title: European shag diet and demography at a North Sea colony over half a century of environmental change
Creators: name: Howells, RJ
Publisher: name: University of Liverpool Repository
Year: 2019

description: The earth’s climate is changing rapidly, with pronounced impacts observed in all well studied ecosystems throughout the globe. Climate-mediated impacts are particularly marked and rapid within marine ecosystems. At the apex of such systems, marine top predators, such as seabirds, are vulnerable to direct and indirect climate-mediated effects, which may alter foraging capacity and prey availability. Consequently, many seabird populations are displaying pronounced changes in diet composition and declining demographic trends, yet the mechanisms underpinning these responses remain largely untested. Although many studies have quantified seabird diet during the breeding season, the climate-related mechanisms determining long-term trends in diet composition are less well understood. Further, as the majority of seabird mortality occurs during winter, the diet of seabirds in winter is a key knowledge gap. Demographic trends have also been observed in numerous seabird species but, as with diet, we have limited understanding of the key mechanisms whereby climate is driving these trends. Finally, analyses of climate have focussed on broad scale processes, yet variation in microclimate may also be a key determinant of fine-scale distribution and demography within seabird colonies, yet this potential factor has been overlooked. As a long-lived species with highly variable demography and plastic foraging habits, the European shag Phalacrocorax aristotelis provides an ideal species to investigate the impacts of environmental change on marine top-predator demography. This thesis uses demographic and diet datasets collected from a shag population breeding on the Isle of May, southeast Scotland, over half a century. By combining these data with environmental covariates, collected over a range of temporal and spatial scales, I quantify the response of this population to pronounced environmental change within the North Sea over the last five decades. My thesis reveals that the diet of nestling shags in this population has changed dramatically over this period, from an almost complete dependence on lesser sandeel Ammodytes marinus to a range of prey types. Crucially, a suite of environmental covariates, including daily wind and long-term ocean warming, has contributed to this change in diet. The diet in winter displayed similar temporal trends (reduced sandeel and greater diversity). However, I found that the reduction in sandeel occurrence was more marked during the non-breeding period, with potential demographic consequences. I also documented a substantial increase in productivity and rapid phenological advancement over the last half a century. Crucially, the productivity trend was linked to this advancing phenology, which in turn was determined by conditions experienced in late winter and in the previous breeding season. Finally, pronounced fine-scale distributional trends occurred in this colony, with an increasing proportion of individuals breeding on the north-east side of the island, showing more rapid improvements in reproductive output than the declining sub-colonies in the south-west. This redistribution may benefit the population since exposure is a key factor in productivity and the prevailing wind direction is westerly. Overall, these results suggest that substantial dietary and demographic plasticity in shags may confer some resilience in this species to predicted future climate-mediated environmental change.