Since those early days, the natural communities have been drastically altered by human activities. When we built our farms, homes, roads and businesses, we cut the prairies and woodlands into fragments, surrounded by concrete and isolated from other natural areas. We changed the hydrology, drastically altering the way the water flows over the land. We introduced competing species from other parts of the world, without bringing along the natural predators of those species. And we stopped the historical fires that had moved across the landscape, shaping the natural communities in this part of the world for the last 8,000–10,000 years. In short, just by our presence here we have profoundly changed the conditions for the natural communities.
Over the last 200 years the wetlands, prairies and woodlands have been changed by these altered conditions, and those changes are not for the better. In the absence of fire, thin-barked trees such as maple and basswood have increased drastically. The once sunny open woodlands have become dark and brush-filled.
Shade-intolerant species of trees such as oaks and hickories, which dominated much of the landscape for thousands of years, are no longer reproducing themselves. Many native flower, grass and shrub species have weakened and dropped out, while others are becoming more and more scarce. Aggressive exotics are moving into these weakened systems much as infections move into a sick patient.
The rich diversity of native plants has been replaced in untended areas by weedy species. In some places, there isn't enough light for even those weedy species to grow, and the exposed soil begins to erode away.
The oak-hickory forest is typical of the Appalachian forest system, of which Illinois is the western-most fringe, and the oaks are the dominant tree in it…In many places these aging trees are not regenerating naturally. More shade in the forest understory means that oak seedlings, which need sun to grow, are being overwhelmed by shade tolerant trees like maples. The result is "maple takeover," the process by which the oak-hickory forest is replaced not by younger oak and hickory trees, but by different tree species.
From The Changing Illinois Environment:
Summary Report of the Critical Trends Assessment Project
A joint project of the Illinois Department of Energy and Natural Resources and The Nature of Illinois Foundation