Host community structure can shape pathogen outbreak dynamics through a phylogenetic dilution effect

Biodiversity loss and anthropogenic modifications to species communities are impacting the frequency and magnitude of disease emergence events. These changes may be related through mechanisms in which biodiversity either increases (amplifies) or decreases (dilutes) disease prevalence. Biodiversity effects can be direct, when contacts among competent hosts are substituted by contacts with sink hosts, or indirect through regulation of host abundances. Whether a disease is diluted or amplified depends on the competence of the host species in the community. Here, we introduce a multi-host compartmental disease model in which we assume that host competence is determined by evolutionary relatedness. By simulating communities of varying phylogenetic structure, and estimating community disease outbreak potential ( R 0), we show how differences in host phylogenetic relatedness can switch host communities from diluting to amplifying a disease, even when species richness is unchanged. We additionally show that phylogenetic dilution can occur simultaneously with amplification through species richness, highlighting the multidimensionality of the disease-diversity relationship. We illustrate our model using empirical data describing the relationship between phylogenetic distances separating hosts and their likelihood of disease sharing. Our study demonstrates how a phylogenetic dilution effect can emerge when we allow transmission to depend on host evolutionary histories.