Analysing size-density relationships of aquatic communities in response to strength of predator-prey interactions and resource subsidy

Size-density relationships within the framework of the Energetic Equivalence Hypothesis (which assumes that each species extracts about the same amount of energy from the environment) can be a central concept in ecosystem ecology both for understanding baseline expectations of community structure and for evaluating the effect of top-down and bottom-up processes and ecosystem services. However, despite strong theoretical foundations, empirical evidence for the Energetic Equivalence Hypothesis is largely missing. Our proposal aims to use existing size data of fish and their prey in lakes obtained for European monitoring programs (EU Water Framework Directive) and from several lake-specific research programs to test whether predator-prey interactions and resource subsidy modify the size-density relationships of fish as predicted by the Energetic Equivalence Hypothesis. For this approach, we can use data from almost 400 lakes sampled in a standardized way which will generate sufficient statistical power for hypothesis-testing. However, before these data can be compared with theoretical predictions, a validation and verification of the optimum statistical approach to calculate size-density relationships from fish monitoring data has to be conducted. Therefore, the proposal addresses a systematic comparison of methods to calculate size-density relationships, and a comparison of results from the same lakes, but based on differing sampling gears. After the superior statistical approach is developed, it will be used to test whether the exponents of size-density relationships shift in a predictable way in response to enhanced top-down control or in response to terrestrial subsidy to lake ecosystems. In the long-term, this approach may help evaluating the strength of ecosystem processes from the size distribution of their interacting populations.