Assisted colonization, also known as assisted migration or managed relocation, refers to the act of deliberately helping plant and animal species colonize new habitats when driven out of their historical habitats due to rapid environmental change, especially climate change. All species have some natural capacity to disperse into new habitats and adapt to change, but ongoing climate change is occurring so rapidly that many species are unable to keep pace naturally. In order to prevent extinctions, some scientists and practitioners are considering assisting the dispersal of species that have poor natural dispersal ability. This idea has sparked intense debate over the potential benefits of performing assisted colonization, which include avoiding many species extinctions, and the risks, which include accidentally introducing new invasive species. Although the debate remains primarily conceptual with few real-world applications, scientists and land managers have already begun to consider several specific assisted colonization projects.
- 1 Background
- 2 Controversy
- 3 Examples
- 4 References
Climate change and species range shifts
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 population declines and extinction due to climate change, many species will need adapt or colonize newly suitable areas. Using a niche modeling approach, scientists have predicted that failure to migrate or adapt would result in eventual extinction of about a quarter of the world’s species this century under moderate climate change. The natural dispersal rates of 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, likely resulting in high rates of extinction by the end of this century in the absence of management. The natural shortcoming of species to migrate in response to human-caused climate change has led some to consider exploring assisted colonization as a means for preventing extinctions.
Assisted colonization vs. species introduction
Assisted colonization is a specific type of species introduction. An 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 colonization acknowledges that the natural dispersal rate of many species may be too slow to naturally respond to rapid human-caused environmental change and asks, “if this species could disperse fast enough to keep pace with the changing environment, where would it establish?” Assisted colonization practitioners consider helping the species disperse into such sites, which are often immediately adjacent to the species’ historical range. Assisted colonization thus represents a small artificial boost to an otherwise natural process, acknowledging that the threat—rapid human-caused environmental change—was produced by humans in the first place. Confusion of assisted colonization with species introduction in general—often much larger in scale and with greater risk of adverse impacts—may be the principal reason why some are reluctant to consider assisted colonization (see Controversy, below).
When first proposed, the idea was referred to as “assisted migration”. The terminology was later criticized for being reminiscent of natural, cyclic animal migrations in response to changing seasons. It was renamed “assisted colonization,” as colonization more accurately describes the natural phenomenon that management would seek to assist. Others have sought to further distinguish this idea from any natural process by referring to it as “managed relocation.” No specific name has yet been unanimously adopted, but within the scientific and conservation community, “assisted migration,” “managed relocation,” and “assisted colonization” are often used interchangeably and are understood to refer to the same idea.
Acclimatization and adaptation as alternatives to colonization
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/or 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 tolerance 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 colonization of new habitats is not an option.
Significant controversy has developed around the idea of assisted colonization since it was first put forth in the scientific literature in 2007. At one extreme of the debate are those who argue that human management of species range shifts should be avoided at all costs due to the high risks of colonizing species becoming invasive. At the other extreme are those who believe that the expected benefits of assisted colonization, primarily in terms of prevented extinctions, may often outweigh the risks. Though the debate has been lively, it has mostly been conceptual and has lacked specificity to real-world applications of managed relocation.
Invasive species risk
Perhaps the principle concern scientists have expressed over assisted colonization is the potential for relocated species to be invasive in their new habitats, driving out native species. Some have gone so far as to call assisted colonization a “planned invasion.” The fear that assisted colonization 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 colonization 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 colonization as a viable management response to climate change.
Those who wish to keep assisted colonization 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 colonization. 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 colonization, 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, conservations 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 colonization decisions and has led some to reject assisted colonization 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 colonization. The reliability of these predictions was supported by the fact that the model accurately predicted suitable sites. The results suggested that if investigators can demonstrate their model makes reliable predictions with real-world data, models might be trusted for informing assisted colonization decisions.
Weighing the 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 colonization programs.
In their rejection of assisted colonization, Ricciardi and Simberloff cite the precautionary principle, stating that any unknown risk, no matter how small, of assisted colonization 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 colonization are probably far worse than the risks of facilitating colonization. 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 colonization will depend on the values of the stakeholders involved in a specific decision rather than the position of scientists on assisted colonization in general. At the very least, some note, scientists should conduct further research into assisted colonization 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 colonization may be moot given that all communities have been altered by humans to some degree whether managers undertake assisted colonization 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 colonization, climate change is already causing many highly mobile species, such as butterflies, to colonize areas they have not previously inhabited. Through assisted colonization, 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.
Forestry in Canada
The British Columbia Ministry of Forests, Lands, and Natural Resource Operations has acknowledged that climate change will likely threaten the health of the millions of trees that are planted in the province every year, with potential environmental and economic consequences:
Approximately 300 million tree seedlings are planted in the western USA, British Columbia (BC) and Yukon each year. Many climatologists are predicting that the climate could be 3–4°C warmer when those trees are harvested 60-80 years after planting. These changes to climate will expose trees to increased stress and health risks, compromising the many goods and services from our forests.
In response to this threat, the Ministry has initiated a large-scale study to determine the long-term health of seedlings of 16 tree species planted beyond their native ranges, into areas expected to become suitable due to changes in climate this century. Results from the study will be used to develop guidelines for when and how assisted colonization of trees should be conducted.
In 2009, British Columbia also altered guidelines for selecting seeds for replanting forests after a timber harvest. Previously, foresters were required to use seeds from within 200 meters downhill and 300 meters uphill, but the new policy allows foresters to obtain seeds from up to 500 meters downhill, taking advantage of the fact that populations in warmer habitats downhill may be better adapted to the future climate of the restoration site.
Dixon National Tallgrass Prairie Seed Bank, USA
Although not actively engaging in assisted colonization, 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 colonization projects in the future.
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