Availability of New Suitable Habitat on a Large Scale

Biological invasion is a prominent example of colonization of new areas of suitable habitat. Invasive species often have life-history characteristics that make them good colonizers. In cases where invasive spread occurs most quickly, many secondary colonization events follow the initial introduction, so that rapid spread occurs by the establishment, increase, and eventual coalescence of many small and widely dispersed populations descended from the initial colonizers of the new habitat. Species that exhibit such rapid, patchy spread include cheatgrass, Bromus tectorum, in western North America; smooth cordgrass, Spartina alter-niflora, in Willapa Bay, Washington State; and the Argentine ant, Linepithema humile, which disperses poorly on its own but covers long distances when assisted by humans. Because many conspicuous invasive species disperse widely, invaders often fall into the 'ruderal' or 'fugitive' life-history categories; but many (such as yellow starthistle, Centaurea solstitialis, and the Argentine ant, L. humile) are very strong competitors, while others (such as saltcedar, Tamarix ramosissina) tolerate extreme environments very well. Whether they are good colonizers because they disperse often, survive well during dispersal, and/or find many types of habitat acceptable for vigorous growth (sometimes despite the presence of a prior resident), invasive species increasingly take advantage of new dispersal pathways opened by the global human economy to reach previously unavailable areas of suitable habitat. These issues are of primary importance in attempts to slow, stop, or prevent biological invasions that are potentially devastating to native communities (see Invasive Species).

Changes in habitat suitability can provide new opportunities for colonization on large spatial scales. One way to be a good colonizer is to enhance the suitability of a new habitat, and some species can facilitate their own spread into new areas by changing the properties of the surrounding ecosystem. For instance, colonization by alien grasses can increase the fuel available to fire and thereby increase fire frequency, area, and intensity. The grasses recover quickly from fire and colonize burned areas, generating a positive feedback loop that favors the grasses' spread. Alternatively, one colonist can open large areas of suitable habitat for another; when the European green crab, Carcinus maenas, arrived in San Francisco Bay, its predation upon two species of native clams reduced their densities dramatically and allowed the rapid spread of the eastern gem clam, Gemma gemma. Finally, climatic change can also drive changes in habitat suitability, with colonization of the newly suitable areas resulting in geographic shifts in species' ranges. Northward plant colonization of newly uncovered habitat after withdrawal of glaciers at the end of the last ice age is well documented in the pollen record. In recent decades, changes in intertidal community composition in the northeastern Pacific reveal northward shifts of several southern species into waters that previously were too cold for them. Climate change can also lead to colonization by facilitating dispersal. Low sea levels may have facilitated the spread of humans from Eurasia to the Americas by revealing the Bering land bridge. Understanding interactions between climate change and colonization is an urgent priority for conserving biodiversity and anticipating likely novel community assemblages, given rapid predicted natural and anthropogenic habitat changes in the future.

One of the most dramatic instances of colonization of new habitat occurs not in physical space but in niche space. Organisms can increase the area of suitable habitat by evolving to use new areas. The global influenza pandemic of 1918, recently shown to be derived from avian influenza, is a spectacular example of a virus' evolution to take advantage of an entirely new host, and other deadly human diseases throughout history (including recent times) may have their origin in niche shifts of animal disease. Colonization of a completely new habitat type frequently occurs after colonization in physical space, as evidenced most clearly by adaptive radiation on species-poor islands. The lag between the initial colonization of a new area by an invasive species and the onset of rapid spatial spread is sometimes also attributed to genetic adaptation to the new environment. Genetic founder effects may also play a role in this adaptation process. In any case, niche shifts precipitated by physical colonization can be an important process in allopatric speciation.

See also: Competition and Coexistence in Model Populations; Competition and Competition Models; Dispersal-Migration; Invasive Species; Island Biogeography; Metacommunities; Metapopulation Models; Succession; R-Strategist/K-Strategists.

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