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A Catastrophe of Cane Toads

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The 200 million cane toads now in Australia provide a cautionary example of the intractable problems that an invasive pest can create in a naive but hospitable environment. In 1935, cane toads (Bufo marinus) were imported to Australia from Hawaii, where they had been established with toads originally sourced from central America. During 1935–1937 the progeny of 101 imported toads were widely distributed in the Australian state of Queensland, in a desperate but misguided effort at biological control of scarab beetle pests of the sugar industry. By 1940, massive beetle infestations were still occurring in places where toads were well established, and it was obvious that the strategy had not been successful. But there was no going back… some 75 years later the beetles still cause problems to the sugar industry – and the toads have created a major problem that continues to unfold across the continent. In hindsight, it seems tragic that early concerns about release of the toads were not given more attention. In January 1936, the President of the New South Wales Naturalist Society, Walter Froggatt, wrote that "immune from enemies, omnivorous in its habits, and breeding all year round, (the toad) may become as great a pest as the rabbit…". It is estimated that there are now 200 million toads in Australia. Their spread across the continent has been well documented, and has now extended by thousands of kilometres to include major areas of the eastern and northern coast, including the iconic World Heritage-listed Kakadu National Park, significant inland areas of the continental north-east, and some off-shore islands. In addition, they are continuing to spread into north-western Australia at a rate of more than 50 kilometres per year; toads reached the Kimberley region of Western Australia and the massive Lake Argyle complex in 2009. On the other side of the continent, a shire council in Sydney is currently attempting to eradicate a breeding population of toads apparently established by animals accidentally transported from infested areas. Predictions of the area over which toads may eventually spread are daunting, and factors such as climate change, as well as the largely unchallenged capacity of the toad for adaptation, are likely to expand this area further. The impacts of toads in Australia are well-documented, and in overview can be considered to occur at domestic and landscape-scale levels. In both instances the major impacting factor is the toads' production of a highly toxic mixture of cardiac glycoside poisons as a means of defence. All stages of the toad's life cycle are poisonous to some degree, with adult toads being highly dangerous.

In adult toads the poisons are stored in glands in the skin, particularly in the enlarged parotid glands above the shoulders, from where they can be secreted in quantity when toads are threatened or attacked. Animals that ingest toad toxins by consuming, or even mouthing, a toad are commonly poisoned. In domestic settings, significant numbers of pet dogs are poisoned by toads; in most economic and social impacts. While achieving this objective frequently involves reducing overall numbers of pests, it can sometimes also be attained in other ways, such as use of exclusion fencing, or direct protection of identified high-value physical or biological assets. Nevertheless, in this instance, the significant biological capabilities and broad geographic distribution of the toad make it extremely difficult to advance this intention. As might be expected with a pest of this status, there have been substantial expenditures on a variety of plans, schemes and activities intended to assist toad management, none of which has yet had broad-scale effect.
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Document Type: Research Article

Publication date: 01 October 2011

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