First things first: if you have never watched the 1988 documentary “Cane Toads: An Unnatural History” by Mark Lewis, do yourself a favor and pick up a copy as soon as possible. You can thank me later.
If you have seen “Cane Toads,” then you know all about the warty, highly toxic, yet strangely endearing scourge of northern Australia. Cane toads (Bufo marinus) are native to Central America and northern South America, but were introduced to the Cairns area in 1936 in an ill-fated attempt to control an outbreak of beetles that were ravaging sugarcane crops. The toads failed to control the beetles, but managed to become serious pests themselves, breeding in prodigious numbers and eating large numbers of endemic reptiles, amphibians, and invertebrates.
Cane toads are among the world’s largest toads; a big female can measure eight inches long or more, and that female might lay 25,000 eggs in a clutch. Cane toads will eat anything they can fit in their mouths. Their skin – particularly the bulbous parotid gland perched on each shoulder – contains a powerful compound called bufotoxin. But despite these less-than-attractive characteristics, cane toads have a certain undeniable appeal. Perhaps it’s their rotund shape and cute hopping gait. Or maybe it’s their trusting nature; they don’t seem to mind humans much, and many individuals even allow themselves be picked up.
In summary: an adorable ecological nightmare.
In a new study, published in the Journal of Evolutionary Biology this week, Benjamin Phillips and colleagues suggest that cane toads might actually be evolving to become even better invaders. Previous research has shown that the rate at which cane toads are spreading across northern Australia has accelerated fivefold since their initial introduction. And individual toads near the invasion front disperse farther and faster than individuals living closer to Cairns, where they were first introduced.
Evolutionary theory predicts that natural selection will favor enhanced dispersal abilities at the fringes of an expanding population. Here’s how: first, imagine that toads produce more offspring at low population densities than at high population densities (this is a common pattern in nature, and it’s called density-dependent population growth). When the population density is high, there aren’t enough resources – food, breeding sites, whatever – for every individual to grow and reproduce at its maximum potential rate. But when the population density is low, there are plenty of resources to go around.
At the geographic center of a large population, dispersing might not gain you much – wherever you go, there will still be other members of your species around. But at the edge of an expanding population (like that of the cane toads), dispersing might mean finding competitor-free space. If this is the case, then the individuals with the greatest dispersal abilities will tend to produce the most offspring. And over time, natural selection will lead to increased dispersal abilities at the edge of an expanding species range.
The existing evidence suggested that this might be happening in the cane toads, but one vitally important question remained unanswered: did the toads at the edge of their expanding range actually evolve their enhanced dispersal ability? Or did they simply disperse farther because they had better opportunities to find competitor-free space? This is the question that Phillips et al. set out to answer.
The researchers began by collecting adult cane toads from four populations that varied widely in distance (zero to 1636km) from the initial introduction site. These adults were fitted with radio-tracking devices and released at a site near Darwin. Their movements were recorded for five nights before they were retrieved. Then these adults were allowed to mate, and their offspring were collected and raised in a common environment. As the young grew, they had to be re-sorted by size fairly often to prevent cannibalism!
By raising all the toadlets in the same conditions, the researchers removed the influence of the external environment; any remaining differences in the dispersal abilities of these young toads could be attributed to genetic differences among their parents. When these youngsters were big enough to be radio-tracked themselves, they were released, just as their parents had been, and their movements tracked accordingly.
The results of this breeding experiment suggest that there is heritable variation in dispersal ability within each of the four toad populations – better-dispersing parents tended to have better-dispersing offspring. This is a prerequisite for evolution by natural selection. And the offspring of “range edge” parents – those collected near the edge of the toads’ expanding range – generally dispersed farther than the offspring of “range core” parents – those collected near Cairns, the initial introduction site. The young toads varied in dispersal distance despite an identical rearing environment. All of this evidence supports the idea that natural selection has favored enhanced dispersal abilities in toads near the edge of the species’ expanding range.
What does all of this mean? Well, it suggests that a rapid range expansion – often a consequence of a biological invasion – can create conditions favoring the evolution of increased dispersal abilities, potentially accelerating the spread of an invasive species. One more reason why invasive species are bad news!
Phillips, B. L., G. P. Brown, and R. Shine. 2010. Evolutionarily accelerated invasions: the rate of dispersal evolves upwards during the range advance of cane toads. Journal of Evolutionary Biology 23:2595-2601.