December 3, 2010

Of mice and opossums: Links between biodiversity and the spread of disease

Article reviewed: Impacts of biodiversity on the emergence and transmission of infectious diseases

By F. Keesing and many other authors. Published in Nature, Vol. 468, pp. 647-652.

The plot line: This article reviews declines in levels of biodiversity and concurrent increases in the spread of infectious diseases, many of which have infected humans (e.g. West Nile Virus, Haanta Virus, and Lyme’s Disease- carried by ticks on mice and opossums). They integrate several recent experimental studies along with observations of biodiversity-disease correlations to explain how decreases in biodiversity (as occurs when forests are cleared for agricultural use, for example) can lead to the spread of disease. While in some cases the loss of biodiversity could actually decrease the spread of certain diseases, they argue that most biodiversity losses are likely to increase the spread of diseases because of the kinds of species that are typically lost when biodiversity decreases.

Relevant quote: Overall, despite many remaining questions, current evidence indicates that preserving intact ecosystems and their endemic biodiversity should generally reduce the prevalence of infectious diseases."

Relevance to landowners and stakeholders:

This article is relevant for anyone who doesn’t like getting an infectious disease that might kill them, which is most people, which is why it is in the widely-read journal Nature and covered by news outlets like NPR. The authors listed land use change as the most common driver of new infectious diseases in humans over the past 65 years. Loss of forests is an important part of land use change, especially considering that forests often house relatively high numbers of species.

Given the link between biodiversity and disease, there is obvious relevance for forest stakeholders. A clear benefit of keeping forests as forests (instead of say, a vineyard or a strip mall) has always been the protection of biodiversity, which is typically thought to be an inherently good thing. But why biodiversity is good is often not articulated by its advocates. This article helps give a specific reason related to human health for why biodiversity is, in general, good. When species are lost from ecosystems, they often are the ones that have resistance to disease. This removal of resistant plants can result in a relative or absolute increase in species that are not resistant to disease. These non-resistors become hosts to pathogens, which can spread rapidly and eventually end up “jumping” on to humans as hosts.

Relevance to managers:

The authors’ discussion of how diversity can be managed to reduce disease was disappointing. Although much of the review focused on natural ecosystems and how their diversity influences transmission of disease, the only recommendation they gave concerning ecosystem management was that conserving larger areas is better than conserving small areas. Forest managers realize, however, that how forests are managed within their boundaries is also just as important for biodiversity. You can have a very large forest with low diversity if management decisions alter the forest in such a way that drives down biodiversity across large landscapes. An example from California is the structural homogenization that has occurred across large forest tracts as a result of fire suppression. The management implications of this paper are best summarized by a colleague who is much smarter and more articulate than I once wrote:

Diversity should be as much our practice as it is our purpose. 

In other words, managers can achieve biodiversity by striving for a diverse mix of management approaches for meeting objectives.

Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:

Like I said above, the management implications were disappointing and too simplistic. They also argue that weedy plant species, which are often the ones left behind when biodiversity decreases, can be more susceptible as hosts to disease. The implication being that increases in weedy species could increase disease transmission. But they do not offer any physiological explanation for why this would be the case, like they do with vertebrates. They do reference an article that may indeed explain the physiological reasoning, but they should have given the reasoning from the cited article.