Ga. Polis et Sd. Hurd, EXTRAORDINARILY HIGH SPIDER DENSITIES ON ISLANDS - FLOW OF ENERGY FROM THE MARINE TO TERRESTRIAL FOOD WEBS AND THE ABSENCE OF PREDATION, Proceedings of the National Academy of Sciences of the United Statesof America, 92(10), 1995, pp. 4382-4386
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.