Ecological roles and importance of sea cows (Order: Sirenia): a review and prospectus

Abstract

Sirenians (manatees Trichechus spp. and the dugong Dugong dugon) are large-bodied and almost exclusively herbivorous aquatic mammals occurring in coastal and freshwater habitats of most tropical and subtropical regions. Although sirenian ecology, habitat use, and abundance have been investigated across their range, little is known about the roles and impacts of these megaherbivores outside of the trophic relationships of 2 relatively well-studied species (T. manatus, D. dugon). This knowledge gap limits our understanding of how sirenians affect communities and ecosystem dynamics. We review the literature on the ecological roles and importance of sirenians in aquatic ecosystems, including the extinct Steller’s sea cow Hydrodamalis gigas. Our review reveals that sirenian herbivory and disturbance can markedly affect species biomass, productivity, and composition in macrophyte communities, thereby indirectly influencing other community members (e.g. invertebrates) and carbon storage in macrophyte biomass and sediments. Sirenians may also couple disparate ecosystems via their movements, compete with other herbivores, and serve as prey for, and be responsive to, predators, including humans. We also identify key knowledge gaps, including (1) the ecology of Amazonian T. inunguis and African T. senegalensis manatees, (2) experimental investigations of the magnitude of trophic impacts and their potential to cascade to other species and carbon dynamics, (3) the roles and impacts of sirenians as omnivores, and (4) the non-consumptive effects of predators on sirenians other than dugongs. Finally, we highlight crucial avenues for future sirenian research, including changes to their ecology in the face of ongoing seagrass declines and climate change.