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.