Assessing marine ecosystem complexity: isotopic integration of the trophic structure of seabird communities from the Southern Ocean

Abstract

Understanding the processes structuring communities is a fundamental goal in ecology and conservation biology. Seabirds are commonly used as sentinels of marine ecosystems, but there is a lack of quantitative information providing a synoptic view of their community structure and of its underlying mechanisms. We used stable isotope analysis of chick feathers to investigate the structure of 2 communities that are representative of the subantarctic (Kerguelen) and Antarctic (Adélie Land) seabird diversity. Total area of the convex hull (a measure of the total δ13C-δ15N niche space) was 8.4-fold higher at the Kerguelen Islands than in Adélie Land, a consequence of the higher seabird diversity at the former locality. Kerguelen seabirds grouped into 2 clusters of oceanic and inshore species, with the latter group not represented in Adélie Land. Communities are primarily structured by the availability of foraging habitats (δ13C) and then of trophic resources (δ15N), with body size being a major driving force of trophic position. Ecological characteristics are more important than phylogeny to shape seabird isotopic niche breadth (standard ellipse area corrected for small sample size, SEAc), with no significant differences between Sphenisciformes, Procellariiformes, and Charadriiformes. By contrast, SEAc varies according to foraging guilds, diet, and a specialist-generalist gradient, with ubiquitous seabirds having a 10-fold larger mean SEAc than pelagic divers. This study sets a baseline against which the effects of long-term environmental changes on seabird community structure can be studied across years and conditions, and provides a relevant starting point for the investigation into the effect of climate change on Southern Ocean ecosystems.