The process by which groups of living organisms expand the space or range within which they live. Dispersal operates when individual organisms leave the space that they have occupied previously, or in which they were born, and settle in new areas. Natal dispersal is the first movement of an organism from its birth site to the site in which it first attempts to breed. Adult dispersal is a subsequent movement when an adult organism changes its location in space. As individuals move across space and settle into new locations, the population to which they belong expands or contracts its overall distribution. Thus, dispersal is the process by which populations change the area they occupy.
Migration is the regular movement of organisms during different seasons. Many species migrate between wintering and breeding ranges. Such migratory movement is marked by a regular return in future seasons to previously occupied regions, and so usually does not involve an expansion of population range. Some migratory species show astounding abilities to return to the exact locations used in previous seasons. Other species show no regular movements, but wander aimlessly without settling permanently into a new space. Wandering (called nomadism) is typical of species in regions where the availibility of food resources are unpredictable from year to year. Neither migration nor nomadism is considered an example of true dispersal. See Migratory behavior
Virtually all forms of animals and plants disperse. In most higher vertebrates, the dispersal unit is an entire organism, often a juvenile or a member of another young age class. In other vertebrates and many plants, especially those that are sessile (permanently attached to a surface), the dispersal unit is a specialized structure (disseminule). Seeds, spores, and fruits are disseminules of plants and fungi; trochophores and planula larvae are disseminules of sea worms and corals, respectively. Many disseminules are highly evolved structures specialized for movement by specific dispersal agents such as wind, water, or other animals.
A special case of zoochory (dispersal using animal agents) involves transport by humans. The movement of people and cargo by cart, car, train, plane, and boat has increased the potential dispersal of weedy species worldwide. Many foreign aquatic species have been introduced to coastal areas by accidental dispersal of disseminules in ship ballast water. The zebra mussel is one exotic species that arrive in this manner and is now a major economic problem throughout the Great Lakes region of North America. Some organisms have been deliberately introduced by humans into new areas. Domestic animals and plants have been released throughout the world by farmers. A few pest species were deliberately released by humans; European starlings, for example.
Some of the most highly coevolved dispersal systems are those in which the disseminule must be eaten by an animal. Such systems have often evolved a complex series of signals and investments by both the plant and the animal to ensure that the seeds are dispersed at an appropriate time and that the animal is a dependable dispersal agent. Such highly evolved systems are common in fruiting plants and their dispersal agents, which are animals called frugivores. Fruiting plants cover their seeds with an attractive, edible package (the fruit) to get the frugivore to eat the seed. To ensure that fruits are not eaten until the seeds are mature, plants change the color of their fruits as a signal to show that the fruits are ready for eating.
Many plants in the tropical rainforests are coevolved to have their seeds dispersed by specific animal vectors, including birds, mammals, and ants. Many tropical trees, shrubs, and herbaceous plants are specialized to have their seeds dispersed by a single animal species. Temperate forest trees, in contrast, often depend on wind dispersal of both pollen and seeds.
Dispersal barriers are physical structures that prevent organisms from crossing into new space. Oceans, rivers, roads, and mountains are examples of barriers for species whose disseminules cannot cross such features. It is believed that the creation of physical barriers is the primary factor responsible for the evolution of new species. A widespread species can be broken into isolated fragments by the creation of a new physical barrier. With no dispersal linking the newly isolated populations, genetic differences that evolve in each population cannot be shared between populations. Eventually, the populations may become so different that no interbreeding occurs even if dispersal pathways are reconnected. The populations are then considered separate species. See Speciation
Dispersal is of major concern for scientists who work with rare and endangered animals. Extinction is known to be more prevalent in small, isolated populations. Conservation biologists believe that many species exist as a metapopulation, that is, a group of populations interconnected by the dispersal of individuals or disseminules between subpopulations. The interruption of dispersal in this system of isolated populations can increase the possibility of extinction of the whole metapopulation. Conservation plans sometimes propose the creation of corridors to link isolated patches of habitat as a way of increasing the probability of successful dispersal. See Population dispersion