to South America, and came to Australia on a tide of enthusiasm for their ability to control beetle pests of sugarcane. This enthusiasm was ill-founded and naturally receded, but by then toads were already ashore in Australia as well as 22 other countries throughout the Caribbean and Pacific.

The introduction of toads in Australia occasioned little fanfare and generated little concern. Following the toads' initial introduction in north Queensland, however, a lone but powerful voice (belonging to one Walter Froggat) raised fears that toads “may become as great a pest as the rabbit or the cactus” and that “there is no limit to their westward spread”. Froggat’s agitation caused an immediate ban to be placed on the further release of toads. This ban, in turn, caused the sugarcane lobby to exert political pressure. Within months word came from the Prime Minister’s Office that the ban was to be lifted, and toads were immediately liberated as far afield as Mackay and Bundaberg. There was no stopping the sugar industry’s saviour now.

Since then, of course, Froggat’s predictions have proved prophetic. Toads now occupy more than 1.4 million square kilometres of the continent. Their range is continuous from northern NSW to the eastern Kimberley, and they continue to invade new areas. Sugarcane beetles are now controlled with insecticides.

Cane toads grow to be larger than any native Australian frog, but they also carry with them powerful cardio-active toxins that no native frog species produces; it is this toxin that inflicts the most environmental damage. There is little doubt that as toads spread they caused the death of countless thousands of native predators - quolls, goannas, crocodiles, snakes, and frogs - which, naive to the toads' toxic nature mistook them for something edible.

Despite this carnage, however, toads have never, to the best of our knowledge, sent a native Australian species to extinction. There have certainly been local extinctions, particularly of quolls, goannas and some snake species, but in at least some places populations of native predators persist. How they do so is a fascinating story of its own, involving a diverse range of responses including rapid evolution and learning. Additionally, when toads remove goanna predators from a system, many other species (whose eggs and adults are preyed upon by goannas) may end up benefiting.

Nonetheless, for some native predators - goannas, snakes, and quolls in particular - the arrival of toads is dire news indeed. We shouldn’t have introduced toads and we should do what we can to limit their impact. Unfortunately, the toads' staggering reproductive potential (5,000 - 30,000 eggs at a sitting) makes them a daunting adversary.

To make matters worse, toads have evolved to become highly dispersive during their invasion of northern Australia, where adult toads will regularly move 50km in a wet season. This combination of high reproductive rate and impressive dispersal ability makes controlling toads a formidable (some might say impossible) challenge. Compared to toads, rabbits, foxes and cats look easy.

Nonetheless, research on the basic ecology of toads has unearthed many surprising facts, some of which can be bent towards toad control. Perhaps the most grisly of these is that toads are cannibalistic, and toad tadpoles (toadpoles) like nothing more than a snack of fresh toad egg. To find fresh toad eggs, toadpoles home in on the very same toxin that makes toads so dangerous to our native predators. Thus, a simple yabby trap, baited with soluble toad toxin, will rapidly catch thousands of toadpoles but won’t attract anything else.

Local control strategies such as this, when executed at the right place, can have profound benefits. By denying toads access to 100 artificial waterbodies in the right place, for example, we can totally prevent the natural spread of toads into Western Australia’s Pilbara region. Doing so would prevent the natural spread of toads into 268,000 square kilometres of suitable habitat (an area the size of Great Britain) and help secure some of the last healthy mainland populations of quolls and goannas that are currently unaffected by toads.

Love them or hate them, toads are here to stay. Their biodiversity impact is probably less than the public imagination would have it, but also substantially more than we would like.

New strategies, built off basic research into the ecology of toads, provide new means of reducing their local abundance, and even of halting their spread across northern Australia. Executing these strategies will require substantial participation and interest from the public. Not the kind of interest one might get for proposals to control dandelions, but interest of the more intense and visceral kind; the kind of interest aroused by toads.

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7 Comments sorted by

  1. George Michaelson


    It surprised me to read that local extinction has not yet equated to species extinction. If true, this poses questions about habitat separation, and the possible defense other species have in being mobile into zones toads don't frequent, and a longer-term adjustment of learned behaviour in some cases (is it the case that mammals are observed to pass on acquired dislike of toad poison to their young?)

    The toad trap reads as promising, but I am led from the pop-sci knowledge of myxy and colisi virus to ask if this is something toads themselves might acquire a survival trait to avoid: if they are cannibals, but have a gene variant which preferences other dietary traits, then traps will over time, preference selection for this gene, leading to localized sub-speciation into non-cannibal preference breeds, which then survive and move on...

    the wheel of life..

    1. Ben Phillips

      QEII Fellow / Senior Lecturer at University of Melbourne

      In reply to George Michaelson

      I agree George, it is surprising, and it does get one thinking. The work done on Quolls in northern Australia (Jonno Webb, Rick Shine and others; is really encouraging regarding the ability of mammals to learn toad avoidance. Word on the street is that some of these trained quolls are doing very well in toad-infested areas and there is the hint that the next generation are surviving well. To my knowledge, nothing published on this latter result yet, but watch that space!

      As for evolutionary responses to toad traps. Maybe. Although you'd have to be trapping a very large proportion of the toad population to really create a selective force strong enough to overcome the inevitable influx of (non-selected) immigrants. Strikes me that toad trapping is never likely to act on the same ubiquitous scale as a biological control agent.

  2. Neville Mattick
    Neville Mattick is a Friend of The Conversation.

    Grazier: ALP Member at A 4th Generation Grazing Station

    A really interesting read, with some hope, thank you Ben for sharing it.

    It is appalling what Humans' have done, only last weekend I was shocked at the distribution of Serrated Tussock near the NSW city of Bathurst, almost right in the City limits - a powerful intruder, Serrated Tussock stands to displace many Native Grasses and the biodiversity dependant on them; how I fight its presence on my Station!

    A sad fact of introduction among many is Gambusia (Mosquito Fish), released into the catchments…

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  3. Peter Hobbins

    PhD Scholar at University of Sydney

    It's good to hear of some innovative approaches to toad control, although at some point we're going to have to acknowledge that they are a permanent feature rather than temporary interlopers.
    Almost all of the research and rhetoric I've read about cane toads, moreover, focuses on their role as 'invasive species'. In other words, the major concern appears to be the expansion of their biogeographic range and the disruption they cause to 'virgin' ecosystems. It's a frontier mentality.
    What's hinted…

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  4. Tim Comber

    logged in via Facebook

    How about the Hudson's Pear, a type of cactus? Escaped from a plant nursery near Lightning Ridge, NSW in the 1960's, it has become a major pest around the Ridge and it is spreading further afield. You'd think that by the 60's we would have enough sense to not introduce new weeds into Australia!

  5. Andrew Smith

    Education Consultant at Australian & International Education Centre

    Is it not ironic that the immigration and internal migration of problem causing pests such as cane toads, rabbits, cats, foxes, various weeds, etc. are ignored by the anti immigration and anti population growth lobbies, (& large parts of the media), who voice concern for the environment, bio diversity and "carry capacity"?

    Science, human ingenuity and innovation have been good at solving problems and issues, provided we are made aware of issues that need to be dealt with, as opposed to politcal sideshows.

  6. Greg Spiers

    Retired Ranger

    I wish you luck and good fortune with lessening the range and destructive influence of what are now 'our' cane toads and their effect on biodiversity, massive local species destruction (if not extinctions yet) of native animals and habitat (habitat relies on its balanced occupants for sustainability in natural ecosystems), but I do have faith that at some time in the, I hope, not too distant future our scientific endeavour will find a way. We must remain optomistic. I commend your initiative and…

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