In 1820, 38% of Illinois was covered by forests, with near complete coverage in the southernmost counties. After the European settlement, much forest land was cleared for agriculture and within a century less than 8% of the original forest remained. Today, less than 1% survives. The importance of forests for biodiversity cannot be emphasized enough, as forests account for over 75% of the wildlife habitat in the state.
Fortunately, the forested area of the state is actually growing. Beginning in the 1940's, the focus of Illinois agriculture shifted from animal husbandry to row-crop production, which is less land-intensive. The abandoned grazing pastures soon witnessed an invasion of woody species, including shrubs and trees. These so-called second-growth forests have spread to fill nearly a third of the area of Illinois' original forest.
Unfortunately, these second-growth forests, are quite fragmented compared to the original forest, and the smaller habitats have made survival difficult for native Illinois species. Add to this the invasion of exotic species and a host of plant diseases, and it's no surprise that Illinois forests are home to nearly 200 threatened and endangered plant species--over half the threatened and endangered plant species in the whole state. The changing composition of the forests has negatively impacted vertebrates as well, because four out of five mammals and three out of five birds need forested land for at least part of their lifecycles.