An Index of biotic condition (IBC) using birds as indicators of coastal wetland quality in North America’s Laurentian Great Lakes

Abstract

To survive in an area, plant and animal species must confront many interacting environmental stressors at different spatial and temporal scales. Consequently, assemblages of resident species, which have successfully negotiated these stressors, are widely recognized as indicators of environmental quality. Many biotic variables have been employed as indicator metrics, but the simplest is presence/absence of sensitive species in a standardized field sample. We describe a multi-species ecological indicator, the Index of Biotic Condition (IBC), which overcomes key concerns with using indicator species or species assemblages to assess environmental quality. Developing an IBC requires substantial ecological knowledge and preliminary data to identify an appropriate set of species or species groups for the area and habitat(s) of interest and to quantify the sensitivity of these species to an explicit environmental reference gradient. Based on objective biotic response (BR) functions describing the relationship between species occurrences and the environmental gradient, we outline a method for deriving species weights (factors that determine the relative contributions of species / species groups), subsequently used to calculate the simple multispecies index. The IBC is more rigorous than structurally similar metrics like the Floristic Quality Index (FQI) and average Coefficient of Conservatism (C¯) and, at least for some applications, is more parsimonious than comprehensive metrics like the Index of Biotic Integrity (IBI) and our previously described Index of Ecological Condition (IEC). We apply information from standardized basin-wide bird surveys conducted between 2011 and 2022 (particularly a subset of data from 2011 to 2015) to develop an IBC for comparing and interpreting the environmental quality of 794 wetlands in the coastal zone of North America’s Laurentian Great Lakes. Implicit in this formulation is the existence of a high-quality reference assemblage that includes 14 obligate or facultative bird species/species groups characteristic of these wetlands. The index ranges from 0 (worst condition) to 10 (best condition) and is correlated with species richness (r = 0.87), although the IBC provides additional information reflecting the taxon-specific environmental sensitivities. The distribution of IBC values for 4,359 point samples in Great Lakes coastal wetlands between 2011 and 2022 is strongly skewed right, with a median value of 2.31. Only 28 wetland points (<1%) yielded an IBC value of 8.0 or greater, suggesting that relatively complete coastal wetland bird assemblages rarely occur at specific points in today’s Great Lakes coastal wetlands, consistent with the region’s well-documented historical reduction in biological diversity.