Refinement of an index of ecological condition for marsh bird communities in lower Great Lakes coastal wetlands

Abstract

Abstract The Laurentian Great Lakes coastal wetlands are highly productive and biologically diverse ecosystems. Ongoing environmental degradation and wetland loss have resulted in a number of restoration and conservation programs. To facilitate reporting on the health and recovery of these wetlands, various indices of ecological integrity have been developed to distill complex biotic community data into results that are easy to interpret and understand. The Index of Ecological Condition (IEC) approach uses a probabilistic framework to derive an index by fitting the observed assemblage of species based on species occurrence along an a priori environmental gradient. This study refines the original IEC by Howe et al. (2007a) using updated methodologies and focusing on marsh bird communities in lower Great Lakes coastal wetlands. The a priori environmental gradient was redeveloped using finer-scale (minimum mapping unit = 50 m2) landscape data than Howe et al. (2007a), including Phragmites australis as an invasive stressor, and incorporating measures of water quality and submerged aquatic vegetation as watershed-level disturbances. The environmental gradient showed a good relationship with known anthropogenic disturbances. Marsh bird communities were surveyed in lakes Erie and Ontario and wetland species with clear responses to the environmental gradient were selected for inclusion in the IEC. Where detection was low, species were grouped into multispecies guilds. The resulting IEC was comprised of 15 biotic response functions: 12 individual species and 3 species guilds, and was highly correlated with the a priori environmental gradient (R2 = 0.54). Species with strong positive responses included Canada Goose (Branta canadensis), Marsh Wren (Cistothorus palustris), Swamp Sparrow (Melospiza georgiana), and American Bittern (Botaurus lentiginosus). Pied-billed Grebe (Podilymbus podiceps) had a particularly strong response and was highly influential in the IEC. Great Egret (Ardea alba) was the only species with a negative relationship to the environmental gradient. The refined IEC performed well and provides easily understood output for coastal wetland practitioners in the lower Great Lakes, tasked with reporting on the ecological integrity of marsh bird communities.