CTAP plots contain an average of 20 plant species, 7.5 of them introduced, while a high quality prairie contains as many as 100-140 different plant species. The most common native species in Illinois grasslands include red top grass, big bluestem, trumpet creeper, switch grass, beadgrass, and common goldenrod. Of the terrestrial habitats, grasslands are the most heavily dominated by introduced species with 60 of 71 monitoring sites dominated by them. Meadow fescue and Kentucky and Canadian blue grasses are the most dominant introduced species. CTAP wetland sites contain an average of 15 plant species, with two of them introduced.
Sites containing introduced species have considerably lower biodiversity.
Reed canary grass, the most commonly encountered introduced species, often completely dominates a site, replacing most native species and almost forming a monoculture. It was the dominant species at 22 of 78 monitoring sites. Some monitoring sites were diverse and still contain a high proportion of native species. Southern Illinois wetlands, in particular, seem to be the least affected by introduced species. The number of wetland-dependent bird species found at the typical site is low, 1.3 species on average.