Abstract
Understanding the trophic niches of marine apex predators is necessary to understand interactions between species and to achieve sustainable, ecosystem-based fisheries management. Here, we review the stable carbon and nitrogen isotope ratios for biting marine mammals inhabiting the Atlantic Ocean to test the hypothesis that the relative position of each species within the isospace is rather invariant and that common and predictable patterns of resource partitioning exists because of constrains imposed by body size and skull morphology. Furthermore, we analyze in detail two species-rich communities to test the hypotheses that marine mammals are gape limited and that trophic position increases with gape size. The isotopic niches of species were highly consistent across regions and the topology of the community within the isospace was well conserved across the Atlantic Ocean. Furthermore, pinnipeds exhibited a much lower diversity of isotopic niches than odontocetes. Results also revealed body size as a poor predictor of the isotopic niche, a modest role of skull morphology in determining it, no evidence of gape limitation and little overlap in the isotopic niche of sympatric species. The overall evidence suggests limited trophic flexibility for most species and low ecological redundancy, which should be considered for ecosystem-based fisheries management.
Highlights
Are maximized by longirostrine and brevirostrine skull morphologies r espectively[14,15]
We review the stable isotope ratios of C and N for biting pinnipeds and odontocetes inhabiting the Atlantic Ocean, from cold temperate Europe to subantarctic South America, to test the hypotheses that (i) the relative position of each species within the isospace is rather invariant and (ii) that common and predictable patterns of resource partitioning exist across communities because of restrictions associated to body size and skull morphology
The results reported here revealed highly consistent isotopic niches for most species of biting odontocetes and pinnipeds and a conserved topology of their community within the isospace across the Atlantic Ocean
Summary
Are maximized by longirostrine and brevirostrine skull morphologies r espectively[14,15]. The fundamental trophic niches of marine mammals would be rather narrow and their trophic positions within food webs would be rather invariant across time and space It should be noted, that skull morphology imposes hydrodynamic constrains that may decouple mouth gape from prey s ize[14,15]. Study of diet through stable isotope analysis lacks the taxonomic resolution of stomach content and scat analyses, but stable isotope ratios in animal tissues integrate dietary information through variable time spans, depending on the tissue turnover rate[25]. This alleviates the extremely short time resolution of stomach and scat contents, which often reveal the composition of just the last few meals. We analyze in detail two species-rich communities from Mauritania and Uruguay to assess the actual degree of overlap in the isotopic niches of species and test the hypothesis that marine mammals are gape limited and, that trophic position increases with gape size
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