The mechanisms behind alternative community states: Understanding producer composition in temporary and semipermanent wetlands

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In many ecological communities, variation in composition appears discontinuous, in that a discrete set of community states (or distinct taxa assemblages) have been observed repeatedly in one ecosystem type. Many hypotheses have been proposed to explain this discontinuous variation in community composition over space and time. I examined the applicability of these hypotheses to understand the structure of producer communities of temporary and semipermanent wetlands in Michigan, USA. I identified several distinct community states, with each characterized by dominance of particular functional groups (submerged, floating, or emergent plants) or the absence of plants throughout a season. I developed a framework for differentiating among these hypotheses by characterizing how community composition varies (1) over space, (2) over time, and (3) with possible drivers. I applied this framework to my survey data of the producer communities of wetlands over four years (n = 21-35 wetlands). I found that several environmental variables (pH, light, depth, and permanence) were associated with community composition based on multivariate analyses in these wetlands. Alternative stable states or interactions among several drivers were the most strongly supported explanations for the expression of multiple community states. I further examined factors driving dominance of free-floating plants in wetland producer communities in two additional studies. I evaluated the roles of nitrogen (N), phosphorus (P), and N:P ratio in driving floating plant dominance in a separate survey dataset and in a field experiment. In the observational work, N, P, and N:P ratios all were associated with producer community composition. I manipulated the three variables in enclosures in two experimental ponds with floating and submerged plants. I found N supply rates drove these patterns more strongly than P and N:P. However, submerged plant composition and pH levels also affected floating plant responses. Finally, I evaluated the role of light in changing the response of floating plants to N. In my observational data, I found that submerged and emergent plants only dominated wetlands with open tree canopies, and that floating plants more consistently dominated systems at intermediate N levels when light availability was low. To investigate this relationship further, I manipulated light, N, and initial densities of floating plants in an outdoor mesocosm experiment with floating plants and algae. In this experiment, I found that floating plants exhibited a light x nutrient interaction. Competition with other plant groups and photoinhibition are discussed as possible mechanisms driving these patterns. In summary, a complex suite of environmental variables drives the expression of multiple community states in temporary and semipermanent wetlands. While there is some support for the hypothesis that nutrients and light drive alternative stable states between floating plants and other producers, several other variables play important roles in regulating dominance of floating plants as well.






Environmental Sciences


University of Illinois at Urbana-Champaign