Is biodiversity bad for your health? 

Richard S. Ostfeld Felicia Keesing, 2017

Natural ecosystems provide services that support human well‐being, but ecosystems may also contain elements that can endanger humans. Some researchers have argued that ecosystems that support high vertebrate diversity pose a danger to human health because they are likely to support a high diversity of zoonotic pathogens, leading to the emergence of infectious diseases. We evaluated the evidence for the three necessary links in the hypothesized causal chain linking high vertebrate diversity to a high probability of emergence of infectious diseases. We found no support for one critical link—that high total diversity of vertebrate pathogens correlates with high diversity of actual or potential zoonotic pathogens. In contrast, there is now substantial evidence that high diversity protects humans against the transmission of many existing diseases. These results have substantial relevance for environmental policy.

 

The necessary logical steps underlying the argument that high host diversity leads to high probability of the emergence of a zoonotic disease. High diversity of vertebrate hosts must result in high total diversity of pathogens within the vertebrate community, which in turn must lead to high diversity of actual or potential zoonotic pathogens (those that can infect humans and cause disease), which in turn must increase the probability of new emergence events. Although a link between host diversity and parasite diversity is relatively well established, effect of host diversity on viral and bacterial pathogens (arrow 1) is not. Evidence does not support a link between overall pathogen diversity and that of actual or potential zoonotic pathogens (arrow 2). Some evidence supports correlations between diversity of zoonotic pathogens and the likelihood of zoonotic emergence (arrow 3), but with important caveats described in the text. 

 

Schematic diagram of how parasite diversity is expected to vary with host diversity when parasites show high host specificity (upper curve) and when they show low host specificity (lower curve). In the latter case, the sharing of parasites between hosts means that the diversity of parasites will saturate as host diversity increases, resulting in little or no additional increases in parasite species at high levels of host diversity.

 

Schematic representation of typical rank–abundance curves, in which the relative abundance of each species is represented on the vertical axis and the rank of each species, from highest to lowest abundance, is given on the horizontal axis. Contrasted are two scenarios, a relatively low‐diversity community in blue and a relatively high‐diversity community in green. The curves represent the common observation that higher‐diversity communities include more species that are rare and fewer that are common. The species added (right‐hand orange circle) in higher‐diversity communities are not likely to be the sources of zoonotic pathogens, whereas the most abundant species in lower‐diversity communities (left‐hand orange circle) are often the sources of zoonotic infection. 

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