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

MORGENTHALER, ANNICK; MILLONES, ANA; GANDINI, PATRICIA; FRERE, ESTEBAN

Hobart

Conferencia; 3rd World Seabird Conference; 2021

The theory of competitive exclusion predictsthat, when the resources are limited, species with similar trophic requirementscan only coexist if there are differences in some dimensions of theirecological niche. The estuary of Ría Deseado, in Argentina, is one of the fewplaces in the world where four species of cormorants/shags coexist: the Red-leggedcormorant (Phalacrocorax gaimardi),the Rock shag (P. magellanicus), the Neotropiccormorant (P. brasiliensis) and the Imperialcormorant (P. atriceps). To determinepossible interspecific differences in one or more of their niche dimensions relatedto the use of resources, during the breeding season, we studied thedietary (pellets and regurgitates), isotopic (stable isotope), spatial(foraging areas) and temporal (breeding calendar) dimensions of the trophicniche of these four species living in sympatry, between 2009 and 2013. We foundthat the Red-legged cormorant was located at a low trophic level; he fed almostexclusively on pelagic prey and segregated its core feeding areas from otherspecies. The Rock shag was located at a high trophic level and presented themost benthic and diverse diet, including several invertebrate taxa. TheNeotropic 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, both benthic and demersal-pelagic. Finally,the Imperial cormorant segregated isotopically, spatially and temporarily from theother species. Its trophic level was situated in between the Red-legged and theRock-Neotropic group. Regarding the breeding calendar, the Neotropic cormorant wasthe most asynchronous species (wide range of laying dates between pairs) andthe one that presented the latest chick-rearing peak. In general, the fourspecies showed a partition of the use of their resources despite occupying arather restricted geographical space. The observed interspecific differencespossibly act by reducing competition during the reproductive period whencormorants are restricted to exploit resources within a limited area. Themultidimensional approach to study the use of the trophic resources of thesefour species allowed us to find that the degree of interspecific segregationvaried according to the dimension examined.