Assisted migration

Assisted colonization, also known as assisted migration or managed relocation, is the act of deliberately moving plants or animals to a different habitat. The destination habitat may have either historically held the species or it may not have hosted the species, but the habitat provides the bioclimatic requirements to support it. Assisted colonization may also supplement an existing population in a site where their numbers are dwindling. All species have some natural capacity to disperse into new habitats and adapt to change, but ongoing climate change is so rapid 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, including avoiding many species extinctions, and the risks, including 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.

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 to 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 inability 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 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).

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