Assisted migration (sometimes referred to as assisted colonization or managed relocation) is the act of moving plants or animals to a different habitat. The destination habitat may or may not have once previously held the species; the only requirement is the destination habitat must provide the bioclimatic requirements to support the species. The goal of assisted migration is to remove the species from a threatening environment and give them a chance to survive and reproduce in an environment that does not pose an existential threat to the species.
In recent years, assisted migration has been presented as a potential solution to the climate change crisis that has changed environments faster than natural selection can adapt to. While assisted migration has the potential to allow species that have poor natural dispersal abilities to avoid extinction, it has also sparked intense debate over the possibility of the introduction of invasive species and diseases into previously healthy ecosystems. Despite these debates, scientists and land managers have already begun the process of assisted migration for certain species.
Climate change is expected to drive many species out of parts of their current ranges while creating new suitable habitats elsewhere. In order to avoid climate change-caused 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, Prehistoric climate change events resulted in massive global extinctions, and the rate of global warming that is projected for the near future is significantly higher than the rate of global warming that occurred in the past 10,000 years. 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.
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. 
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 acclimatization and adaptation. The potential for acclimatization or adaptation to allow persistence in the face of climate change varies by species and is generally poorly understood. One study determined that evolution of higher temperature tolerances in some species of amphibians and reptiles will likely occur fast enough to allow these species to survive a 3 °C temperature increase over 100 years, consistent with low- to mid-range projections of global warming. By contrast, many species, such as most temperate trees, have longer generation times and therefore may adapt more slowly; they may take thousands of years to evolve a similar increase in temperature tolerance. Adaptation this slow would be insufficient for keeping up with expected future global warming if migration of new habitats is not an option. In addition to acclimatization and adaption, assisted evolution is an alternative to assisted migration that has been growing in popularity recently due to the worldwide coral reef crisis. Assisted evolution is the practice of using human intervention to accelerate the rate of natural evolutionary processes. There are three main types of assisted evolution.
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. More recently, stress conditioning is being studied as a potential solution for the preservation of coral reefs as they are continually exposed to ocean warming and acidification.
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, 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 refers to the process where an egg and sperm from two different species can fertilize and produce young. Hybridization was studied in the 1800s by Johann Gregor Mendel, who posthumously has been credited with the discovery of genes and alleles and their impact on an offspring's genotype. Benefits of hybridization include the increase in genetic diversity and the potential for genetic combinations which are able to adapt to, and reproduce in, increasingly difficult environments. Hybridization of coral reefs during the annual coral spawning is being experimented with to create hybrid offspring that will hopefully have higher survival and growth rates in a variety of climate change related conditions.
Generally speaking, there are three accepted ways that assisted migration can take place, each one of them with specific benefits and situations in which it applies. They can be defined as reintroduction, introduction and augmentation processes.
In augmentation, a population is identified with a small number of mating individuals. This can lead to many problems, including inbreeding depression, and often leads to a dwindling number of individuals. Further complicating matters, with such a small population and consistent inbreeding depression, genetic drift is of worry as well, leading to high levels of homozygosity. To combat these problems, individuals are reintroduced to the population. This can be done via ex situ breeding of individuals or by physically relocating a separate population to join the identified, problematic population.
In introduction, a species is brought to a habitat in which it has never before existed. This can be done for a number of reasons, ranging from climate change associated habitat loss to the introduction of predator species that cannot be controlled. Generally speaking, this is the type of assisted migration that contains the most potential for harmful effects, like those described elsewhere in this article. Currently, a number of introductions of endangered populations from Australia have been made with varying degrees of success to small islands near the mainland where the only reason that the population had not dispersed before was due to the physical waterway.
Reintroductions involve restoring a species to its native range. The species may no longer be found there due to any number of reasons, though most common is often the introduction of predators or habitat loss due to either climate change or other human factors. This is generally done to broaden the range of threatened populations and to reconnect fragmented populations.
Significant controversy has developed around the idea of assisted migration since it was first put forth in the scientific literature in 2007. The two sides can be separated roughly as follows. 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 continues 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.
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 (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.
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 (e.g., become invasive) or worse (e.g., fail to establish) 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 (e.g., minimum January temperature, average annual precipitation) 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.
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. Those wary of moving species instead suggest expanding networks of habitat corridors, allowing species to naturally migrate into newly suitable areas. Under the rates of climate change projected for the coming decades, however, even perfectly connected habitats will probably be insufficient. Species that cannot naturally keep pace with shifting climates will be at risk regardless of habitat connectivity. Evidence suggests that slowly evolving and slowly dispersing species (including species that are dispersal-limited due to habitat fragmentation) will decline or go extinct in the absence of assisted migration programs.
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. 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. 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. 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.
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. 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.
Due to climate change, accidental species introductions, and other global changes, there is nowhere on the planet free of human disturbance. 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. 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. 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. 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. 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.
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. 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 
Forestry in North America
The assisted migration of forests in North America is an ongoing process of human-facilitated forest migration applied to a variety of tree species on the continent. Programs created by public and indigenous governmental bodies, private forest owners, and land trusts have been researching, testing, evaluating, and sometimes implementing forest assisted migration projects as a form of adaptation to climate change. Assisted migration in the forestry context differs from assisted migration as originally proposed in the context of conservation biology, where it is regarded as a management tool for helping endangered species cope with the need for climate adaptation. The focus in forestry is mitigating climate change's negative effects on the health and productivity of working forests.
Forestry assisted migration is already underway in North America because of the rapidly changing climate and the forestry industry's reseeding practices. It is now standard practice for governmental and industrial harvests of trees to be followed by the planting of seeds or seedlings in the harvested areas. Hence, an opportunity automatically arises post-harvest to select seeds (and sometimes different species of trees) from areas with climates that are expected to arrive in the harvested sites decades into the future. The government of British Columbia in Canada was the first federated state on the continent to make the decision to change their seed transfer guidelines accordingly in 2009.
Longer distance forms of assisted migration were not, however, considered prior to climate modelling and within-forest evidence of the increasing pace of climate change. Serious discussion and debate ensued in the forestry profession beginning around 2008. The debate focuses around the ethical implications of artificially migrating ecosystems, the risks and benefits of such endeavors, and the values at the heart of assisted forest migration projects.There are also recorded instances of inadvertent assisted migration of North American trees. Beginning in the early 20th century, two trees famously endemic to California, the giant sequoia and coast redwood, have been planted for urban forestry purposes northward in cities along the Pacific coast of Oregon, Washington, and British Columbia. Today these specimens are not only thriving; they are prominent along urban skylines and often outrank the native trees in sizes achieved. As well, several kinds of Magnolia native to the southeastern United States have dispersed into the forest understory, thanks to ornamental plantings producing seeds beyond their native ranges.
Dixon National Tallgrass Prairie Seed Bank, US
Although not actively engaging in assisted migration, the Dixon National Tallgrass Prairie Seed Bank seeks to collect seeds from populations of species expected to decline or disappear due to climate change. They prioritize collections from populations at greatest risk of disappearance and for which suitable habitat is projected to occur elsewhere in the general region, keeping open the possibility of using collected seeds for assisted migration projects in the future.
Great Barrier Reef
The Great Barrier Reef's health has been in jeopardy in recent years due to rising sea temperatures caused by climate change. The Australian Institute of Marine Science has been at the forefront of attempting to save the reefs using various forms of assisted evolution and assisted migration. Assisted evolution is believed to be a temporary solution to save many threatened species from global warming and other climate change related environmental changes.
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.
Florida torreya, US
The Florida torreya (Torreya taxifolia) is an critically endangered tree of the yew family, Taxaceae, found in the Southeastern United States, at the state border region of northern Florida and southwestern Georgia. A self-organized group of conservationists called the Torreya Guardians was created in 2004 to facilitate the assisted migration of the endangered tree by rewilding it in more northern parts of the United States.
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