Linking biodiversity loss and infectious disease in human and wildlife populations

Pieter T. J. Johnson¹ and David W. Thieltges²

¹University of Colorado, Boulder, USA and ²University of Otago, New Zealand

Although most emerging diseases can be linked to environmental change, the ecological mechanisms responsible for changes in infection are often unknown. Growing interest in ecology has recently focused on the hypothesis that community diversity can mediate infection levels and disease (‘dilution effect’). In turn, biodiversity loss – a widespread consequence of environmental change – can indirectly promote increases in disease, including those of medical and veterinary importance. While this work has focused primarily on correlational studies involving vector-borne microparasite diseases (e.g. Lyme disease, West Nile virus), we argue that macroparasites with complex life cycles (e.g. helminths) offer an excellent additional model in which to experimentally address mechanistic questions underlying the dilution effect. Here, we integrate recent ecological research on the dilution effect in microparasites with parasitological research on the decoy effect in macroparasites to explore key questions surrounding the relationship between community structure and disease. We find consistent evidence that community diversity significantly alters parasite transmission and pathology under laboratory as well as field conditions. This occurs due to reductions in susceptible host density or in the encounter rate between susceptible hosts and infectious stages. The degree of transmission reduction typically increases additively with increases in species richness but varies with parasite life cycle stage, spatial scale, and host species. Taken together, this synthesis highlights the broad link between community structure and disease while underscoring the importance of mitigating ongoing changes in biological communities owing to species introductions and extirpations.