The function of biodiversity in the ecology of vector-borne zoonotic diseases

This is a critical evaluation of the influence of species diversity within communities of vertebrates on the risk of human exposure to vector-borne zo onoses. Vertebrates serve as natural reservoirs of many disease agents (vir al, bacterial, protozoal) that are transmitted to humans by blood-feeding a rthropod vectors. We describe the natural history of the Lyme disease zoono sis to illustrate interactions among pathogens, vectors, vertebrate hosts, and risk to humans. We then describe how the presence of a diverse assembla ge of vertebrates can dilute the impact of the principal reservoir (the whi te-footed mouse, Peromyscus leucopus) of Lyme disease spirochetes (Borrelia burgdorferi), thereby reducing the disease risk to humans. Exploring the l ogic of what we call the dilution effect reveals four conditions that are n ecessary for it to apply generally to vector-borne zoonoses: (1) the feedin g habits of the vector are generalized; (2) the pathogen is acquired by the vector from hosts (as opposed to exclusively transovarial transmission); ( 3) reservoir competence (the ability of a particular host species to infect a vector) varies among host species; and (4) the most competent reservoir host tends to be a community dominant, as defined by the proportion of the tick population fed by that species. When these conditions are met, vertebr ate communities with high species diversity will contain a greater proporti on of incompetent reservoir hosts that deflect vector meals away from the m ost competent reservoirs, thereby reducing infection prevalence and disease risk. Incorporating the likelihood that the abundance of competent reservo irs is reduced in more diverse communities, owing to the presence of predat ors and competitors, reinforces the impact of the dilution effect on the de nsity of infected vectors. A review of the literature reveals the generalit y, though not the universality, of these conditions, which suggests that th e effects of diversity on disease risk may be widespread. Issues in need of further exploration include (i) the relative importance of diversity per s e versus fluctuating numbers of particular species; (ii) the relevance of s pecies richness versus evenness to the dilution effect; (iii) whether the d ilution effect operates at both local and regional scales; and (iv) the sha pe of empirically determined curves relating diversity to measures of disea se risk. Further studies linking community ecology with epidemiology are wa rranted.