Climate change is a naturally occurring event. However, within the past several decades, anthropogenic factors have caused this natural process to accelerate (Karlén 1997). Warming trends associated with climate change have impacted the Antarctic marine ecosystem by influencing the extent of annual sea ice cover (Croxall et al. 1988). Over the past 20-50 years, warming trends have caused a decrease in the amount of sea ice surrounding Antarctica (Poncet and Poncet 1987). Antarctic krill, a shrimp-like organism towards the bottom of the Antarctic marine food chain, is a staple dietary component for top marine predators whose distribution is influenced by sea ice. Observations and diet sampling have shown that Antarctic krill is the primary component of Adelie penguin (Pygoscelis adeliae) diet (Ainley et al. 1984). In addition to Antarctic krill, mesopelagic squid (Psychroteuthis glacialis) and two Antarctic fish (Pleuragramma antarcticum and Electrona antarctica) also are important components of the Adelie penguin diet (Emslie et al. 1998).
Early dietary studies of Adelie penguins suggested that krill contibuted upwards of 90% of the breeding season diet (Volkman et al. 1980; Lishman 1985). However, a study by Ridoux and Offredo (1989) found that the diet of northern breeding localities of Adelie penguins consisted of nearly 100% krill; whereas, southern breeding localities relied more equally on both krill and fish. The contribution of krill in the diet of penguins at southern breeding localities had also decreased to 60% in comparison to previous studies by (Volkman et al. 1980) and Lishman (1985). In addition, Ridoux and Offredo (1989) found that the contribution of fish in the diet of penguins at southern breeding localities increased, in comparison to previous studies (Volkman et al.1980; Lishman 1985), from nil to nearly 39%. These authors proposed that penguins at southern breeding localities compensated for krill scarcity by altering their diet to include more fish. The proportion of these prey items in past Adelie diet appears to vary with climate change (Emslie et al.1998). Adelie penguin populations and other krill-dependent species have fluctuated over the last 50 years in response to these environmental and dietary changes (Fraser et al. 1992). However, detailed information on these fluctuations is lacking. Here we test the hypothesis that prey diversity in Adelie penguin colonies varies in relationship to past climates.
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