A study published by a team of biologists suggests that as a nation's population life expectancy increases, so does its percentage of invasive and endangered birds and mammals. It suggests that rather than population density alone being the largest threat to wildlife, it's the quality of life that matters most.

The very presence of humans is often blamed for the increasing numbers of extinct species on the planet. But a team of biologists from the University of California-Davis examined 15 economic, ecological, and social variables to judge which factors of that human presence are the biggest contributors to the downfall of species.

Expanding the survey to include the number of invasive species in a country revealed even more interesting results. Introducing a non-indigenous species to any given ecosystem can have devastating effects. Controlling the number of invasive species was identified as the greatest challenge threatening the Galapagos when the World Heritage Committee placed it on its danger list, for instance, and in the UK the introduction of the eastern gray squirrel from North America is blamed for devastating the red squirrel population, now down to under 200,000. What the report found, however, is that a combination of economic and social factors is responsible for this devastation—just as, in reality, deforestation and other factors have contributed to the fall of the red squirrel.

Taking in data from 100 countries, representing 87 percent of the world's total population and 43 percent of global GDP, the study focused on the following: GDP, export-import ratio, tourism, undernourishment, energy efficiency, agricultural intensity, rainfall, water stress, wilderness protection, biodiversity, life expectancy, adult literacy, pesticide regulation, political stability, and female participation in national government. It is a comprehensive look at the human impact on the landscape, but it became clear that life expectancy was the greatest correlating factor responsible for the damage.

"It's not a random pattern," says lead author Aaron Lotz, a postdoctoral scholar in the Department of Wildlife, Fish, and Conservation Biology at the time of the study. "Out of all this data, that one factor—human life expectancy—was the determining factor for endangered and invasive birds and mammals."

The fact that the team took another 14 human-based factors into account, however, gives a much more detailed picture of why this might be. For instance, as GDP per capita increased, so did the percentage of invasive birds and mammals. The more affluent of the 100 countries featured naturally engage in more international trade, which may be contributing to that increase.

New Zealand is a good example of this, though it is described as an "outlier." It's a wealthy country but with few indigenous land mammals. Thus, it turns out, there has been a massive influx of invasive species since colonization around 800 years ago, resulting in the nation having the highest percentage of both endangered and invasive species combined. The same thing happened in the Galapagos—being a remote island with few native mammals, those who came to settle brought with them livestock and other species that would help in building a new civilization. On the Galapagos, the other unfortunate side to that was diseases, vermin, and other unintended visitors that came along for the ride. This is obviously unusual, however—hence New Zealand's designation as an "outlier." "New Zealand's invasive species crisis may be due in large part to its isolation, high endemism, and recent human colonization," write the authors. "Island ecosystems are often the most invaded and consequently the most threatened worldwide."

Altogether, New Zealand, the US, and the Philippines had the highest percentages of endangered and invasive birds, while countries in Africa had the lowest percentage of invasive and endangered birds and mammals. This, the authors behind the study hypothesize, might be linked to the fact that they engaged in some of the lowest rates of import/export activity in the world.

"These countries have had very little international trade relative to a majority of the countries in this analysis because of their closed trade policy," explain the authors. Across the 26 nations surveyed, there were only 29 invasive bird species and 39 invasive mammal species. With little international trade, diversity remains as it was designed to.

Despite all these contributory factors, life expectancy remained the best indicator. If only life expectancy were included in a prediction model, the correct percentages of invasive and endangered birds and mammals could be calculated.

"Increased life expectancy means that people live longer and affect the planet longer; 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."

Considering the fact that we'd all like to live longer, the authors demand that we provide communities with "incentives, tools, and capacity to manage ecosystems sustainably."

"Conservation will only be successful if local communities… understand that they are living on environmental capital rather than on interest."

In contrast, another paper published earlier this year by biologists at Ohio State University blamed the growing population for species extinction, claiming that the average growing nation should expect an increase of 10.8 percent more species threatened with extinction by 2050. The original prediction model was published in 2004, but this year the team used it to see if it had correctly predicted 2010 extinction data. It had. They then moved on to extend their predictions to the next decade and beyond.

"Our projection is based on human population density alone. It doesn't take into account climate change, industrialization, or wars. So the actual numbers that we predict for 2050 will be very different because everything we do will exacerbate the problem," said Jeffrey McKee, professor of anthropology at Ohio State and lead author of the study, who is basically telling us it will only get worse. "You can do all the conservation in the world that you want, but it's going to be for naught if we don't keep the human population in check."

Human density can often correlate to GDP, of course, but perhaps not as often to quality of life, and therefore life expectancy, as resource and amenity levels struggle to keep up with the demand.

This story originally appeared on Wired UK.