PHILADELPHIA (MCT) — It's windy. Rain is imminent. The path is muddy.

But Patricia Zaradic is loving it all. What's important is that she is out in nature, a place her research tells her fewer and fewer Americans are heading.

In the last two decades, park visits, permits for camping or fishing and other data show a fundamental, pervasive shift away from outdoor activities, the Bryn Mawr ecologist concludes in a study published recently in the Proceedings of the National Academy of Sciences.

National parks are still popular, to be sure. Last year, 275 million people visited them. But adjusting for population increases, Zaradic says attendance is 70 million short of a 1987 peak.

Overall, participation in outdoor activities has declined 18 percent to 25 percent, according to the study by Zaradic and co-author Oliver R.W. Pergams, a University of Illinois conservation geneticist.

They link the decline to a term they coined, videophilia, that is, doing stuff indoors in front of a screen — watching TV, sitting at computers, playing Xbox — instead of taking a hike.

The authors say the trend has impact well beyond the nation's expanding waistline: It could blunt conservation efforts and other activities that depend on an appreciation of Mother Nature.

"It's striking and significant," Zaradic said recently, as she paused along a woodsy Haverford trail she often walks with her husband and three children.

"There's this whole other body of research that indicates it's time spent in nature, especially as a child, that leads to environmental sensitivity as an adult," said Zaradic.

And not just experiences at Yellowstone. Even the small realm of bugs and earthworms and fallen leaves in the American back yard has import, Zaradic said. It's something "you just can't get from a flat screen, no matter how high-D it is."

The trend also worries leaders of environmental organizations who are noticing all the gray hair among their supporters.

"There's a strong correlation to whether people have nature experiences as they grow up and whether, as adults, they will be ... concerned about policies that affect nature," said Bill Kunze, Pennsylvania state director for the Nature Conservancy, the national group that funded Zaradic's research.

Kunze called it "an interesting paradox" that people are participating less in nature activities even as interest in the environment is heating up, prompted by concerns over global warming.

"I don't think they've made the full connection between climate change and nature," he said.

Growing concern has led to a renewed interest in developing hands-on nature programs for children, with names such as "Ranger Ricks" and "No Child Left Inside."

For instance, the Schuylkill Center for Environmental Education in the city's Roxborough neighborhood has instituted nature programs for children as young as 2. "Nature Tots" come with their parents, and "we just take them out for walks," said Erin Johnson, program development manager. "Little excursions out to meadows and ponds to just look."

Among older kids, she can tell the ones who aren't familiar with the great outdoors. "They are constantly wondering what's going to get them. Even bugs. They're very scared of bugs."

Some become antagonistic, squishing tadpoles or insects.

Zaradic has a doctorate in ecology and evolution from the University of Pennsylvania and is a regional fellow with the Environmental Leadership Program, a national nonprofit that seeks to nurture environmental leaders. She and Pergams began studying how people relate to the natural world several years ago.

In their current paper, amid all the downhill data, they found but one tiny increase: People went from hiking once every 12.5 years to once every 10 years.

A recent 15-year trend analysis by the U.S. Fish and Wildlife Service echoes their findings. It shows declines of 16 percent among anglers, 11 percent among hunters, and 23 percent among those taking trips to watch wildlife.

National Park Service spokesman Gerry Gaumer acknowledges park officials don't see "as much of the family vacations as we used to," although he noted, "I don't see that there's any crisis as to visitation."

David Brown, executive director of America Outdoors, a national organization of oufitters, resorts and tour operators, said his group's research shows that primitive activities have, indeed, declined. Fund-strapped parks can't keep the trails open. Women don't like the bathrooms — or the lack thereof.

However, he said, demand is strong for "outfitted trips," especially near popular destinations.

"You would assume from the study that everything is crashing," he said.

But the Nature Conservancy's chief scientist, Peter Kareiva, based in Seattle, thinks the country is in the midst of a vast social change.

"I have kids," he said. "Almost everyone I knew growing up wandered around in the woods. There really is less of it."

Some say video games and TV aren't the only problems.

"When I talk to parents and kids, it's very clear there are other major things going on," said Richard Louv, author of a 2005 book, "Last Child in the Woods: Saving Our Children From Nature-Deficit Disorder."

One is access. "It's pretty tough to go out in the woods if it's been cut down."

Homework has increased. Ditto scheduled activities.

And neighborhood covenants. "Just try to put up a basketball hoop, let alone a treehouse," Louv said.

Parents worry about Lyme disease from ticks, West Nile virus from mosquitoes, and, most of all, the chance, however slim, that some stranger will harm their child.

Zaradic knows all the parental impulses. Her children are 2, 7 and 11.

But, "out of some of this work, I've realized how important it is to have them play out in the backyard with the dog and just run around and do what kids do," she said.

Zaradic grew up in concrete-bound New York City. But most weekends, the family took to the Long Island beaches, where they went fishing, got mussels, and found "all these amazing things," she said. "It seemed like such a magical world."

She has noticed that her own children seem much different after their annual summer vacations, in an Adirondacks cabin without electricity.

"The kids come back so energized and enlivened, and at the same time calmer and more settled with themselves," she said.

Pergams and Zaradic hope to focus more studies on what happens if you do give kids a grounding in nature.

"It looks like that's the way to go, but there's no good data on it," Zaradic said. She hopes it will tell them what works, what doesn't.

She glanced up at the cloudy sky, tapped her boots against the ground and nodded. "We're getting to a point where I think it's pretty critical to turn this trend around."