Infectious Disease and Species Diversity

Although diseases can cause diminishment or extirpation of populations of certain species, and thereby reduce biodiversity, infectious diseases can also be important in maintaining diversity of hosts in natural systems. In addition, parasites (micro- and macroparasites) themselves make up a substantial proportion of existing biodiversity. Conservation of parasite diversity can be justified on purely intellectual grounds, but parasites also could have utilitarian value as biological control agents, models for human disease, or indicators of ecosystem function. Our objective is to evaluate the effect of disease on host diversity, and the effect of host diversity on disease. In each of these cases, effects may be positive or negative. We aim to use or modify existing theory, and develop novel theory, to predict the conditions likely to favor positive effects and those favoring negative effects. These predictions will be tested by reviewing existing data.

Effect of disease on diversity: By promoting death and inhibiting reproduction, diseases can severely reduce populations of certain species and, therefore, to reduce local species richness and diversity. However, parasites, like other natural enemies, can enhance diversity by mediating the coexistence of competitors that would otherwise exclude one another. Community ecologists have well-developed theory of how and when shared natural enemies can prevent competitive exclusion and enhance local diversity. We will review this body of theory and explore how it can be applied to host-pathogen systems. Our approach will be to examine a variety of scenarios in which a parasite is imposed upon a pre-existing community of species competing for a common resource. These scenarios will take the form of idealized model systems, and for each scenario, we will discuss empirical examples for which it can be applied. Several scenarios have strong parallels with well-studied concepts in community ecology. For example, parasitism on non-interacting or weakly interacting hosts can produce apparent competition, and is likely to reduce diversity. On the other hand, if strong competitors are attacked by specialist parasites then parasitism can enhance diversity, with conceptual links to Janzen and Connell's hypothesis for tropical tree diversity. If a dominant competitor is most susceptibile to a generalist parasite, that parasite can enhance diversity in a manner similar to a keystone predator.

Effects of diversity on disease:Much of epidemiological theory has concentrated on the one-parasite, one-host case. However, hosts have ecological interactions with other species, and many parasites infect >1 host species that vary in their susceptibility and transmission rates. Consequently, the diversity of hosts or host competitors has a potential to strongly influence establishment, dynamics, and equilibrium levels of parasites and pathogens. The impact of disease may be enhanced or inhibited by increasing diversity of hosts, mediated through effects on abundance of individual host species or within- and between-species transmission. Increasing diversity may mean that incidental hosts may be exposed to a greater variety of parasites and pathogens. The potential of biodiversity to reduce the incidence or impact of diseases of humans, crops, livestock, or fisheries provides a utilitarian justification for conserving diversity. We are developing novel theoretical approaches to predict the effect of host diversity on disease. These predictions can then be tested using existing data on disease in natural and agricultural systems.

Chair: Robert Holt (

Rapporteur: Eric Schauber (