Is biodiversity bad for your health?

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.