Population Regulation in Magellanic Penguins: What Determines Changes in Colony Size?

POZZI, L.; GARCIA BORBOROGLU, P.; BOERSMA, D.; PASCUAL, M.A.

PLOS ONE

PUBLIC LIBRARY SCIENCE

Lugar: San Francisco; Año: 2015 vol. 10

1932-6203

Seabirds are often studied at individual colonies, but the confounding effects of emigrationand mortality processes in open populations may lead to inappropriate conclusions on themechanisms underlying population changes. Magellanic penguin (Spheniscus magellanicus)colonies of variable population sizes are distributed along the Argentine coastline. Inrecent decades, several population and distributional changes have occurred, with somecolonies declining and others newly established or increasing. We integrated data of eightcolonies scattered along*600 km in Northern Patagonia (from 41°26´S, 65°01´W to 45°11´S, 66°30´W, Rio Negro and Chubut provinces) and conducted analysis in terms of theirgrowth rates, production of young and of the dependence of those vital rates on colony age,size, and location. We contrasted population trends estimated from abundance data withthose derived from population modeling to understand if observed growth rates were attainableunder closed population scenarios. Population trends were inversely related to colonysize, suggesting a density dependent growth pattern. All colonies located in the north?which were established during the last decades?increased at high rates, with the smallest,recently established colonies growing at the fastest rate. In central-southern Chubut, wherecolonies are the oldest, the largest breeding aggregations declined, but smaller colonies remainedrelatively stable. Results provided strong evidence that dispersal played a majorrole in driving local trends. Breeding success was higher in northern colonies, likely mediatedby favorable oceanographic conditions. However, mean foraging distance and body conditionof chicks at fledging were influenced by colony size. Recruitment of penguins in thenorthern area may have been triggered by a combination of density dependence, likely exacerbatedby less favorable oceanographic conditions in the southern sector. Our resultsreaffirm the idea that individual colony trends do not provide confident indicators of populationhealth, highlighting the need to redefine the scale for the study of population changes.