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Cambridge University Science Magazine

If you have ever flown to Australia, watched the TV series Nothing to Declare, or heard about the saga of Johnny Depp’s dogs in 2015, you will know that Australia has some of the strictest biosecurity laws around. But just how dangerous is your egg salad sandwich or favourite pet that you are trying to sneak in?

Invasive species introduced during early British occupation have wreaked havoc on Australia’s biodiversity, with these issues still ongoing today. Red foxes, brought in for hunting, carried disease and caused a decline in small mammal and bird populations. Rabbits, also introduced as game, had no natural predators, and despite thousands of miles of ‘rabbit-proof’ fence, they spread to the furthest reaches of the country. Wild horses, deer, and even feral camels continue to contribute to the erosion and depletion of Australia’s native plant species.

One of the most interesting cases is that of the cane toad in the 1930s. Australia’s northern sugar cane industry was faced with a ravenous population of cane beetles, eating away at its crops. Despite some early concerns about environmental impact, industry-mounted political pressure won out in 1935, successfully lobbying to bring in the Central American cane toad as a form of biological control. The great hope was that the introduced cane toads would simply eat the cane beetles, keeping beetle numbers at bay, the sugar cane intact, and solving the industry’s pest problem. There was even a successful case study to work off — a Nature article at the time, entitled ‘Toads Save Sugar Crop’, hailed their triumphant introduction as pest control in Puerto Rico.

To say the plan failed is a massive understatement. The stomachs of cane toads found near sugar cane crops did in fact contain cane beetles, but they also contained the remains of plenty of other species, as they left the dry open fields for more plentiful food-diverse areas. They feasted on unintended insects, like ants that consume cane beetle larvae, cancelling out some of their intended pest-control benefits. But the problem was not only due to what the toads did and did not eat, but also what ate the toads. Large native species that eat cane beetles, like lizards and goannas, started to eat the cane toads and suffered poisoning from the toxic glands on the toads’ backs. Another indirect consequence was that rats, which ate both the cane toads and the valuable sugar cane, thrived. Rats were one of very few species immune to the cane-toad toxins and increased in number, while the predators of rats, such as goannas, suffered severe population declines. Overall, the cane-beetle populations in Australia were left essentially unchanged, while the cane toads flourished.

Their numbers boomed in Australia from those first few thousand released to now well over 200 million — that’s eight cane toads for every person in Australia. Cane toads were found to have some odd commercial uses, like as a vehicle for early pregnancy testing in the 1950s and cane toad leather, which you can still buy today. Cane toads are so prolific that they have even become part of the Australian identity: a state rugby team is nicknamed after the pest, ‘toad golf’ is a questionable eradication method, and even toad-themed festivals exist with competitions for the largest one caught. This boom in cane toad numbers has been accompanied by rapid evolutionary developments, including longer legs on toads found at the frontier as their territories expand at approximately 60 km per year. The species has also developed cannibalistic tendencies in this short span of time, likely due to their large numbers and lack of predators, leading to competition for resources.

This competition for resources, including shelter, extends to native frog and bird species, like the ground-nesting rainbow bee-eater, which now produces one third fewer fledglings due to nest usurping in some areas. While the cane toad has not been directly linked to any species extinction, it has a distinct effect on animal populations when it encroaches into new areas. A 75% reduction in northern quoll populations has largely been attributed to their consumption of cane toads. 

Intriguing evolutionary adaptations have also occurred in native Australian species to cope with the cane toad. Crows and kites are known to turn cane toads over onto their backs to avoid contact with their poisonous glands, leaving the skin uneaten. ‘Australia’s otters’, known as rakali, use almost surgical precision, making incisions only near the edible organs and thigh muscles, expertly extracting and discarding toxic organs like the gall bladder, leaving in their wake what looks a rogue outdoor dissection class. Rakali also specifically target only the largest toads to get the greatest food reward for their hard work, with 75% of carcasses found in this way belonging in the top 3% of cane-toad size.

But if the cane toads were such a failure, why then were the toads promoted as a success story in Puerto Rico? It could be mostly due to the vast differences in ecosystems between the two locations. Another theory is that the initial Nature article failed to account for external factors that would reduce cane-beetle numbers, unique to the situation in Puerto Rico. Following the cane toad's introduction, there were several years of uncommonly heavy rainfall, which reduced the survival of cane beetle larvae underground. In either case, differences between results in Australia and Puerto Rico demonstrate that success in one location does not necessarily predict success elsewhere. The sugar industry failed to heed this lesson multiple times, as it had previously introduced other species to Puerto Rican cane fields in an attempt to control pests. Introduction of the small Indian mongoose (initially praised as a success in reducing rats in Jamaican cane fields) later led to a boom in ticks, declines in native species, and the mongoose becoming a prominent rabies carrier. Similarly, the initial, hailed success of cane toads as a pest-control species led to ‘cane-toad fever’ and their introduction and proliferation in different ecosystems all over the Pacific Region, with varying results.

Cane toads have since been named by National Geographic as one of the most damaging invasive species in the world. Research into control methods such as trapping, release of sterilised males, genetic modification, physical barriers, and training of native species with ‘taste aversion’ is ongoing, but reports by the Australian government conclude that the eradication of the species is unlikely and some of its damage irreversible.

Something to declare?

Actively preserving native biodiversity is increasingly important as climate change threatens habitats and the spread of disease becomes easier across the globe. Maintaining the delicate balance of our environment has intrinsic value, as we depend on it for clean air and water, pollination, waste breakdown, food, and, of course, natural pest control. One driver of biodiversity loss that could be curtailed is instances of large corporations and industries, through a desire for ever greater profits, ignoring the potential environmental impacts of their decisions. Parallels can be drawn between the role of the sugar industry in lobbying for cane-toad introduction in Australia and pesticide companies such as Monsanto-Bayer lobbying for relaxed pesticide regulations worldwide, despite application being linked to declining bee populations. The cane toad example also illustrates how overstated claims, oversimplification of complex processes, and lack of publication of negative findings in major science journals can have far-reaching, unintended consequences.

More than ever, we need further research and legislation to safeguard our natural ecosystems for future generations. Even though we have come far with conservation and research to reverse past damage, you cannot unscramble the egg.

Monica Killen is a final-year PhD student in clinical neurosciences at Selwyn College. Illustration by Biliana Tchavdarova Todorova.