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The eyed brown may have been a little sleepy. When John Wagner picked it up the insect didn't offer much of a struggle. Wagner pointed out that this was probably a newly emerged adult, an interpretation based on the flawless condition of the animal's wings. A butterfly that had been around for a while would look a little more beat-up. As the name suggests, the brown wings of this butterfly are decorated with eyelike spots.

Starting life this late in the year, this eyed brown may not have time to reproduce. We found it along a path in an oak woodland, which is not the sort of place the books describe as habitat for this species. The eyed brown ought to be in a wet prairie. The caterpillar eats sedges, the grasslike plants that are especially common in wet areas.

We were walking through Spears Woods, one of my favorite forest preserves. Spears is a glimpse of the natural landscape of northeastern Illinois, with a diverse, healthy oak woods bordering a prairie. Several marshes are scattered around the preserve. It was the third stop on the itinerary for our group, students in a class on fall ecology offered by the Field Museum. We were looking for patterns in the landscape and for any pleasant surprises--such as the eyed brown--that happened to show up.

Wagner and his colleague Phil Hansen, both members of the museum's education staff, were our guides. The interpretive skills required for this exercise combined botany, ecology, and history with entomology and anything else we could bring. Hansen is an expert on the history--ecological and otherwise--of this region. Wagner's expertise starts with insects but includes a broad range of stuff that can be categorized as "things that live under logs." We started the morning at the Swallow Cliff toboggan slide and went on to the neighboring Cap Sauers Holdings before finishing the trip at Spears.

The history part of the trip was essential, because every acre of ground in northeastern Illinois has been heavily assaulted by human action in the past 180 years. Often you can read the details of this assault in the wreckage left behind in the wake of the disaster. Part of the time we were studying these serious issues, and part of the time we were just enjoying what there was to be seen on a pleasant day as summer turned to autumn.

The newly emerged eyed brown could still find lots of nectar in the mid-September landscape. The woods were full of the flowers of white snakeroot. Out on the prairies the purple flowers of ironweed and the yellow of tall coreopsis were everywhere. An assortment of goldenrods bordered the paths through the woods.

Starting our trip at the toboggan slide gave us a chance to enjoy a panoramic view of one of the largest blocks of undeveloped land remaining in northeastern Illinois. The Palos preserves add up to more than 12,000 acres. Roads run through them, but there are no other interruptions. A half dozen Illinois nature preserves are scattered through them. Standing at the top of the slide, we were on the south slope of the Sag Valley, the former outlet of Lake Michigan. We looked out over miles of forest preserve. A great blue heron, its wings flapping slowly, crossed the valley as we watched.

Cap Sauers Holdings--at more than 1,500 acres, the largest roadless area in Cook County--is named for the man who ran the forest preserve system for more than 30 years. The preserve has everything from the most pristine landscapes to old fields now dominated by weeds.

We started our hike into Cap Sauers along a trail that had once been a narrow but paved road that led to a Girl Scout camp. Foundations are about all that remains of the camp, but the greenery around us spoke of major disturbance in the past. Exotic species were everywhere. Common buckthorn shrubs lined the path. Under the trees the brown skeletons of garlic mustard plants covered the ground. Their growing season had ended months ago, and the thousands of seeds each plant had produced were now scattered and ready to push native species out of the woods. There was even a large catalpa towering over everything else. Catalpas are native to the gulf coast, but they have been planted over a much larger area. They are unlikely to establish themselves in the wild, so when we come across one in the forest preserves, we usually can find an old homesite or other structure nearby.

The native trees around the catalpa were all small and all specialists in occupying disturbed ground. This landscape was easily read: recent disturbance so heavy that everything native had probably been destroyed.

But as we continued walking the landscape began to change. A few large old white oaks appeared, their broad crowns speaking of a past when this land was more open. Why it was open is a question that would take more research to answer. Maybe it was an open savanna sustained by fire. Maybe somebody had a flock of sheep nipping everything off just above ground level.

As we got farther from the disturbed areas the big old oaks were joined by young hickories and a scattering of black cherries. If we had come earlier in the season, we might have seen a great wildflower display.

What we saw at Cap Sauers was a buildup to the beauties of Spears Woods. The decline of oak woods is a major problem throughout the midwest. Various species of oaks are the dominant trees in the kinds of woodlands that have been characteristic of the midwest for thousands of years. Under current conditions there is a widespread failure of reproduction by the oaks, a failure that has inspired the search for ways to manage our oak woods in a way that will keep them alive. Spears, which is on La Grange Road between 87th and 95th streets, is a lovely oak-hickory woods with a vigorous population of small, young trees.

Spears also gave Wagner a chance to give us lessons in how much life there is in a dead log. Ecologists have generated many a master's thesis from careful surveys of the details of the process of decay in dead logs, and Wagner filled us in on the major steps from live wood to total decay.

A stage-one log is newly dead. Its bark still clings firmly to the underlying wood. In stage two the bark is loose. Small critters--beetle grubs and isopods--move into the space between bark and wood and begin digesting the wood. Insect eaters from woodpeckers to raccoons may start searching under the bark at this point. If they remove the bark and expose the wood they can retard the process of decay by many years. The exposed wood dries and becomes a much less hospitable environment for both animals and fungi.

In stage three the sapwood, the younger outer layers of wood in the log, begins to get soft. Fungi and bacteria are the only organisms that can digest wood. The fungi do it on their own. The bacteria often live in the guts of the beetles, ants, and other insects that eat wood.

Green plants can take root on logs at this point. Some species specialize in the microhabitat of the dead log. In the north woods you can often find yellow birch trees whose trunks stand on roots that rise two or three feet above the ground. These trees started life on dead logs. When the logs rotted away, they left the trees high above the ground.

Logs can go through this process while they are standing, and as the wood softens, cavity-nesting birds such as chickadees may be able to excavate nest holes.

Stage four in the recycling of a log occurs when the heartwood, the very hard wood at the center of the log, is softened. Of course many living trees are afflicted with heart rot, so stage four can sometimes happen when the tree is still alive.

In stage five the log has been reduced to a low mound of earth. The energy that was concentrated in its tissues has been transferred to the tissues of a host of small organisms. The essential minerals have been liberated from the wood and are ready to be taken up again by growing plants. The last beetles go in search of a new log where they can lay their eggs. While they last, dead logs are among the liveliest places in the woods.

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