The role of mammals as key predators in marine ecosystems

Abstract

Marine mammals are believed to exert a strong influence on ecosystems, though the best methods for measuring their ecological role have long been debated. Here, we used an ecosystem modelling approach to explore the underlying forces that define their ecological role, with a focus on predatory effects. We used the outputs and metadata from 55 food web models built using Ecopath with Ecosim software to analyse the effects of several likely explanatory variables on 2 indices of trophic significance: the relative trophic impact (TI) and the ‘keystoneness’ index (KS). Results showed that both TI and KS are positively related to trophic level. We also found an additional effect of group biomass in both TI and KS, but this response was clade-dependent: the higher the biomass, the higher the TI and KS of odontocetes and pinnipeds, suggesting that food intake contributes to the predatory effects of these clades through biomass variation. We did not detect a latitudinal pattern in the ecological role of any clades. We also found no effects of metabolic rates on TI and KS. In sum, these findings suggest that (1) the predatory effects of marine mammals are mainly defined by their trophic levels and food intake, and (2), since odontocetes and pinnipeds feed on higher trophic levels and have higher trophic impact, these 2 clades can be more vulnerable to competition from fisheries overexploitation. We can conclude that odontocete declines can cause significant changes in marine communities. Our meta-analytic approach contributes to the understanding of predatory effects of marine mammals on different ecosystems.