Dispersal: Ballooning and Other Mechanisms
The meaning of the word dispersal both in a general and in a biological context, is somewhat equivocal. Confusion originates from the fact that dispersal may indicate either the event of being dispersed or the act of dispersing (Longman 1978). In order to obtain useful definitions for various migrational processes, Pielou (1979) distinguished three kinds of dispersal. Her two classes of diffusion and secular migration, both concerned with slow and gradual shifts in the distribution range of a species, are not relevant here. Only the events falling into her classification of jump-dispersal are relevant to the type of dispersal considered here. Quoting Pielou’s definition: “Jump-dispersal is the movement of individual organisms across great distances, followed by successful establishment of a population of the original dispersers’ descendants at the destination. The salient points are that the whole journey is completed in a short period of time, usually very much shorter than the life-span of an individual; and that the journey usually takes the dispersers across totally inhospitable terrain as, for example, when spiders are carried by air currents far across the open sea.” As interpreted here, dispersal does not necessarily involve an extension of the original distribution range of a species. This is in contrast to Platnick’s (1976b) view, in which the use of the term dispersal is specifically restricted to indicate events of range extension.
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