Smithsonian.com

Infectious diseases increase when biodiversity decreases. (Credit: Nicolle Rager-Fuller, National Science Foundation)

We’re losing species at an alarming pace. The current rate of loss has been estimated to be 100 to 1,000 times the background extinction rate, and that’s expected to rise by a factor of 10 to 100 over the next 50 years. But if simple disappearance isn’t enough to get you worried, a new study in Nature finds that declining biodiversity frequently results in an increase in infectious diseases.

Many species that are lost provided buffers from pathogens, according to the study. The species left behind are the ones that are disease transmitters, though the reasons for this are not yet understood. Take the West Nile virus, for example, which is transmitted by mosquitoes and for which several species of birds act as host. When the variety of bird species is low, the population tends to include many species that harbor West Nile. In these areas, humans are more likely to contract the disease.

This pattern holds for parasites, animal infections, plant pathogens and coral diseases. The need for biodiversity even plays out at the microbial level. A rich diversity of microbes may regulate which become pathogenic in some cases, and in others high microbial diversity may protect against dangerous invasive pathogens.

Higher biodiversity could increase the potential sources for diseases, so losing this variety may seem like a good thing, the scientists say. But that is outweighed by the increase in contact between source and hosts (such as domesticated pigs and human, with swine flu) that comes when only a few species dominate a landscape.

The scientists say that in light of these findings, preserving biodiversity is now an even more urgent concern. “When a clinical trial of a drug shows that it works, the trial is halted so the drug can be made available,” says the study’s lead author Felicia Keesing, an ecologist at Bard College. “In a similar way, the protective effect of biodiversity is clear enough that we need to begin implementing policies to preserve it now.”