As long as it has been known to science, the diminutive western swamp tortoise has been in peril. By the time it was formally named in 1901—using a decades-old museum specimen—Pseudemydura umbrina was presumed extinct. And since it was rediscovered in the 1950s, biologists have struggled to protect it from the twin threats of habitat loss and introduced predators, which drove its numbers to bottom out at just 30 individuals in the 1980s. Now that climate change poses an even more urgent threat to the endangered tortoise, biologists have a controversial plan to safeguard its future—by moving it to new sites outside of its known historical range. The translocation, which took place today, makes the tortoise the first vertebrate to be deliberately relocated because of climate change.
The yearlong trial, several years in the planning, will track 12 captively bred juvenile tortoises released to each of two sites roughly 250 kilometers south of their native habitat on the outskirts of Perth, Australia. Although the sites aren’t ideal for the tortoises now, detailed modeling of rainfall, temperature, swamp hydrology, and tortoise biology predict they will be in half a century.
The trial will be a contentious test case in conservation circles. Introducing nonnative species into new ecosystems has an ignominious history, not least in Australia, where the deliberately introduced European red fox and domestic cat have wreaked havoc on native wildlife. But the idea of assisted colonization has gained some favor over the past decade as conservationists grapple with the impacts of rapidly changing climate on habitat suitability for numerous flora and fauna.
For the western swamp tortoise, whose numbers in the wild are now estimated at just 50 breeding adults, declining rainfall is the primary concern. The 15-centimeter-long tortoises feed when rains fill swampy habitats in winter, and then enter a state of dormancy known as estivation when the swamps dry out to clay pans in the late spring or early summer. The less rain in winter, the more likely the hatchlings and juveniles will starve before the next winter rains arrive. Swamps that were wet for 5 to 7 months of the year in the 1960s are now often dry for most of the year, and rainfall is set to decline further in the future. Hemmed in by urban sprawl and agricultural land, the tortoises can’t up and move, either. “It’s a double whammy,” says conservation physiologist Nicola Mitchell from the University of Western Australia in Perth, who is leading the trial.
Herculean conservation efforts—carving out nature reserves, establishing captive breeding programs, pumping water into swamps in dry years—have done little to shore up the tortoise’s future in the wild, which is why Mitchell and her colleagues have turned to assisted colonization. “It was time to try something a little bit out there,” she says.
“It’s a bold thing to do and it’s a good thing to try," says conservation biologist Hugh Possingham from the University of Queensland, St. Lucia, in Brisbane, Australia, who is not involved in the trial.
But others worry that such introductions could disrupt existing species, and that negative impacts may not become apparent for decades. Anthony Ricciardi, an invasive species biologist at McGill University in Montreal, Canada, sees assisted colonization as “ecological gambling.” The impacts of assisted colonization—“planned invasions,” he calls them—are notoriously difficult to predict.
“Any conservation action you take is in itself questionable and should be questioned,” says Gerald Kuchling from the Western Australian Department of Parks and Wildlife in Perth, who has headed the swamp tortoise recovery efforts for the past 28 years. “But at some stage, you have to do a trial to actually find out what it means to assist them in colonizing completely new areas.”
Even if the tortoises prove benign, as Mitchell and Kuchling anticipate, Ricciardi fears that such trials set an alarming precedent. “Other people are going to be looking at [this trial] and they're going to be saying, ‘OK, what about my species?’” he says. “I wouldn’t like to spin that roulette wheel very many times.”
Assisted colonization has already been used for plant species, such as Torreya taxifolia, an endangered conifer native to the Florida panhandle. Conservationists in New Zealand are also weighing the strategy to save the endangered hihi, or stitchbird (Notiomystis cincta), which faces an uncertain future if not moved to cooler climates farther south.
But Possingham doubts the floodgates will open any time soon. “It’s not open slather,” he says, and scientists will judge each proposed colonization on its merits. For Possingham, who thinks assisted colonization will become a “pragmatic” conservation tool to prevent extinctions, results of the trial can’t come soon enough. “Unless we try these things, we’ll never learn how to do them, so the sooner the better.”