Cannibalism regulates densities of young wolf spiders: evidence from field and laboratory experiments

Ecology, March, 1996 by James D. Wagner, David H. Wise

INTRODUCTION

Spiders are a ubiquitous and diverse group of terrestrial predators that often consume a substantial fraction of herbivore and detritivore populations (Van Hook 1971, Moulder and Reichle 1972, Manley et al. 1976). Descriptive and experimental studies have revealed the importance of prey availability and habitat structure in determining spider growth, fecundity, and/or abundance, and experimentalists recently have begun to uncover the impact of natural enemies on spider densities (cf. reviews by Uetz 1991, Wise 1993). What is often lacking is information on the degree of density dependence of the various mortality factors, and the extent to which they ultimately influence spider population size.

Most studies on the regulation of spider populations have been restricted to web-building species. Examples include the extensive documentation of resource-based territoriality in a desert funnel-web spider (Riechert 1978) and the experimental evidence for (Spiller 1984a, b) and against (Wise 1981, 1983, 1993, Horton and Wise 1983) intra- and interspecific competition for prey. The nomadic lifestyles of wandering or cursorial spiders make it more difficult to manipulate their densities, and most methods of estimating their population size (e.g., sifting leaf litter, pitfall trapping) are disruptive to the system. Despite the challenges they impose on experimentalists, cursorial spiders deserve closer scrutiny because they represent a major component of the spider fauna.

Recent studies with two different species of cursorial wolf spiders have revealed strong density-dependent (DD) declines in spider abundance within fenced field plots (Oraze and Grigarick 1989, Wise and Wagner 1992). Oraze and Grigarick postulated cannibalism to be the DD mortality factor in their study of Pardosa ramulosa in rice fields. We uncovered convergence in densities of young instars of Schizocosa ocreata, which is abundant in leaf litter of the forest floor. Although we could not factor out the possible contribution of emigration, the overall pattern of results strongly suggested that DD mortality from natural enemies and/or cannibalism was the major cause of the density convergence (Wise and Wagner 1992).

Cannibalism has been postulated to be an important mechanism regulating populations of spiders (e.g., Riechert and Lockley 1984) and other generalist arthropod predators (Fox 1975a, Polis 1980). Cannibalism involving wolf spiders has been observed in the field (e.g., Edgar 1969, Hallander 1970, Yeargan 1975). We have occasionally observed cannibalism within S. ocreata spiderlings in both forest and laboratory. Directly measuring rates of cannibalism in the field is difficult, particularly in structurally complex habitats such as leaf litter. Cannibalism can be demonstrated to occur in the laboratory, but researchers usually have not related the results of laboratory studies directly to the dynamics of the natural population (an exception: Leonardsson [1991]: isopods).

We used both field and laboratory experiments to evaluate the magnitude of mortality from cannibalism among newly dispersed spiderlings of S. ocreata. In 1992 we conducted a predator-reduction field experiment, in which we manipulated spiderling densities in fenced plots that were modified to prevent emigration and reduce access by natural enemies. By reducing the influence of these factors on spiderling density we obtained an indirect measure of the contribution of cannibalism to the decline in spiderling densities. In 1993 we examined spiderling survival in laboratory arenas in which we could eliminate mortality from natural enemies completely and thereby measure rates of cannibalism directly. The goals of the laboratory studies were to determine if cannibalism among S. ocreata spiderlings is a DD mortality factor, and if cannibalism can occur at a magnitude large enough to explain the density decline observed in the field.

METHODS

Study species

Schizocosa ocreata (Hentz) is a medium-sized (adult females 73.3 [ or -] 1.3 mg [n = 131]) wolf spider (Lycosidae) common in forests throughout the eastern United States (Dondale and Redner 1978, Stratton 1991). Wolf spiders are keen-sighted, wandering spiders (Land 1985) that do not rely on webs to capture food, but instead ambush or actively pursue their prey. S. ocreata is an annual species, maturing in spring and producing its first egg sacs in early summer. Female wolf spiders carry the egg sac attached to their spinnerets until the young emerge. The emerging spider-lings climb onto the female's abdomen, where they remain for 7-14 d until they begin to disperse into the leaf litter. In our study site, the young stages of S. ocreata occur at high densities, [approximately equal to]60-90 individuals/[m.sup.2] (Wise and Wagner 1992).

Predator-reduction field experiment

The goal of the field study was to examine the impact of natural enemies and cannibalism on the rates of mortality of young-instar S. ocreata. We stocked fenced plots, from which emigration was prevented, with known densities of S. ocreata, and monitored the rate of decline in spider density after excluding and reducing a suite of natural enemies. Comparison of spider mortality in the predator-reduction plots with control plots allowed us to obtain an indirect measure of the rate of cannibalism in S. ocreata. The study was conducted in a mature beech - oak forest at the Patuxent Wildlife Research Center, U.S. Fish and Wildlife Service, Laurel, Maryland, USA (39 [degrees] N, 77 [degrees] W).