Factors affecting body mass of prefledging emperor geese

Abstract

Body mass of pre fledging geese has important implications for fitness and population dynamics. To address whether interspecific competition for forage was broadly relevant to prefledging emperor geese, I investigated the factors affecting body mass at three locations across the Yukon-Kuskokwim Delta, Alaska. From 1990 - 2004, densities of cackling geese more than doubled and were 2-5x higher than densities of emperor geese, which were relatively constant over time. During 2003 - 2004, body mass of emperor geese increased with net above-ground primary productivity (NAPP) and grazing lawn extent and declined with interspecific densities of geese (combined density of emperor and cackling geese). Grazing by geese resulted in consumption of 90% of the NAPP that occurred during the brood rearing period, suggesting that interspecific competition was due to exploitation of common food resources. At six sampled locations, grazing lawn extent varied among- and within-locations, and was stable or declined slightly during 1999-2004, indicating reduced per capita availability. I conclude that negative effects of interspecific goose densities on body mass of pre fledging geese are partially responsible for recent declines in the fall age ratio of emperor geese because of a positive correlation between body mass and survival to fall staging areas. Management to increase the population size of emperor geese should consider interspecific densities of geese and interactions between interspecific densities and forage.