Divergent trophic responses of sympatric penguin species to historic anthropogenic exploitation and recent climate change

The Southern Ocean is in an era of significant change. Historic overharvesting of marine mammals and recent climatic warming have cascading impacts on ecosystem function, including shifting environmental conditions and altered species interactions. Pygoscelis penguins act as sensitive indicators of these ecosystem changes in Antarctica. We examined trophic responses of sympatric chinstrap (P. antarctica) and gentoo (P. papua) penguins to nearly 100 years of shared environmental change in the Antarctic Peninsula region using compound-specific isotope analyses of historic museum specimens. Our molecular isotope approaches allow us to disentangle consumer trophic dynamics and baseline biogeochemical cycling using differentially fractionating amino acid nitrogen isotopes. We found that a century ago, gentoo penguins fed almost exclusively on low trophic level prey, such as krill, during the peak of historic overexploitation of marine mammals, which was hypothesized to have produced a “krill surplus.” In the last 40 years, gentoo penguin trophic position has increased a full level as krill declined in response to recent climate change, increased competition from recovering marine mammal populations, and the development of a commercial krill fishery. A shifting isotopic baseline supporting gentoo penguins suggests a concurrent increase in coastal productivity over this time as waters warm and ice decreases. In contrast, chinstrap penguins showed no changes in trophic position, despite changes in krill availability over the past century. The specialized foraging niche of chinstrap penguins likely makes them more sensitive to changes in krill availability, relative to gentoo penguins, as evinced by their declining population trends in the Antarctic Peninsula over the past 40 years. Over the next century, similarly divergent trophic and population responses are likely to occur among Antarctic krill predators if climate change and other anthropogenic impacts continue to favor generalist over specialist species.