Human "Footprint" Seen on 83 Percent of Earth's Land

Hillary Mayell
for National Geographic News
October 25, 2002
Scientists have produced the first map that traces human influence on
the natural world, and the numbers are big. Overall, 83 percent of the
total land surface and 98 percent of the areas where it is possible to
grow the world's three main crops—rice, wheat, and maize—is
directly influenced by human activities.

"Yes, humans have a huge influence on the Earth's ecosystems," said Eric Sanderson, a landscape ecologist with the Wildlife Conservation Society and one of the co-authors of the study.

"But instead of being discouraged or depressed," he added, "we want people to understand they can actually make choices; that it's possible to live with wildlife in ways that allow us to make a living and at the same time coexist with wildlife."

The map was designed to illustrate the extent of human influence and identify opportunities for conservation. "What can't be measured can't be managed," said Marc Levy, a data specialist at Columbia University's Center for International Earth Science Information Network (CIESIN).

The figures are not a surprise to scientists, said Sanderson.

"Wildlife biologists and ecologists are really pretty discouraged about the inability to get the message out," he said. "In the United States, people have the idea that there are these really big wild spaces still out there. It's not true. One study showed that 20 percent of the continental U.S. land mass is within 500 meters of a paved road"—an area equal to about five and a half football fields.

"I think we're really missing the boat on this," said Sanderson. "It's going to be the most important issue in the 21st century, yet no one is talking about it." The study by Sanderson, Levy, and colleagues is published in the latest issue of the journal BioScience.

Mapping Human Impact

Trying to measure the extent of human impact is not new. As far back as 1864, author and naturalist George Marsh posed the question, asking to what degree the processes of nature were threatened by human activities. Biologists and ecologists have been trying to make that measurement ever since.

The authors of the study used four variables to measure human influence: population density, access from roads and waterways, electrical power infrastructure, and land transformation.

"There's a lot of evidence that these individual factors have effects, but this is the first time someone's tried to put them all together as one index, to try and see what the biological effects are," said Sanderson. "What we tried to do is actually map human influence and the gradients of human influence."

A growing global human population—six billion today and likely to reach eight billion by 2020, according to the United Nations—is the engine that drives the various influences. People build settlements and roads, transform land to grow food and graze animals, and manufacture goods in ways that frequently lead to pollution, climate change, and ever-increasing consumption of natural resources.

Even in the wildest, most remote places, conditions were shown to be directly linked with the number of people who live in these areas and draw on local resources.

One study showed that 98 percent of the variation in extinction rates in national parks in Ghana over a 30-year period could be explained by the size of the park and by the number of people living within 50 kilometers (30 miles) of it: The higher the density and the smaller the park, the higher the extinction rate.

Other studies have shown that road development leads to a rise in invasive species, a decline in native species populations, and changes in the natural landscape that negatively affect ecosystem functioning. One researcher has estimated that a million vertebrates are killed every day on U.S. roads.

The researchers scored each of the variables on a one to ten scale, combined the numbers, and mapped the results.

Preserving Nature Far and Near

Not surprisingly, the places most heavily influenced by human activities are the world's largest cities, such as New York, Mexico City, Calcutta, Beijing, and London.

The least affected places include regions such as the Arctic Tundra, forests in northern Canada and Russia, and the deserts of Africa and central Australia.

The researchers note that 60 percent of the world's land surface lies along a continuum between those two extremes, with this range of conditions offering many opportunities for targeted conservation efforts.

Conservation International, the Sierra Club, and other groups concerned with preserving wilderness and biodiversity have identified "hot spots" representing areas where preservation is crucial to maintain long-term biodiversity.

Sanderson and his colleagues say it's equally important to conserve nature close to areas where people actually live.

"The map can be used to identify giant tracts of land that might be preserved, as well as tiny pockets in highly urbanized areas, where conservation efforts might be most usefully targeted," said Sanderson.

To help identify areas most likely to benefit from conservation dollars, the scientists looked at the world in terms of biomes, or communities of living organisms in a single major ecological region. The northeastern United States, for instance, is a biome of temperate broadleaf and mixed forests.

While the entire region is heavily urbanized, some spots are still relatively undisturbed—places the researchers call the "last of the wild." The Catskill Mountains, for instance, are considerably wilder than downtown New York City. These wilder places are natural candidates for conservation measures.

The least disturbed places in comparable biomes of Africa are generally larger and much wilder, but would benefit similarly from conservation efforts.

"The last-of-the-wild areas are almost by definition a guide to opportunities for effective conservation," said Sanderson. "They're the places where the widest range of plants and animals are living with the least amount of human conflict."

Conservation methods will vary depending on the landscape. Wilderness, countryside, suburbs, and cities require different approaches.

In New York City, efforts might focus on restoring the Hudson River; in Africa, on setting aside protected areas; and in South America, on reconnecting habitat fragments.

Whether the strategy is preservation, conservation, restoration, or a combination, knowing what's out there will help, say the authors.

Our understanding of how human activities affect the global environment is in its infancy, they note, adding that determining the extent of the human footprint is a first step toward acknowledging that people must act as stewards of Earth.

"We have the means if we choose to make changes," said Sanderson. "Scientists know a lot about how to preserve and restore nature. But we need to get people as individuals, institutions, and governments to realize we can make those choices."br>Join the National Geographic Society

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