Biogeography of marine mammals and their vulnerability to climate change

Although extinction due to climate change are still uncommon, they might surpass those caused by habitat loss or overexploitation over the next few decades. Among marine megafauna, mammals fulfil key ecological roles in the ocean, and the collapse of their populations may have irreversible consequences for ecosystem functioning. However, the biogeography, essential step for predicting effects of climate change on marine megafauna was not so far explored in the literature. We compiled a functional trait database for 121 species of marine mammals characterized by 14 functional traits and the associated range maps extracted from IUCN website. We estimated marine mammal species richness (SR) as well as functional (FD) and phylogenetic diversity (PD) per grid cell (1°) and assessed the spatial congruence of these facets of biodiversity hotspots with human threats at the global scale. Thanks to this database and by using a trait-based approach we assessed the vulnerability of all marine mammals to global warming under high and low greenhouse gas emission scenarios for the middle and the end of the 21st century. We highlighted that FD was weakly correlated with both SR and the PD and observed a lack of spatial congruence between current human threats and the distribution of biodiversity hotspots. This finding calls for caution when using only species richness as a benchmark for defining marine mammal biodiversity hotspots. We observed that the North Pacific Ocean, the Greenland and the Barents Seas host the most vulnerable species to global warming. Future conservation plans should focus on these regions with long histories of overexploitation. Beyond species loss, we showed that the potential extinction of the marine mammals that were most vulnerable to global warming might induce a disproportionate loss of functional diversity, which may have profound impacts on the future functioning of marine ecosystems worldwide.