Watch Out, Kangaroos: Poisonous Cane Toads Are Evolving Into Even Deadlier Invaders
Not everything about evolution is good. Case in point: Invasive cane toads in Australia have started evolving to jump straighter and farther than ever, allowing them to quickly expand into new regions and creating an even greater threat to Australia's native wildlife.
Cane toads were first imported to Australia from Hawaii nearly 80 years ago, when the first 100 amphibians were brought to northern Queensland to fight a native beetle devastating the sugarcane crop. That didn't work. There are now an estimated 200 million cane toads in Australia, and they have spread across thousands of miles. The massive invaders not only eat anything smaller than them but also secrete a highly toxic venom that is deadly to anything that tries to consume them, whether a kangaroo that inadvertently swallows one or a snake on the hunt for dinner.
The toxic toads have proved particularly devastating to native predators such as the northern quoll, a cat-size marsupial, and a monitor lizard called the goanna. By the 1980s, the cane toad had reached Kakadu National Park, a cradle of biodiversity in the Northern Territory. “It is evident that a major decline of the northern quoll has occurred in Kakadu National Park, and will continue to occur, and that northern quolls may disappear from Kakadu National Park altogether within the foreseeable future due to the invasion of the cane toad,” the Australian Department of the Environment stated in a 2005 report. One recent study found that populations of Argus monitor lizards declined by 90 percent after cane toads arrived in their habitat. Even large predators such as freshwater crocodiles are not immune.
At first the cane toads' spread was relatively slow, but some have picked up the pace in recent decades, and the Australian government predicts they might reach the west coast of the country. According to research published on Oct. 8 in the Proceedings of the Royal Society B, that's because the toads are rapidly evolving. Those on the furthest edges of the "invasion vanguard" now hop in a relatively straight line and travel nearly twice as fast as those in areas where the toads have lived for years.
This vanguard expands its territory 34 to 40 miles a year. The frogs closer to the original Queensland infestation site, however, still meander in search of prey, meaning they expand their territory more slowly, about 25 miles per year.
Lead author Gregory Brown, a postdoctoral researcher with the School of Biological Sciences at the University of Sydney, explained that this evolutionary adaptation is likely because cane toads breed so prodigiously—a single female can lay thousands of eggs at a time. During the species' life cycle, the period when they metamorphose from tadpoles into small toads poses the greatest risk.
"There is likely strong pressure to disperse away from breeding sites," he said. One threat comes from the toads themselves: "Little cane toads are highly cannibalistic and will happily eat smaller siblings as they emerge from the water." Other predators may also take advantage of the high density of young toads to get an easy if potentially deadly meal.
This means the toads that jump the farthest and the straightest get away from predators most easily. They then pass that trait to their offspring, which also jump straighter and farther. This magnifies the effect of the invasive species as it expands into new territories over successive generations.
Brown and his fellow researchers came to their conclusion after capturing and breeding toads from several regions. Those they collected in Queensland gave birth to frogs that hopped in a more "tortuous path," as Brown put it, indicating they were putting more effort into finding prey.
The toads collected at the edges of the species' spread, in the Northern Territory, produced toads that hopped so straight and so fast they quickly disappeared. "We put transmitters on them and lost all our toads because they went hurtling over the horizon," fellow researcher Rick Shine told the Australian Broadcasting Corporation.
This speeding-up trend is unlikely to slow, Brown noted. He said the "front line toads" in all likelihood have lost their meandering mode and now just jump straight forward. "The meandering toads still exist, but it takes them a few years post-invasion to start showing up." By then, the vanguard toads have already hopped farther forward, leaving more ecological devastation in their wake.