Multifactor Population Limitation: Variable Spatial and Temporal Control of Spiders on Gulf of California Islands

Abstract

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 (10–160× in plant cover) with heavy rains from El Niño 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 Niño 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.

Seasonal shift from bottom-up to top-down impact in phytophagous insect populations

“…Since, in these studies, predators primarily responded to changes in prey density, there was an initial period of rapid herbivore population growth followed by eventual suppression resulting from the delayed numerical response of predators. Polis et al (1998) documented a 1-year lag between prey (spider) increases and eventual predator (pompilid wasp) suppression. In contrast, our study found no appreciable lag in predator effect on herbivore populations since predator abundance was already high in fertilized plots due to an aggregative response rather than a numerical response.…”

“…Hence, when the bottom-up effects on the temporal development of natural enemies (via dispersal) are taken into account, different outcomes on the dynamics of prey are observed. Thus, elucidating those bottom-up related factors that mediate predator dispersal and aggregation is essential to unraveling the temporal and spatial dynamics of predator-prey interactions (Polis et al 1998;Nakano et al 1999) and could also be exploited in developing conservation biological control strategies for pest management (Settle et al 1996;Landis et al 2000).…”

“…Despite the recognized importance of such interactions, we know little about how the direct and interactive effects of host plants and natural enemies on herbivore dynamics vary either temporally throughout the year or spatially across habitats (but see Polis et al 1998;Polis 1999;Denno et al 2002). Recent studies suggest that the relative importance top-down and bottom-up effects in natural communities can vary both within a season as well as among years (Walker and Jones 2001).…”

“…Walker and Jones (2001) showed that, even for the same species, the relative importance of natural enemies and host plants change from year to year. Notwithstanding a growing body of empirical studies examining temporal variation in top-down and bottom-up effects on terrestrial communities (Power et al 1996;Polis et al 1998;Ernest et al 2000;Ostfeld and Keesing 2000), and an increasing recognition of its importance to food web dynamics (Hunter and Price 1992;Holt 2000), little is known of the processes that influence this variation.…”

“…We anticipated that enhanced vegetation structure and increases in alternative prey might promote the accumulation and retention of predators earlier in the season (Vince et al 1981;Döbel and Denno 1994;Settle et al 1996). Thus, understanding how bottom-up factors advance or delay the temporal onset of top-down impacts is an under-explored aspect of the interactive effects of plant resources and natural enemies on herbivore dynamics (Polis et al 1998). …”