2 - Hungry spiders

Summary

Food limitation of terrestrial carnivores

The concept of resource limitation is so central to ecological thinking that it might seem unnecessary to justify examining the impact of prey supply upon spider populations. However, attempts to uncover broad patterns in resource limitation frequently have generated controversy. One that directly engulfs spiders started with a brief, carefully argued communication by Hairston, Smith & Slobodkin (1960). They concluded that the carnivore trophic level of terrestrial ecosystems is ‘resource-limited in the classical density-dependent fashion.’ In particular, they argued that predators are food limited, and that competition occurs on this trophic level. Hairston et al. did not propose that every group of terrestrial carnivores is food limited. Nevertheless, because spiders are major terrestrial predators, it follows that a shortage of prey should frequently affect spider densities. As a model terrestrial predator the spider cannot escape the controversy created by the sweeping predictions made by Hairston et al.; indeed, continuing interest in testing their predictions in conjunction with disagreement over the prevalence of competition has already drawn spiders into the fray (Wise 1975, Schoener 1983a, 1986a).

Prey is conventionally defined to be a limited resource if an increase in the prey supply increases the predator survival and/or fecundity. If increases in one or more of these parameters cause the average population density of the next generation to increase, the population is food limited. Thus, food limitation is defined ultimately in terms of population density, but evidence that food is limiting frequently comes from within-generation measurements of individual survival rates or fecundity.