Improving representations of higher trophic-level species in models : using individual-based modelling and dynamic energy budget theory to project population trajectories of southern elephant seals

Higher trophic-level marine predators are of particular interest to ecologists, as these species can be used to observe direct and indirect effects of changes to ecosystem dynamics and physical environments. The population dynamics of these species and their breeding and foraging traits can inform ecosystem-based fisheries management, such as that which has been adopted by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) for the Southern Ocean. This thesis presents and applies a framework for representing marine mammals in ecosystem models, using dynamic energy budget (DEB) theory incorporated in individual-based models (IBMs). It investigates effects at a population level while taking into account individual traits, as well as energetic requirements of the selected species. The model is the first of its kind for a large marine mammal. Chapter 1 justifies the selection of the southern elephant seal Mirounga leonina as the study species, for which good demographic data are available, and summarises the thesis approach. Chapter 2 discusses how detailed representations of apex (non-fish) predators are generally lacking from ecosystem models. This is likely due to the challenges involved in representing complex life histories (i.e. capital or income breeders, intermittent reproduction, site fidelity) of mammals and birds in ecosystem models. This chapter uses a nested qualitative network model to show that the level of detail with which higher trophic-level species are represented in ecosystem models can have implications for ecosystem-level predictions. Chapter 3 develops a DEB-IBM for southern elephant seals, using data from longitudinal studies from Macquarie Island. This model i) simulates energy use and life histories, as well as breeding traits in an emergent manner, ii) projects a stable population over time, and iii) has realistic population dynamics and structure based on emergent life history features. Chapter 4 investigates whether the DEB-IBM can assist in evaluating hypotheses regarding drivers of the decline of the southern elephant seal population on Macquarie Island. Results from this chapter suggest a number of interacting drivers for population decline. Finally, Chapter 5 discusses and addresses limitations of the model. It discusses future versions of the model that could be made spatially explicit; include males; be adjusted for other species; and be incorporated in end-to-end ecosystem models to improve higher trophic level species representations.