What’s the trouble with toads?
Currently ranked at 16th out of 100 by the International Union for the Conservation of Nature’s (IUCN) Invasive Species Group, cane toads are one of the world’s worst invasive species. They are naturally found in Central and South America, where a range of natural predators keeps their populations under control.
The toads have a voracious appetite, eating almost anything that will fit in their mouths. They mostly eat invertebrate and small vertebrates but have also been found to eat plants and even pet food.
Their appetite was recognized by twentieth century scientists as being potentially useful in controlling insect pests that were decimating crops. As a result, the toads were initially introduced to islands such as Puerto Rico and Hawaii to protect farms.
Encouraged by these early experiments and despite some early warnings, in 1935 the toads were introduced into Queensland, Australia, in an attempt to control pests on sugar cane plantations.
Since being introduced to the northeast of Australia, the toads have spread in all directions. As the amphibians are toxic at all stages in their lifecycle and with no natural predators, they reached Brisbane within a decade and have since spread across much of northern Australia.
Despite efforts to destroy their eggs, amid suggestions of using viruses, walls and even handbags to stop them, the cane toads continue to advance across the island. The Australian government estimates that their range is expanding by as many as 60 kilometers a year as they adapt to life right across the country.
As they’ve spread, the toads have had a devastating impact on Australia’s wildlife. Their poison has killed many native predators, leaving their prey to proliferate. Anything the amphibians don’t poison they try to eat, with birds such as the rainbow bee-eater particularly hard hit as the toads eat their chicks.
However, Australian species are fighting back. Some crows have learned to flip the toad onto its back so they can eat its soft, non-toxic underbelly. Some snakes are adapting to the threat by becoming more resistant to the toad’s toxins, while others have become smaller so that their mouths are no longer big enough to swallow the amphibian.
Now numbering among these species are the freshwater crocodiles, which have been observed to change their behavior in response to the cane toads. Following anecdotal evidence, a group of Australian scientists decided to investigate the claims more closely.