Dispersal, dispersion,
and distribution
7
This chapter explores some of the reasons why populations are found where they
are. We describe the finer-scaled pattern as the dispersion and the broader scale as
the distribution. We offer examples of how different factors such as temperature
and seasonality limit the distribution of wildlife. We then discuss the causes for dis-
persal, and finally methods of modeling rates of dispersal of populations.
Dispersalis the movement an individual animal makes from its place of birth to
the place where it reproduces. Dispersal is not to be confused with migration(move-
ment backward and forward between summer and winter home ranges) or with local
movement(movement within a home range). The terms immigration and emigra-
tion are used in mark–recapture studies to mean movement into and out of a study
area of arbitrary size and location. Migration is used by population geneticists to mean
“the movement of alleles between semi-isolated subpopulations, a process that by
definition involves gene flow between subpopulations” (Chepko-Sade et al. 1987).
Although these uses differ from their ecological uses, the difference is usually obvi-
ous from context and causes little confusion.
Dispersionis the pattern of spatial distribution taken up by the animals of an area.
Dispersions may be fixed if the animals are sessile but more commonly they change
with time under the influence of a changing dispersion of resources. A dispersion at
a given time may be changed by dispersal, or local movement, or both.
The distributionof a population or species is the area occupied by that popula-
tion or species. It is depicted as the line drawn around the dispersion. The distribu-
tion can be subdivided into gross range and breeding range, and it can be mapped
at different scales.Dispersal is an action performed by an individual ( Johnson and Gaines 1990). An
animal disperses or it remains within its maternal home range. If it disperses, it may
move only that distance sufficient to bring it to the nearest unoccupied and suitable
area within which to establish its own home range, or it might move a considerable
distance, crossing many areas that look suitable enough, before settling down.
The mechanism of dispersal may also vary. The individual may be pushed out of
the maternal home range by a parent or it may move without any prompting save
for that supplied by its genes. The young of some species never meet their parents
(e.g. frogs, reptiles, the mound-building birds of the family Megapodidae) and so must
provide their own motivation. In mammals, at least, there are two forms of dispersal
that have been recognized (Stenseth and Lidicker 1992). Presaturationdispersal
is seen in some species of small mammals where juveniles leave their natal range
even when the density of the population is low. The mechanism is either that90