Biodiversity and disease risk: The case of lyme disease

Utilitarian arguments concerning the value of biodiversity often include the benefits of animals, plants, and microbes as sources of medicines and as laboratory models of disease. The concept that species diversity per se may influence risk of exposure to disease has not been well developed, however. We present a conceptual model of how high species richness and evenness in communities of terrestrial vertebrates may reduce risk of exposure to Lyme disease, a spirochetal (Borrelia burgdorferi) disease transmitted by ixodid tick vectors. Many ticks never become infected because some hosts are highly inefficient spirochete infections to feeding ticks. In North America, the most competent reservoir host for the Lyme disease agent is the white-footed mouse (Peromyscus leucopus), a species that is widespread and locally abundant. We suggest that increases in species diversity within host communities may dilute the power of white-footed mice to infect ticks by causing more ticks to feed on inefficient disease reservoirs. High species diversity therefore is expected to result in lower prevalence of infection in ticks and consequently in lower risk of human exposure to Lyme disease. Analyses of states and multistate regions along the east coast of the United States demonstrated significant negative correlations between species richness of terrestrial small mammals (orders Rodentia, Insectivora, and Lagomorpha), a key group of hosts for ticks, and per capita numbers of reported Lyme disease cases, which supports our "dilution effect" hypothesis. We contrasted these findings to what might be expected when vectors acquire disease agents efficiently from many hosts, in which case infection prevalence of ticks may increase with increasing diversity hosts. A positive correlation between per capita Lyme disease cases and species richness of ground dwelling birds supported this hypothesis, which we call the "rescue effect." The reservoir competence of hosts within vertebrate communities and the degree of specialization by ticks on particular hosts will strongly influence the relationship between species diversity and the risk of exposure to the many vector-borne diseases that plague humans.