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Proc Natl Acad Sci U S A. May 9, 1995; 92(10): 4382–4386.

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; many small islands support 50-200 spiders per m3 of cactus. Energy for these spiders comes primarily from the ocean and not from in situ productivity by land plants. We explicitly connect the marine and terrestrial systems to show that insular food webs represent one endpoint of the marine web. We describe two conduits for marine energy entering these islands: shore drift and seabird colonies. Both conduits are related to island area, having a much stronger effect on smaller islands. This asymmetric effect helps to explain the exceptionally high spider densities on small islands. Although productivity sets the maximal potential densities, predation (by scorpions) limits realized spider abundance. Thus, prey availability and predation act in concert to set insular spider abundance.

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Selected References

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  • Schoener TW, Toft CA. Spider populations: extraordinarily high densities on islands without top predators. Science. 1983 Mar 18;219(4590):1353–1355. [PubMed]
  • Schoener TW, Spiller DA. Effect of lizards on spider populations: manipulative reconstruction of a natural experiment. Science. 1987 May 22;236(4804):949–952. [PubMed]
  • Uetz GW, Kane TC, Stratton GE. Variation in the social grouping tendency of a communal web-building spider. Science. 1982 Aug 6;217(4559):547–549. [PubMed]

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