Food Web Ecology -- individual life-histories and ecological processes shape complex communities
Abstract: This thesis sets out a food web framework for size-structured populations. The framework enables an ecological approach to food web modelling as the individual life-history from birth, through maturation, and ultimately death is explicitly resolved with the use of bioenergetics based on individual body size. Each population resolves size-structure through a size-spectrum containing the individual abundance as a continuous function of body size. Individuals select prey items of a suitable size, which can be popularised as "big ones eat smaller ones". This allows individuals to change diet throughout life (life-history omnivory). In the framework individual food consumption leads to growth in body size and allocation to reproduction, which drives the population dynamics as opposed to instantaneous population increase in unstructured food web models. Paper I introduces the framework and shows how a simple realistic parametrisation is possible when a trait-based species characterisation is used. An analytical approximation of the food web framework is derived, and validated through comparison with dynamic simulations. Paper II extends the dynamic framework by also considering space, and demonstrates how large food webs can be formed through sequential community assembly. The resulting communities resemble the topology of natural food webs as well as complying with empirical data on diversity and biomass distributions -- demonstrating that individual-level food encounter and prey-selection from the rule "big ones eat smaller ones" lead to complex and realistic food webs. Paper III uses the analytical solution of the framework to show the conditions under which the many-small-eggs strategy of the fishes is a viable strategy. Paper IV utilises the trait-based species description to show that coexisting species pairs involved in intraguild predation exist for all resource levels. The model thus explains empirically observed coexistence at high resource levels contrary to contemporary theoretical models. Paper V demonstrates how harvesting initiates a trophic cascade that may propagate both downwards and upwards in trophic levels, and that the harvesting pattern may influence whether or not trophic cascades are empirically detected. In Paper VI the analytical solution is used to provide a theoretical understanding of empirically observed relationships between natural mortality, growth, and production rates.
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