Ecological and evolutionary effects of predation in environmental gradients

Understanding species interactions and how they affect densities and distributions of organisms is a central theme in ecological research. Studying such interactions in an ecosystem context is challenging as they often depend on species-specific characteristics and rates that not only change during the ontogeny of an organism, but also are affected by the surrounding environment. This thesis focuses on two separate questions and study systems that highlight different aspects of how effects of predation can depend on environmental conditions. In the first part of the thesis, we studied how temperature affects attack rate and handling time, two ecological rates with profound importance for predator-prey dynamics. Using a metaanalysis, we first show that the currently dominating model for temperature dependence in predator-prey interactions, i.e. the Arrhenius equation, has weak support in available empirical literature. This suggests that we need new rules for how and when we can generalize on the temperature-dependence of intake rates. We then use a simple model and a series of experiments to demonstrate that differences in the relative physiological capacity between predator and prey can impose strong non-linear effects on temperature-response curves of attack rate. In the second part of the thesis, we study the role of predation along a benthic-pelagic habitat gradient in promoting divergence and resource polymorphism among prey. We show that presence of a large piscivorous predator, the northern pike (Esox lucius), induces dwarfs, giants or divergence into both ecotypes in populations of European whitefish (Coregonus lavaretus) depending on lake characteristics. Using dated introductions of whitefish as controlled natural experiments, we show that pike presence induces rapid life history divergence between pelagic and littoral habitat use strategies, and that this divergence can translate into partial reproductive isolation in a matter of decades. Our results demonstrate the potential for thresholds in a crucial ecological rate, setting the stage for tipping points with potentially far reaching implications for effects of warming on predator prey dynamics and ecosystem stability. Moreover, they illustrate the potentially drastic consequences of such tipping points by demonstrating the importance of a single predator species as a driving force behind the creation and maintenance of biodiversity in a natural system.