DISPERSAL

Introductions, colonisations of new habitats and extinctions are all closely related to the dispersal ability of a species. It is seldom appreciated that a colonisation event and an extinction event may be indistinguishable at the moment they are viewed: from the perspective of a parasite, the same strategy is involved both in establishing a population from a colonising propagule and in retrieving a population from the point of extinction. Introductions and extinctions are in effect the two sides of the same coin and the success or failure of each process is closely linked to the dispersal ability of the parasite.

In his review of parasite dispersal, Kennedy (1976) concluded that the so-called dispersal stages of parasites were not particularly effective in achieving dissemination in space or time. The duration and timing of the reproductive period and the release of eggs of a parasite, especially if reproduction exhibits a seasonal cycle, is more closely related to the opportunities for, and probability of, infecting the next host in the life cycle than to anything else. If the intermediate host of an acanthocephalan itself exhibits a seasonal cycle in reproduction such that the new generation appears at a particular time of year, then parasite reproduction will normally be synchronised with this. The activities of any short-lived, active, free-living stages of a parasite are also far more likely to be related to the probability of locating and infecting the next host than to disseminating the parasite in space or time, as they cover only very short distances in their lifespan.