Longer and less overlapped food webs in anthropogenically disturbed marine ecosystems: confirmation from the past

SAPORITI, FABIANA; BEARHOP, STUART; SILVA, LAURA ALEJANDRA; VALES, DAMIÁN GUSTAVO; ZENTENO, LISETTE; CRESPO, ENRIQUE ALBERTO; AGUILAR, ALEX; CARDONA, LUIS

Liège

Conferencia; 28th Conference of the European Cetacean Society; 2014

Human exploitation of marine resources is characterized by the preferential removal of the largest species. Accordingly, the current pattern of distribution and the ecological role of marine megafauna, including marine mammals, do not necessarily correspond to those natural. Retrospective studies allow assessment of shifts. The marine ecosystems off Argentina have suffered major anthropogenic changes during the past two centuries due to sealing and industrial fishing. These impacts caused a reorganization of the entire ecosystem that could have led to a new equilibrium state. Here we used stable isotope analysis and quantitative methods (i.e SIBER) to analyze changes in the topology of the marine food web during the last 6000 years in northern and southern Patagonia, with a particular focus on pinnipeds. The stable isotope ratios of carbon and nitrogen in the shell of intertidal molluscs were used to infer changes in the isotopic baseline and those in modern and archaeological bones of South American sea lions, South American fur seals and Magellan penguins were used to understand changes in trophic level and topology of top predators within the food web. We found that ancient food webs were shorter, more redundant and more overlapped than current ones, with today pinnipeds and penguins foraging at higher trophic levels. These counter-intuitive results are the consequence of the intense exploitation of pinnipeds during the 19th and 20th centuries, as their current population number is still much lower than originally. In this scenario, fur seals and sea lions remain well below the carrying capacity of the ecosystem; they were relieved from intraspecific competition and consume larger prey than 6000 years ago. On the other hand the penguins may have taken advantage of decreased competition from pinnipeds (especially fur seals) thus feeding better prey. Research carried out with the support of the Fundación BBVA.