OREGON STATE UNIVERSITY

Nature study: Loss of biodiversity can increase disease transmission

12/02/2010

CORVALLIS, Ore. – The loss of biodiversity in ecosystems ranging from marine coral reefs to terrestrial forests can increase the transmission of infectious diseases in humans, other animals and plants, according to a new scientific analysis just published in the journal Nature.

This connection between two developing crises – emerging novel diseases and unprecedented declines in biodiversity – has long been suspected, but has been difficult to quantify, scientists say.

“In theory, the loss of biodiversity could increase or decrease transmission of disease,” said Anna Jolles, a disease ecology specialist in the College of Veterinary Medicine at Oregon State University and one of the authors of the study. “Certainly, having naturally high biodiversity should increase the potential pool of sources for new pathogens. But the evidence suggests that in most cases, biodiversity loss actually ramps up transmission of disease. Now we need to find out why.”

Jolles and her colleagues examined dozens of published, peer-reviewed studies for their analysis, which found that when biodiversity declined, the species most likely to disappear often were those that served as a buffer to infectious disease transmission. Species that remained often magnified the disease, such as with West Nile virus, Lyme disease and hantavirus.

“We knew of specific cases like West Nile virus and hantavirus in which declines of biodiversity increase the incidence of disease,” said Felicia Keesing, an ecologist with Bard College in New York, and lead author of the Nature study. “But we’ve learned that the pattern is much more general.

“Biodiversity loss tends to increase pathogen transmission and infectious disease,” she said.

As part of their analysis, the authors looked at three recent studies of mosquito-transmitted West Nile virus, for which several species of birds act as hosts. All three studies found a strong correlation between low bird diversity and increased risk of transmission to humans in the United States. Communities with low avian diversity tend to be dominated by species that amplify the virus, the authors note, “inducing high infection prevalence in mosquitoes and people, while communities with high avian diversity contain many species that are less competent hosts.”

That tendency isn’t always the case, Jolles acknowledged. For novel diseases, greater diversity may provide a larger potential pool of hosts.

“Hotspots for novel disease emergence sometimes center around areas where growing human populations come in contact with lots of wild animal species,” Jolles said. “The more host species are around, the more pathogen species they will harbor, and the more opportunity for transmission to people.”

Nevertheless, the Nature analysis found overwhelming evidence tying the loss of biodiversity with increases in disease transmission. And that pattern, the authors say, is consistent with different types of pathogens – including viruses, bacteria and fungi – and for many types of hosts, whether they are human, other animals or plants.

One possible explanation can be found in a study of a trematode that causes schistosomiasis in humans. This parasite alternatively infects snails and humans in different stages and in the study, host snails were placed in tanks of the same density, but with non-host snails as well in some tanks. In the tanks with a single species, transmission rates were higher than in tanks with multiple species.

“The difference was that in multi-species treatments, the parasites frequently ended up in dead-end hosts,” Jolles said. “That is a pattern that seems to repeat itself with different pathogens and in different ecosystems.”

The study notes that global biodiversity continues to shrink at an alarming pace, with extinction rates estimated at 100 to 1,000 times higher than have occurred throughout most of history. And extinctions are projected to rise dramatically in the next 50 years.

“If there is one message we hope comes out with this paper,” Jolles said, “it is the clear need to conserve biodiversity to the greatest extent possible – because it may be our best insurance policy against infectious diseases.”

Much of the research was funded by the National Science Foundation, the National Institutes of Health, and the U.S. Environmental Protection Agency.