| Shoreline wetlands of large lakes are ecotones between terrestrial and aquatic systems, and many ecological characteristics (e.g. water chemistry, vegetation) vary along the lake-to-upland elevation gradient in these systems. In Great Lakes coastal wetlands, this variation is enhanced by annual and multi-annual changes in water level. Aquatic animal communities are likely to respond to this spatial variation in habitat conditions, but few studies have sought such faunal variation or its causes. In this study I collected animal community data along the elevation gradient in a northern Lake Huron coastal wetland over a 3-year period during which the water level changed dramatically.; Replicate samples were collected along the slope at 10 cm elevation intervals, covering a 60 cm elevation range in a ∼120 m horizontal distance. In the first year, multivariate analysis revealed that animal community composition varied gradually with elevation, and seasonal changes appeared to result from up-slope migrations of some animal taxa. During the second year water levels were over 30 cm higher, and in the third year water level dropped back toward initial levels. Most insect taxa were able to respond rapidly to these changes, while many non-insects migrated more slowly. These differences led to increasing invertebrate densities and changes in community composition at different points on the gradient, and led to a loss of wetland diversity, as many rare taxa declined and generalist animals thrived. Flooding duration was significantly correlated with densities of the most common invertebrate taxa.; Because predator densities also varied over the gradient, I ran field experiments to determine whether they (small fish and larval dragonflies) partly caused the observed faunal differences over space and time. Fish reduced invertebrate densities, favoring certain generalist taxa, while dragonfly effects were less dramatic. Taxon-specific effects suggested that predation can prevent some prey taxa from occurring at gradient points where predators are dense. Results led to a conceptual model of temporal and spatial shifts in flooding and predation controlling animal community composition in coastal wetlands. |
