Complexity and stability in small aquatic systems

Despite years of research exploring the effects of complexity on community and ecosystem stability, there is little consensus on the magnitude or direction of these effects. Much of this confusion stems from disparity in the meaning of the terms complexity and stability. While dividing the term stability its components has rendered it more precise, an analogous treatment of complexity has not yet occurred. Here, I parse complexity into two fundamentally distinct components and explore the effects of changing each. Complicatedness is elaboration at a single hierarchical level, and hierarchical organization is elaboration of hierarchical levels. An example of complicatedness is species number within a trophic level, while an example of hierarchical organization is the number of trophic levels in a food web. In this dissertation, I explore the meaning and impacts of these two types of complexity in several ways. First, I examine the conceptual construct of dividing complexity into its components and propose a statistical technique for empirically distinguishing between these components. Second, I test this and a second statistical method in a series of 42 freshwater rock pools across a gradient of ecosystem size. Third, I create artificial communities and perturb them with two different types of species additions, each representing a different component of complexity. Results revealed: (1) comparing variation across size classes is a promising method for distinguishing between complicatedness and hierarchical organization, and (2) that increasing hierarchical organization changed stability and values and/or rates of change more often than variation across size classes is a promising method for distinguishing between complicatedness and hierarchical organization, and (3) that increasing hierarchical organization changed stability and values and/or rates of change more often than did increasing complicatedness. These results suggest that hierarchical organization and complicatedness have fundamentally different effects on community and ecosystem structure, function and stability, and that reducing the complexity-stability debate to the diversity-stability debate may be an unwarranted simplification.; Finally, in an independent study, I examined the relationship between environmental variables and plankton community structure and change in 42 freshwater rock pools. Ecosystem size and location, but not permanent sediment, were important in determining plankton community structure. In contrast, phytoplankton species turnover rate did not correlate with any environmental variables, while zooplankton species turnover rate responded only to location.