SPECIES-HABITAT RELATIONSHIPS AMONG ANTARCTIC SEABIRDS - A FUNCTION OF PHYSICAL OR BIOLOGICAL FACTORS

We employed a ''natural experiment'' to evaluate the hypothesis that a major physical feature of high-latitude marine habitat, the percentag e of the sea covered by pack ice, affects species composition among An tarctic seabirds. Our experiment entailed replicate transects through markedly altered physical habitat in the Scotia-Weddell Confluence: a series of storms caused the pack ice to advance and retreat rapidly an d repeatedly over a 200-km-wide area. Regardless of where their habita t moved, pack-ice and open-water species occurred at significantly hig her densities in the ice and open-water habitats, respectively. There were no time lags in the response of species to habitat alteration. In addition, pack-ice and open-water species had identical diets regardl ess of where their preferred habitat was located. These results suppor ted the hypothesis and showed that physical rather than biological var iables affect species composition among pelagic assemblages of Antarct ic seabirds. Results supported the conclusion that a lack of appropria te adaptations constrain open-water species to reside away from the pa ck ice and that unremarkable prey availability fails to attract pack-i ce species to open waters.