Thursday, June 08, 2006

The Importance of Biological Diversity to Human Health

A new essay from PLoS Medicine talks about the relationship between biodiversity and public health issues, especially the link between vector-borne diseases like malaria and the rate of human infection. Some of the ways that disease risk might be reduced are:

  • Zooprophylaxis: Another host animal living in close proximity to humans absorbs the brunt of attacks by the disease vector, hence reducing the risk of disease transmission to humans.

  • Poor 'reservoir species': Disease vectors usually have several hosts (e.g. ticks that feed on mice and squirrels) and these hosts have varying degrees of success in getting the vectors infected. In more biologically diverse communities, the proportion of hosts which are poorer 'reservoirs' tends to be greater, hence reducing the proportion of potential vectors which actually bear the disease.

  • The dilution effect: from the article -- "the vectors that transmit the pathogen only take a limited number of bites in their lifetime; when some of these bites are taken from hosts that are not competent to amplify the pathogen, these bites are wasted. This reduces the rate at which the pathogen is transmitted." This might be compounded by non-viremic hosts (i.e. where a diseased vector must be feeding at the same time as an undiseased one to transmit the pathogen, because the host does not get infected) and reliance of the vector upon noncompetent hosts for part of its life cycle.

Not just animals and humans but plants too suffer from diseases, and it has been found that interspersing fields planted with a single variety of crop with other species tends to reduce transmission rates of diseases that specifically infect the crop of interest, because the other species act as buffers and barriers to transmission. Predators are also shown to play a role in disease reduction by removing the sickest and weakest animals and so reducing infection rates. The authors end by highlighting the possible effect that climate change might have on disease, as the temperate zones become warmer and vector-borne diseases from the tropics move into them. As they encounter a less biologically diverse landscape, the proportion which end up infecting humans and domestic plants and animals may likely increase.

However, a few questions remain which they do not discuss: why is it that when biological diversity is reduced, the species that remain tend to be more competent reservoirs of vector-borne disease than those which do not survive? Is it merely a sampling artifact of the limited number of cases of which we are aware, or does it reflect a grimmer ecological process at work? Is high diversity always a good thing? The authors point out that for zooprophylaxis against malaria using cows, mosquito bites may be diverted away from humans in the short term, but they may contribute to mosquito abundance in the long term. Because of the innate complexity of ecological cause-and-effect, it's likely that general statements linking more biodiversity with better public health are tenuous at best, and what benefits should be examined carefully case by case.

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