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

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.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)(1991)(1992)(1993)(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 Nifio 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 yearto-year oscillations.El Nifio 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. can vary greatly in space and time. Freshwater ecologists have most clearly analyzed how variable predation and productivity interact to form a rich array of dynamics (Carpenter and Kitchell 1993, Rosemond et al. 1993, Osenberg and Mittelbach 1996). The concurrent analysis of top-down and bottom-up forces on the structure and control of terrestrial populations and communities is less common (e.g., Schmitz 1993, 1994, Polis and Hurd 1995; see Strong 1992). Pol...

The influence of top–down, bottom–up and abiotic factors on the moose ( Alces alces ) population of Isle Royale

“…Some terrestrial studies suggest that both top-down and bottom-up influences are important (e.g. de Ruiter et al 1995;Polis et al 1998;Sinclair et al 2000;also Dyer & Letourneau 1999). However, consistent with previous speculation (e.g.…”

“…Brett & Goldman 1997;Schmitz et al 2000). An equally important, but less studied aspect of population regulation is how interannual variation in population dynamics is affected by natural levels of interannual variation in top-down, bottom-up and abiotic processes (Polis et al (1998) is an exception). To further our understanding of these relationships, we assessed how interannual variation over a 40 year period in a large, terrestrial herbivore (moose, Alces alces) population was affected by interannual variability in predator abundance, forage availability and several abiotic factors.…”

Multiple density–dependence mechanisms regulate a migratory bird population during the breeding season

“…source-sink, buffer effect, site dependence); and (iii) those arising from intrinsic differences among individuals. Past tests for regulation have almost invariably focused, mostly for practical reasons, on single mechanisms (Rodenhouse et al 1999), even though it has been suggested that several negative feedback mechanisms may operate simultaneously (den Boer & Reddingius 1996;Polis et al 1998).…”

Insects and climate change: processes, patterns and implications for conservation.

“…The climatic or biotic limiting factors for particular interacting species may also be difficult to predict. For example, after increases in plant productivity associated with heavy rains in desert islands in the Gulf of California, spider densities doubled in 1992 in response to increased insect prey, but then were greatly reduced in 1993 because of parasitism by wasps whose populations increased because of increased nectar and pollen resources (Polis et al, 1998 …”