Population dispersal is the movement of individuals out of or into a population. Dispersal can take two separate forms: emigration, a movement of individuals away from a population and immigration, movement of individuals into a population. Migration, a separate type of movement, is a cyclic movement of individuals back and forth from and to a population area.
Population structure may be affected by dispersal in events such as mass movement of individuals resulting in drastic changes within a short period of time. In most cases dispersal of individuals away from a population is balanced by individuals entering the population, as well as natality and mortality, which keeps the overall population size and structure constant. Population structure is also greatly affected if it is at or near carrying capacity. 'Carrying capacity' is defined as the maximum number of individuals a specific area is able to support. If a population is stable (in balance) and near or at its carrying capacity, moderate dispersal will have little effect on the population since changes in natality and mortality will compensate for any fluctuations. If a population is greatly above or below its carrying capacity, dispersal such as immigration into a population below carrying capacity may increase population size even to the extent of extinction prevention. Mass immigration into a population that is already above its carrying capacity may result in slowing of population growth.
In 1946, Wolfenberger studied a great number of works published regarding organism dispersal and concluded that most small organisms disperse as the logarithm of the distance from the source (Figure 1). The x-axis plots the distance the dispersers are found a ~o c
Distance from the source
Figure 1 Organisms dispersal: most small organisms disperse as the logarithm of the distance from the source. Relative abundance of the dispersers decreases with increased distance from the source of dispersal.
from the source and the j-axis portrays the relative abundance of the dispersers. Wolfenberger pointed out that this pattern is found regardless of the type of dispersal that takes place. Organisms that are active in their movement or organisms that disperse through wind, water currents, or via animal vectors show the same type of dispersal pattern. It should be pointed out that not all species fall within the above dispersal pattern and many species such as bees and birds follow a directed movement that may change movement due to encountering a richer food source further away from the source.
Dispersal is affected by barriers (geographical and human made) as well as the vagility of individuals. 'Vagility' is the intrinsic ability of an individual for movement. In addition to affecting population size and structure, dispersal is an important force in gene flow and prevention of inbreeding.
As a result of human expansion into previously unoccupied habitat, many populations previously enjoying free movement between areas have been separated due to habitat fragmentation, where unsuitable habitat separates suitable habitat, preventing or greatly restricting dispersal and exchange of genetic variability. Construction of 'migration corridors' between population areas is one way to increase this exchange. These migration corridors, if identified and used by the species, will allow safe dispersal, migration, and gene flow from one population to another.
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