All organisms in nature are where we find them because they have moved there. This is true for even the most apparently sedentary of organisms, such as oysters and redwood trees. Their movements range from the passive transport that affects many plant seeds to the apparently purposeful actions of many mobile animals. Dispersal and migration are used to describe aspects of the movement of organisms. The terms are defined for groups of organisms, although it is of course the individual that moves.
Dispersal is most often taken to mean a spreading of individuals away from others, and is therefore an appropriate description for several kinds of movements: (i) of plant seeds or starfish larvae away from each other and their parents; (ii) of voles from one area of grassland to another, usually leaving residents behind and being counterbalanced by the dispersal of other voles in the other direction; and (iii) of land birds amongst an archipelago of islands (or aphids amongst a mixed stand of plants) in the search for a suitable habitat.
Migration is most often taken to mean the mass directional movements of large numbers of a species from one location to another. The term therefore applies to classic migrations (the movements of locust swarms, the intercontinental journeys of birds) but also to less obvious examples like the to and fro movements of shore animals following the tidal cycle. Whatever the precise details of dispersal in particular cases, it will be useful in this chapter to divide the process into three phases: starting, moving and stopping (South et al., 2002) or, put another way, emigration, transfer and immigration (Ims & Yoccoz, 1997). The three phases differ (and the questions we ask about them differ) both from a behavioral point of view (what triggers the initiation and cessation of movement?, etc.) and from a demographic point of view (the distinction between loss and gain of individuals, etc.).
The division into these phases also emphasizes that dispersal can refer to the process by which individuals, in leaving, escape from the immediate environment of their parents and neighbors; but it can also often involve a large element of discovery or even exploration. It is usefUl, too, to distinguish between natal dispersal and breeding dispersal (Clobert et al., 2001). The former refers to the movement between the natal area (i.e. where the individual was born) and where breeding first takes place. This is the only type of dispersal possible in a plant. Breeding dispersal is movement between two successive breeding areas.
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