Illinois Natural History Survey - University of Illinois

Grassland for Prairie Chickens: How Much is Enough?

 

 The first of scattered sanctuaries for prairie chickens were acquired with extremely limited funds in the early 1960s by the Prairie Chicken Foundation of Illinois and The Nature Conservancy. Little information was available on the amount of grassland that would be required to preserve the species in Illinois. Also, estimates of the number of individuals of a species that constitute a minimum viable population ranged from 50 to 500; the latter allowed for evolutionary processes to continue over the long term.

Initial goals called for 1,000 acres in each of two counties (Jasper and Marion) to develop and maintain two breeding populations of about 500 birds from extant remnant flocks. A sex ratio of approximately 50:50 was assumed. These goals were believed to be realistic because prairie chicken numbers soared from about 80 to 400 (40- 206 cocks) between the mid-1960s and early 1970s in Jasper County. This dramatic response occurred with only 660 acres of sanctuary grassland available to the birds by spring 1972. Nevertheless, the sanctuary goals were raised to 1,500 acres in each of two counties in 1973 to allow for probable intensification in farming practices.

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Praire chicken hen at INHS sanctaury in southern Illinois.

By spring 1982, a similar encouraging response occurred in Marion County with an increase to about 230 prairie chickens (116 cocks) with only 450 acres of sanctuary grassland. For 19- year periods, densities averaged 93 and 83 cocks per square mile of managed land for Jasper and Marion counties, respectively. Thus, for nearly two decades, 100 prairie chicken cocks per square mile of sanctuary grassland appeared to be a realistic density goal for Illinois. Two sanctuary systems, each with 1,500 acres, well-situated, properly managed, and well- used by the birds, appeared to be at least minimum goals with which to achieve long- term preservation of the species.

Unfortunately, land acquisition goals were not attained and the favorable responses did not continue into the current decade. By spring 1994 in Jasper County, the count of prairie chickens on booming grounds had declined to six Illinois cocks plus two translocated Minnesota cocks. This brink of extinction occurred despite a new record of nearly 1,000 acres of sanctuary grassland available in 1992 to the prairie chickens at Bogota. The situation was not much better in Marion County where the cock count ranged from 9 to 18 in the past five springs with approximately 500 acres of sanctuary grassland.

Factors documented to have decimated prairie chicken numbers include (1) poor nest success due to predation in some years, (2) intense interactions with pheasants, (3) declining egg quality symptomatic of genetic deficiencies, and (4) intensifying land use on private cropland adjacent to the sanctuaries (the list of other probable negative factors is too long for this report). Illinois Department of Natural Resources (IDNR) sanctuary managers have successfully controlled nest predators and pheasants in recent years. So far, genetic management via translocation of prairie chickens from large populations in Minnesota, Kansas, and Nebraska also appears successful. From only 6 Illinois cocks in spring 1994, numbers increased to 70 cocks by spring 1996 on at least four well-established booming grounds in Jasper County. Limited data on egg fertility and hatchability suggest that egg quality has returned to normal. Moreover, new sanctuary acquisitions by the IDNR of 60 and 215 acres in 1995 bring the total to 1,636 acres in Jasper County. These changes enhance the survival prospects for prairie chickens in Illinois.

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Prairie chicken boomer.

Still, an answer to our initial question, How much grassland is enough?, remains evasive. Attainment of 1,500 acres of quality grassland may soon become a reality, at least in Jasper County. Sanctuary land in Marion County remains at 760 acres, only half the minimum goal.

Estimates derived from one predictive model suggest a need for about 4,000 acres of suitable grassland to sustain a population containing 200- 250 prairie chicken cocks. The estimate from this model was based on prairie chicken research in Minnesota and Wisconsin conducted on range acquired after European settlement, that is, not the species' original range. Application of the same formula to Illinois data (using means for interbooming ground distances and cock numbers per booming ground) suggests that only 1,500 acres may indeed do wonders on Illinois' original prairie chicken range. This quantity of grassland appears especially workable if brome (Bromus inermis), a preferred grass, is emphasized in sanctuary management. However, using several other estimation approaches, a need for several thousand acres of grassland is indicated. Currently, some geneticists calculate that more than 10,000 individuals might be needed to ensure long- term species survival. Researchers are clearly challenged to identify quality habitats and determine how much will be enough.

Ron Westemeier, Center for Wildlife Ecology



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