After investigating 15 likely suspects such as tourism, political stability, and water stress, researchers from UC Davis determined human life expectancy was was the major link to the decline of certain species and the rise of others. 

The study is unique in the way it studied human life expectancy, which is rarely used as a measure of humanity's impact on the environment, according to the study.

"It's not a random pattern," said lead researcher Aaron Lotz of the Department of Wildlife, Fish and Conservation Biology. "Out of all this data, that one factor - human life expectancy - was the determining factor for endangered and invasive birds and mammals."

Researchers noted their study's approach was "exploratory" and that they "did not seek mechanisms," meaning conclusions drawn by their work should be used as a pathway into discussion about the effects of human life span - and the direct effects of humans - on the environment.

"Some studies have this view that there's wildlife and then there's us," Lotz said. "But we're part of the ecosystem. We need to start relating humans to the environment in our research and not leave them out of the equation. We need to realize we have a direct link to nature."

"Increased life expectancy means that people live longer and affect the planet longer," researchers wrote. "Each year is another year of carbon footprint, ecological footprint, use of natural resources, etc. The magnitude of this impact is increased as more people live longer."

Other interesting results of the study, as summarized by UC Davis' website: The U.S., New Zealand, and the Philippines had the highest proportion of invasive and endangered species, and New Zealand had the highest of all three. African countries tended to have the lowest proportions of both.