Scientists at the University of California, Davis, report their examination of 15 social and ecological variables -- from tourism and per capita gross domestic product to water stress and political stability -- correlated with the numbers of invasive and endangered birds and mammals, two indicators of what conservationists have dubbed "land sickness."
Although rarely included in indexes examining human impacts on the environment, human life expectancy is a key predictor of global invasions and extinctions, they said.
"It's not a random pattern," UC Davis postdoctoral scholar Aaron Lotz said. "Out of all this data, that one factor -- human life expectancy -- was the determining factor for endangered and invasive birds and mammals."
Other factors considered were agricultural intensity, rainfall, pesticide regulation, energy efficiency, wilderness protection and total population.
The study's results suggest the need for a better understanding of the complex interactions among humans and their environment, Lotz said.
"Some studies have this view that there's wildlife and then there's us," he 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."