Dispersal

The absence of a species in a particular area may simply be due to the fact that the species has failed to reach that area. The conditions of the habitat in question may be suitable for the species and there may be no problems with other species already occupying that habitat. Ecologists call the ability and propensity of a species to disperse a measure of the species' vagility. Species with high vagility. are both willing and capable of traveling long distances. We'd place most birds in that category (see below)

Orioles have high vagility and are able to disperse over great distances.

Species with low vagility are either not willing to move from their home site or are unable to move long distances. An example of a species with low vagility is shown below.

Snails have low vagility. Even an alley can limit their dispersal.

Introduced species can give us a handle on how dispersal is related to colonization of a habitat. The European Starling (Sturnus vulgaris) is such an introduced species. They were brought over from Europe by immigrants that wanted to bring some song birds so they'd feel more at home. Early attempts at introductions failed (Pennsylvania 1850, Cincinnati 1872, Portland 1890). Not until introductions were started in New York City did they grab hold. Eighty birds were released in Central Park in April, 1890. This was followed by another 80 in March of the following year. The releases continued for 10 years. By 1930 the Starling had spread west to St. Louis. Within 60 years they have spread over most of North America (including most of Canada). Early attempts failed because of the irregular wanderings of 1-2 year old non-breeding juvenile birds. During their first year or two the juvenile birds would wander away from the main population and then wouldn't be able to find any mates when they matured. By having multiple introductions the population density was maintained so that these birds could then find a mate. The initial problems introducing the European Starling were not due to habitat problems, but were only due to the particulars of their dispersal behavior.