People often ask me, "Why should I care if a species goes extinct? It's not essential to my daily life, is it?"

Well, according to new research published December 2 in Nature, the answer is yes—healthy biodiversity is essential to human health. As species disappear, infectious diseases rise in humans and throughout the animal kingdom, so extinctions directly affect our health and chances for survival as a species. (Scientific American is part of Nature Publishing Group.)

"Biodiversity loss tends to increase pathogen transmission across a wide range of infectious disease systems," the study's first author, Bard College ecologist Felicia Keesing, said in a prepared statement.

These pathogens can include viruses, bacteria and fungi. And humans are not the only ones at risk: all manner of other animal and plant species could be affected.

The rise in diseases and other pathogens seems to occur when so-called "buffer" species disappear. Co-author Richard Ostfeld of the Cary Institute of Ecosystem Studies points to the growing number of cases of Lyme disease in humans as an example of how this happens. Opossum populations in the U.S. are down due to the fragmentation of their forest habitats. The marsupials make poor hosts for the pathogen that causes Lyme disease; they can also better defend themselves from the black-legged ticks that carry the affliction to humans than can white-footed mice, which, on the other hand, are thriving in the altered habitat—and along with them disease-carrying ticks. "The mice increase numbers of both the black-legged tick vector and the pathogen that causes Lyme disease," Ostfeld said.

The authors focused on diseases—including Lyme, West Nile virus, hantavirus and nine others—around the world. In each case they found that the maladies have become more prevalent during the time in which local biodiversity shrank.

Three of the cases they studied  found that the rise of West Nile virus in the U.S. corresponded to decreases in bird population density.

The researchers also conclude that humans and wildlife really shouldn't interact. Direct contact with wildlife—say, in the form of the often illegal bushmeat trade—could in turn cause more diseases to jump from animals to humans.

The best solution to both situations: "Preserving large intact areas and minimizing contact with wildlife would go a big step of the way to reducing disease," Keesing said in Nature.

So should you care? Yes you should, if you value your health. A healthy planet equals healthy humans, a lesson it's really time we learned.

Photo: West Nile virus, via Wikipedia