frog on a lily pad

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

Illinois land was once famed for its wetness as much as its richness. It is conservatively estimated that at the time of European settlement more than eight million acres in Illinois (roughly one acre in five of its total area) were wetlands of one kind or another. Other estimates suggest that this figure is low. (Figure 7-1 )


"Wetlands" describes land where the water table is at or near the surface and the soils are hydric (wet and low in oxygen) and occupied by hydrophytes (plant species adapted to life in water or in saturated soils.) That definition encompasses bogs, marshes, sedge meadows, wet prairies, fens, swamps, bottomland forest, ponds, sloughs, mudflats, and areas having frequent river overflows. Wetlands in Illinois may be fed by runoff, rainfall, seepage from groundwater, or a combination of all of these sources.

The value of wetlands to the environment has only recently been widely recognized. For example, wetlands filter and purify water that flows through them. They also store water during flood events and trap sediments that otherwise would enter streams. Wetlands are thought to provide natural flood control by slowing the movement of rainfall and snowmelt into streams and by storing excess water that streams cannot accommodate during high flows. They are also thought to contribute to increased low flows in streams, in part because they help recharge shallow aquifers that feed streams during low-rainfall periods. Wetlands provide habitat to an impressive diversity of plants and animals.

Wetlands of all types have been substantially reduced in extent, mainly because of agriculture. A study using the Illinois Wetlands Inventory found some 918,000 acres of "natural" wetlands (that is, not diked, impounded, or excavated) remained statewide in the 1980s--less than a tenth of their original extent. (Illinois is one of the ten states that have lost more than 70% of their original wetland acreage.) Remaining natural wetlands cover only about 2.6% of the state's land area. (Figure 7-2 ) These are concentrated in the northeast (along major rivers such as the Fox, Illinois, Des Plaines, and Kankakee) and in southern Illinois. Of the surviving wetlands, only about 6,000 acres are high in ecological quality and undisturbed.

In 1992 Illinois had some 330,000 acres of wetlands modified or created by dikes, impoundments, or excavations (e.g., farm ponds and municipal reservoirs). Under federal rules, the destruction of wetlands in certain cases such as dredging or filling must now be mitigated by the construction of a like amount of wetlands elsewhere. In the first half of 1993, the U.S. Army Corps of Engineers approved the filling of 91 acres of wetlands, and mitigation was completed for 93 acres. However, created or restored wetlands have not generally been able to function at the levels of biological and hydrologic complexity of their natural models.


Wetlands are ecologically complex, and plants living in them have made complex adaptations. Of the 172 families of vascular plants that occur in Illinois, 108 contain species that thrive in aquatic or moist-soil environments. The sedges are the most species-rich of these families, followed by grasses, sunflowers, orchids, and mints. According to a conservative definition of wetland plants, a total of 952 species are found in Illinois wetlands and these constitute about 42% of the state's native flora.

Those wetland plant species considered endangered or threatened in Illinois as of 1993 and known from extant populations were most numerous in Lake (with 66 species), Cook (with 38 species), and McHenry (with 41 species) counties. Cook leads all counties in the number of species listed as threatened or endangered that were believed to be extirpated from the county, with 36; Cook was followed by Kankakee (17 species), McHenry and Peoria (16 each), Kane (15), and Lake and Winnebago (14 each). Approximately 35 wetland species are thought to have become extirpated from the state overall. Thismia americana, which is known to have appeared only in Illinois (near Chicago's Lake Calumet), was last seen here about 75 years ago and may be extinct, although efforts to locate Thismia continue.

The rarest types of wetland habitats tend to support the greatest number of rare plants. Illinois bogs and fens are a minor part of the Illinois wetland inventory in terms of acreage (only 350 undisturbed acres of each are thought to survive), but those peatlands support some three dozen endangered and threatened plant species.

Less than 10% of the plant species now found in Illinois wetlands are not native to the state. Though few in number, the non-native wetland species are aggressive. Glossy buckthorn infests sedge meadows, bogs, fens, and floodplain forests, especially in northeast Illinois, where it quickly overtops native species; seeds of its fruits are widely dispersed by birds.

Purple loosestrife, a northern European native that arrived in North America nearly 200 years ago and reached Illinois sometime before 1940, was found in at least 25 Illinois counties by 1985, mostly in the northeast. Sale of purple loosestrife is now illegal in Illinois but it reproduces abundantly from roots, stems, or its copious seeds.


Because they are rich habitats for animals, wetlands are the venue for a disproportionate amount of Illinois' animal-based recreation, from fishing to waterfowl hunting. Wetlands also harbor disproportionate numbers of rare animals; 64% (61 of 95) of the endangered and threatened animal species listed in Illinois as of 1993 use wetlands in some way. (Figure 7-3)

Figure 7-3. Wetland Use by Endangered and Threatened Vertebrate Species

Source: Ecological Resources, Illinois Natural History Survey, 1994

Birds. In all, 274 bird species commonly observed in Illinois can use wetlands opportunistically for nesting, foraging, and resting, but 105 typically depend on or are strongly associated with these highly specialized habitats for nesting and foraging. Their populations have become imperiled as these habitats shrink in size. King rails, for example, are much less commonly sighted than they once were.

Many nesting birds such as egrets, herons, and double-crested cormorants build colonies in wetlands (mainly floodplain forests). The total numbers of some birds, such as great egrets, increased during the 1980s, while those of the night heron declined. Fluctuations in colony size and species composition are common among such birds; however, the apparent increased number of colonies may be partly the result of more diligent field surveys.

Of the 43 bird species listed as endangered or threatened in Illinois as of 1993, 30 are strongly associated with wetlands, especially during the breeding season. Some species are dependent on more than one type of wetland; the great egret, for example, typically nests in floodplain forests but prefers to forage in shallow-water wetlands.

As recently as the 1880s, many bald eagles nested in Illinois, mainly along the Illinois and Mississippi rivers. However, poisoning by the insecticide DDT, hunting, and habitat destruction throughout its range caused a decline in eagle populations, and from 1978 to 1987 only two to four such nests were recorded in Illinois. While numbers of nesting bald eagles seem to be increasing in recent years--17 nests were recorded in 1992--Illinois remains mainly a wintering ground. A total of 1,211 bald eagles were counted along the Illinois and Mississippi rivers in 1990. (Figure 7-4)

Figure 7-4. Bald Eagles Along Major Illinois Waterways 1958-1993*

Source: Ecological Resources, Illinois Natural History Survey, 1994

The double-crested cormorant is another bird that seems to be rebounding from a mid-century decline. Combined populations documented during the fall migrations through the Illinois and Mississippi river valleys rose from fewer than 100 birds around 1970 to nearly 6,000 in 1992.

Mammals. Although few mammals are adapted specifically to wetlands, eight of the ten mammal species considered endangered or threatened in Illinois as of 1993 use wetlands to some extent. The swamp rabbit (whose populations are in decline in Illinois), and the marsh rice rat are among them. Such commercially valuable furbearers as the raccoon, mink, muskrat, and beaver also inhabit wetlands.

Reptiles and amphibians. Not surprisingly, 37 of Illinois' 41 amphibian species--salamanders, frogs, tree frogs and toads--use wetlands at least part of each year. Because of their permeable skin and because they are exposed to both terrestrial and wetland environments, amphibians are especially susceptible to environmental stresses. Their highly specific adaptations also leave some species vulnerable to habitat changes; the only natural Illinois population of the silvery salamander occurs in Vermilion County, where the animals breed in a single vernal pool. The eastern newt, once thought to occur across Illinois, is no longer found in the state's central counties due to the draining of prairie marshes.

Reptiles as a group are less dependent on water than are amphibians, but at least 47 of the 60 Illinois reptiles use wetlands to some extent. Seven of the nine species listed as endangered or threatened in Illinois as of 1993 (three turtles and four snakes) use wetlands. The draining of wetlands in heavily farmed parts of the state is thought to have contributed to declines in Blanding's turtle and the massasauga snake significant enough to put both species on the state's watch list.

Fish. Twelve of the 29 fish species listed as threatened or endangered in Illinois as of 1993 either occur in wetlands (mainly swamps, oxbow lakes, and sluggish backwaters) or breed in them. The widespread drainage of such habitats reduced the range for seven Illinois wetland species that were subsequently listed as threatened or endangered. The blacknose shiner, for example, had been recorded throughout the northern two-thirds of Illinois prior to 1905, but was seen in only eight counties in the 1950s and 1960s and has been found only in Lake County since 1980.

Invertebrates. Wetlands are rich grounds for invertebrates, from protozoa to clams and snails. A significant number of insects are adapted for life in water; they include water beetles, mayflies, dragonflies, and water scorpions. Several faunal studies have examined species groups, mainly pest insects such as mosquitoes, deer flies, and horse flies. But as is true of most other wetland-dwelling species, data that might demonstrate population trends for these macroinvertebrates in Illinois wetlands are largely lacking.

Relatively few (four of 51) of the invertebrates endangered or threatened in Illinois as of 1993 are wetland species. One of these, Hine's emerald dragonfly, demonstrates how complex are the survival needs of many species. Known in Illinois only since 1983, this dragonfly is limited to specific habitats in the Des Plaines River watershed. It requires clean water during its immature stage, which lasts three years, and is restricted to marsh communities where calcareous water seeps from between overlying glacial till and limestone bedrock.


Because they are low-lying, wetlands function like sinks that collect polluted runoff or sediments from adjacent lands. Wetlands have long been favored as places to dump wastes because they were considered "waste" land. (A prime example is Chicago's Lake Calumet area, which functions improbably as a combination landfill and nature area.) Many of the more than 3,000 Illinois sites known to have been used for land disposal of wastes are located in wetlands. A 1988 survey found that 8% of Illinois' surviving wetlands and deep water acreage--more than 100,000 acres in all--are located within one mile of a known landfill or open dump site and thus potentially are at risk from contamination.

Disappearing Ducks | Agricultural Lands |

Return to Table of Contents