Biota Migration and Dispersal

In general, migration and dispersal both refer to the movement of plants or animals over distances beyond their regular home range. Migration is typically seasonal, long distance, and repeated annually, such as the movement of birds between breeding and wintering grounds. Dispersal is more permanent, varies in distance, and is only done once or a limited number of times in the life of an organism. The timing, distance, and effects of movement are intrinsically linked to the habitat features being moved from, through, and to.

Animal migration takes an individual from one physical and ecological environment to another, typically, in search of resources or mates or in response to an environmental shift. The most common examples include birds that breed in northern latitudes during the Northern Hemisphere summer and then travel south toward or into the tropics before the weather turns cold [Page 277](e.g., Purple Martin, Mississippi Kite). Ungulates and other large mammals in the African savannas travel in enormous herds tracking water during the dry season, creating amazing spectacles of mass migration. Rather than seasonal, migration could take a lifetime to complete its cycle. Pacific salmon, for example, are born in freshwater streams, travel to sea for most of their lives, and then to their natal stream to spawn and then die. Migration can take two lifetimes. In some insect species, a parental generation migrates, reproduces, and dies, and the offspring return.

The triggers to initiate this movement behavior are often linked to either seasonal cues such as day length or resource availability such as access to water or food. Several species of migratory birds (e.g., the European quail and the white-crowned sparrow) have been shown to respond to circadian rhythms with changes of flight activity, suggesting day length as a cue for their seasonal migration. Insectivorous birds respond instead to a reduction in the abundance of insects as a cue for travel. If the target of the migration is finding a mate or mating, as in the case of the salmon, then the cue may be the reaching of sexual or hormonal maturity. In the case of the following resources, migration may be less structured and more nomadic, without return to the original area specifically. This type of movement is seen in the large mammals of the African savannas (e.g., the wildebeests and the zebras) as they follow the availability of water and forage.

As opposed to migration, dispersal is typically permanent and unidirectional. It is a phenomenon of both plants and animals and is the main mechanism for gene flow among populations. The dispersal process involves three somewhat distinct stages: (1) emigration, (2) transfer, and (3) settlement. Emigration, or departure from the predispersal area or group, often but not always, occurs at or near sexual maturity. Movement or transfer between sites or social groups, typically, involves travel through unsuitable or unavailable habitat that can include increased mortality risk. Finally, settlement or immigration in a secondary area or group may be as simple as haphazardly landing somewhere and surviving if it is suitable or sequentially investigating several areas and then choosing the most suitable. The dispersing individual can be entirely passive in this process, as seeds are expelled from a parent plant and then carried on the wind. Or the individual could be entirely active, as are female birds that leave the parental nest and investigate the territories of several males before choosing one with whom to nest and mate.

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