By V. DE VOS, Veterinary Ecologist.
Introduction
Populations of all living organisms may maintain parasitic populations. Older definitions conveyed the idea that a parasite is a form that inevitably causes harm to its host. This is not always true and it would seem much more satisfactory to define parasitism as a state in which an organism (the parasite) is metabolically dependent to a greater or lesser extent on another (the host). Within this framework parasites can then be categorised as those which are harmful and those which are harmless, or even beneficial, to the host. Damage to the individual host by the parasite and overreaction by the host is known as “disease”. The ill effects of disease are usually visible. This is however, looking at parasitism and disease from an individual’s point of view. Looking broader at population and even ecosystem levels, a parasite which is harmful to the individual host may at the same time be of benefit to the species (survival of the fittest and survival of better gene material) or ecosystem (prevention of overutilization by acting as a culling mechanism). In a nature reserve, which is big enough to accommodate the natural ecological processes, disease is usually looked at from an ecological point of view, rather than the individual.