Assisted migration
Assisted migration is "the intentional establishment of populations or meta-populations beyond the boundary of a species' historic range for the purpose of tracking suitable habitats through a period of changing climate...." It is therefore a nature conservation tactic by which plants or animals are intentionally moved to geographic locations better suited to their present or future habitat needs and climate tolerances — and to which they are unable to migrate or disperse on their own.
In conservation biology, the term first appeared in publications in 2004. It signified a type of species translocation intended to reduce biodiversity losses owing to climate change. In the context of endangered species management, assisted colonization and managed relocation were soon offered as synonyms — the latter in a paper entailing 22 coauthors.
In forestry science and management, assisted migration is discussed in its own journals and from perspectives different from those of conservation biologists. This is, in part, because paleoecologists had already concluded that there were significant lags in northward movement of even the dominant canopy trees in North America during the thousands of years since the final glacial retreat. In the 1990s, forestry researchers had begun applying climate change projections to their own tree species distribution modelling efforts, and some results on the probable distances of future range shifts prompted attention. As well, translocation terminology was not controversial among forestry researchers because migration was the standard term used in paleoecology for natural movements of tree species recorded in the geological record. Another key difference between forestry practices and conservation biology is that the former, necessarily, was guided by "seed transfer guidelines" whenever a timber or pulp harvest was followed up by reforestation plantings. The provincial government of British Columbia in Canada was the first to update their guidelines with, what they call, "climate-based seed transfer."
Overall, debate concerning the ethics of assisted migration in forestry practice was both short-term and muted compared to that which prevails in conservation biology. For this reason, a separate Wikipedia page titled Assisted migration of forests in North America was launched in 2021 and made into a useful teaching tool for climate adaptation education and decision-making in the forestry profession.
The remainder of this page therefore focuses on the topic of assisted migration in conservation biology and especially its applications for management of endangered species.
Background
is expected to drive many species out of parts of their current ranges while creating new suitable habitats elsewhere. In order to avoid population declines and extinction, many species will need to either adapt or colonize newly suitable areas. Using a niche modeling approach, 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 those needed to keep pace with projected habitat shifts in many regions of the world.Prehistoric climatic changes have resulted in massive global extinctions, and the rate of warming projected for the near future is many times faster than changes in the past 10,000 years. Climate change is increasingly altering geographic ranges where species survive, which can lead to rapid habitat loss and population decline. The inability of some species to naturally keep pace with human-caused climate change has led scientists and land managers to consider assisted migration as a means for preventing extinctions. Trees, amphibians, and corals are projected to disperse too slowly to match climate change already underway. Geographic or human-caused barriers to natural dispersal amplify the problem and thus contributed to the listing as "critically endangered" two small-range endemic species for which assisted migration is now underway: Australia's western swamp tortoise and America's Florida torreya tree. As of 2023, however, there have been few other examples of assisted migration experiments: A review paper concludes, "Assisted migration was most common for plants, followed by birds, and was rarely implemented for other taxa."
Assisted migration v. species introduction
Assisted migration is a specific type of species introduction. Species introduction is any act of establishing a species in a habitat it does not currently occupy. It often refers to a long-distance relocation, such as the accidental introduction of an invasive species from one continent to another, or the intentional relocation of a species in decline to a habitat where it can persist. By contrast, assisted migration acknowledges that the natural dispersal rate of many species may be too low to naturally respond to rapid human-caused climate change and instead focuses on where the species would be able to disperse fast enough via natural selection to keep pace with the changing environment. Assisted migration practitioners consider helping the species disperse into such sites, which are often immediately adjacent to the species' historical range. In their eyes, assisted migration represents a small artificial boost to an otherwise natural process.Controversy
While assisted migration has the potential to allow species that have poor natural dispersal abilities to avoid extinction, it has also sparked debate over the possibility that the migrated species might spread diseases or even become too successful in the recipient ecosystems. Even so, several assisted migration projects or experiments have begun for several critically endangered species.Beginning around 2007, opposing pro and con positions became apparent in the field of conservation biology, while still relatively unknown to public promoters of conservation and managers of conservation lands. Supporters generally believe that the expected benefits of assisted migration, including saving and strengthening species, outweigh the potential harm of any project. Detractors generally believe that other conservation techniques which do not include the high risk of invasive species are not only better suited but are also more likely to succeed. This debate continued throughout the literature, generally due to a lack of real-world applications and follow-ups. Though these conservation efforts are becoming increasingly common, few long term looks at their success have been conducted.
In 2022 a review paper by seven researchers in the United States assessed shifts in what they called "conservation strategies for the climate crisis." Among the "novel strategies" surveyed was "climate-adaptive assisted migration." The team found that academic publications became less focused on the pros and cons of the concept through the years. Instead, more attention was given to modeling or mapping where particular species could be moved. While plants had been the focus of most of the early publications, animals took the lead in recent years. Corals, invertebrates, mammals, and birds were the leading types of animals assessed for assisted migration needs and prospects. Even so, "most authors presented assisted species migration as appropriate only for species under exceptionally high threat from climate change." By 2023, a news article in the journal Nature reported, "attitudes towards assisted migration are slowly shifting as conservationists realise just how fast the climate is changing."
A 2023 review paper analyzed 204 species that had been subject to intentional, experimental, or inadvertent assisted migration. Among their conclusions: "Despite hesitancy around the tactic, humans have a long history of relocating plants and animals for a variety of reasons. Further, Indigenous peoples have been translocating species for millennia. As such, it is somewhat surprising that assisted migration for the purpose of conservation has been subject to so much controversy."
Invasive species risk
Perhaps the principal concern scientists have expressed over assisted migration is the potential for relocated species to be invasive in their new habitats, driving out native species. The fear that assisted migration will facilitate invasions stems mostly from observations of the vast numbers of species that have become invasive outside their native ranges by introduction by humans. Although most agree that assisted migration efforts, unlike accidental introductions, should involve detailed planning and risk assessment, for some, any threat of introducing invasive species, no matter how small, disqualifies assisted migration as a viable management response to climate change.Those who wish to keep assisted migration on the table often note that the vast majority of historical species invasions have resulted from continent-to-continent or continent-to-island transportation of species and that very few invasions have resulted from the comparatively short-distance, within-continent movement of species proposed for assisted migration. For example, Mueller and Hellman reviewed 468 documented species invasions and found that only 14.7% occurred on the same continent where the species originated. Of the 14.7%, the vast majority were fish and crustaceans. Terrestrial species that became invasive on the same continent where they originated were often transported across large biogeographic barriers, such as mountain ranges. These long-distance, within-continent translocations are unlike expected uses of assisted migration, which generally involve helping species colonize habitats immediately adjacent to their current ranges.
Uncertainty in the planning process
To identify populations at risk and locate new potential habitats, conservationists often use niche models. These models predict the suitability of habitats in the future based on how closely their climates resemble the climate currently inhabited by the species. Though useful for describing broad trends, these models make a number of unrealistic assumptions that restrict the usefulness of their predictions. For instance, they do not consider the possibility that species may be able to develop tolerance of new climates through acclimatization or adaptation. Further, they do not account for the fact that a given species may perform better or worse in a new habitat than in its current range if the community of competitor, predator, and mutualist species is different there. Additionally, because different climate variables rarely shift in unison, it is possible that few areas will exactly match the historical climates of species threatened by climate change. Such multi-directional climate shifts will make it especially difficult to determine the species that are at greatest risk of habitat loss due to climate change and to predict future suitable habitat. The uncertainties in predictions of future suitable habitat limits confidence in assisted migration decisions and has led some to reject assisted migration entirely.Despite the uncertainty inherent in predictions of future suitable habitat, some studies have demonstrated that predictions can be quite accurate. A study of Hesperia comma butterflies in Britain identified unoccupied habitat sites that were likely to support the species under a warmer climate based on their similarity to occupied sites. As the climate warmed, the butterfly colonized many of the sites; most of the sites it did not colonize were located far from existing populations, suggesting they were uncolonized because the butterfly could not reach them on its own. The data suggested that the suitable, uncolonized sites could be good targets for assisted migration. The results suggested that if investigators can demonstrate their model makes reliable predictions with real-world data, models might be trusted for informing assisted migration decisions.