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No. The 2025 characterizations replace previous versions; the newest version uses additional data gathered since the 2007 version, as well as an entirely new approach to evaluating biological assemblages that emphasizes intactness (i.e., proportion of maximum species diversity retained in contemporary period) rather than diversity or index scores as with previous versions.
Biological Stream Characterization and Biologically Significant Streams
The Illinois Department of Natural Resources, Division of Natural Heritage, in partnership with the University of Illinois-Illinois Natural History Survey, updated its Biological Stream Characterization (BSC) in 2025. This is the third revision of the BSC (1984, 1992, 2007 are previous versions) and, like previous versions, this updated BSC categorizes streams based on condition of biological assemblages. For the first time, however, the 2025 BSC incorporates species distribution models (SDMs) that leverage species occurrence records to expand the spatial scope of characterizations to include all stream segments in Illinois.
The BSC project includes two categorizations: BSC and Biologically Significant Streams (BSS) grades. Both the BSC and BSS are tools for evaluating intactness (i.e., proportion of maximum species diversity retained) of aquatic life assemblages in Illinois’ streams and rivers. The BSC grading compares species-level predictions of historical (all records) and contemporary (1990 to current) habitat suitability and then applies a grade (A, B, C, D, F) to all Illinois stream reaches. Grade letters represent decreasing degrees of assemblage intactness (i.e., streams with grades of “A” or “F” are most or least intact, respectively). The BSS is applied only to reaches with species observations and is the ratio between contemporary occurrence records to either historical occurrence records (BSS 1) or historical modeled species presence (BSS2). Reach-level BSS1 or BSS2 status was assigned based on the reach-level value of each ratio relative to all reaches graded.
Both assessments evaluate and grade fish, freshwater mussel, and EPT (Ephemeroptera [mayflies], Plecoptera [stoneflies], and Trichoptera [caddisflies]) taxa individually, as well as all taxa combined. Each taxon and the combined all taxa assessment can be viewed as individual layers under assessment headings (grading or BSS).
Frequently Asked Questions
A SDM uses species occurrences and environmental variables at the locales of those occurrences to identify correspondences between occurrence and environment and then predicts habitat suitability for individual species across large spatial extents (e.g., all Illinois stream segments). SDMs have extensively been used by scientists to evaluate species ecology and biodiversity. They are used in the 2025 BSC to fill information gaps created by incomplete survey coverage in space, time, and stream types.
Two criteria were used to identify taxa suitable for characterizing streams within the context of this project: 1) Occurrence records are accessible in digital format and contain precise locale and date information, and 2) taxa respond to gradients in environmental characteristics at scales of time and space relevant to this project. Fish, mussel, and EPT records meet these criteria.
The BSS 1 recognizes reaches of high intactness by considering correspondence of species diversity between contemporary and historical occurrence records. In reaches with survey effort insufficient for estimating historic species diversity SDMs were used for the historic diversity value. Those reaches with high intactness under this second method were categorized BSS 2. The two BSS methods were differentiated to maintain methodological transparency.
BSC and BSS characterizations may be used to identify relative diversity of stream reaches or patterns of relative diversity within watersheds or Illinois. Conservation agencies and organizations may use these characterizations to target conservation efforts or identify relevant resources. But, variation of aquatic life assemblages occurs rapidly in space and time, so the 2025 BSC and BSS characterizations should be viewed as one tool among many in the conservation toolbox.
The final report for the BSC/BSS project, which includes analytical methods, can be found at https://www.ideals.illinois.edu/items/135089. Additional questions may be directed to Brian Metzke, IDNR Aquatic Ecologist, at Brian.Metzke@illinois.gov.
