May 19, 2009 -- Maintaining the diversity of life on Earth does more than just keep the world interesting. It also keeps us healthy.
At a field site in Panama, scientists found that rates of hantavirus tripled in rodents as the number of rodent species dropped. Hantavirus is an often-fatal disease that can spread from rodents to people.
The new study strengthens a growing sense that reducing biodiversity increases the risk that diseases will jump from animals to people.
"We are undergoing a massive episode of extinction by human hands," said Richard Ostfeld, an ecologist at the Cary Institute of Ecosystem Studies in Millbrook, New York. "If we have a less diverse planet, it means we will have a sicker human population."
In the last few years, scientists have increasingly noticed that, when biodiversity dips, rates of Lyme disease, West Nile virus, SARS and other infectious diseases rise. Called zoonotic diseases, these illnesses also spread from animals to people.
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For most zoonotic diseases, there is usually a small number of host species that act as a reservoir for the infection. Mice and chipmunks, for example, are the main reservoirs of Lyme disease, even though the ticks that carry Lyme-causing bacteria will bite pretty much any warm-blooded vertebrates they encounter -- including raccoons, foxes and squirrels, which don't carry the disease.
When there are lots of species around, chances are greater that ticks will bite animals that don't carry Lyme disease, making them less likely to be infectious themselves when they later bite people. As diversity drops, on the other hand, mice and chipmunks are the types of species that tend to be left behind, allowing the disease to proliferate.
The theory is called the "dilution effect," and most evidence for it so far comes from observations that show spikes in disease where biodiversity is lost. Ostfeld and colleagues wanted to see if they could show that one was, in fact, causing the other.
Led by Gerardo Suzan of the University of New Mexico in Albuquerque, the team investigated hantavirus, which people catch from rodent droppings and urine. In Central America, two main species act as reservoirs for hantavirus: the pygmy rice rat and the cane mouse.
At the edges of forests in southwestern Panama, the researchers set up 24 experimental plots. They left some sites alone. In others, they mimicked diversity loss by regularly trapping and removing rodents, but allowing pygmy rice rats and cane mice to stay put.
Five months later, the scientists report this month in the journal PLoS ONE, both pygmy rice rats and cane rats had increased in abundance in the experimental plots. Even more striking, the proportion of these animals infected in with hantavirus nearly tripled in those plots -- from 12 percent to 35 percent.
"That's a bad thing as far as hantavirus is concerned," Ostfeld said. "We know that human risk is related to the number of infected rodents running around."
The new study is the first field-based experimental evidence for the link between diversity and disease, and that's a big step, said Pieter Johnson, a disease ecologist at the University of Colorado, Boulder.
Still, he said, the relationship is complex. Simply having more species in an area is not as important as having a certain combination of species, and scientists haven't yet worked out all of those details.
It's been hard to convince people to care about biodiversity, added Johnson, whose recent lab experiments have linked fluke worm diversity with rates of schistosomiasis, a parasitic disease. These new studies might help convince people to protect species they may not have had the energy to care about before.
"The potential link that biodiversity loss could actually hurt human health or wildlife health is a pretty eye-opening concept for many people," Johnson said. "If we don't want to get sick, then we need to keep more species around."
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