Abstract
Penguins are a monophyletic group in which many species are found breeding sympatrically, raising questions regarding how these species coexist successfully. Here, the isotopic niche of three sympatric pygoscelid penguin species was investigated at Powell Island, South Orkney Islands, during two breeding seasons (austral summers 2013–2014 and 2015–2016). Measurements of carbon (δ13C) and nitrogen (δ15N) stable isotope ratios were obtained from blood (adults) or feather (chicks) samples collected from Adélie Pygoscelis adeliae, chinstrap P. antarctica, and gentoo P. papua penguins. Isotopic niche regions (a proxy for the realized trophic niches) were computed to provide estimates of the trophic niche width of the studied species during the breeding season. The isotopic niche regions of adults of all three species were similar, but gentoo chicks had noticeably wider isotopic niches than the chicks of the other two species. Moderate to strong overlap in isotopic niche among species was found during each breeding season and for both age groups, suggesting that the potential for competition for shared food sources was similar during the two study years, although the actual level of competition could not be determined owing to the lack of data on resource abundance. Clear interannual shifts in isotopic niche were seen in all three species, though of lower amplitude for adult chinstrap penguins. These shifts were due to variation in carbon, but not nitrogen, isotopic ratios, which could indicate either a change in isotopic signature of their prey or a switch to an alternative food web. The main conclusions of this study are that (1) there is a partial overlap in the isotopic niches of these three congeneric species and that (2) they responded similarly to changes that likely occurred at the base of their food chain between the 2 years of the study.
Highlights
The coexistence of sympatric species and the extent to which their ecological niches overlap are fundamental issues in both theoretical and applied ecology (Silvertown, 2004; Vellend, 2010)
A moderate to strong overlap was measured in the isotopic niches, and assumed in the trophic niches, of the three species, both in adults and chicks
This could be a consequence of a more diverse diet among individual gentoo penguins compared to Adélie or chinstrap penguins and supporting what has been found for that species at other study sites (Camprasse, Cherel, Bustamante, Arnould, & Bost, 2017; Lescroel, Ridoux, & Bost, 2004; Polito et al, 2015; Ratcliffe & Trathan, 2011); this assertion is borne out by diet samples collected at nearby Signy Island (BAS unpublished data; Figure 1)
Summary
The coexistence of sympatric species and the extent to which their ecological niches overlap are fundamental issues in both theoretical and applied ecology (Silvertown, 2004; Vellend, 2010). Such mechanisms can involve temporal separation of chick-rearing periods among species (Lynch, Fagan et al, 2012; Trivelpiece, Trivelpiece, & Volkman, 1987), use of spatially distinct foraging habitats in two or three dimensions (Cimino, Moline, Fraser, Patterson-Fraser, & Oliver, 2016; White & Conroy, 1975; Wilson, 2010), or specialized feeding on different types of prey when sharing the same areas (Hinke et al, 2015; Negrete et al, 2017; Polito et al, 2015) Such closely related species can be expected to reduce the overlap in their ecological niches when resources are limited, as has been observed for instance between Adélie and chinstrap penguins (Lynnes et al, 2002). Interannual variation in isotopic niche has been shown to occur concurrently in pygoscelid penguins in other areas (Negrete et al, 2017), and it was expected that potential changes in isotopic niche would be reflected in all three species
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