Illinois Natural History Survey - University of Illinois

INHS Reports Autumn 2000

Exotic Vegetation in Illinois Wetlands

The loss of wetland habitat across the United States has been well publicized in recent years. Here in Illinois the losses have been particularly devastating, with well over 90% of our original wetland acreage already lost and the majority of the remaining substantially degraded. Surprisingly to many people, much of this degradation can be attributed, at least in part, to invasion by exotic plant species. As used here, the term "exotic" refers to any species not native to Illinois, either native to another country or to another part of the United States. With human impacts to wetlands, such as increased levels of sedimentation (caused by development, construction, and agricultural activities), agricultural runoff, and alterations in wetland hydrology (e.g., changes in water depth, lack of water level fluctuation), hardy, disturbance- tolerant exotic plants often come to dominate. Although these invasive exotics often take advantage of disturbed conditions, this is not a requirement and many species readily spread into relatively undisturbed, high- quality, natural communities, displacing native plants. Community composition and diversity can be dramatically altered, often threatening or eliminating rare or endangered species.

Garlic mustard (Alliaria petiolata) is now a serious problem in forested
wetlands of northeastern Illinois.

In an effort to quantify the prevalence of exotic plant species in Illinois wetlands, data from over 2,500 wetlands were analyzed, based principally on information collected from wetland delineation projects conducted throughout Illinois. In this analysis, only "dominant" plant species were considered. Dominant plant species are major components of the plant community, contributing more to the character of the community than other species present. Therefore, this discussion of exotics can be considered a fairly conservative survey of their impact because they are considered in this analysis only when they were very prevalent (dominant) in a given wetland community.

Seventy- seven plant species not native to Illinois were identified as dominant in wetlands throughout the state. Reed canary grass (Phalaris arundinacea), common in a variety of wetland habitats, was clearly the most prevalent species overall, occurring as a dominant in 30% of all wetlands and in 48% of the northeastern Illinois (Chicago area) wetlands. Narrow- leaf cattail (Typha angustifolia), primarily a pond and marsh species, was dominant in about 10% of all wetlands. Overall, 52% of all wetlands had at least one dominant, exotic species. In the Chicago region of northeastern Illinois, an amazing 76% of wetlands contained dominant exotic species, compared to only 37% for the rest of the state. Within the Chicago area, marshes and herbaceous wetlands (such as wet meadows and sedge meadows) showed the highest rates of exotic dominance, an alarming 81%, followed by forested wetlands, also very high at 71%. Buckthorn (Rhamnus cathartica) and garlic mustard (Alliaria petiolata), although not typically thought of as wetland species, were, unfortunately, common dominants in northeastern Illinois forested wetlands. Two nonwetland, non- native, lawn and pasture grasses, bluegrass (Poaspp.) and fescue (Festuca spp.), were fairly common wetland dominants around the state, as were two smartweeds (Polygonum persicaria and P. hydropiper), moneywort (Lysimachia nummularia), and curly dock (Rumex crispus). Purple loosestrife (Lythrum salicaria) was a common exotic dominant in Chicago area marshes.

Reed canary grass (Phalaris arundinacea) is the most prevalent exotic
wetland plant species in the northern half of Illinois.

Overall, levels of exotic species dominance are very troublesome, particularly in the Chicago region where prevalence is extremely high. Although more well-known, problematic exotics, such as purple loosestrife and garlic mustard, are a concern, reed canary grass appears to be the biggest threat to wetlands statewide. Forming dense stands capable of excluding virtually all other plant species, this perennial grass readily spreads into most wetland habitats and is already likely beyond control throughout much of Illinois.

Brian Wilm, Center for Wildlife Ecology

Charlie Warwick, editor