Figure 2.

The effect of increasing genetic diversity on R₀ by adding new hosts. Suppose we have 100 hosts, with 10 genotypes in equal frequency (Point A). Now suppose that we experimentally double the host population size, without affecting the genetic diversity (Point B). We see an increase in R₀, but the increase is not large. Now suppose we double the host population size, but also increase the number of genotypes from 10 to 20 (Point C). We now see a large decrease in R₀, but it does not go below 1, so the disease can still spread. Finally, suppose that we experimentally double the host population size, but we also increase the number of genotypes by fourfold (Point D). Now R₀ not only deceases, but it goes below 1, so the disease will die out. Redrawn from Lively (2010a).