Biodiversity and disease risk: Dilution effect or simply habitat change?

Background/Question/Methods

There has been an explosion of interest in the possible role that biodiversity plays in the transmission of zoonotic diseases shared by humans and other animals.  A mounting list of studies finding negative correlations between the diversity of host species and some measures of disease risk have been offered as evidence that species interactions and the hosts themselves play a direct role in variation in disease risk. However, in addition to an also growing list of studies showing an opposite trend, there are strong reasons to worry about the causal role of species rather than correlated environmental factors in decreasing disease risk.  We examined detailed aspects of transmission ecology for a vector-borne pathogen, West Nile virus, across a land use gradient. 

Results/Conclusions

In agreement with previous studies we found a negative relationship between host species diversity (and richness) and disease risk. However, additional data not collected/presented in other studies of this system, and in many other dilution effect studies, shows that the correlation with host diversity is likely to be spurious. Instead differences in habitat that cause the changes in host communities are instead responsible for modulating disease risk. Our results challenge a growing paradigm, and offer a path for moving the field forward to determine exactly when biodiversity provides an ecosystem service for human health.