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 intensity (top-down effects). Data from 20 islands and five years (1990-1994) show that the cumulative productivity of islands, a sum of the input of marine detritus and terrestrial plant productivity, varies spatially and temporally. Marine inputs are a function of the perimeter/area ratio and are thus relatively greater on smaller islands. Land plant productivity, a function of precipitation, is negligible in most years but can increase greatly (10-160X in plant cover) with heavy rains from El Nino events (e.g., 1992-1993). Consumer intensity is a function 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 some islands and because parasitoid densities show extreme year-to-year oscillations. El Nino rains produced large changes in the quantity and quality of plant resources and the control of spider populations. Spider densities doubled in 1992 in response to high levels of prey that followed heavy rains but crashed in 1993, despite continued high plant productivity and prey availability. The appearance of copious nectar and pollen, food of adult parasitoid wasps, caused wasp populations to erupt in 1993. Wasps emerged as a major, but previously hidden, trophic influence and greatly suppressed spider populations. Pompilids act as nonequilibrium agents-they are regulated by factors other than prey (spider) availability. In general, smaller spider populations were more variable year to year in the magnitude of their increases and decreases. In spite of the great temporal and spatial variability, consistent changes in plant, prey, spider, and wasp dynamics occurred repeatedly and apparently independently on islands throughout the archipelago. Such patterns of concerted change strongly suggest that general processes related to productivity and consumption are key to understanding the dynamics of this system.