A predator’s perspective on causal links between climate change, physical forcing and ecosystem response

The mechanisms by which variability in sea ice cover and its effects on the demography of the Antarctic krill Euphausia superba cascade to other ecosystem components such as apex predators remain poorly understood at all spatial and temporal scales, yet these interactions are essential for understanding causal links between climate change, ecosystem response and resource monitoring and management in the Southern Ocean. To address some of these issues, we examined the longterm foraging responses of Adélie penguins Pygoscelis adeliae near Palmer Station, western Antarctic Peninsula, in relation to ice-induced changes in krill recruitment and availability. Our results suggest that (1) there is a direct, causal relationship between variability in ice cover, krill recruitment, prey availability and predator foraging ecology, (2) regional patterns and trends detected in this study are consistent with similar observations in areas as far north as South Georgia, and (3) largescale forcing associated with the Antarctic Circumpolar Wave may be governing ecological interactions between ice, krill and their predators in the western Antarctic Peninsula and Scotia Sea regions. Another implication of our analyses is that during the last 2 decades in particular, krill populations have been sustained by strong age classes that emerge episodically every 4 to 5 yr. This raises the possibility that cohort senescence has become an additional ecosystem stressor in an environment where ice conditions conducive to good krill recruitment are deteriorating due to climate warming. In exploring these interactions, our results suggest that at least 1 ‘senescence event’ has already occurred in the western Antarctic Peninsula region, and it accounts for significant coherent decreases in krill abundance, predator populations and predator foraging and breeding performance. We propose that krill longevity should be incorporated into models that seek to identify and understand causal links between climate change, physical forcing and ecosystem response in the western Antarctic Peninsula region.