| Summary: | The authors employed a "natural experiment" to evaluate the hypothesis that a major physical feature of high-latitude marine habitat, the percentage of the sea covered by pack ice, affects species composition among Antarctic seabirds. he 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 and repeatedly over a 200-km-wide area. egardless of where their habitat moved, pack-ice and open-water species occurred at significantly higher densities in the ice and open-water habitats, respectively. here were no time lags in the response of species to habitat alteration. n addition, pack-ice and open-water species had identical diets regardless of where their preferred habitat was located. hese results supported the hypothesis and showed that physical rather than biological variables affect species composition among pelagic assemblages of Antarctic sea-birds. esults supported the conclusion that a lack of appropriate adaptations constrain open-water species to reside away from the pack ice and that unremarkable prey availability fails to attract pack-ice species to open water.
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