Imagine the scene; the camera pans from a Petri dish of wriggling larvae to a sexy blonde scientist who is peering at the animals through a microscope. She looks up, terror-stricken, and says into the camera `My God, they're evolving!' While TV and movie scenes like this make most biologists chuckle,Australian conservation biologists who are working to contain an invasion of exotic cane toads may not find them funny. This is because a recent paper by Phillips and colleagues has demonstrated that introduced cane toads in Australia have evolved in the 70 years since their arrival in ways that are accelerating their conquest of the continent.
Cane toads (Bufo marinus) are a South American species that was introduced to Queensland in 1935 as a strategy for controlling insect pests in sugar cane fields. Like many of the stories involving intentional introduction of species by humans, the cane toad story is a textbook illustration of how ecological short-sightedness can have disastrous long-term consequences. Not only did the cane toads fail to control the cane beetles they were recruited to keep in check, but they are currently eating their way across Australia,outcompeting native frog species (Australia has no endemic toads), and pushing many endangered species to the brink of extinction. The toads continue to expand their range at an alarming rate in spite of intense efforts to control their spread to the rest of the country.
To investigate toad dispersal, the authors of this paper tracked cane toad movements using radio telemetry and found that they can move up to 1.8 km per night during the rainy season. Cane toads are one of the world's largest anuran species, but this kind of movement is truly remarkable among anurans,and it got the authors wondering if natural selection has resulted in a population of toads that is better adapted at dispersal. Their reasoning was that the introduction of cane toads to Australia has resulted in a unique opportunity for the toads, where prey is plentiful and experienced predators are rare. Because the proliferation of toads in a given area will eventually diminish prey availability, those toads that can continuously move into virgin territory will be richly rewarded for their efforts in terms of reproductive output.
It is known that leg length in anurans correlates positively with hopping speed, so the authors first tested whether this was true in cane toads, and indeed it was - longer legged toads covered more ground per night. To test whether long-legged toads reach virgin territory first, the researchers set up shop just in front of the invasion front and measured leg lengths of the toads as they swept past. Sure enough, the toads with the longest legs tended to arrive first, and the ones with the shortest legs tended to arrive last,suggesting that long-legged individuals enjoy the benefits of dispersal into new territory more than their lagging short-legged conspecifics. While these results strongly suggest positive selection for long-leggedness in the toads,it doesn't prove that the population is actually evolving. For this, the authors looked to historical records and preserved specimens, and concluded that leg length has increased significantly in cane toads since their arrival. The authors suggest that increased leg size has resulted in a population of cane toads that is better adapted for dispersal than their founding ancestors. This is consistent with historical records that show a fivefold increase in the rate of range expansion by the toads since they were introduced, from 10 km per year originally to over 50 km per year now. This study sends a sobering message to conservation biologists that exotic introductions are best dealt with swiftly, before the founder population has a chance to evolve into an even more formidable opponent.
