A note on species richness and the variance of epidemic severity

The commonly observed negative correlation between the number of species in an ecological community and disease risk, typically referred to as “the dilution effect”, has received a substantial amount of attention over the past decade. Attempts to test this relationship experimentally have revealed that, in addition to the mean disease risk decreasing with species number, so too does the variance of disease risk. This is referred to as the “variance reduction effect”, and has received relatively little attention in the disease-diversity literature. Here, we set out to clarify and quantify some of these relationships in an idealized model of a randomly assembled multi-species community undergoing an epidemic. We specifically investigate the variance of the community disease reproductive ratio, a multi-species extension of the basic reproductive ratio R 0 , for a family of random-parameter community SIR models, and show how the variance of community R 0 varies depending on whether transmission is density or frequency-dependent. We finally outline areas of further research on how changes in variance affect transmission dynamics in other systems.