Assisted migration

• 1. Introduction

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  Assisted migration is the intentional establishment of populations beyond the boundary of a species' historic range, for the purpose of tracking suitable habitats through a period of changing climate. It sounds like science fiction: humans deliberately picking up a tree or tortoise and carrying it hundreds of kilometers to a new home, because the climate where that creature evolved is no longer safe. Yet this is precisely what conservation biologists, government agencies, and even self-organized citizen groups have been doing in recent decades. The question driving them is urgent. Scientists have predicted that a failure to migrate or adapt will result in about a quarter of the world's species dying out this century under moderate climate change. The natural dispersal rates for many species are far slower than the pace of projected habitat shifts. For trees, amphibians, and corals especially, the gap between how fast they can move and how fast the climate is moving may already be insurmountable without human help. Two species illustrate the stakes most starkly: Australia's western swamp tortoise and America's Florida torreya tree, both listed as critically endangered partly because geographic barriers prevent them from moving on their own. Their stories, and the fierce debate surrounding the very idea of moving species on purpose, will shape how conservationists respond to the coming decades of climate disruption.

• 2. Where the Idea Came From

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  The term assisted migration first appeared in conservation biology publications in 2004. It described a type of species translocation intended specifically to reduce biodiversity losses driven by climate change. Within a few years, two near-synonyms joined the vocabulary: assisted colonization arrived in 2007, and managed relocation in 2009, the latter introduced in a paper with 22 coauthors. The parallel conversation in forestry science was older and less contentious. Paleoecologists had already documented significant lags in the northward movement of dominant canopy trees across North America in the thousands of years since the final glacial retreat. By the 1990s, forestry researchers were applying climate change projections to tree species distribution models, and the probable distances of future range shifts were drawing attention. Forestry practitioners were also accustomed to "seed transfer guidelines" that governed which seed sources could be used after a timber harvest. The provincial government of British Columbia became the first to update those guidelines with what they called "climate-based seed transfer." Because migration was already the standard term in paleoecology for natural tree movements recorded in the geological record, the concept arrived in forestry with far less controversy than it stirred in conservation biology. Debate in forestry was, in the assessment of those who observed both fields, both short-term and muted by comparison. The divergence between the two fields was distinct enough that a separate entry on assisted migration of forests in North America was launched in 2021 as a teaching tool for climate adaptation education.

• 3. The Heart of the Debate

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  Beginning around 2007, pro and con positions became clearly visible in conservation biology, even while the broader public remained largely unaware the debate was happening. Supporters argued that the expected benefits of assisted migration, including saving and strengthening species, outweigh the potential harm of any project. Detractors countered that other conservation techniques, ones that do not carry the risk of introducing invasive species, are both better suited and more likely to succeed. The principal fear is that a relocated species could spread disease or become too successful in its new habitat, crowding out native species. This concern draws force from the long history of accidental introductions that turned invasive. Yet those who favor keeping assisted migration available point to a revealing dataset. Mueller and Hellman reviewed 468 documented species invasions and found that only 14.7% occurred on the same continent where the species originated, and within that fraction, the vast majority involved fish and crustaceans. Terrestrial species that became invasive on the same continent typically crossed large biogeographic barriers such as mountain ranges, circumstances quite unlike the short-range within-continent moves proposed for most assisted migration projects. Ricciardi and Simberloff invoked the precautionary principle, arguing that any unknown risk of creating invasive species, no matter how small, is disqualifying. Many scientists pushed back, noting that the precautionary principle cuts both ways: the risks of extinction from not facilitating migration must be weighed against the risks of acting. A 2022 review by seven researchers in the United States tracked how the academic literature had shifted. Publications became less focused on the pros and cons of the concept over time, and more attention moved to modeling or mapping where particular species could actually be moved. Animals, including corals, invertebrates, mammals, and birds, had overtaken plants as the focus of recent years. Even so, the 2022 review found that most authors presented assisted species migration as appropriate only for species under exceptionally high threat from climate change.

• 4. Florida Torreya: Citizens Move First

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  The Florida torreya, Torreya taxifolia, is a critically endangered tree of the yew family found at the state border region of northern Florida and southwestern Georgia. A self-organized group of conservationists called the Torreya Guardians formed in 2004, taking advantage of a plant-specific exception in the Endangered Species Act of 1973 to begin moving this glacial relict tree northward, even while the official recovery plan had not yet authorized such action. Their project unfolded before governmental programs caught up to the idea, making it an unusual case where citizens led and agencies followed. By 2018, the Torreya Guardians had documented the species thriving at a dozen legacy horticultural planting sites, including seed production and next-generation saplings at several sites in North Carolina. That same year, their own plantings in Cleveland, Ohio, began producing seeds. The group also gathered documentation of historic groves in northward states and used that evidence in a 2019 petition to downlist the torreya from endangered to threatened. A decision came two years later with no change in the species' imperiled status, but Factor E of the official decision, titled "Documentation of Historical Groves," acknowledged the citizens' work. Early scholarly debates over assisted migration as a climate-adaptation tool frequently cited the Florida torreya project, and so did international media, making this citizen-led experiment one of the most-discussed examples in the entire field.

• 5. Western Swamp Tortoise: Planned and Precise

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  The western swamp tortoise, Pseudemydura umbrina, is endemic to a small portion of southwestern Australia. It was considered extinct until rediscovery in 1954, and reported in 1981 to be "a relict species of a monotypic genus, of very restricted range and specialized habitat." Its significance in conservation history is specific: it became the first endangered vertebrate experimentally translocated to a distant location expressly because of climate change, moved 300 kilometers poleward. By the time assisted migration trials began, the species' sole remaining original refuge held only captive-bred tortoises that had already been reintroduced there. The first trial launched in 2016 with the release of 24 captive-raised juveniles, preceded by detailed planning and research. A generally positive result, despite impediments to statistically significant data, was reported in a journal article four years later. A second trial began in 2022 in Scott National Park. Lead scientist Nicola Mitchell, an associate professor of conservation physiology at the University of Western Australia, addressed the ethical question directly in an interview with the International New York Times: should humans let nature run its course and doom this species to extinction because of climate change, or do humans have an ethical responsibility to act? Mark Schwartz, a conservation scientist at the University of California, Davis, acknowledged the scale of the problem, saying that moving enough species to resolve the threat "basically seems untenable." He also noted that climate-responsive translocations were more acceptable than another approach under consideration: gene editing to make species more climate-proof. Additional translocations of baby turtles continued into 2023.

• 6. Monarch Butterflies and the Two-Layer Problem

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  In 2024, positive results of a combined plant-and-animal assisted migration project were reported in the volcanic region near Mexico City. A native high-altitude tree favored by the overwintering population of Monarch butterflies had been test-planted on an extinct volcano offering higher altitudes than the existing monarch sanctuary 75 kilometers away. Thirteen scientists coauthored the paper, concluding that establishing the tree Abies religiosa at 3,600 and 3,800 meters is feasible, and that planted stands could eventually serve as overwintering sites for the Monarch butterfly under projected future climates. To make the tree plantings survive at all, the team first had to establish a native shrub, Baccharis conferta, as nurse plants to shield newly planted fir seedlings from frost and drought during their early years. The lead author noted that Monarch butterflies had already begun appearing near the translocation site in recent years, establishing new colonies at colder places within the Nevado de Toluca, behavior suggesting they were already searching for new overwintering locations. This project represented what observers called a second-level complication beyond single-species work: securing a slow-growing tree to serve as a mature forest sanctuary by century's end, specifically to support a migratory insect whose needs depend on that tree being there when it arrives.

• 7. Alternatives and Genetic Tools

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  Not every species facing climate change needs to move. One study determined that evolution of higher temperature tolerances in some amphibians and reptiles could occur fast enough to allow those species to survive a 3 degrees Celsius temperature increase over 100 years, consistent with low- to mid-range projections of global warming. Temperate trees present a very different picture. Their longer generation times mean they may take thousands of years to evolve similar tolerance increases, far too slowly for the pace of projected warming. For species where movement is also not possible, researchers have turned to assisted evolution. Stress conditioning exposes organisms to sublethal stress, inducing physiological changes that increase tolerance to future stress. A 1989 experiment used heat shock stress conditioning on rat kidneys to extend their safe cold storage time to 48 hours. More recently, stress conditioning is being studied for coral reef preservation as reefs face ongoing warming and acidification. Assisted gene flow, or AGF, works differently: it identifies naturally occurring genes that produce desired traits and increases the likelihood those genes are passed to offspring, without inserting artificial genetic code. Different coral colonies of the Great Barrier Reef are currently being interbred to test whether offspring display increased resistance to warmer living conditions. Hybridization introduces a different set of complications. Cupressus abramsiana, a threatened cypress tree endemic to a small region along the California coast, illustrates the risk. The 2016 federal update of its recovery plan warned that nearby horticultural plantings of Monterey cypress, whose native range lies on the opposite side of Monterey Bay, posed a hybridization threat. Arizona cypress, native to Arizona, compounds the danger further. In the case of outbreeding depression, hybrid offspring with reduced fitness can push a small population's growth rate below replacement, wasting the reproductive potential of two populations and potentially driving the very extinction that conservation was meant to prevent.

• 8. Law, Policy, and What Changed in 2023

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  A review paper published in the journal Science in 1989, titled "Translocation as a Species Conservation Tool: Status and Strategy," compiled translocation efforts for rare birds and mammals in the United States, Hawaii, Canada, Australia, and New Zealand from 1973 to 1989. Habitat destruction, habitat fragmentation, and hunting were listed as the primary causes of decline. Climate change was not mentioned as a cause for concern. Three decades later, the International Union for the Conservation of Nature reported that climate change currently affects at least 10,967 species on the IUCN Red List of Threatened Species. A 2021 IUCN report by its Conservation Translocation Specialist Group ran 355 pages and mentioned climate change 20 times, yet mentioned assisted colonization as an adaptive response just once. Although the Endangered Species Act of 1973 did not restrict assisted migration in itself, a regulatory change in 1984 regarding experimental populations made translocations outside a species' historical range more difficult to justify. In June 2022, the U.S. Fish and Wildlife Service published a proposed rule that would allow the introduction of listed species to suitable habitats outside their historical ranges, acknowledging that climate change and invasive species were causing historical habitats to shift and become unsuitable. More than 500 comments arrived during the comment period that closed in August 2022. The U.S. Department of Interior announced its decision in June 2023: the section 10(j) rule would be modified by deleting reference to "historical range" as a parameter for where experimental populations could be located. This effectively authorized assisted species migration for endangered or threatened plants and animals, with the stated rationale that the potential impact of climate change had not been fully realized when the original regulations were written, but had become even more dramatic in the decades since.