Adélie penguins have been breeding on the West Antarctic Peninsula for centuries, but the region is one of the fastest warming places on Earth. Over the last couple of decades, this changing climate has ushered in a newcomer: gentoo penguins, who enjoy the same krill. Now, researchers studying why Adélie populations have abruptly dropped reveal that increased competition with gentoo penguins hasn’t exacerbated their declines. The two coexist by foraging in different habitats, according to findings published in Scientific Reports this week.
In the 1970s, there were 15,000 breeding pairs of Adélie penguins (Pygoscelis adeliae) around Palmer Station on Anvers Island. Today, only a few thousand pairs are left. These penguins are migratory, leaving their breeding colony during the winter for life at sea. Meanwhile, gentoo penguins (Pygoscelis papua) stay at the breeding colony all winter. During the summer, they live less than 20 kilometers (12.4 miles) apart.
Because rapid warming in the area is happening at the same time as gentoo increases and Adélie decreases, a team led by Megan Cimino from the University of Delaware wanted to test for competition between the two during the chick-rearing period. They compared how the penguins foraged for Antarctic krill (Euphausia superba) from January 4 to 31 in 2011. The team attached satellite transmitters to 10 Adélie penguins from Humble Island and eight gentoo penguins from Biscoe Point. During this phase of their breeding cycle, when the parents are provisioning for chicks, the foraging ranges of the two may overlap.
The team also deployed a propeller-driven, autonomous underwater vehicle called the REMUS for 11 days during the study period. The REMUS swims about as quickly as a penguin and can dive about as deep. And it was equipped with sensors to measure things like temperature, light, salinity, chlorophyll, and relative acoustic backscatter – which helps identify aggregations of penguin prey. Zooplankton aggregate in groups of varying densities, and penguins typically search for those rare, high concentrations.
Krill aggregations were detected within the foraging ranges (horizontally and vertically) of both penguin species. These highly mobile prey must balance the need to consume food with predation avoidance when picking their locations: someplace dark with just enough phytoplankton to eat.
For the most part, Adélie and gentoo penguins foraged in different places. Both can forage down to depths of over 150 meters (492 feet) below the surface, though the Adélies mostly stayed in the upper 50 meters (164 feet). Meanwhile, gentoo penguins generally foraged in the upper 100 meters (328 feet), but went as deep as 150 meters. This might have to do with differences in body size, buoyancy, oxygen stores, and how they attack krill.
But an interesting thing happened in the places where foraging areas of the two penguins overlapped: Four of the gentoos switched to foraging at deeper depths, further limiting competition with the Adélies above.
Declines in Adélie penguins along the West Antarctic Peninsula, the authors write, are more likely due to direct and indirect climate impacts on their life histories.
Chris Linder
Image in the text: Megan Cimino/University of Delaware
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