| Summary: | Tolerance and avoidance are the two contrasting strategies that plants may adopt to cope with herbivores. Tolerance traits define the degree to which communities remain unaffected by herbivory. Trade-offs between herbivore avoidance and competitive strength and between avoidance and colonization ability may shape population traits and communities under herbivory. In this thesis I present comparative and experimental studies on populations and communities of aquatic plants and how they deal with herbivory by waterfowl. At the population level I show that the degree of compensation to herbivory by the clonal pondweed \emph{Potamogeton pectinatus} strongly depends on the timing of herbivory. Herbivory on aboveground biomass in summer reduces plant biomass and tuber production severely whereas predation on tubers in winter by Bewick 's swans \emph{Cygnus columbianus bewickii} is largely compensated for. Despite this high tolerance to winter herbivory, fennel pondweed may exhibit a flexible avoidance strategy towards tuber predation by burying tubers deeper in the sediment in the presence of swans. Since deeper burial in the sediment is costly, pondweed trades competitive strength for safety from herbivores. I present support for this idea from field studies and an exclosure experiment. The extent to which species can adjust to changes in herbivore pressure is constrained by the limits of phenotypic plasticity and microevolution. Therefore, at the community level, trade-offs may allow coexistence of different strategies thus stimulating functional diversity when herbivory is heterogeneous in space and time. In addition, herbivory may influence interspecific competition itself possibly leading to drastic changes within aquatic plant communities. This may especially be the case when species are ecosystem engineers. This may be the case in the competition between charophytes (macroalgae) and pondweeds which is shown from mesocosm studies.
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