A mechanistic basis for the disease-diversity relationship for directly transmitted microparasites

Background/Question/Methods

Changes in host community biodiversity can dramatically impact parasite transmission. Several empirical studies have found negative associations between host biodiversity and disease risk (often measured as parasite prevalence in reservoir hosts and/or vectors). However the mechanisms underlying this disease-diversity relationship (termed the “dilution effect”) remain unclear. General theory and testable predictions for the dilution effect are undeveloped. Here, we present a formal framework to mechanistically explore disease-diversity relationships for directly transmitted parasites. We explicitly include host regulation via intraspecific and interspecific competition, where the latter can be dependent or independent of interspecific contact rates (covering a spectrum of resource utilization overlap, habitat selection preferences, and temporal niche partitioning).  We examine how these factors interact with frequency-dependent and density-dependent transmission, along with transmission competencies and life history traits of the hosts in the assemblage, culminating in the derivation of a relationship describing the propensity for parasite fitness to decrease in species assemblages relative to in single host species.

Results/Conclusions

The relationship reveals that predictions are more variable than commonly acknowledged: increases in biodiversity do not necessarily suppress frequency-dependent parasite transmission and regulation of hosts via interspecific competition does not always lead to a dilution effect.  Moreover, adding a less competent host to the assemblage does not inevitably lead to a dilution effect, and adding a more competent host does not invariably lead to amplification of disease risk. In general, the transmission potential of the resident species must be greater than the transmission potential of the invading species for the dilution effect to manifest but this is not sufficient when per-capita interspecific contact rates are high relative to intraspecific contact rates (e.g., when species are highly territorial). Our results suggest that when host species are of similar abundance, elements of species composition such as host life history traits and contact patterns override the impact of species richness on the disease-diversity relationship.