Advancing the Sea Ice Hypothesis: Trophic Interactions Among Breeding Pygoscelis Penguins With Divergent Population Trends Throughout the Western Antarctic Peninsula

Abstract

We evaluated annual and regional variation in the dietary niche of Pygoscelis penguins 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 the sea 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 of Pygoscelis penguins 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 that Pygoscelis penguins 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 of Pygoscelis penguin 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.

Cyclical prey shortages for a marine polar predator driven by the interaction of climate change and natural climate variability

“…Adélies are ice‐obligate in winter, and previous work has suggested that declines in winter sea ice trigger declines in Adélies (Fraser et al 1992; Forcada and Trathan 2009; Gorman et al 2021). Therefore, we examined whether availability of sea ice could explain cycling in Adélie populations.…”

“…Modeling work has suggested that more fish in the diet promotes Adélie chick growth and survival (Chapman et al 2011), but existing empirical work exploring this hypothesis is limited. Research spanning the summers of 2008-2010 found that chicks located at a more southern colony on the western Peninsula, where they were provisioned with a higher trophic level diet (i.e., a greater ratio of fish to krill), had higher fledging weights than chicks around Anvers Island (Gorman 2015). However, within each site, the proportion of fish in the diet had no relationship with chick fledging weight.…”

Spatial link between Adélie penguin foraging effort and krill swarm abundance and distribution

“…Adeĺie penguins (Pygoscelis adeliae) are a key indicator species under CEMP because they are important consumers of krill (Croxall et al, 2002). However, since the establishment of CEMP, there is increasing recognition Adeĺie penguin diets are more diverse than traditionally believed, with substantial spatial and temporal variability in the proportion of krill and fish consumed (Clarke et al, 2002;Ainley et al, 2003;Lynnes et al, 2004;Tierney et al, 2009;Gorman et al, 2021). The diet of Adeĺie penguin populations in the Scotia Sea, East Antarctic and regions of the Antarctic Peninsula are generally dominated by krill, while populations in the Ross Sea have a more varied diet and (Ribic et al, 2008) consume a mixture of fish and euphausiid species (Ratcliffe and Trathan, 2012).…”

Light-level geolocation reveals the short-distance non-breeding movements and distribution of tufted puffins throughout the Northeast Pacific Ocean

“…In this context, it might be expected that seabird populations would be particularly resilient to variation in the marine environment. However, seabirds are among the most threatened vertebrates (Dias et al, 2019), and a large literature has documented changes in demographic rates, abundance, or community structure with ocean climate variability (La Cock, 1986;Ainley et al, 1994;Montevecchi and Myers, 1997;Gjerdrum et al, 2003;Morrison et al, 2011;Oro, 2014;Gorman et al, 2021) with recent reports of mass mortality events in association with extreme marine heatwaves (Jones et al, 2018;Jones et al, 2019;Piatt et al, 2020;Arimitsu et al, 2021).…”