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...."[1] 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.[2][3] It signified a type of species translocation[4] intended to reduce biodiversity losses owing to climate change. In the context of endangered species management, assisted colonization (2007)[5] and managed relocation (2009)[6] were soon offered as synonyms — the latter in a paper entailing 22 coauthors.[6][2]
In
Overall, debate concerning the
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
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.[25] Geographic or human-caused barriers to natural dispersal may already be at cause for the listing as "critically endangered" two small-range endemic species for which assisted migration is now underway: Australia's western swamp tortoise[26] and America's Florida torreya tree.[27] The inability of species to migrate in response to human-caused climate change has led to some scientists and land managers to consider exploring assisted migration as a means for preventing extinction of species.[28] As of 2023, however, there have been few examples of assisted migration projects underway: A review paper concludes, "Assisted migration was most common for plants (particularly trees), followed by birds, and was rarely implemented for other taxa."[29]
Assisted migration v. species introduction
Assisted migration is a specific type of species introduction.
Alternatives
Even under rapid climate change, dispersal into new areas may not be necessary for some species to persist. Instead of tracking climate shifts through space, some species may be able to survive in their present locations by developing tolerance to new conditions through
Stress conditioning
Stress conditioning consists of exposing organisms to sublethal stress, with the goal of inducing physiological changes that increase tolerance to future stress events. There has been documented evidence that some changes can be passed throughout generations in both plants and animals. Stress conditioning can be artificially induced in a laboratory environment to create desired responses based on their environments. Notable examples include a 1989 experiment which used stress conditioning via heat shock on rat kidneys to extend their safe cold storage time to 48 hours.
Assisted gene flow
Assisted gene flow (AGF) works to increase the presence of desired naturally-occurring genes in offspring. AGF relies on pre-existing genes within the species' genome, rather than the artificial creation and insertion of genetic code within the genome of the species. Assisted gene flow can also introduce related species' genomes into the gene pool to allow for the introduction of previously impossible behaviors into the new species. AGF identifies genes that produce desired behaviors or tolerance to environmental conditions, and works to increase the chance that parental transmission of the gene in question occurs (also known as heritability). Determining which genes within the genome produce desired behaviors or environmental tolerance consist of experiments which measure the growth, survival, and behavior exhibition of offspring with varying genotypes. AGF is one possible strategy to preserve species that are threatened by climate change,[35] and can be applied to both plants (e.g. forest restoration) or animal populations. Currently, different coral colonies of the Great Barrier Reef are being interbred to test whether offspring display increased resistance to warmer living conditions. Increased resistance to warmer living conditions allow for the preservation of the Great Barrier Reef even as water temperatures continue to rise.
Hybridization
Hybridization refers to the process where an egg and sperm from two different species can
In contrast, for
Controversy
While assisted migration has the potential to allow species that have poor natural dispersal abilities to avoid
Beginning around 2007,[28] 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.[18]
In 2022 a review paper by seven researchers in the United States assessed shifts in what they called "conservation strategies for the climate crisis."[39] 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."[39] 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."[40]
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.[41] 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 (often inadvertent) 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.[41]
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.[30][42][43] 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.[30] 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.[5]
Uncertainty in the planning process
To identify populations at risk and locate new potential habitats, conservationists often use
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.[48] 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.
Risks and benefits
The science is clear that climate change will drive many species extinct, and a traditional, land-preservation ethic will not prevent extinctions.
In their rejection of assisted migration, Ricciardi and Simberloff cite the precautionary principle, stating that any unknown risk, no matter how small, of assisted migration resulting in the creation of new invasive species is enough to require that it not be undertaken.[41] Many scientists reject this position, however, noting that in many cases where extinctions due to climate change are likely, the risks of extinction from not facilitating migration are probably far worse than the risks of facilitating migration.[42][51] They argue that the precautionary principle cuts both ways, and the risks of inaction must be compared against the risks of action. Others note that the ethics of assisting migration will depend on the values of the stakeholders involved in a specific decision rather than the position of scientists on assisted migration in general.[52] At the very least, some note, scientists should conduct further research into assisted migration and improve our capacity to predict specific outcomes instead of outright rejecting it.[51]
Because confidence in expected outcomes is often greater in the short-term (e.g., 20 years) than the long-term future, it may be more reasonable to use short-term projections to guide actions.[53] However, it is also important to consider whether the climate will remain suitable long enough for colonizing species to mature and reproduce, if that is the management goal.[54]
Due to climate change, accidental species introductions, and other global changes, there is nowhere on the planet free of human disturbance.[55] Thus, the idea that land managers should refrain from creating human-altered communities through assisted migration may be moot given that all communities have been altered by humans to some degree whether managers undertake assisted migration or not.[56][57][58] Given the reality of global change, it will be impossible to maintain past ecological communities indefinitely. Many therefore believe we should strive to maintain biodiversity and functioning ecosystems in the face of climate change, even if it means actively moving species beyond their native ranges.[57] In the absence of assisted migration, climate change is already causing many highly mobile species, such as butterflies, to colonize areas they have not previously inhabited.[57] Through assisted migration, managers could help rare or less-mobile species keep pace, possibly preventing future extinctions due to a their inability to colonize new areas fast enough. Though some argue that nature often responds to challenges more effectively in the absence of human intervention, others note that current climate change, itself, is a human intervention.[57] Many species that would have been effective dispersers under slower, natural climate change may be left behind by more mobile species under current rates of human-caused climate change. Thus, through changing the climate, humans may already be artificially segregating species even without actively relocating them.[57]
Critics may also have major concerns about different genetic issues when considering assisted migration such as maladaptation to novel environmental conditions and hybridization with similar species. These often depend on the genetic structure and level of genetic variation in the source populations. The environmental conditions in which these populations are being introduced must also be taken into account. In order to enhance genetic variation, and thus adaptive potential, material could be sourced from multiple populations. This is known as composite provenancing.[59] However, if the environmental gradient is well known, such as predictable changes in elevation or aridity, source populations should be ‘genetically matched’ to recipient sites as best as possible to ensure that the translocated individuals ae not maladapted. This strategy of moving species beyond their current range has been suggested for those that are severely threatened or endangered. By moving them outside their native range, hopefully the immediate threats of predation, disease, and habitat loss can be avoided. However, these species are usually already suffering from some sort of genetic issue resulting from low effective population size such as inbreeding depression, loss in genetic diversity, or maladaptation. Therefore, caution must be taken with what few individuals remain and rapid population growth must be the primary goal. In the case of some species, this can be accomplished with a captive breeding program [60]
Governmental policies
Global conservation policy
A review paper published in the journal Science in 1989, titled "Translocation as a Species Conservation Tool: Status and Strategy," compiled the use of translocations for rare species (of birds and mammals) from 1973 to 1989 in the United States, Hawaii, Canada, Australia, and New Zealand.[61] Habitat destruction, habitat fragmentation, and hunting were the primary causes of decline listed in that paper. Climate change was not mentioned as a cause for concern.
Three decades later, the
In 2022, the update of the United Nations Convention on Biological Diversity agreement recognized "climate change" as the third most significant threat to global biodiversity. "Changes in land and sea use" along with "direct exploitation of organisms" were regarded as greater threats. Because the agreement was at the level of "goals" and "targets," no mention was made of conservation tools, such as translocation, nor its climate-adaptive form (assisted migration).[64]
USA Endangered Species Act
Although the Endangered Species Act of 1973 did not in itself restrict assisted migration, a regulatory change in 1984 regarding "experimental populations" made prospective translocations more difficult to justify.[65] June 2022, the U.S. Fish and Wildlife Service published a proposed rule in the Federal Register that would "revise section 10(j) regulations under the Endangered Species Act to better facilitate recovery by allowing for the introduction of listed species to suitable habitats outside of their historical ranges. The proposed change will help improve the conservation and recovery of imperiled ESA-listed species in the coming decades, as growing impacts from climate change and invasive species cause habitats within their historical ranges to shift and become unsuitable."[66] The comment period ended August 2022, with more than 500 comments posted online by supporters and opponents.[67] The final decision is scheduled for publication June 2023.[68] A 2010 paper in Conservation Letters had pointed out that, while no statutory changes appeared necessary to facilitate this newly proposed form of climate adaptation, "current regulations are an impediment to assisted colonization for many endangered animal species, whereas regulations do not necessarily restrict assisted colonization of endangered plants."[69]
The U.S. Department of Interior in June 2023 announced its decision to modify the section 10(j) rule by deleting reference to "historical range" as a parameter for where "experimental populations" were authorized to be located. This effectively authorized assisted species migration for endangered or threatened plants and animals.[70] The press release summarized the reason for the change as, "At the time the original 10(j) regulations were established, the potential impact of climate change on species and their habitats was not fully realized, yet in the decades since have become even more dramatic. These revisions will help prevent extinctions and support the recovery of imperiled species by allowing the Service and our partners to implement proactive, conservation-based species introductions to reduce the impacts of climate change and other threats such as invasive species."[71]
Implementation
A number of scholarly reports have documented natural poleward range shifts of mobile species — notably, butterflies and birds, during the past several decades of global warming. This is especially the case in the United Kingdom, where natural history observations are reputable and reach back several centuries.[72] It has also been documented that plants are being sold in nurseries in Europe far north of their historically native ranges, and with apparent success in the colder habitats.[73] Evidence of such "inadvertent assisted migration", owing to the horticultural trade, has also been documented for plants in the United States.[74]
In the North American context, assisted migration is most often discussed in the context of the relocalization of the continent's forests. In the late 2000s and early 2010s, the Canadian provinces of Alberta and British Columbia modified their tree reseeding guidelines to account for the northward movement of forest's optimal ranges.[75] British Columbia even gave the green light for the relocation of a single species, the Western Larch, 1000 km northward.[76]
In the series below of actual and prospective assisted migration projects, all but one (Florida torreya tree) are being advocated and implemented by professional scientists, and usually with oversight by governmental endangered species programs. Taxonomic significance in successfully translocating plant and animal species range from (a) maintaining the genetics of an isolated population (American pika), to (b) preventing extinction of a subspecies (Quino checkerspot butterfly), to (c) preventing the extinction of a species (Florida torreya tree), and to (d) preventing extinction of a genus (Western swamp tortoise).
First projects
• Florida torreya, US
The
By 2018 the citizens had accomplished documentation of species thrival in a dozen legacy horticultural plantings — including seed production and next-generation saplings at several sites in North Carolina.[80] In 2018 their own plantings in Cleveland, Ohio, began producing seeds[81] (turquoise star in image at right).
Early scholarly debates on the pros and cons of assisted migration as a climate-adaptation tool for endangered species conservation often mentioned the Florida torreya project.,[82][83] as did international media[84][85]
• Western Swamp Tortoise, Australia
The
By the time assisted migration trials began, the sole remaining original refuge for this species was inhabited only by captive-bred tortoises that had been reintroduced.[88] The first trial began in 2016, with the release of 24 captive-raised juveniles.[89] In contrast to the Florida torreya tree example, this first experiment in assisted migration of an endangered species in Australia was "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.[88]
A second trial began in 2022 in the same region, this time in Scott National Park. Lead scientist is Nicola Mitchell, an associate professor of conservation physiology at the University of Western Australia. She openly spoke to the International New York Times about the ethical imperative: Should humans just let nature run its course, thereby dooming this species to extinction because of climate change? "Or do we have an ethical responsibility" to act in its behalf?[26]
Mark Schwartz, a conservation scientist at the University of California, Davis, was quoted in the article. Speaking about the scale of biodiversity threats posed by climate change, versus using assisted migration as an adaptive strategy, Schwartz said: "To move enough species to resolve this threat basically seems untenable." Yet he also noted that climate-responsive translocations were more acceptable than another approach under consideration: initiating gene editing to make species more climate-proof. Lead scientist Mitchell acknowledged the risks, while offering "we can potentially undo our mistakes by recapturing them."[26] Additional translocations of baby turtles continued into 2023.[40]
Early advocacy without implementation
• Quino Checkerspot butterfly, US
The
Notably, it became the first endangered species for which climate change was reported as a current threat and thus a factor to be considered in its recovery plan.[91] However, as reported in The Guardian April 2014
A butterfly species whose population collapsed because of climate change and habitat loss has defied predictions of extinction to rapidly move to cooler climes and change its food plant. The quino checkerspot (Euphydryas editha quino), found in Mexico and California, has shifted to higher altitudes and surprisingly chosen a completely different species of plant on which to lay its eggs, according to research presented at the Butterfly Conservation's seventh international symposium in Southampton.... "Every butterfly biologist who knew anything about the quino in the mid-1990s thought it would be extinct by now, including me," said Prof Camille Parmesan of the Marine Sciences Institute at Plymouth University...."[92]
In a paper titled, "Endangered Quino checkerspot butterfly and climate change: Short-term success but long-term vulnerability?", the authors acknowledged the butterfly's surprising ability to utilize a new larval plant food in a cooler nearby habitat and concluded: "Quino appears resilient to warming climate. However, projections indicate that most or all of Quino’s current range in the USA, including the new high elevation expansion, will become uninhabitable. Our most frequent projected future range (circa 2050) is c. 400 km northward from current populations, hence conservation of Quino may eventually require assisted colonization."[93]
• American Pika (rodent) and Joshua Tree, US
Within the first decade of the terms assisted migration and assisted colonization appearing in the journals of
At the time, when climate-adaptation tools were originally discussed, scientific applications to well-known species garnered media attention.[96] Because successfully capturing, transporting, and releasing an alpine mammal would require planning and "considerable financial resources,"[95] serious advocacy for launching such a project for the pika did not occur. As for Joshua tree, in 2019 the U.S. Fish and Wildlife Service ruled against listing this desert plant as a "threatened" species,[97] and California state government did the same in 2022.[98] During this time, three U.S. Fish & Wildlife Service scientists aggregated existing research (including range shift climate modelling) into a report titled, "Examining the Past, Present, and Future of an Iconic Mojave Desert Species, the Joshua Tree."[99] It was published in December 2020. No mention was made of any form of translocation, including assisted migration, for securing the species against future climate change.
• Stitchbird (hihi), New Zealand
The stitchbird, also known as the Hihi, is a bird endemic to New Zealand. Changes in climate have shown to have a profound effect on the hihi's ability to thrive and reproduce. As a result, human caused climate change is an existential threat to the species. The hihi's current native habitat is becoming unstable due to rising temperatures, and suitable temperatures are shifting further south. Assisted migration is being considered as a means of ensuring the hihi can remain in its current natural habitat. Critics, however, argue the risks that are presented to the new host environments are not worth the potential benefits assisted migration may present.[100]
Inadvertent assisted migration
Mature
Reports of full naturalization of poleward horticultural plantings of other native trees have been used as support for intentional deployment of assisted migration at larger scales as a tool for climate adaptation.
See also
- Climate change adaptation
- Forest management
- Effect of climate change on plant biodiversity
- Hemerochory
- Escaped plant
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