A growing global human populationsix billion today and likely to reach eight billion by 2020, according to the United Nationsis 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 undisturbedplaces 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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