Last Resort: Moving Endangered Species in Order to Save Them

Original publication by Zach St. George for e360.yale.edu on 27 October 2022

Scientists have long warned that climate change and other threats will require relocating some endangered species outside their historic ranges. Now, U.S. officials are proposing rules that would enable them to use this new — and potentially controversial — conservation tool.

A critically endangered Guam rail. The species was relocated to boost its odds of survival.
SBS ECLECTIC IMAGES / ALAMY STOCK PHOTO

The Guam rail, also known as the ko’ko’, has all the architectural grace of a wood-paneled Buick station wagon. Mostly brown, with white longitudinal stripes on its head and a houndstooth pattern around its midsection, it has a crow-sized chassis, tiny wings, and long, chickenish legs. It is narrow in profile, designed for a life of scurrying through the underbrush. The rail cannot fly, and it nests on the ground.

This way of life suited the rail fine until humans released brown tree snakes onto its native island of Guam. In the early 1970s, the bird began a “drastic numerical and distributional decline,” according to the U.S. Fish and Wildlife Service, and by 1983, fewer than 100 individual birds remained.

In the mid-1980s, Fish and Wildlife and a consortium of zoos gathered up what birds they could find and began breeding them. Guam rails, it soon became clear, are willing captive breeders. In 1989, the agency proposed releasing some of the birds back into the wild — not on Guam but on the island of Rota, some 40 miles away, which had not, to anyone’s knowledge, ever had Guam rails.

The planet’s wildlife is in precipitous decline. The World Wide Fund for Nature and the Zoological Society of London published a study this month estimating that the populations of some 5,200 vertebrate species have declined by an average of roughly 70 percent since 1970. A 2019 report from the United Nations warned that human activity threatened as many as a million species with extinction. The leading cause of this decline is habitat loss. Humans have displaced countless species, directly or indirectly modifying much of the Earth’s surface — more than 70 percent of its land, according to another recent United Nations estimate.

With increasing numbers of species at risk of extinction, says one conservationist, “we may need this tool a lot more often.”

At the same time, people have purposefully or accidentally introduced invasive species to habitats around the world. The brown tree snake, which devoured Guam rails, along with other native birds, lizards, and bats, is a classic example.

Now climate change is further altering the habitats of the world’s species —warming lakes and oceans, transforming forests to grassland and tundra to woodland, and sending glaciers flooding into the sea. This spring, the Intergovernmental Panel on Climate Change released a report estimating that as much as 14 percent of the tens of thousands of terrestrial species in its assessment could face extinction if the world warms 1.5 degrees Celsius above pre-industrial levels, which it is now on track to do.

In response to these changes, lifeforms are rearranging themselves, migrating to follow shifting conditions. But many species, especially those that have already lost much of their habitat, like the Guam rail, may need human help to reach places where they can survive.

Currently, the U.S. Fish and Wildlife Service — which, along with the National Marine Fisheries Service, enforces the Endangered Species Act — rarely moves threatened and endangered species beyond where they are known to have occurred in the past, as it did with the Guam rail. Its regulations allow it to establish “experimental populations” of species only in their “probable historic range,” except in extreme cases.

A rare Florida Torreya. Native to Florida and Georgia, the endangered tree has been planted throughout the eastern
U.S. JASON SMITH

Recently, though, Fish and Wildlife proposed a revision to its regulations that would allow it to move species beyond their historical range, calling this a “necessary and appropriate” step in response to the twin threats of climate change and invasive species. With increasing numbers of species at risk of extinction, says Tim Male, founder of the nonprofit Environmental Policy Innovation Center, “we may need this tool a lot more often.”

The ability to move species beyond their historical range would be a small change on paper, but one that both supporters and opponents of the revision say could significantly impact American conservation.


When Congress passed the Endangered Species Act in 1973, it gave the National Marine Fisheries Service and the Fish and Wildlife Service the ability to move species from one place to another, a practice called “translocation.”

“The agency knew for species like the bald eagle, they were going to have to translocate individuals from Alaska to the Lower 48, where it had been extirpated in many locations,” says Patrick Shirey, an environmental scientist and legal scholar at the University of Pittsburgh. “Congress gave the explicit authority for translocation.”

It further clarified this authority in a 1982 revision to the Endangered Species Act, giving the wildlife agencies the ability to create “experimental populations” of listed animals and plants. While Congress offered no limits on where the agencies might establish experimental populations, stipulating only that they must be physically separate from “non-experimental,” or wild, populations of the same species, Fish and Wildlife provided limits of its own. In regulations it adopted in 1984, the agency restricted itself to establishing experimental populations only within a species’ “probable historic range.”

The fossil record shows the arrangement of species is continually in flux, especially during times of climatic change.

“Historic range” was both ill-defined and Eurocentric, in a country where written records extend back less than six centuries, but where human history — and ecological impact — extends back tens of thousands of years. It also took a static view of the living world, seeming to assume that the past range of a species represents that species’ ideal range. This view was at odds with the fossil record, which showed that the arrangement of species is continually in flux, especially during times of climatic change. It was also increasingly at odds with the reality of climate change in the present.

Since at least the 1980s, scientists have debated the merits of a very specific type of translocation called “assisted migration” (and sometimes “assisted colonization” or “managed translocation”), which aims to conserve species by moving them to climatically suitable places outside of the range that they currently or recently occupied. Some scientists argued that habitat destruction would make it impossible for many species to keep up with the pace of climate change, and that without human help those species could face extinction. Others, pointing to the destruction wrought by invasive species, maintained that moving species to new habitats as a conservation method was too risky.

Decades later, despite hundreds of academic papers and countless media reports on the topic, though, there are relatively few real-world experiments in assisted migration. Scientists have moved butterflies, lichens, and rock lobsters, along with a handful of other creatures. A group of private citizens planted the endangered Florida Torreya, an evergreen in the yew family that is native to riparian areas in Florida and Georgia, far to the north, throughout the eastern United States. A number of timber companies, state and federal forestry agencies, and tribes in the U.S. and Canada have moved trees, most of them common species.

Gray wolves were reintroduced to Yellowstone National Park in the 1990s.
YELLOWSTONE NATIONAL PARK

The Fish and Wildlife Service’s regulations appear to provide for the possibility of assisted migration experiments. In “extreme” cases where the agency’s director determines that a species’ critical habitat has been “irreversibly altered or destroyed,” the agency may establish an experimental population outside of the species’ historical range. The agency relied on this exception to move the Guam rail to Rota.

“That’s pretty clearly unsuitable and irreversible,” Shirey, of the University of Pittsburgh, says, speaking of the rail’s habitat in Guam. “A nonnative, invasive predator that eats everything, and a bird that can’t fly away.”

But few cases are so clearcut or so legally defensible. In a 2010 paper with Gary Lamberti, an ecologist at the University of Notre Dame, Shirey argued that while this exception seemed to allow for assisted migration experiments of rare, endangered species, in most cases it would be too difficult to prove that a species’ habitat is irreversibly altered or damaged, or that its situation was “extreme.” As the biodiversity crisis progresses, the barrier for designating cases as extreme is getting higher. Fish and Wildlife spokesperson Karen Armstrong noted that it is increasingly routine for climate change and invasive species to threaten species’ habitat. “These are no longer ‘extreme cases,’” she wrote.


In 2018, Fish and Wildlife began to consider changing its regulations, and this past June it published its proposal to remove the words “historical range” from its regulations on experimental populations.

“What we think is extreme today may be common in a fairly short period,” an ecologist says of the concept of moving species.

The proposed rule drew more than 500 comments. Many were critical of the proposed changes, calling them unnecessary or misguided, suggesting that they would allow the service to release endangered species — and the laws and regulations they bring — nearly anywhere in the country.

Some were concerned about the possible reintroduction of wolves and other predators, noting that the wolves that Fish and Wildlife reintroduced into Yellowstone and parts of the Southwest in the 1990s have killed ranchers’ cattle and sheep. Other commenters echoed longstanding arguments against the use of assisted migration, suggesting that translocating creatures carried too much risk of unintended consequences.

Ben Novak, lead scientist at Revive & Restore, a nonprofit focused on using cloning, gene editing, and other genetic technologies in conservation, says that fears that new experimental populations might turn into invaders are overblown. While translocated species have caused ecological disasters — as when people brought cane toads from Hawaii to Australia, for example, or introduced Indian mongooses to the West Indies — nearly all of those species were translocated for economic or cultural reasons, not as part of conservation efforts, Novak says.

Last year, Novak and his colleagues published a paper analyzing the purposeful translocations of more than 1,000 species in the U.S. over the last 125 years as part of conservation efforts. They found only one conservation translocation — of a fish — that resulted in the loss of significant biodiversity.

An Indian Mongoose. Brought to the West Indies in the 19th century, the species devastated native reptile populations.
PHOTOTRIP / ALAMY STOCK PHOTO

In the late 1980s, Fish and Wildlife moved 200 endangered watercress darters from its native Black Warrior River drainage, near Birmingham, Alabama, into Tapawingo Springs, 15 miles northwest. But Tapawingo Springs, it turned out, was home to another rare species, the rush darter, which was only described as a species in 1998. By 2001, the now-thriving population of watercress darters had wiped out the spring’s population of rush darters (although the species persists elsewhere). Such mistakes are rare, Novak says. “I actually think ecologists have a strong history of making predictions. We can do this well.”

In its proposal, Fish and Wildlife did not make an explicit connection between its proposed ability to establish experimental populations outside of species’ historical ranges and the decades-old debate over assisted migration. But evolutionary geneticists Janna Willoughby and Avril Harder did, in a comment they submitted to Fish and Wildlife with other members of the lab that Willoughby leads at Auburn University. The biodiversity crisis means regulators “need to seriously consider conservation actions that are currently deemed too extreme,” Harder said. Willoughby agreed: “What we think is extreme today may be common in a fairly short time period.”

Mark Schwartz, a conservation scientist at University of California, Davis who was an early skeptic of assisted migration, said that it’s hard to say exactly what effect Fish and Wildlife’s rule change will have on conservation efforts. On the one hand, he said, the relative dearth of assisted migration experiments may be due less to the current legal barriers and more to a lack of scientific and societal consensus on the practice, making wildlife managers reluctant to use it. On the other hand, he said, “I do think the lack of policy has hindered experimentation, in that it is often not an option on the table because of a lack of policy that permits it.”

A Guam kingfisher. Extinct in the wild, the species is being introduced to an island near Guam. 
RGB VENTURES / SUPERSTOCK / ALAMY STOCK PHOTO

Schwartz was part of a team that developed a risk-analysis framework for the National Park Service to use in considering potential assisted migration experiments, published last year. He is now working with the Fish and Wildlife Service’s Pacific Region subdivision to develop a similar framework for that agency.

Gwen Iacona, a conservation scientist at Arizona State University, said that Fish and Wildlife’s proposed rule change is useful, but what would really make the Endangered Species Act more effective is more funding. “Lack of funding is far and away the biggest limitation to ESA recovery,” she said. “The ESA has the capability — still — to do its job well, but we as a society have to give it a chance by funding the actions that its implementation demands.”

Karen Armstrong, at Fish and Wildlife, said that the agency could not say when it would publish a final version of the rule change, nor did it formulate the rule with any particular listed species in mind. But she did note that it recently proposed moving another species outside of its historical range, citing extraordinary circumstances: the Guam kingfisher, or sihek.

The kingfisher is as boldly appointed as the Guam rail is unassuming. It has dark blue wings, an orange head and body, and a dark stripe over its eyes like an old-fashioned burglar’s mask. The kingfisher was driven to extinction in the wild by the brown tree snake and has persisted since the early 1980s only in zoo-run breeding programs. But these programs are at capacity. To make space, and prepare for the day when Guam is rid of brown tree snakes, Fish and Wildlife plans to release a small population of kingfishers into the wild — not on Rota, but on Palmyra Atoll, 3,650 miles to Guam’s east.

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