The challenge is to find a nuanced balance between enjoying nature and protecting it

From this past February in the New York Times, Leaving Only Footsteps? Think Again by Christopher Solomon.

Solomon hits many of the issues that underpins the MLPA Urban Wildlife initiative and its Park Portfolio Approach.

When we think of injuring nature, it is easy to point an accusing finger at mining companies and their strip mines or timber barons and their clear-cuts. But could something as mellow as backcountry skiing or a Thoreauvian walk in the woods cause harm, too?

More and more studies over the last 15 years have found that when we visit the great outdoors, we have much more of an effect than we realize. Even seemingly low-impact activities like hiking, cross-country skiing and bird-watching often affect wildlife, from bighorn sheep to wolves, birds, amphibians and tiny invertebrates, and in subtle ways.

Impacts from outdoor recreation and tourism are the fourth-leading reason that species are listed by the federal government as threatened or endangered, behind threats from nonnative species, urban growth and agriculture.

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You’d be surprised by the ripples left by a day-hiker’s ramble through the woods. In 2008 Sarah Reed, an associate conservation scientist at the Wildlife Conservation Society, and her colleagues found fivefold declines in detections of bobcats, coyotes and other midsize carnivores in protected areas in California that allowed quiet recreation activities like hiking, compared with protected areas that prohibited those activities.

“That is the kind of difference that you don’t see often in ecological studies,” Dr. Reed said. Dogs, a frequent villain, weren’t the issue for these carnivores; people were, according to her research.

Birds get ruffled, too. Researchers who studied trails around Boulder, Colo., found that populations of several species of songbirds, including pygmy nuthatches and Western meadowlarks, were lowest near trails. “There’s something about the presence of humans and their pets when they go on hikes that causes a bit of a ‘death zone’ of 100 meters on either side of a trail,” said Prof. Rick Knight of Colorado State University. Running, canoeing, cycling and similar activities negatively affected birds in nearly 90 percent of 69 studies that researchers reviewed in 2011. Reductions were seen in the number of nests built, eggs laid and chicks hatched or fledged.

In Connecticut, wood turtles, labeled a “species of special concern” in the state, vanished from one wildlife preserve over 10 years after the area was opened to activities like hiking, researchers found.

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The uncomfortable fact is, we’re all complicit. In a not-yet-published review of 218 studies about recreation’s impacts on wildlife, researchers found more evidence of impacts by hikers, backcountry skiers and their like than by the gas-powered contingent.

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A century ago, nature had elbow room. Now, there’s a lot less of it, while recreational activities and nature tourism are growing in most parks, wilderness areas and other protected areas around the world.

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Conflicts with nature are a result. Still, scientists insist they don’t want to lock people out of nature. Spending time on a mountainside, or hip-deep in a trout stream, is tonic for brain and body. Research bears this out. And people who recreate outdoors are among nature’s most ardent constituents. Without them, “our landscapes would erode even faster than they are now,” said Dr. Heinemeyer, the wolverine researcher.

The challenge is to find a nuanced balance between enjoying nature and protecting it, recognizing that recreation does not necessarily complement conservation or preservation.

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And in the case of future parks and protected areas, we need to carefully consider the goals for such places and how recreation fits in or doesn’t, because once it is allowed, it is tough to restrict. “Whether or not to allow public access is probably the most important decision that gets made,” Dr. Reed said.

Of course not all wildlife is the same. Some species flee; others habituate. Some populations might be healthy enough to withstand disturbance; others, too fragile. We now know recreation is having impacts in ways that we hadn’t imagined. We must plan accordingly.