Parasite pathogenicity and the depression of host population equilibria

Abstract

THE use of pathogens as agents for the biological control of pest species has increased rapidly in recent years with the realisation that chemical pesticides are not the panacea they were once thought to be^1–6. The selection of a suitable parasite is often based on the assumption that highly pathogenic organisms will be most effective in minimising pest population growth^6–8. Recent theoretical studies of the dynamics of host–parasite associations provide a template for testing this assumption^9–12. This paper examines the relationship between parasite pathogenicity, as measured by the severity of the pathogen's influence on host survival, and the consequent depression of the host population from the size it would have achieved in the absence of infection. The maximum degree of depression is achieved by direct life cycle parasites of moderate to low pathogenicity, a prediction which has important implications for the use of pathogens as biological control agents. In agreement with traditional beliefs^13,14, theory suggests that highly pathogenic species will cause their own extinction but not that of their host. The relationship between population depression and host reproductive potential is shown to be critically dependent on the biological characteristics of the pathogen.