Date of Award:


Document Type:


Degree Name:

Master of Science (MS)


Watershed Sciences

Committee Chair(s)

Edward Hammill


Edward Hammill


Timothy Walsworth


Michael Cortez


To grasp the functioning and stability of ecosystems, it is important to understand species interactions. With many ecosystems becoming more imperiled from urbanization and anthropogenic influences it is important to understand ways in which species can adapt to rapid changes in their environment. Phenotypic plasticity is one such tool at nature’s disposal to initiate rapid change, where species with the same genetic makeup can have different expressed traits depending on their environment. Inducible defenses are one such form of phenotypic plasticity in which prey can express different levels and forms of defense depending on the threat of predation present in their environment. In this thesis, I work to determine the mechanisms by which P. aurelia balance the costs and benefits of producing defenses through the manipulation of predator and prey densities to encourage a better understanding of this form of phenotypic plasticity. Using an experimental framework, I show that prey density leads to a reduction in base morphology but may be linked to increased defense induction in this protist. At the top of the food chain predators are important in controlling prey density and increasing mortality of predators can reduce the constraints on prey growth leading to a cascading effect through a food chain. However, adaptable prey responses to predation can lead to counterintuitive reactions of the trophic levels to predator mortality. Furthermore, the death of predators plays a key role in the cycling of nutrients in a system. Energy flows up the trophic levels of a food chain through consumption, but recycling of dead biomass and excretion allows for some of those resources to be reclaimed by the lowest trophic level. In this thesis, I also investigate inducible defenses in a theoretical setting to better understand how adaptable traits may interact with nutrient recycling and foraging costs to influence responses to predator mortality and system stability. I found that nutrient recycling led to an increased negative response of predators to their own mortality while also providing an observable increase to predator density due to bottom-up. Overall, I further our understanding of inducible defenses in natural and theoretical settings.