The effect of parasites on wildlife.

Populations of animals which live in the wild are regulated by many biotic and abiotic factors. Parasites are one of the biotic factors. Parasites may influence their hosts in different ways. They may cause the death of the host due to a direct lethal effect or an indirect effect. Direct lethal effects may occur if killing is a part of the life cycle of the parasite or if hosts and parasites have not developed an equilibrium. The introduction of hosts or parasites into a new environment with suitable hosts or parasites is an example. Death by parasitism may also be caused by a combination of the emaciating effects of parasites combined with factors such as bad weather conditions, environmental pollution or human handling. Parasites may also influence the behaviour of their hosts. If the hosts are intermediate hosts in the life cycle of the parasites, the alterations in behaviour may make them an easier prey for their predators, the final hosts. Parasites may also influence the reproductive success of the hosts. In this respect the relationship between the red grouse (Lagopus lagopus scoticus) and the caecal nematode Trichostrongylus tenuis has been well worked out.