The relationship between parasite virulence and environmental persistence: a meta-analysis.

Why some parasites evolve and maintain extreme levels of virulence is a question that remains largely unanswered. A body of theory predicts that parasites that form long-lived spores able to persist in the environment evolve higher virulence, known as the sit and wait hypothesis. Such parasites can obliterate their local host population and wait in the environment for further hosts to arrive, reducing some of the costs of high virulence. On the other hand, some models predict the opposite to be true, that virulence and environmental persistence are both costly and traded off, the resource allocation hypothesis. I conducted a meta-analysis on published data on the relationship between environmental persistence and virulence collected to date. I first examined all data available to date and then conducted a smaller analysis focussing on just those studies testing the specific predictions of the sit and wait hypothesis. Empirical work supports both hypotheses; however, the direction of the effect is largely associated with parasite type. In both analyses, viruses tend to show evidence of resource allocation trade-offs, these traits are positively correlated in bacterial and fungal parasites.