Maritime mammals: terrestrial mammals as consumers in marine intertidal communities

The phenomenon of terrestrial mammals as predators in marine intertidal communities, and thus as agents of energy transfer from sea to land, is poorly understood. We review here the evidence for terrestrial mammals intentionally entering the ocean shore at low tide in order to prey on living marine invertebrates, fish, algae, and seagrasses. We introduce the term 'maritime mammals', defined as coastal mammalian predators that utilize living intertidal energy resources and transfer these resources to the land. We document 135 records of predation among 45 species of terrestrial mammals in 8 orders feeding in marine intertidal zones. Most predation events are by carnivores (59%, mostly by raccoon, mink, black bear, and Arctic fox), followed by rodents (20%) and artiodactyls (14%). Maritime mammals occur on all continental coastlines of the world except Antarctica. Most records are from the Eastern North Pacific Ocean, with 21 species of maritime mammals. Twelve maritime mammal species occur in the Western North Atlantic Ocean, and 8 maritime mammals are known for the Eastern South Pacific Ocean. These 3 regions account for 42% of the diversity of maritime mammals. A total of 228 different prey taxa are known to be consumed, representing 12 phyla of marine organisms; 2/3 of these taxa are bivalve and gastropod mollusks, crabs, and fish. Introduced populations of 17 species of mammals have been recorded as maritime predators; 15 of these are recorded on islands where endemic mammals were absent or rare. Maritime mammals are widespread globally, with often repeated cases of predation being observed for the same species. We suggest that predation by maritime mammals is a rarely studied, rather than rare, phenomenon, and maritime mammals are thus a largely overlooked guild of intertidal predators. Quantitative field observations (using techniques such as infrared night vision, radiotelemetry, and stable isotope analysis) and experimental studies (using exclosures, or by taking advantage of the removal of insular introduced mammal populations) are required. The importance of intertidal resources in supplying energy to terrestrial populations of many species of mammals world wide may have been underappreciated.