The Tide of Toads Plaguing Australia
Chris - Now another invasive species is the cane toad. They were introduced to Australia in 1935 from South America and the idea was to try and control insect pests that were laying waste to Queensland's cane fields. Unfortunately, the result has been a massive disaster. Bufo marinus, which is the Latin name for a cane toad, can live for fifteen years and it produces 30 000 offspring per toad every single year. As well as being highly invasive, it's also extremely toxic. This is having a devastating effect on Australian native species that are particularly partial to a toad-sized meal. Sydney University's Rick Shine has been watching the advance of the tide of toads as they move west across Australia.
Rick - In studies at the invasion front of this feral toad that's roaming across Australia, we discovered that they're moving much faster now than they were when the invasion started. Looking at the movement patterns of toads up in the bush near Darwin, we've found that longer legs help toads to move faster. Sure enough, the toads at the invasion front have got substantially larger legs than the ones further behind.
Chris - And it's taken 50 or 60 years for them to develop this trait?
Rick - The toads were introduced to Australia about 70 years ago on the other side of the continent from where we're working on them, so they've covered about a million square kilometres over 70 years. From our work, it looks as though they're getting faster and faster.
Chris - So how fast does a toad hop in the average day?
Rick - If you look at the toad, it really doesn't look as though it's made for speed. When we first started, I'd expected that a toad would be pretty tired after moving 50 metres in a night. But when we strapped little radio transmitters onto these guys at the invasion front, we found that they were moving often half a kilometre or a kilometre in a single night. They keep moving in the same direction. Basically, they seem to have a compass in their pocket and they're just heading west.
Chris - That's an extraordinary distance for something which is five or six inches long to have covered in a night.
Rick - I'm astonished at how far the toads can move. These first toads that arrive at the invasion front are really incredible active animals. They spend the day hidden in grass, but as soon as the night falls, they go straight back to the main road, face west and start hopping.
Chris - Now you mentioned the invasion front. What have you actually done to watch these guys as they make their progress west?
Rick - Well myself and my colleagues have been mostly studying the snakes at an area not too far from Darwin for about 20 years. We've got a study site that we actually understand pretty well. The toads have just arrived. As a toad arrives at our study site, we're out there every night looking around catching the toads, we strap a small radio transmitter with a little waist band around to hold it on and let the toad go again. We then locate the toad every day. That way we build up a picture of where they travel, how far they travel and what direction they're going in.
Chris - And what about the study on the leg length? How have you made that finding?
Rick - Well we were interested in the fact that toads seem to run along the road rather than in the bush. The obvious reason was that you can move much faster on the road. So we ran some very simple trials over short distances and we discovered that toads are quicker in the open ground but also a toad with longer legs is quicker than a toad with the same body length but shorter legs. We thought that in an invasion, the animals at the front will be the fastest ones. There will be continuous selection for the fastest animal. Now if that's the case, what you should find is that the toads at the front should be faster than the toads further back, and from our little series of race way trials, we thought it might turn out that the toads at the front might have longer legs than the guys at the back. Rather to our astonishment, that's exactly the pattern that we found.
Chris - Are there any disadvantages that go with longer legs?
Rick - It's really intriguing that leg length decreases consistently with how long toads have been in an area. If we look at the samples that have been taken and put in museums over a 60 year period in Queensland, it's clear that relative leg length is decreasing year by year. Now that really does suggest that there's some cost to having long legs but we have no idea at the moment what that cost might be and we're' very interested in what that might be. We're trying to run a series of trials to test ideas.