An Index of Ecological Condition Based on Bird Assemblages in Great Lakes Coastal Wetlands

We use bird distributions in non-forested coastal wetlands of the Great Lakes to illustrate a new, conceptually explicit method for developing biotic indicators. The procedure applies a probabilistic framework to derive an index that best “fits” an observed assemblage of species, based on preliminary information about species’ responses to human environmental disturbance. Among 215 coastal wetland complexes across the U.S. portion of the Great Lakes, 23 bird species were particularly sensitive (positively or negatively) to a multivariate environmental disturbance gradient ranging from 0 (maximally disturbed) to 10 (minimally disturbed). Species like Sandhill Crane (Grus canadensis) and Sedge Wren (Cistothorus platensis) showed strong negative relationships with human disturbance, while others like Common Grackle (Quiscalus quiscula), American Robin (Turdus migratorius), and European Starling (Sturnus vulgaris), showed strong positive relationships with disturbance. The functional shapes of these biotic responses were used to determine indices of ecological condition (IEC) for new sites. Values of IEC were highly correlated with the environmental gradient, but deviations from a 1:1 relationship reveal novel insights about local ecological conditions. For example, sites dominated by invasive plant species like Phragmites australis tended to yield IEC values that were lower than expected based on the environmental gradient. This framework for calculating ecological indicators holds significant potential for other applications because it is flexible, explicitly linked to a disturbance gradient, and easy to calculate once standardized biotic response functions are documented and made available for a region of interest.