Australasian Science: Australia's authority on science since 1938

Expansion of Invasive Species Underestimated

By Stephen Luntz

The rapid expansion of invasive species through spatial sorting is not being incorporated into predictions of their spread, according to A/Prof Rick Shine of the University of Sydney’s School of Biological Sciences.

Shine identified spatial sorting as a new form of evolution through research on cane toads entering his study area at Fogg Dam (AS, June 2011, pp.16–19). He revealed that toads at the invasion front have longer legs and were more mobile than those further back. Inevitably this means fast-moving toads interbreed, producing even faster offspring.

“It is a very common observation that invasions accelerate,” Shine says. “Often after a species arrives nothing much happens for quite a while and then expansion becomes rapid. Cane toads are better documented than most, with lots of records of when they first arrived in towns.”

The phenomenon extends beyond animals, with pine trees at an invasion front having unusually light seeds that disperse great distances.

Despite these longstanding observations, Shine’s team reported in PNAS that predictions for invasion rates are still being made based on movement patterns of individuals that are far from the front. “People don’t usually look for species in locations where the chance of finding them is small, because they have only just arrived,” Shine says.

The lack of studies on other species means that the differences in speed have not be quantified elsewhere, but Shine observes: “Toads at the invasion front had more than twice the overall yearly displacement of cane toads tracked a few years later at the same site. So, studies that only look at dispersal rates of animals in established populations may underestimate the rate that they can spread into new territory.”

Shine also showed that while toads in established territory travel randomly, the pioneers “spend a larger proportion of their time covering long distances in a consistent direction”. He describes this as “a real puzzle. One can see how a gene that makes them move in a straight line encourages dispersal, but what makes Harry the Toad move west rather than north or south isn’t clear.”

There is a price to pay for such movement: Shine found that pioneer toads suffer spinal arthritis and were more likely to be eaten by predators, but says that confirmation has not been found in other species – probably because the research has not been done.

One positive from this research is that estimates of a species’ capacity to keep up with its preferred temperature range in the face of global warming are also likely to be too low.