The modern world would barely be recognizable to the mammoth and bison herds of ages past. Roads subdivide large stretches of land, and clusters of buildings and people have sprung up nearly everywhere. Indeed, humans have modified the environment so much, they may have cut the distance by which mammals—large and small—roam by some two-thirds, according to a novel analysis published today. That lack of movement could upend ecosystems and increase the number of human-animal conflicts, researchers say.
“We’re moving into an era where humans have changed natural environments extensively,” says Oscar Venter, an ecologist at the University of Northern British Columbia in Prince George, Canada, who calls the new paper “very important” with significant implications. “What’s not exactly clear is what this is going to mean.”
Scientists have tried to figure out how human activity affects animals for decades. For nearly 20 years, for example, they have used GPS collars to track threatened species living in national parks, in farmlands, and near suburbs and cities. But such studies typically follow a single species or population over time, limiting how the results can be applied.
A few years ago, Marlee Tucker, a biologist at the Senckenberg Biodiversity and Climate Research Centre at Goethe University in Frankfurt, Germany, and other researchers decided to launch a much wider scale study. They wanted to compare the movements of as many mammal species as possible—from pocket mice to grizzly bears—and find out how much human actions affect those activities.
So Tucker started collecting data on the whereabouts of animals equipped with GPS collars from previous studies. She and her team assembled one of the largest data sets—more than 800 animals from 57 species—to date. They then compared the movements of those creatures with a previously published index of human activity, which included everything from the presence of roads, buildings, and nighttime lights to population density and land devoted to farming in different areas.
The team found that mammals in areas with a large human “footprint” moved half to a third as far as those in areas with a low human footprint. In areas most heavily influenced by people, the animals’ maximum roaming range averaged about 7 kilometers; for low-footprint areas, the average roaming range was 22 kilometers, Tucker and colleagues report in Science. “Such dramatic reductions in the movement of species are surprising and very important for what’s going to happen in the future,” Venter says.
That’s because movements of mammals are critical not only for the animals themselves, but for the ecosystems they inhabit. “Animals act as mobile links, linking different areas of a landscape together,” Tucker says. One of the best examples of potential ecosystem collapse is near the border of Kenya and Tanzania, in the Serengeti–Masai Mara ecosystem, says Jared Stabach, an ecologist at the Smithsonian Conservation Biology Institute in Washington, D.C. Wildebeests there carry seeds and nutrients as they traverse the landscape during their annual migration. But now that roads, agriculture, and poaching threaten their migration, that could mean those resources don’t get distributed throughout the ecosystem. The loss could trickle down to local economies that depend on tourism, Stabach says.
Humans are changing the landscape in other ways. Farmed crops provide ample food sources that encourage small and large mammals alike to take up a sedentary lifestyle. Those clusters of nonmigrating creatures in turn provide a rich breeding ground for diseases like avian flu. And the closer wildlife strays to human habitation, the greater the opportunity for human-animal conflict. “Every animal is balancing on a knife’s edge to get resources while minimizing risk,” says Grant Hopcraft, an ecologist at the University of Glasgow in the United Kingdom who was not involved in the work.
Venter would like to see a longer-term version of the study, because Tucker’s analysis examines movement over just 10 days. “That’ll tell us something about the trajectory of change as we go forward,” he says. That’s if there’s a future to go toward. Hopecraft says that built environments are increasingly widespread, and as for once-dynamic ecosystems, “We’re essentially wrapping them in plastic shrink wrap and hoping they’re going to survive.”