High trophic overlap within the seabird community of Argentinean Patagonia: a multiscale approach

1. Food web interactions in animal communities can be investigated through the measurement of stable isotopes (e.g. delta(15)N, delta(13)C). We used this approach in a community of 14 species of seabirds breeding on the Argentinean Patagonian coast. Tissue samples were collected from nestling and adult seabirds, as well as prey, during three consecutive breeding seasons in 28 breeding colonies. 2. Relative to those in other temperate and polar regions, this seabird community showed a high degree of overlap in trophic level (TL) among species (93% of species within a TL range of 0.7) and also a comparatively high mean trophic level (4.1). 3. Relative positions of seabirds in relation to prey suggest that most species feed on pelagic fish and to a lesser extent on invertebrates. Stable isotope values of specialist feeders, Olrolg's (Larus atlanticus) and dolphin gulls (Leucophaeus scoresbii), which were previously assumed to feed mainly on crabs and sea lion excrement, respectively, suggested a broader diet than expected. 4. Based on stable isotope values of individuals, groups of phylogenetically related species generally showed a high degree of overlap within each group. 5. Given the degree of isotope overlap in this species-rich community, coexistence could be interpreted as a consequence of superabundance of food or species diversification in morphology and foraging strategies. The short range of trophic level makes these seabirds vulnerable to the reduction of fish stocks due either to commercial fishing or stochastic fluctuations.