Illinois Natural History Survey - University of Illinois

Grassland Habitats in Illinois

Tallgrass prairie once dominated much of the Illinois landscape, but these native prairies have been nearly extirpated. What grassland remains is fragmented within an agricultural and human-altered landscape. Additionally, non-native species have replaced native prairie species. Gone are the oceans of big and little bluestem, replaced by isolated fields and remnants. These grasslands currently constitute the habitats of species such as the endangered Henslow's Sparrow that are native to prairies. How native prairie species utilize the current landscape is not well understood.

We analyzed the distribution of grasslands throughout Illinois to better understand the current status of this habitat. Our goals were to identify existing large blocks of grassland, to identify areas with a high density of grasslands, and to characterize selected regions based on the size and abundance of grassland tracts. The derived distribution of grasslands was also compared with known locations of Henslow's Sparrows.

Analyzing the existing grasslands for all of Illinois' 56,000 square miles required the use of computer-based geographic information system technology. The land cover database of Illinois (INHS Reports, Nov./Dec. 1996) was used to create the initial map of grasslands. This database has a spatial cell resolution of 28.5x28.5 meters, requiring over 180 million such cells to cover all of Illinois. Approximately 17% of Illinois is classified as grassland, including roadways, railroad corridors, strip mine areas, old fields, pastures, and remnant prairies.

The current distribution of grasslands in Illinois forms an interconnected web of thin corridors, mirroring roadways, greenways, fields, and railway corridors. Our analysis began by isolating 12 contiguous tracts of grassland from thin connecting corridors and characterizing each tract by interior area and size. Tracts with 10 or more hectares (ha) of interior grassland were retained for analysis and classified as being larger than 10, 40, or 100 ha (Figure 1). All subsequent analyses were based on this representation of grasslands.

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The amount of grassland within a county varies widely across the state. Will County has the greatest amount of grassland in tracts larger than 100 ha and also has the largest single tract (2,990 ha) (Figure 2). This tract lies within the Midewin Tall-grass Prairie. Another large, single tract (1,078 ha) occurs in Jo Daviess County at the site of the former Savanna Army Depot. Grasslands account for over 45% of the land cover of Jo Daviess county; however, only 1.1% of this is in tracts larger than 100 ha. Johnson County has the greatest number of grasslands larger than 100 ha but all are of moderate size (148-384 ha).

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Figure 2. Grassland tracts near Goose Lake Prairie, Il., classified by size. Continuous gray indicates grassland; darker gray indicates larger tracts.

General locational data for 25 Henslow Sparrow sitings statewide were incorporated to evaluate our grassland model. This grassland bird species is listed as endangered in Illinois and is known to prefer large, idle tracts of grassland. Fourteen of the 25 Henslow Sparrow locations occurred in grasslands with interiors of at least 10 ha. Of these 14, 7 occurred in grasslands with interiors larger than 100 ha.

This is the first time the land cover database has been used for this type of analysis. Field studies will help validate the distribution of grasslands and the large tracts identified in our analysis. The results of this research, which was funded by the Heritage Division of the Illinois Department of Natural Resources, will help guide field work and restoration efforts by state biologists. The next step is to understand the ecological importance and functioning of these grassland areas as habitat for native prairie fauna in Illinois.

Mark Joselyn, Jocelyn Aycrigg, and Tony McKinney, Center for Wildlife Ecology; James Herkert, Division of Natural Heritage, Illinois Department of Natural Resources, Springfield.



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