Why do oceans shelter fewer species than land? A review on the role of sinking, dispersal and body size

Oceans, covering 71% of the Earth's surface, have sheltered life over longer time compared to land. In 1994, Robert M. May posed the following question: why do oceans hold a smaller number of extant species compared to land? Here, we review this unsolved question by focusing on the limitations of marine pelagic diversification, and compiling global data. We explore the extent to which physical differences across pelagic, sunlit benthic and terrestrial realms, and related processes including dispersal and speciation, determine the low biodiversity in pelagic primary producers, in contrast to the more diverse sunlit benthos. We assess inter-realm differences in geographic patterns of species turnover, with terrestrial plants and benthic invertebrates showing highest and pelagic plankton showing the least turnover with distance. These results support that differential dispersal abilities of species groups, associated with dispersive versus sessile life-styles, contribute to drive the land–ocean disparity in diversity. We discuss the importance of particle sinking in the marine plankton body size and the implications to the biogeographic ranges. Using global ocean data, we find a negative trend between species' body size and biogeographic ranges for phytoplankton and zooplankton, and a positive trend for fishes. This suggests that negative relationship is associated to the low plankton diversity observed in open sea. Advection and sinking likely impose demographic thresholds in plankton linked to small body size, higher density and reproduction rate, which helps maintain the population cohesion in pelagic systems. This may cause reduced allopatric speciation and small body size of primary producers, limiting diversification, habitat complexity and coevolution in higher trophic levels. Our synthesis of literature and data compilation highlights that differences in dispersal modes and body size across the sunlit benthos and pelagic domain, imposed by sinking and ocean drift in plankton, emerge as primary factors limiting the overall marine diversity.