Endothermy, ectothermy and the global structure of marine vertebrate communities

Birds and mammals are the leading marine predators at high latitudes, while sharks and other large fish occupy top positions in tropical waters. The present study proposes that temperature-dependent predation success (TPS) explains global patterns of marine vertebrate community structure. Burst speed increases with temperature in ectotherms but is independent of temperature in endotherms. If capture success depends on relative swimming speeds of predator and prey, ectothermic prey will be more vulnerable to attack by endothermic predators at low temperatures. Conversely, high temperatures should enhance the ability of ectothermic predators to prey on endotherms. Pursuit-diving seabirds (penguins, auks and some cormorants) and pinnipeds (seals and sea lions) are ubiquitous in ocean waters with summer surface temperatures cooler than the mid-teens to low 20s (degrees C) but are virtually absent in warmer regions. We suggest that the near-absence of these animals at low latitudes is due to TPS, as warm water increases the difficulty of capturing fish prey and increases vulnerability to predation by large ectothermic and partially endothermic sharks. Pursuit-diving birds and pinnipeds are virtually absent from warm temperate and tropical waters, even where primary productivity and fisheries data suggest that food supplies are ample. This indicates that the low productivity that prevails in much of the tropical zone cannot explain the worldwide distributional patterns of pursuit-diving birds and pinnipeds. Endothermy in marine communities increases with cooler temperatures and with animal size. Pursuit-diving birds and pinnipeds are sensitive to temperature limits and may suffer important range contractions as oceans warm.