Dividing the pie: the use of trophic resource by four sympatric species of cormorants in a saltwater estuary from Southern Patagonia

Abstract

**Abstract:** The theory of competitive exclusion predicts that, when the resources are limited, species with similar trophic requirements can only coexist if there are differences in some dimensions of their ecological niche. The estuary of Ría Deseado, in Argentina, is one of the few places in the world where four species of cormorants coexist: the Red-legged cormorant (Poikilocarbo gaimardi), the Rock shag (Leucocarbo magellanicus), the Imperial shag (L. atriceps) and the Neotropic cormorant (Nannopterum brasiliensis). To determine possible interspecific differences in one or more of their niche dimensions related to the use of resources during the breeding season, we studied the dietary (pellets), isotopic (stable isotope), spatial (foraging areas) and temporal (breeding calendar) dimensions of the trophic niche of these four sympatric species, between 2009 and 2013. We found that the Red-legged cormorant was located at a low trophic level; he fed almost exclusively on pelagic prey and segregated its core feeding areas from other species. The Rock shag was located at a high trophic level and presented the most benthic and diverse diet, including several invertebrate taxa. The Neotropic cormorant was situated at a similar trophic level as the Rock shag, with which it overlapped its isotopic niche, as well as its core feeding areas. The Neotropic cormorant fed only on fishes. Finally, the Imperial cormorant segregated isotopically, spatially and temporarily from the other species. Its trophic level was situated in between the Red-legged and the Rock-Neotropic group. Regarding the breeding calendar, the Neotropic cormorant was the most asynchronous species, and the one that presented the latest chick-rearing peak. In general, the four species showed a partition of the use of their resources despite occupying a rather restricted geographical space. The observed differences possibly act by reducing competition during the reproductive period when birds are restricted to exploit resources within a limited area. The multidimensional approach to study the use of the trophic resources of these four species allowed us to find that the degree of interspecific segregation varied according to the dimension examined. **Authors:** Annick Morgenthaler¹, Ana Millones¹, Patricia Gandini², Esteban Frere² ¹Centro de Investigaciones de Puerto Deseado, Universidad Nacional de la Patagonia Austral, ²Centro de Investigaciones de Puerto Deseado, Universidad Nacional de la Patagonia Austral, and CONIC