Abstract
We evaluated annual and regional variation in the dietary niche ofPygoscelispenguins including the sea ice-obligate Adélie penguin (Pygoscelis adeliae), and sea ice-intolerant chinstrap (Pygoscelis antarcticus) and gentoo (Pygoscelis papua) penguins, three species that nest throughout the western Antarctic Peninsula (AP) to test thesea ice trophic interaction hypothesis, which posits that penguin breeding populations with divergent trends, i.e., declining or increasing, are reliant on differing food webs. Our study relies on values of naturally occurring carbon (13C/12C, δ13C) and nitrogen (15N/14N, δ15N) stable isotopes as integrated proxies of penguin food webs measured over three years at three different breeding colonies. At Anvers Island in the north, where reductions in sea ice and changes in breeding population trends among sympatric sea ice-obligate (Adélie) and sea ice-intolerant (chinstrap and gentoo) penguins have been most notable, our analyses show that all three species ofPygoscelispenguins became more similar isotopically over the reproductive period. By late chick-rearing at Anvers Island, crèched chicks at 5-weeks-old for all species occupied similar trophic positions. Isotopic mixing models indicated that the proportions of prey provisioned by adult penguins to 5-week-old chicks at Anvers Island were generally similar across species within years, consisting primarily of Antarctic krill (Euphausia superba). Crèched Adélie chicks had higher δ13C and δ15N values at Avian and Charcot Islands, southern breeding colonies where sea ice is more prominent and populations of Adélie penguins have increased or remain stable. Trophic position increased with latitude, while the proportions of prey provisioned by Adélie penguin adults to chicks at southern breeding colonies included species typical of high Antarctic marine food webs, especially crystal krill (Euphausia crystallorophias). A Bayesian metric for dietary niche width, standard ellipse area (SEA-B), indicated thatPygoscelispenguins with greater population changes in the north had more variability in dietary niche width than stable populations further south. Our results lend insight on marine food web drivers ofPygoscelispenguin reproduction at the regional scale and question the long-standing paradigm that Antarctic krill are the only food web component critical to penguin reproductive survival in this region of the Southern Ocean.
Highlights
Polar marine ecosystem studies have demonstrated broad correlations between climate-driven variation in physical oceanography and associated change in the abundance and distribution of key trophic levels including predator populations (Grebmeier et al, 2006; Ducklow et al, 2007)
Elucidating relationships between sea ice change and associated food web effects on penguin demography has been highlighted as a key component to understanding population responses to climate variability in the Southern Ocean (Forcada and Trathan, 2009)
Our study considered regional breeding population trends among western Antarctic Peninsula (AP) Pygoscelis penguins from an integrated food web perspective based on variation in δ13C and δ15N values of penguin blood tissue measured at the individual level, as well as stable isotopes (SI) values of dominant prey
Summary
Polar marine ecosystem studies have demonstrated broad correlations between climate-driven variation in physical oceanography and associated change in the abundance and distribution of key trophic levels including predator populations (Grebmeier et al, 2006; Ducklow et al, 2007). Sea ice was recognized as important in mediating many trophic interactions, the physical extent of this critical winter habitat was considered the key ecological factor that allowed penguins to fully exploit their prey, Antarctic krill (Euphausia superba) (Fraser et al, 1992) This paradigm of species’ opposing life history affinities for the presence of sea ice has been invoked to explain population change, at range margins, by Antarctic penguins including gentoo (Pygoscelis papua) (Ducklow et al, 2007; Forcada and Trathan, 2009; but see Trivelpiece et al, 2011) and emperor (Aptenodytes forsteri) (Trathan et al, 2011) penguins of the western AP. Few studies have considered this issue from an integrated food web perspective as most have exclusively focused on variability in Antarctic krill stocks as the mechanistic link between climate and sea ice change in this region of the Southern Ocean and facets of Pygoscelis penguin demography that underlie breeding population change (Fraser and Hofmann, 2003; Hinke et al, 2007; Trivelpiece et al, 2011; Ducklow et al, 2013)
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