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Niche restriction is universal among parasites, and it has been suggested that it is at least partly the result of selection which favours mate finding and thus cross-fertilization, leading to greater genetic diversity ('mating hypothesis'). In this chapter, I review evidence for niche restriction at the level of hosts (host range and host specificity), populations (aggregation) and host individuals (microhabitat specificity). I also discuss mating behaviour and particularly mate finding, the morphological basis of mating, using some recent light- and electron-microscopic studies, evidence for the role of niche restriction in facilitating mating, and the relative importance of niche segregation and differences in copulatory organs for reinforcing reproductive barriers. No attempt is made to discuss all these aspects for each of the parasite groups in detail. Instead, I select examples from those groups that have been examined best, in order to clarify patterns and to test the mating hypothesis of niche restriction and segregation. It is emphasized that only a holistic approach can clarify the significance of mating in niche restriction, i.e. an approach that considers all evidence jointly. |
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