What do rats, lions and humans all have in common? For one thing, they could all be playing a role in the demise of homo sapiens. According to a new study out last week, declines in the number of large wild animal species -- such as giraffes, zebras, elephants and, of course, lions -- could lead to an increase in the number of rodents carrying zoonotic diseases, or illnesses that are transmissible from animals to humans. Basically, with fewer large animals in existence, more rodents -- and more dangerous, even deadly diseases -- are able to thrive.

"Wildlife has been declining really since the Pleistocene, since early human arrival and interaction with large animals," Hillary Young, an ecologist and one of the study's lead authors, told Smithsonian.com. "It's one of the major impacts of humans on the world today."

Young and her colleagues went to Kenya to conduct several experiments that separated wildlife from their usual grazing and breeding grounds. Over the course of two years, the team measured the difference in rodent populations on plots of land that did have large animals and plots that didn't. They found that on plots without animals, the number of mice and a certain kind of flea doubled -- and so did the number of disease-carriers among them.

"In this case," Young explained, "what we have is a group of rodent-borne pathogens that look like they respond in a really simple way to large wildlife loss." And, as Smithsonian.com writer Helen Thompson points out, the ramifications of that response could be many. "The paper makes it difficult to avoid the idea that, as large animals continue to face extinction, we may see rising waves of diseases," Thompson writes. "So preserving the Earth's biodiversity may, in a very real way, help preserve us."

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