Longer-legged toads are rapidly evolving and threatening an eco-nightmare, say scientists. Dick Ahlstrom reports.
More than a million square kilometres of northern Australia have been overrun, literally, by an invading species of toad. Alarmingly, the wide-open spaces of Australia seem to be having an effect on the toads' evolution, encouraging longer leg growth in support of a quicker-paced invasion.
The introduction of the cane toad (Bufo marinus) to Australia more than 70 years ago has been "disastrous", according to researchers from the University of Sydney, who describe the changing "invasion front" posed by the toad this morning in the journal Nature.
An outsider brought in to help combat insect pests on sugar cane crops, the toxic and highly invasive toads have adopted Caesar's dictum, Veni, Vedi, Vici.
"The disaster looks set to turn into an ecological nightmare because of the negative effects invasive species can have on native ecosystems, and over many generations, rates of invasion will be accelerated owing to rapid adaptive change in the invader," the authors write.
The adaptive change they speak of is longer legs. In B marinus, longer legs mean faster legs and more rapid colonisation. The researchers sought to confirm the evolutionary changes supporting a more rapid invasion by putting themselves in the path of the toad's "invasion front" 60km east of Darwin in north Australia.
These toads can put in an "astonishing locomotor performance" despite their substantial size, the authors state. They weigh up to two kilograms but can still manage to cover almost two kilometres in a single night.
There they sat waiting for B marinus's arrival and as expected they found that the first toads to show up had longer legs than those that arrived later. "As the toad invasion front passed our study site, we measured relative leg lengths of all toads encountered over a 10-month period," they said. "Longer-legged toads were the first to pass through, followed by shorter-legged conspecifics. Longer-legged toads therefore moved faster through the landscape." If their assumption that the tall ones were the faster ones was correct, then toads at the invasion front should have longer legs than those bringing up the rear, they stated. "As predicted, longer-term historical analysis within Queensland populations shows that relative leg length is greatest in new arrivals and then declines over a 60-year period." All of this points to more trouble in the future, because the authors' third premise was that the rate of progress of the toad invasion "should increase through time" as evolutionary change supported the emergence of fleeter-footed toads.
This in fact is happening, they found. "Toads expanded their range by about 10km a year during the 1940s to 1960s, but are now invading new areas at a rate of more than 50km a year." This five-fold increase in the annual rate of progress of the toad invasion has played havoc with assumptions about the spread of the pesky toad. "Accordingly, previous predictions about the time course of future expansion of the toads' range seriously underestimate their actual rates of movement," the authors say.
The rush to conquer new territory is exerting an evolutionary force that is "likely to fine-tune organismal traits in ways that facilitate more rapid expansion of the invading population", the authors conclude.
"These rapid shifts in toad morphology, locomotor speed and invasion velocity indicate that conservation biologists and managers need to consider the possibility of rapid adaptive change in invading organisms." If the toads can do it, so can other invading organisms in other ecosystems. "Hence, control efforts against feral organisms should be launched as soon as possible, before the invader has had time to evolve into a more dangerous adversary," the authors conclude.