In the 1930s, Australia released cane toads to fight the beetles; Today 200 million control the country

Australia’s inland edges remain indifferent to change, with heat remaining above ground for long periods and movement appearing slow even when it is not. However, in these same places, something introduced nearly a century ago was steadily rewriting its boundaries.

The cane toad arrived with a very specific mission attached to it, carrying within it both scientific optimism and agricultural urgency. This intention did not survive contact with the landscape. What followed was a spread that never stopped, just adjusted its pace, finding water wells, roadside ditches, suburban lawns, and everything in between. And somewhere along this expansion, the animal itself began to transform in small but measurable ways.

Australian cane toads were released to control beetles in sugarcane fields

As Australian Geographic reported, in the 1930s, cane toads were released in northern Queensland as a form of biological control. Sugarcane fields were under pressure from the beetles, and the idea was simple enough on paper: introducing a predator might keep the pests under control. The account did not hold up for long. The frogs ignored the beetles that prompted their journey and instead made use of whatever was available, while quickly adapting to a country that offered far fewer natural controls than their native range.

What began in a narrow agricultural context expanded almost immediately. Releases doubled in the first years, and animals moved outward faster than anyone had anticipated, crossing ecological boundaries against which they had no real defense.

How does reproduction feed? The cane toad has expanded into Australia

Over time, their presence extended from the tropical north to vast inland corridors. Queensland became an established region, and from there, it penetrated into parts of the Northern Territory, the Kimberley region and extensions of New South Wales.

Numbers are now counted in the hundreds of millions, although the exact number changes depending on rainfall and breeding conditions.Part of the explanation lies in how quickly they reproduce when conditions allow. The reproductive cycle is short and intense. The tadpoles congregate in dense groups in shallow waters, feeding and growing in conditions that would limit many native species. Once they reach adulthood, females can release thousands upon thousands of eggs in a single clutch, and this can happen more than once a year.Native frogs in the same areas tend to operate on a much smaller scale. Their reproductive output is lower, and their timing is often closely linked to specific seasonal periods. Cane toads don’t seem to have the same self-control. When there is water they use it, and when it is gone they wait for it in the soil or shelter until it returns.

How did cane toads in Australia begin to evolve on the move?

In recent decades, attention has turned not only to how widespread cane toads are, but how fast they are now moving.

There is a marked difference between populations at the center of their range and those at the edge of expansion. One feature that attracted particular attention was leg length. Frogs at the increasing limits of their distribution have been observed with longer hindlimbs compared to those found in older populations. It’s a subtle transition and not a dramatic one, but it’s an important one in terms of movement.

Longer legs allow longer jumps and greater sustained travel across open ground.The result of these changes appears in the prevalence rate. It has reportedly spread to a population of 200 million frogs. Previous estimates estimated movement at about ten kilometers per year in some areas. More recent observations indicate that the leading edge can now cover distances several times greater than that.

Australia’s ongoing struggle to manage the spread of the cane toad

Efforts to manage cane toads in Australia have taken many forms over the years, most with limited long-term success.

Physical removal can succeed locally, but rarely extends across the vast areas these species now inhabit. Barriers and traps tend to provide only temporary relief.More experimental approaches have emerged in recent years. One of these involves what is sometimes described as adaptation of local predators to avoid cane toads. In parts of northern Australia, scientists have experimented with exposing baby goannas to small, controlled encounters with frogs. The idea is not to completely eliminate the threat, but rather to allow the reptile to experience the frog’s toxicity without fatal consequences, so that it learns to avoid it later in life.