| Summary: | Unraveling the proximate physiological, ecological and evolutionary mechanisms that underlie population responses to environmental variability is an increasingly important challenge facing conservation biologists and is the primary motivation of this thesis. I studied Pygoscelis penguins that breed west of the Antarctica Peninsula (AP). Here, breeding populations of Adélie (P. adeliae), chinstrap (P. antarctica) and gentoo (P. papua) penguins are undergoing pole-ward range shifts that correlate with long-term warming trends of the region. However, mechanistic relationships that link marine ecosystem variability with demographic parameters that drive population change are rarely quantified. My data support the following conclusions. At Anvers Island, where reductions in sea ice have been notable, Pygoscelis penguins became more similar isotopically throughout the breeding season, due to a depletion in δ13C and δ15N of blood tissue. Here, crèched chicks of all species occupied similar trophic levels and isotopic mixing models indicated that the proportions of prey provisioned to crèched chicks were generally similar across species within years. Crèched Adélie chicks were isotopically enriched at Avian and Charcot Islands, southern rookeries where sea ice is more prominent, and their trophic level increased with latitude. Penguin food-webs are isotopically wider in regions with greater sea ice coverage. Molecular primers (P2/P8) amplified DNA of all species to discern sex. At Anvers Island, I detected little evidence of trophic foraging influencing body and egg mass of adult penguins and chick body mass. However, regional analyses suggested that a provisioning diet enriched in δ15N, such as that found at southern rookeries, resulted in heavier five-week-old Adélie penguin chicks. Corticosterone hormone did not mediate individual variation in Pygoscelis breeding performance. I found no overall genetic structure among Adélie penguins based on microsatellite and mitochondrial DNA markers, however, pairwise comparisons including Charcot Island were significant. Gene flow was asymmetrical from the species’ regional core to its northern range. In conclusion, variability in sea ice-associated food-webs of the western AP is an important determinant of Pygoscelis penguin reproductive performance, but not at the physiological level. A lack of genetic structure implicates dispersal as an important demographic factor that may structure population responses to future ecosystem change.
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