West Nile Virus is an infectious disease that arrived in New York City in 1999, and subsequently spread across the country to the west coast. It is transmitted to humans from passerine (perching) birds via mosquito vectors. This study tested the dilution effect hypothesis, which posits that greater diversity (of birds in this case) reduces the concentration of species that are the primary disease reservoirs (American robin, American crow, blue jay, western scrub jay, common grackle, house finch, and house sparrow), thus reducing vector contact with infected individuals, and ultimately transmission to humans. The study analyzed the incidence of human infection during 2003-2004, and found that biodiversity was indeed associated with reduced WNV infection rates among humans.
For all 3 years, the county-level human incidence of WNV disease was strongly, and significantly, negatively correlated with bird diversity within that county [Ostfeld 2009: 41].
Similar results are reported for studies of the dilution effect of biodiversity on Lyme disease risk. Furthermore, having collected data on the competence of various mammalian hosts to infect ticks with Lyme disease, as well as each host species’ average tick burden, the authors state that “we can project the number of ticks that will feed on them and the proportion of those ticks that will become infected” [Ostfeld 2009: 42].
We conclude from these studies that high vertebrate diversity is negatively correlated with human risk of exposure to Lyme disease. Furthermore, knowledge of the species composition of these communities, beyond simple measures of species richness or evenness, strongly enhances our ability to predict risk [Ostfeld 2009: 42].
In summary,
Evidence for a protective dilution effect of high diversity has been obtained for numerous infectious diseases of humans, wildlife, and plants. The weight of evidence suggests that protection against exposure to infectious diseases should be added to the list of utilitarian functions of biodiversity [Ostfeld 2009: 42].
“Evidence for a protective dilution effect of high diversity has been obtained for numerous infectious diseases of humans, wildlife, and plants. The weight of evidence suggests that protection against exposure to infectious diseases should be added to the list of utilitarian functions of biodiversity [Ostfeld 2009: 42]. |
Ostfeld, R.S., 2009, Biodiversity loss and the rise of zoonotic pathogens, European Society of Clinical Microbiology and Infectious Diseases 15 (Suppl. 1), https://onlinelibrary.wiley.com/doi/full/10.1111/j.1469-0691.2008.02691.x.