Interactions between sources of mortality and the evolution of parasite virulence

A well-known result from the theory of the evolution of virulence is the pr ediction that the virulence of a pathogen (i.e. the rate of parasite-induce d host mortality) always evolves to higher levels when host background mort ality rates increase. This prediction, however, is derived from models that assume that host mortality sources combine additively to determine the ove rall host mortality rate. In this paper, we suggest that such additivity is probably rare for many host-pathogen systems, and explore how the predicti ons for the evolution of virulence are altered when interactions between ho st mortality sources are incorporated into the theory. Our results indicate that if mortality-source interactions are sufficiently strong then the evo lutionarily stable level of virulence can actually decrease as the backgrou nd mortality rate increases. Consequently, a detailed mechanistic descripti on of how parasites and other mortality sources combine to cause host morta lity is required before reliable predictions about virulence evolution can be made. Moreover, mortality-source interactions make empirical comparisons of the virulence of different parasites a much more subtle issue.