Biodiversity May Not Reduce Disease Despite Popular Theory
March 21, 2013

Theory That Biodiversity Can Reduce Disease Is An ‘Oversimplification’ Say Researchers

redOrbit Staff & Wire Reports - Your Universe Online

Despite the predictions of a popular theory, maintaining biodiversity in an ecosystem does not necessarily reduce the transmission of diseases from animals to humans, say researchers from the Stanford University Woods Institute for the Environment.

Writing in the journal Ecology Letters, co-authors James Holland Jones and Dan Salkeld challenge what is known as the dilution effect — a widely held hypothesis claiming that the risk of human illness resulting from animal pathogens decreases as the variety of different species in an area increases.

Those ailments, which are known as “zoonotic diseases,” are responsible for more than 75 percent of new, emerging or re-emerging human diseases, according to the World Health Organization (WHO).

The dilution effect predicts that the risk of such disease transmissions decreases proportionally as biodiversity in an area increases. However, Jones, Salkeld and their colleagues believe that the theory is most likely incorrect.

In the first study to formally review the dilution effect, the Stanford-led team conducted a meta-analysis of studies to evaluate links between host diversity and the risk of human infection from disease-causing pathogens.

They were able to poll estimates from studies and test for biases, and still found “very weak support, at best” for the dilution effect, Woods Institute Communications writer Rob Jordan reported on Wednesday.

In fact, Jordan wrote that the researchers “found that the links between biodiversity and disease prevalence are variable and dependent on the disease system, local ecology and probably human social context. The role of individual host species and their interactions with other hosts, vectors and pathogens are more influential in determining local disease risk, the analysis found.”

Jones, Salkeld and their colleagues determined that “broadly advocating for the preservation of biodiversity and natural ecosystems to reduce disease risk is ℠an oversimplification of disease ecology and epidemiology,´” Jordan added.

They believe that a greater understanding of exactly how pathogens are transmitted is necessary in order to better keep these zoonotic diseases in check. In addition, they advise researchers to focus more of their attention on the link between disease risk and both species characteristics and ecological mechanisms.

“Specifically,” Jordan said, they “recommend that researchers focus more on how disease risk relates to species characteristics and ecological mechanisms. They also urge scientists to report data on both prevalence and density of infection in host animals, and to better establish specific causal links between measures of disease risk — such as infection rates in host animals — and rates of infection in local human populations.”

It is important to note that the Jordan and Salkeld-led team was only able to review a total of 16 different studies (13 published, three unpublished) for their meta-analysis. They note that the investigation is still in its earliest stages, and because of the limited information currently available, drawing any concrete conclusions regarding the relationship between biodiversity and zoonotic disease transmission would be premature.

Nonetheless, Jones said that he is “very confident in saying that real progress in this field will come from understanding ecological mechanisms. We need to turn to elucidating these rather than wasting time arguing that simple species richness will always save the day for zoonotic disease risk.”