Effects of Host Diversity on Infectious Disease

The dynamics of infectious diseases can be affected by genetic diversity within host populations, species diversity within host communities, and diversity among communities. In principle, diversity can either increase or decrease pathogen transmission and disease risk. Theoretical models and laboratory experiments have demonstrated that a dilution effect (decreased disease risk with increasing diversity) can occur under a wide range of conditions. Field studies of plants, aquatic invertebrates, amphibians, birds, and mammals demonstrate that the phenomenon indeed does occur in many natural systems. A dilution effect is expected when (a) hosts differ in quality for pathogens or vectors; (b) higher quality hosts tend to occur in species-poor communities, whereas lower quality hosts tend to occur in more diverse communities; and (c) lower quality hosts regulate abundance of high-quality hosts or of vectors, or reduce encounter rates between these hosts and pathogens or vectors. Although these conditions characterize many disease systems, our ability to predict when and where the dilution effect occurs remains poor. The life-history traits that cause some hosts to be widespread and resilient might be correlated with those that promote infection and transmission by some pathogens, supporting the notion that the dilution effect might be widespread among disease systems. Criticisms of the dilution effect have focused on whether species richness or species composition (both being metrics of biodiversity) drives disease risk. It is well established, however, that changes in species composition correlate with changes in species richness, and this correlation could explain why the dilution effect appears to be a general phenomenon.