How close did you pay attention to the landscape the last time you went for a hike in a national park? Did you notice the variety of trees or plants, the birds, the insects? Could you tell where there had been a fire or a flood? More subtly, could you see where the nutrient sinks in the ecosystem were? Most of us probably can't answer that last question, but researchers at Isle Royale National Park have uncovered some intriguing contributions to the park's biodiversity.

For those who see the national parks as being preserved time immemorial under gigantic bell jars, there is evidence that each succeeding generation makes an imprint or two on the parks. It wasn't too terribly long ago, for instance, that you climbed into the bleachers to watch the bear buffets in Yellowstone National Park, or that the El Tovar Hotel at the Grand Canyon National Park had separate dining areas for men and women.

There is perhaps no more ethereal place than the Upper Geyser Basin in Yellowstone National Park. There, when the night skies are thick with scudding clouds, ghostly phantoms promenade on the geysers' drifting zephyrs.

Fall can be a downright gorgeous time to take a hike in a national park. The air is crisp, the bugs are gone, the crowds have vanished, and the landscape is gorgeous. Just don't forget that the bears are trying to put on some extra calories to get through the winter. And a hungry bear can be an ornery bear.