From The Changing Illinois Environment: Critical Trends, 1994 , Summary Report of the Critical Trends Assessment Project

What wasn't forest or open water in presettlement Illinois was prairie. The exact extent of these grasslands is disputed, but it is safe to say that in 1820 at least 60% of Illinois' land area was grasslands of one type or another.

Modern scientists recognize six main subclasses of prairie in Illinois. These are distinguishable mainly by differences in soils and topography; further subdivisions based on soil moisture produce a total of 23 distinct prairie types in the Prairie State.


Flat terrain and deep loess soils made most Illinois prairies ideal for agriculture. The breaking of the Illinois prairies began in earnest with the invention in 1837 of a self-scouring steel plow both strong enough to slice through the dense mat of prairie plant roots and slick enough to slip through sticky loam soils. Vast stretches of prairie were destroyed between about 1840 and 1900. According to one account, the 60-square-mile Fox Prairie in Richland County was reduced from more than 38,000 acres to 160 acres of prairie between 1871 and 1883. McLean County once had 669,800 acres of prairie; today it has five of high quality. Champaign County once had 592,300 acres of prairie; today it has one of high quality. Prairie remnants in such counties probably have escaped natural areas surveyors (especially along railroad rights-of-way) but including them would still leave Illinois with only a very small number of acres of surviving prairie.

The Illinois Natural Areas Inventory completed in 1978 found that only 1/100 of 1% (2,352 acres) of high-quality original prairie survives. Most sites of relict prairie occur on hilly land along the northern and western edges of the state and other places where plows and bulldozers can't reach, such as wetlands, cemeteries, and railroad rights-of-way. Four out of five of the state's prairie remnants are smaller than ten acres and one in three is smaller than one acre. Of the 253 prairie sites identified by the inventory, four out of five are not protected as dedicated nature preserves.

Illinois prairie remnants are often less than one acre in size, and the entire local population of some plant and animal species may be only a few individuals. The smaller such local populations are, the more vulnerable they are. Extremely isolated populations of plants and animals can develop so-called inbreeding depression, or an inability to reproduce, especially if they are not wind-pollinated species whose widely dispersed seed gives them ample opportunities for cross-breeding with distant populations.

To date Illinois has few examples of inbreeding depression in its prairie preserves, although that may also reflect the lack of appropriate studies looking for it. Many prairie plants are long-lived, producing only a few generations per century, and thus are unlikely to quickly show the effects of inbreeding.

Because they tend to be inaccessible to the plow, hill prairies are among the last "living windows" into the presettlement Illinois ecology. Illinois hill prairies hold only half the acreage they did 50 years ago. Without periodic fires to check their growth, woody species invade hill prairies from adjacent lands. Comparing aerial photos from 1940 to the present shows that Revis Hill Prairie has decreased in size from 39.2 acres in 1939 to 17.4 acres in 1988.

The tallgrass prairie (where it survives) is a nitrogen-limited system, meaning that the exuberant growth of grasses and other plants consume most of the nitrogenous nutrients in the soil. This chronic nitrogen shortage helps prevent plant species not adapted to it from invading the grasslands. However, supplemental nutrients can enter prairie ecosystems in various ways and in various forms and are likely to alter their species composition. Nitrate and ammonium compounds are delivered from the air by both wet and dry deposition. Total nitrogen deposition on Illinois soils through most of the 1980s ranged from 17 kilograms per hectare per year to less than ten in the Chicago area; nitrogen pollution from runoff and groundflow is locally even more concentrated than that from the air. Prairie plants have been shown to vary in their ability to capitalize on atmospheric carbon dioxide, another nutrient, which some experts expect to double during the next century.


The number of "prairie species"--those plants capable of living at least part of their life cycle in that habitat--is quite large. The Illinois Plant Information Network counts 851 species of plant native to Illinois prairies. However, by no means do all of these occur in any one site. More than 100 species are seldom found in any one prairie, although (as is true of forest land) the larger patches are host not only to more plants, but to more kinds of plants.

Of the 497 plant and animal species considered endangered or threatened in Illinois as of 1993, 117 occur in prairies. Because nearly all species found in prairie occur in other states, or in habitats other than prairies, there are few species endemic to the Illinois prairie ecosystem.

Because of the peculiar conditions under which many Illinois prairie remnants survive, they are vulnerable to peculiar threats. Plants native to blacksoil prairies of the sort that flourish atop undisturbed country graveyards are being overtaken by non-native species planted by mourners as landscape ornamentals. Many exotic plants are little more than nuisances when they root in prairie patches, but species such as white sweet clover and giant teasel are very aggressive. Some can be eliminated by such approved management techniques as periodic burning.


The conversion of prairie to "secondary grasslands" in the form of hay fields and pastures actually enhanced Illinois' habitat for certain birds such as the dickcissel and the prairie chicken. But more recent changes in agricultural practice led to the decline of even these surrogate prairies. Three species of birds once common on the prairies have been extirpated in Illinois (the sandhill crane, once thought extirpated, recently reappeared in Illinois) and another thirteen species were endangered or threatened in the state as of 1993.

Insects have proven more adaptable, although some species may be struggling in Illinois. Typical is the Karner blue butterfly. A native of the Great Lakes and Northeast, the caterpillar of the Karner blue is uniquely adapted to feeding on the leaves of the wild lupine once common in northern Illinois savannas. Nationwide, populations of the Karner blue have declined by 99%, and in 1992 the U.S. Fish and Wildlife Service added it to the list of endangered species. The insect had not been seen for a century in Illinois until the summer of 1992, when five (perhaps a windblown "tourist" and its progeny) were spotted in Lake County.

The distribution of 255 of 640 prairie insect species surveyed since 1982 is restricted to prairie/savanna remnants. Perhaps one-fifth of these are found in only a very few, usually small, sites and must be considered imperiled. Among these is the loosestrife root-borer, which today is found at fewer than six sites; a handful of other species, such as the Dakota skipper, have not relocated to nonprairie habitat and are assumed to be extirpated in Illinois. Separate surveys of the protected prairie at Illinois Beach State Park have found that two butterfly, one moth, and a dozen leafhopper species that once inhabited it have not been seen there for many years.

"The appropriation of habitat by habit" | Wetlands |

Return to Table of Contents