| Previous studies examining aquatic snail community structure have used a wide variety of explanatory variables including water chemistry, area effects, habitat diversity, and biotic interactions. Almost all have focused on within-patch characteristics and have generally ignored the surrounding landscape context. The northern shoreline of Lake Huron includes a dynamic wetland complex containing numerous Great Lakes coastal wet meadows. I sampled aquatic snails from 26 coastal wet meadows in 1997 and 1998, and measured 28 within-patch characteristics used to describe the water chemistry, habitat structure, and plant community. A GIS database was used to calculate 27 landscape-level characteristics describing the adjacent and nearby patches and the complexity of the shoreline. I used canonical correspondence analysis (CCA) to identify patterns in the distribution and abundance of snails associated with the within-patch characteristics. Landscape effects were examined by CCA after accounting for within-patch effects. Additional associations with water depth were examined using multiple regression analyses and laboratory experiments.; Nineteen species of snails were collected with six species occurring abundantly in most wet meadows. The occurrence of various plant species explained the greatest amount of within-patch variation in the snail community, followed by characteristics of habitat structure. Water chemistry explained the least amount, as expected. Significant associations with multiple landscape context variables, especially bulrush, cattail, and upland meadows, were observed after accounting for within-patch effects. In 1997 only, there were significant associations with shoreline complexity as measured by sinuosity and fractal dimension at multiple scales. A suite of variables that were related to urban land use and habitat fragmentation was included in a 1998 model, suggesting some concern for anthropogenic impacts on the snail community and the need for conservation of these wetlands. In an analysis of water levels, mean depth was the most common predictor, followed by spatial and temporal variation in water levels. Laboratory experiments showed interspecific variation in snail motility and desiccation resistance that were consistent in explaining snail community structure.; A conceptual model of the Great Lakes coastal wet meadow snail community was developed based on a depth gradient from deeper wet meadows that remain permanently flooded to shallow and hydrologically semi-isolated wet meadows in which periodic drying occurred. The model included interactions at various scales, from within-patch to shoreline complexity, and the proximate behavioral and physiological differences among snail species. Further testing of the pathways in this model will help to clarify the mechanisms behind the observed patterns. |
