POPULATION CHANGES IN MAGELLANIC PENGUINS: WHAT FACTORS DETERMINE COLONY TRENDS?

POZZI, LUCIANA; BOERSMA, DEE; PASCUAL, MIGUEL; GARCÍA BORBOROGLU, PABLO

Bristol

Conferencia; 8va. International Penguin Conference; 2013

University of Bristol

Population changes result from several interrelated factors influencing abundance, but disentangling their relative effects is a challenging goal. Magellanic penguin (Spheniscus magellanicus) colonies are broadly distributed along the Patagonian coastline. Breeding aggregations vary in size, ranging from a few breeding pairs (new colonies) to hundreds of thousands (largely established colonies). New colonies have extended the distribution range of Magellanic penguins northward. To understand what factors are driving these population and distributional changes, we collected and integrated ecological, reproductive and demographic data simultaneously from eight Magellanic penguin colonies scattered along North and Central Patagonia from 41°26´S,65°01´W to 45°11´S,66°30´W. We assessed population trends based on abundance data and contrasted them with those derived from population models, to understand if the observed growth rates were possible from internal dynamics of the colonies, or if were they more consistent with metapopulation dynamics. We evaluated the local productivity of colonies by estimating breeding success and weighting chicks at fledging. We determined foraging trips distances using satellite tags to track breeding adults at sea and using published information. Northern populations increased steadily, with annual rates ranging from 1.09 to 3.11, with the smaller, most recently established colonies growing the fastest. In Central Patagonia the largest colonies declined at growth rates between 0.95 to 0.99, but smaller populations were stable for the last 15 years. Despite interannual variability, mean breeding success was higher in the North (range 0.74-1.27) than in Central Patagonia (range 0.12-0.95). Chicks fledged were heavier from smaller colonies (Mean=3164kg) than from larger ones (Mean= 2643). Foraging trip distances were shorter in smaller colonies. Overall, mean trip distance increased with colony size (R2=0.81, p-value=0.015). The comparison among population trends revealed that the steady increase in northern colonies could not be sustained by internal dynamics. These results provide evidence that immigration played a major role, suggesting colonies are not closed and are controlled by metapopulation dynamics. Factors triggering migration remain unclear but the proximity of food might be tightly related making the smaller, northern populations more attractive to potential recruits. Finally, we cannot discard that the largest populations from Central Patagonia may have declined due in part to density dependent processes.