The dynamics of size-structured ecosystems

Understanding the function of ecosystems and their response to human and environmental impacts is necessary for improved management of the aquatic environment. Size-based predation and the very wide scope for growth in many aquatic species are fundamental processes that define the structure and function of aquatic ecosystems. In this thesis, I focus on marine ecosystems, where widespread over-fishing has led to profound changes in size-structure resulting from the removal of large predatory fish.

The relative importance of natural processes, environmental effects, and exploitation on size-structure are often difficult to quantify. Here, fishing effects are shown to have more influence than sea temperature on the observed temporal changes in fish community size-structure, using the Celtic Sea as an example. Distinct trophic interactions can also lead to different patterns in community sizestructure, even in the absence of exploitation. I show that different patterns in the relationship between abundance and body-mass of individual organisms, sampled in the North Sea, emerge from communities that are dominated by predators as opposed to organisms competing for a shared resource. This finding is validated with a dynamic size-spectrum model that accounts for trophic interactions within and across the two distinct size-structured communities.

The degree of coupling between the two communities alters its response to human induced perturbations and overall ecosystem stability. Furthermore, a large difference between the sizes of predators and their preferred prey is destabilising and produces oscillations in size-spectra models. Empirical evidence for a relationship between the observed temporal variability of 26 fish populations and their respective mean predator-prey mass ratios is consistent with the theoretical findings reported here. Identifying these and further mechanisms that control natural variability of populations and communities will help to inform natural resource managers of the necessary levels of precaution required to support an ecosystem-based approach.