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Effects of Productivity on Aquatic Food Webs

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Title: Effects of Productivity on Aquatic Food Webs
Author: Ward, Colette
Department: Department of Integrative Biology
Program: Integrative Biology
Advisor: McCann, Kevin
Abstract: Ecologists have long sought to understand the effects of productivity on community structure, and the question remains an issue of pressing importance given contemporary patterns of anthropogenic change. Extensive debate has revolved around bottom-up (resource limitation) and top-down (predation) hypotheses for community response to productivity, with the latter now dominating our conceptualization of this question, especially in aquatic ecosystems. Key to this discourse is the principle that top-down control is a fundamental response of communities to rising productivity, and, in the absence of bottlenecks to energy flux, becomes stronger across productivity gradients. I argue that this principle, when projected onto commonly occurring food web motifs, readily predicts (a) common violations of assumptions of classical top-down hypotheses, and (b) that community responses to rising productivity are not conserved across productivity gradients, but are instead context-dependent. Here I use this principle to predict and explore the context-dependent nature of aquatic food web structure across large gradients of productivity. I show that food webs increasingly depart from the fundamental assumptions of classical top-down hypotheses with rising productivity (i.e. strong consumer-resource (especially producer-herbivore) interactions, linearity and singularity of food chains, rarity of omnivory, and productivity-driven lengthening of food chains). Alternative energy channels which arise with productivity subsidize generalist predators, which in turn mediate community structure. I demonstrate this at the scale of large, whole-ecosystem food webs, where increasing productivity is directed into bottom-up controlled detritus channels as primary producers become less edible, subsidizing generalist predators, which in turn exert top-down control on herbivores in an apparent trophic cascade. I also demonstrate this phenomenon in food webs at a sub-ecosystem scale, where subsidies from an alternative energy channel facilitate community compositional turnover from edible to less edible consumers across productivity gradients. I further show that rising productivity causes food chain length to decline as increasing energy flux begets top-heavy biomass pyramids, which favour omnivory. Overall this thesis suggests that, in contrast to conventional thinking, mechanisms of community response to productivity are not conserved across productivity gradients, and instead may be readily predicted by a simple framework for energy flux.
Date: 2014-04
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