Human activities are dramatically reducing biodiversity, and the frequency and severity of infectious disease outbreaks in human, wildlife, and domesticated species are increasing. These concurrent patterns have prompted suggestions that biodiversity and the spread of diseases may be causally linked. For example, the dilution effect hypothesis proposes that diverse host communities inhibit the abundance of parasites through several mechanisms, such as regulating populations of susceptible hosts or interfering with the transmission process. Thus, diverse communities may inhibit the proliferation of parasites, thereby promoting the stability of ecological communities and ecosystem services (e.g., nutrient cycling, carbon sequestration, and natural product production) [Civitello 2015: 8667].
This meta-analysis concludes that as a general rule across ecosystems, biodiversity inhibits parasitism. Previous studies had focused on particular host-parasite systems, and found that greater host diversity dilutes, or limits, the spread of disease. “Consequently, anthropogenic declines in biodiversity could increase human and wildlife diseases and decrease crop and forest production” [Civitello 2015: 8667].
Civitello, D., et al., 2015, Biodiversity inhibits parasites: broad evidence for the dilution effect, PNAS 112(28), https://www.pnas.org/content/112/28/8667.short.