Uncertainty over the role of top-down and bottom-up forces influencing marine top-predator populations often constrains their use as indicators of marine-ecosystem change. This chapter reviews historic and contemporary data to explore how different potential drivers have shaped abundance trends and distribution of the North Atlantic fulmar Fulmaris glacialis. Previously, debate on the causes underlying this classic example of range expansion has centred on alternative hypotheses; each championing single drivers of change. In contrast, studies now suggest that fulmar populations have responded to multiple drivers, each with varying influence depending both upon the population parameters being investigated, and the scale at which these investigations are made. These findings highlight how attempts to identify a single driver of change may be misplaced, and efforts should instead be made to understand how different drivers interact to influence the dynamics of these and other marine top predators.

Many marine top predators have shown dramatic changes in abundance over the last century, potentially providing useful clues to the state of the marine environment. However, in most cases, there is uncertainty over the drivers of observed abundance changes, constraining attempts to predict future trends or to incorporate such information into ecosystem management. Most fundamentally, it is often unclear whether changes are driven by top-down processes or bottom-up influences on food supplies (e.g. Springer et al. 2003, Trites & Donnelly 2003).