Now there’s one more reason to protect animals’ natural habitats: They can prevent you from getting sick. A new study shows that as biodiversity — the diversity of species with an ecosystem — decreases, the level of infectious disease within humans increase. Growing agriculture demands, clear-cutting forests and climate change are pushing wildlife from their habitats and are driving many species to local extinction. Some estimates put the current extinction rate at anywhere from 100 to 1,000 times higher than what scientists consider the normal extinction rate based on fossil evidence. Ecologists expect the current extinction rate to increase a thousand times more over the next 50 years.
Previous studies have hinted at a link between biodiversity loss and human sickness, but the study published this week in the journal Nature provides the strongest evidence yet, says the study’s lead author, biologist Felicia Keesing of Bard College in Annandale-on-Hudson, N.Y.
Keesing and her colleagues pored over the available literature on biodiversity loss and found that in every case they looked at, the incidence of disease in the surrounding human population rose. “The results are shockingly consistent,” Keesing said.
There are a number of reasons why this occurs, she said, but the most common factor seems to be that when biodiversity drops, the ecosystem loses “buffer” species that ordinarily would take the brunt of diseases. For example, hantavirus in the southwestern United States is spread among rodents and causes deadly hantavirus pulmonary syndrome in humans. If fewer species are crowding their habitat and using resources, rodents are more likely to spread, encounter each other and spread the virus.
In other cases, the loss of some species leaves room for even more of a disease-bearing species. Opossum are particularly susceptible to forest clearing, making way for white-footed mice to take their place. More mice means more blacklegged ticks, which carry Lyme disease.
Keesing said it’s unclear why the hardiest species also seem to be the most effective at spreading disease, but it could be that the features that make species survivors also make them suitable hosts for diseases. Living fast and dying young makes for a thriving species but reduces the reasons to evolve strong defenses against disease. Or it could be that diseases simply evolve to best fit the strongest survivors.
Either way, Keesing said the findings are a call to action for conservationists and environmental officials. “Clearly we need to make more efforts to preserve biodiversity,” she said.
Especially important is carefully monitoring areas where land use is quickly changing from its natural state, she said, as these will likely by hot spots for infectious disease.
Margie Lee, a biologist at the University of Georgia, said she expects the paper to be a landmark study. “Rather than offering vague recommendations to simply protect the environment, it’s very specific in its recommendations,” she said.
Gregory Glass, a microbiologist at Johns Hopkins University, is cautious about overinterpreting the findings, though. It’s not obvious that this pattern holds true for every disease, he said, and many studies have found that more biodiversity breeds more diversity within diseases and parasites. “Do I think biodiversity is important? Yes,” Glass said. “Do I think it’s clear-cut … that biodiversity is always good for human health? No, and it almost certainly isn’t true all the time.”
