How do different taxa respond to landscape stressors in Great Lakes coastal wetlands?
Urban development, point source pollution, agricultural runoff, and introductions of invasive species are among many landscape stressors that affect the ecological condition of Great Lakes coastal wetlands. Different types of organisms respond to these stressors in diverse ways. To better understand the responses of biotic communities to human-induced stressors, we compared the occurrences of species representing 5 taxa (birds, diatoms, fish, invertebrates, and vascular plants) in Great Lakes coastal wetlands across the U.S. portion of the Laurentian Mixed Forest Province from 2001 to 2004. Using linear regression we modeled the observed relationships between the abundances of coastal wetland species (wetland obligates or species strongly associated with wetlands) and 3 stressor gradients associated with the adjacent watershed: percent agricultural land cover, percent developed land cover, and a multivariate metric (SumRel) combining these variables with population density, road density, and point source pollution data. Predictor variables were modeled as polynomials (up to third order) of the original gradients, allowing for non-linear responses. We classified species’ responses as positive, negative, or intermediate with respect to each stressor and compared the proportions of species exhibiting each type of response within each taxon. We also compared overall responses of each taxon to the stressors.
Many different responses were exhibited by species in each taxonomic group, but in general more species responded negatively than positively to the environmental stressor gradients. More than half of the species in each taxon had no significant response to the gradients. Birds had the highest proportions of negative (26.9%) and positive (11.5%) responses to SumRel and the highest proportion of negative responses (34.6%) to the agricultural gradient. Very few species in each taxonomic group exhibited a response to the development gradient (less than 20% per taxa). Biotic responses to landscape stressors such as these have already been used to develop an ecological indicator of ecosystem health called the Index of Ecological Condition (IEC). This metric has been applied to coastal wetlands, inland lakes, and forests in the Laurentian Great Lakes region using several taxa. Our results can be used to develop a multi-taxa and multi-gradient IEC that will inform land managers about the health of coastal wetlands and the landscape stressors that affect these ecosystems.