According to a document produced by The Nature Conservancy, the tallgrass prairie is "the most diverse repository of species in the Midwest [and] ... habitat for some of the Midwest's rarest species."
It is rather difficult to give a total number of species that occur on prairies in Illinois. Since the tallgrass prairie ecosystem is recently evolved, there are few endemic species and few species that occur on prairies are restricted to the prairie habitat. Most prairie species also occur outside the prairie region in habitats other than prairies.
Prior to European settlement, the landscape of the tallgrass prairie in Illinois was a complex matrix with specialized communities embedded in the prairie:
The borders between these communities and the prairie fluctuated on both short and long term bases depending on rainfall, drought, and fire frequency. This ever changing matrix adds to the problem of placing some species into the "prairie species" category. For this discussion we include all species that occupy or utilize during some stage of their life cycle the types of habitats recognized as prairie by the Illinois Natural Areas Inventory; excluded are species restricted to open grass, sedge, and forb-dominated communities classified as wetlands, such as sedge meadows and fens.
The soil underneath the prairie is a dense tangle of roots, rhizomes, bulbs, corms, and rootstocks. While the above ground part of most prairie plants die back each year; the plants are kept alive from year to year by these underground structures. The roots of prairie plants often extend deeper into the ground than the stems rise above it. For instance, the roots of big bluestem may be 7 feet or more deep, and switchgrass roots more than 11 feet deep. Some of the roots die and decompose each year, and this process has added large quantities of organic matter to the soil. This is one reason why the prairie soils are so fertile.
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