Marine reptiles, birds and mammals and nutrient transfers among the seas and the land: An appraisal of current knowledge

There is convincing evidence that large land mammals were formerly responsible for substantial transfers of nutrients across the freshwater and land system. Many of them were made extinct, probably by human hunting, around 10 000 to 14 000 BP, and the populations of the survivors were severely reduced as the Holocene progressed. Evidence is examined that such transfers were equally important among the oceans, coastal waters and inland systems as a result of the activities of anadromous fishes, marine reptiles, seabirds and marine mammals. Numbers of all of these have also been greatly reduced as a result of human activities, though largely in the past few hundred years. Their past and present status and potentialities for nutrient recycling and transfers are assessed. Large marine animals have potentially substantial top-down effects in structuring marine communities, though there is good evidence only for sea turtles and inshore habitats. There are many potential pathways for nutrient transfer, reflected in large accumulations on land of guano from seabirds. Great whales, because of their size and movement range, also offer large potentialities for transfer of nutrients across the oceans and from deep water towards the surface. However, from the very limited evidence available, it appears that the absolute global effects of these are likely to have been and to be minor. Transfers on land benefited from large nutrient supplies available in soils, from high productivity arising from light availability, from a dense and diverse mammal community and from a much smaller ratio of freshwater volume to land area. The huge volume of ocean water, its much greater mixing by wind and currents, and lower overall productivity preclude against globally significant changes in nutrient concentrations as a result of the movements of large animals, despite sometimes dramatic local effects of seabirds nesting on land and anadromous fish moving from the ocean to their freshwater spawning grounds. The intense scientific interest in these models, and the supportive vocabulary created to describe these processes, may have led to an overemphasis of their global significance.