EXTRAORDINARILY HIGH SPIDER DENSITIES ON ISLANDS - FLOW OF ENERGY FROM THE MARINE TO TERRESTRIAL FOOD WEBS AND THE ABSENCE OF PREDATION

Some islands in the Gulf of California support very high densities of spiders. Spider density is negatively correlated with island size; man y small islands support 50-200 spiders per m(3) of cactus. Energy for these spiders comes primarily from the ocean and not from in situ prod uctivity by land plants. We explicitly connect the marine and terrestr ial systems to show that insular food webs represent one endpoint of t he marine web. We describe two conduits for marine energy entering the se islands: shore drift and seabird colonies. Both conduits are relate d to island area, having a much stronger effect on smaller islands. Th is asymmetric effect helps to explain the exceptionally high spider de nsities on small islands. Although productivity sets the maximal poten tial densities, predation (by scorpions) limits realized spider abunda nce. Thus, prey availability and predation act in concert to set insul ar spider abundance.