Good medicine for conservation biology: The intersection of epidemiology and conservation theory

Infectious disease can be a concern for several aspects of conservation biology, such as determining threats to species, estimating population viability, and designing reserves, captive breeding, and recovery programs. Several measures are useful for describing infectious diseases in host populations, but it is not straightforward to determine the degree to which a particular disease may affect a host population. The most basic epidemiological theory suggests that populations should be least subject to host-specific infectious disease when they are at low abundance (paradoxically, the state at which they are in most need of conservation action). There are important exceptions, however, such as when a reservoir host exists or when Allee or stochastic effects occur. Several of the key threats to biodiversity - habitat alteration, introduced species, pollution, resource exploitation, and climate change - can facilitate and/or impair transmission of infectious disease. Common management tools such as population viability analysis rarely address infectious disease explicitly. We suggest that such an inclusion is both possible and warranted. Considerations of infectious disease may influence the way we determine whether a species is in need of protection and how we might design reserves and captive breeding programs. Examples from the literature suggest that (1) introduced pathogens can make abundant species rare and (2) diseases of domestic animals can dramatically affect rare species. For both scenarios, conditions that cause stress or reduce genetic variation may increase susceptibility to disease, whereas crowding and cross-species contact can increase transmission. Southern sea otters (Enhydra lutris nereis) make an interesting case study for consideration of the intersection of epidemiology and conservation because disease may be an important factor limiting the growth of otter populations. We conclude that pathogens are of increasing concern for conservation. Because many newly emerging pathogen dynamics often do not conform to the simplifying assumptions used in classic epidemiology, a detailed understanding of pathogen life history will illuminate the intersection of epidemiology and conservation theory.