Despite everything that had been done to save them, the quolls were still dying. And this time, it wasn’t for the expected reason.
Quolls are small Australian mammals that superficially resemble ferrets, but are more closely related to kangaroos and koalas. Being hunters, they tackle all kinds of small prey. But if they attack cane toads—large invasive amphibians that were introduced to Australia in the 1930s—they get a mouthful of poison, and inevitably die from violent seizures and heart attacks. As the toads spread through the continent, the northern quoll—the smallest of the six species—started disappearing. In some places, 90 percent of them vanished. In other sites, they went completely extinct.
To save the species, conservationists collected 64 northern quolls in 2003 and relocated them to two toad-free Australian islands: Pobassoo and Astell. In these sanctuaries, the animals flourished. Within a decade, they had become so abundant that scientists began to think about reintroducing them to their former homes. Ben Phillips, from the University of Melbourne, started training the island-born quolls to avoid cane toads by feeding them with toad sausages laced with noxious chemicals. Sure enough, when released on the mainland, the trained quolls avoided the toads, while untrained individuals quickly died. The plan was coming together.
“But we encountered a new problem,” says Chris Jolly, who’s also from the University of Melbourne and works with Phillips on the reintroduction project. “We were losing the quolls to predators. I would be radio-tracking the quolls and sometimes, I’d end up radio-tracking a dingo with a GPS collar in its belly.”
Dingoes are feral dogs that have been in Australia for around 3,500 years, and quolls have had plenty of time to adapt to their presence. But island-born quolls, which had never encountered dingoes, have no instinctive fear of them—or of other lethal threats, like cats. In an experiment, Jolly showed that mainland quolls mostly stayed away from food sources that were laced with hair from dingoes or cats, while island-born quolls were largely unperturbed.
More surprisingly, the offspring of the mainland and island quolls showed roughly the same pattern as their parents, even though all of them had been raised in captivity and none of them had experience of predators. Those born to mainland parents were wary of eau de dingo; those born to island parents were not. This suggests that the quolls’ aversion to predators is at least partly genetic. It’s an innate trait that the island population has lost.
“From a conservation perspective, this isn’t good news,” says Melanie Massaro, from Charles Sturt University. “When these predator-naïve animals are reintroduced into an environment where predators are present, they are easy prey.”
On predator-free islands, animals often evolve into tamer versions of their mainland ancestors. But scientists tend to assume that this process is slow, taking place over timescales that are irrelevant for conservation. The quolls suggest otherwise. Northern quolls live for just a year, and 13 generations have come and gone on Astell and Pobassoo Islands. That’s all it took for these animals to lose their fear of dingoes, creating a population that is now ill-suited to life on the mainland. “It’s surprising,” says Jolly.
How could this change have happened so quickly? Jolly suspects that the quoll’s anti-predator instincts were linked to other psychological traits, like timidity and risk aversion, that proved too costly on their island homes. Astell and Pobassoo are small places with few types of prey. The relocated quolls soon multiplied beyond the ability of the islands to sustain them, and many were forced to find new sources of foods.
“When we collected these quolls, many of them had completely worn down their teeth from eating hermit crabs and foraging out in the reef,” says Jolly. “Only the very boldest of these animals would have survived and passed on their genes.” Perhaps this unforgiving filter—this survival of the boldest—kept out the genetic variants that made quolls wary of dingoes and other threats.
“In Australia, we’re focusing too much on separating native species from exotic predators, and putting them on islands and behind exclusion fences,” says Katherine Moseby, from the University of New South Wales. “These are great insurance populations, but unless we aim for future coexistence, we are just exacerbating prey naivety and making the problem worse.”
Moseby and her colleagues have been trying to address this problem in an unusual way. They raised endangered marsupials like bilbies and bettongs in fenced enclosures that also contain a small number of neutered cats. The cats kill some of the marsupials, but the survivors retain the ability to avoid these predators, and are more likely to survive when reintroduced.
This strategy “removes the least capable of those animals from the population,” says Jolly. “In the short term, it’s not the prettiest thing to lose some of the animals you’re trying to conserve. But if it’s the animals’ interests in the long term, I think it’s a necessary evil.”
Moseby says that people should consider doing the same thing on islands: introducing neutered cats to direct the evolution of the sheltered species. “While this is unpalatable to many conservationists, the evidence is mounting for a change in the way we manage our threatened species,” adds Moseby.
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