An ecological approach to the concept of parasitism

Inaugural lecture--Department of Zoology, Rand Afrikaans University, 27 May 1986The existing definitions of parasitism revolve around two elements, i.e. damage to and feeding on a host. These two elements are, however, inadequate to explain the phenomenon of parasitism, since they are in certain respects, too inclusive and in other respects too exclusive. Parasitism is an ecological concept, a form of interspecies association where small animals occupy the micro-habitats and niches on other life forms. The role of parasites in an ecosystem is discussed and elucidated by examples from an aquatic environment. Parasitic animals can be divided into different categories, based on the particular form of interspecies interaction. These categories are facultative parasitism, pseudo-parasitism, temporary parasitism, periodic parasitism and permanent parasitism. These types of host/parasite associations are discussed by means of different examples. A hypothesis on the origin of these associations is formulated: parasite/host association can originate either by a niche shift, or by a micro-habitat shift. It is further postulated that parasite/host associations do not develop due to an evolutionary association between the parasite and the particular host, but rather to an association between the parasite and the particular niche. Specialization in parasites is therefore aimed at niche reservation in order to eliminate inter- and intra-species competition