Botanists and plant ecologists sometimes use a quantitative measure called the Floristic Quality Index (FQI) to express the "quality" of a natural area. This standardized tool, introduced by Floyd Swink and Gerald Wilhelm of the Morton Arboretum, replaces subjective assessments, and although approximate, provides a useful number for comparing various natural areas. FQI is not a stand-alone value, but is used together with other assessments to evaluate the quality of a site.

Coefficient of Conservatism The basis of the FQI calculation is the coefficient of conservatism (C), a value given to each species on a state-wide basis. Each native species is assigned a value from 0 to 10, that represents the probability that this plant species is likely to occur in landscapes relatively unaltered from those of pre-settlement times. Plant species with high C values are relatively specialized in their requirements, and thus are found in more restricted habitats.

Thus, a widespread species such as Canada goldenrod (Solidago canadensis), found in numerous habitats from roadsides to remnant prairies, is assigned a value of 1 whereas prairie dropseed (Sporobolus heterolepis), found almost exclusively on high-quality remnant prairies, has a value of 10.

The C values for northern Illinois are given in the important book by Swink and Wilhelm Plants of the Chicago Area (published by the Indiana Academy of Sciences). A similar set of C values has been established for all members of the Wisconsin flora, and has been published by the Wisconsin Department of Natural Areas. The C value for each native species is also given on the University of Wisconsin-Madison Herbarium web site and in Wildflowers of Wisconsin and the Upper Midwest, by Merel Black and Emmet J. Judziewicz, University of Wisconsin Press, Madison, Wisconsin.

The mean C for Pleasant Valley Conservancy was calculated from a detailed check list which combines all our observations for the 4 years 2002 through 2005. This check list has a total of 319 native species (out of a total of 401 identified species), found in the various habitats (oak woods, oak savanna, marsh, prairie remnant, etc.).

Some examples of species at Pleasant Valley with high C values are: Lithospermum canescens (hoary puccoon), Panicum leibergii (prairie panic grass), and Sporobolus heterolepis (prairie dropseed), C=10; Asclepias purpurascens, Asclepias viridiflora, Ceanothus americanus, Cypripedium parviflorus, Eupatorium sessilifolium, and Viola pedatifida, C=9, Aster sericeus, Baptisia alba, Bromus kalmii, Hasteola suaveolens, Lithospermum incisum, Oxalis violacea, and Pedicularis lanceolata, C=8. In total, there are 190 species at Pleasant Valley Conservancy which have C values of 5 or higher.

The mean coefficient of conservatism for Pleasant Valley Conservancy is 5.1, which is an impressively high value for a landscape that has been under restoration for such a short period of time. Using these data, the Floristic Quality Index calculated using the Swink and Wilhelm formula, is 91.3, again, an impressively high number. (According to an assessment by Michigan workers, an FQI greater than 50 represents an area of high conservatism and an area with an FQI greater than 35 is considered floristically important in the state.)

Obviously, a high FQI depends on the presence of species with high C values. Such plants show strong fidelity to specific habitats. Such fidelity goes hand-in-hand with rarity. However, a high C value does not necessarily mean that the species is "endangered" or "threatened". This depends on the commoness of the required habitat of a plant with high C value. For instance, Lithospermum canescens has a C value of 10 but is not endangered or threatened, because its preferred habitat, dry prairie remnants, is fairly common in the state.

Floristic Quality Index for Each Management Unit

The table below lists the species found at Pleasant Valley Conservancy in order of their C value, from high to low, based on data from 4 years.

Coefficient of Conservatism for Plant Species