Why Are Cane Toads a Problem in Australia? An Invasion Story

A pest control plan that backfired

In the early 20th century, the sugar industry in Queensland had a serious pest: the grey-backed cane beetle, whose white grub larvae chew the roots of sugarcane. The cane toad, native to Central and South America, had a reputation as a beetle-eater. It had been introduced to Puerto Rico in the 1920s and to Hawaii in 1932, and sugar growers there believed the toads helped.

In June 1935, 102 cane toads arrived at a sugar research station at Gordonvale, near Cairns, from Hawaii. They were bred, and from August 1935 thousands of young toads were released into cane fields around Cairns, Innisfail and other towns. After a brief pause while officials debated the risks, releases resumed in 1936, and tens of thousands more toads were set free.

The plan did not work. Adult cane beetles live on the upper parts of the cane, where toads cannot reach them, and the grubs are underground. Cane fields also offered little daytime shelter for the toads. There is no convincing evidence that the toads ever reduced beetle damage. Instead, they found Australia almost perfect: warm, wet summers, abundant insects and almost no predators adapted to their poison.

How far have cane toads spread?

The invasion started slowly and then accelerated.

Today, cane toads occupy well over a million square kilometers, from northern New South Wales through Queensland and the Northern Territory into the Kimberley. Estimates around 2011 put the total population at more than 200 million.

In the early decades, the front advanced about 10 km (6 mi) a year. By the time toads crossed the Northern Territory, it was moving roughly 50 to 60 km (30 to 37 mi) a year. Researchers discovered why: toads at the leading edge had evolved longer legs and traveled farther and in straighter lines each night than toads in older populations. The fastest toads kept ending up at the front together and breeding with each other. This speed has a cost, though: some front-line toads develop spinal arthritis.

Why are cane toads so deadly to native animals?

The toad’s defense is a milky toxin made by large parotoid glands behind its eyes and by smaller glands across the skin. It contains bufadienolides, compounds that disrupt the heart. All stages carry toxin, including eggs and tadpoles.

Predators in the Americas evolved alongside the toad, and many either avoid it or can tolerate its toxin. Australia has no native toads at all, so its predators had no such defenses. A goanna, quoll or snake that grabs a large toad can be dead within minutes to hours.

The hardest-hit groups are:

• Northern quolls. These cat-sized marsupial predators have vanished from many areas after toads arrived, including parts of Kakadu National Park. The species is now listed as Endangered.
• Monitor lizards (goannas). Populations of yellow-spotted monitors dropped by up to 90 percent in some areas after the toad front passed, and Mertens’ and Mitchell’s water monitors also declined sharply.
• Snakes. Frog-eating snakes such as king brown snakes (mulga snakes) and death adders are especially vulnerable.
• Freshwater crocodiles. Some river populations suffered heavy die-offs as crocodiles ate toads.

Ripple effects through the ecosystem

Removing top predators changes everything below them. When goannas declined, some of the animals they normally eat, such as certain smaller lizards, became more common. In parts of the Northern Territory, fewer goannas raiding nests meant that more saltwater crocodile eggs survived. These knock-on effects show how one invasive species can reshape a whole food web.

Cane toads also eat enormous numbers of insects and other small animals and compete with native frogs for food and breeding pools, though these effects seem much smaller than the poisoning of predators. Their eggs and tadpoles can poison native fish and aquatic insects that eat them. In towns, pet dogs that bite toads are often poisoned too.

Not all bad news: Australian wildlife fights back

The story is not one of total collapse. Several predator populations that crashed when toads arrived have partly recovered in the following years or decades. Some native animals have learned to avoid toads, and some have even evolved.

• Red-bellied black snakes living with toads for decades have become more resistant to the toxin and less likely to eat toads, and studies found that some toad-exposed Australian snakes evolved relatively smaller heads, which stops them from swallowing the largest, most toxic toads.
• Some crows and kites flip toads over and eat only the nontoxic organs.
• Rakali, the Australian water rat, learned within a couple of years to remove the toxic skin and glands before eating.
• Meat ants attack and kill young toads near water.

Scientists are also testing ways to teach vulnerable predators such as quolls and goannas to avoid toads before the invasion front reaches them, with promising results in some trials. The Australian government has listed the toad’s impact on native wildlife as a “key threatening process” under national environmental law.

Lessons from the cane toad

The cane toad has become a textbook warning about introducing a species without understanding how it will behave in a new place. Australia has faced similar problems with other introduced animals, from European rabbits to red foxes; see how did rabbits get to Australia? for a parallel story. For the native animals at risk, browse our list of Australian animals.

The toad is not just an Australian issue. It is also invasive in Florida, Hawaii, the Caribbean and many Pacific islands; read are there cane toads in Florida? for the American side of the story.