MULTIFACTOR POPULATION LIMITATION - VARIABLE SPATIAL AND TEMPORAL CONTROL OF SPIDERS ON GULF-OF-CALIFORNIA ISLANDS

Spider density on desert islands in the Gulf of California is a joint function of variable productivity (bottom-up effects) and consumer int ensity (top-down effects). Data from 20 islands and five years (1990-1 994) show that the cumulative productivity of islands, a sum of the in put of marine detritus and terrestrial plant productivity, varies spat ially and temporally. Marine inputs are a function of the perimeter/ar ea ratio and are thus relatively greater on smaller islands. Land plan t productivity, a function of precipitation, is negligible in most yea rs but can increase greatly (10-160X in plant cover) with heavy rains from El Nino events (e.g., 1992-1993). Consumer intensity is a functio n of the continual influence of predaceous scorpions and the sporadic occurrence of parasitoid spider wasps (Pompilidae); the importance of these factors varies greatly, both because scorpions are absent on som e islands and because parasitoid densities show extreme year-to-year o scillations. El Nino rains produced large changes in the quantity and quality of plant resources and the control of spider populations. Spid er densities doubled in 1992 in response to high levels of prey that f ollowed heavy rains but crashed in 1993, despite continued high plant productivity and prey availability. The appearance of copious nectar a nd pollen, food of adult parasitoid wasps, caused wasp populations to erupt in 1993. Wasps emerged as a major, but previously hidden, trophi c influence and greatly suppressed spider populations. Pompilids act a s nonequilibrium agents-they are regulated by factors other than prey (spider) availability. In general, smaller spider populations were mor e variable year to year in the magnitude of their increases and decrea ses. In spite of the great temporal and spatial variability, consisten t changes in plant, prey, spider, and wasp dynamics occurred repeatedl y and apparently independently on islands throughout the archipelago. Such patterns of concerted change strongly suggest that general proces ses related to productivity and consumption are key to understanding t he dynamics of this system.