Cannibalism regulates densities of young wolf spiders: evidence from field and laboratory experiments
James D. Wagner
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 : 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.
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).
The field experiment consisted of eight treatments set up as a randomized-block design, with one replicate for each treatment in each of four blocks. The treatments were three spiderling densities (Zero, Low, and High) crossed with predator-reduction treatments. A single block consisted of two groups of four contiguous fenced enclosures (plots) arranged in a 2 x 2 design with two shared walls. The two groups of plots within a block were 2-5 m apart, and the blocks themselves were 20-100 m apart.
The basic experimental unit was a 2-[m.sup.2] area (1.4 x 1.4 m) of the forest floor enclosed with 36-cm metal flashing buried 8 cm in the soil. Though wolf spiders lack true scopulae on their tarsal claws and thus cannot climb smooth vertical surfaces (Fuelix 1982), they can climb metal barriers to which dirt has adhered (J. D. Wagner, personal observation). To ensure the fence was an effective barrier to S. ocreata migrations, we installed a horizontal 5-cm metal lip barrier along the top of both sides of the wall. Each enclosed plot contained 15 pitfall traps made from plastic jars (4.7 cm wide and 6.5 cm deep) placed flush with the soil surface within a polyvinyl chloride sleeve. Each trap contained a moistened piece of foam stopper to maintain high humidity. The pitfall traps were closed between sampling periods by replacing the traps with capped jars.
Manipulation of spider density: Zero, Low and High treatments. – Enclosures and pitfall traps were installed 4-12 June 1992, and for the next month adult S. ocreata females were removed from all plots by live trapping and visual searches during day and night. All encountered spiders were adults; most were females that had egg sacs or produced them shortly after being collected. None carried emerged spiderlings, nor were spiderlings or older immature stages collected. A few mature male S. ocreata were encountered, but were not collected since they die by the time spiderlings disperse (J. D. Wagner, personal observation). Females removed from the enclosures, and other females with egg sacs collected from surrounding areas, supplied spiderlings to stock the enclosures. Adult females were housed in the laboratory in individual jars with a moistened plaster-of-Paris base to maintain a high humidity. After several of a female’s progeny had started to disperse, we gently coaxed the remaining spiderlings from their mother and weighed the brood to determine average spiderling mass. Spiderlings were then isolated in individual vials and randomly assigned to either a low-density (90 individuals/plot = 45 spiderlings/[m.sup.2] = 0.75 x normal density) (Low) or high-density (360 individuals/plot = 180 spiderlings/[m.sup.2] = 3x normal) (High) treatment. Broods were distributed proportionately across treatments to remove any genetic or mass bias on future spiderling growth and survival. Normal spiderling field density (60 spiderlings/[m.sup.2]) was calculated as the product of adult female density in the fenced enclosures (1.4 [+ or -] 0.2 females/[m.sup.2]) and fecundity for all spiders collected (40.2 [+ or -] 1.3 spiderlings/egg sac, n = 78 females). Spiderlings were introduced into the Low and High enclosures from 20 July through 29 July 1992, with 29 July being designated Day 0 for all data analysis. No S. ocreata spiderlings were added to the zero-density (Zero) enclosures. The zero-density plots allowed us to evaluate the effectiveness of the fence barriers in preventing immigration of S. ocreata into the experimental plots. The experiment was terminated on 19 October 1992 (Day 84).
Manipulation of natural enemies: Predator-Removal treatment, Sift and No Sift controls. – To remove predators from the Predator-Removal plots, all the leaf litter was removed, hand-sifted to isolate predators, and returned. We removed all generalist arthropod predators that could feed upon wolf spiders, e.g., carabid and staphylinid beetles, centipedes, and all spiders (Edgar 1969). Immediately after removing the predators we covered the plots with 3-mm mesh netting (mesh-type C-80045; Tenax Corporation, Jessup, Maryland 20794 USA) to exclude ground-foraging birds and predaceous wasps. Litter was sifted and netting was installed 8-10 July 1992, 10 d before we started stocking the plots with spiderlings. Every 2-3 wk the plots were searched without disturbing the litter, and any discovered predators were captured and preserved. Predators were also removed if trapped during one of the four sampling periods when spider densities were estimated.
We established two types of controls. In the Sift control, litter was removed, sifted, and returned, but captured predators were returned to the plot. Since removing and sifting the litter could affect the prey of S. ocreata, Zero and Low-No Sift plots were established. High-No Sift plots were not created since this treatment would have required obtaining and processing an additional 1440 spiderlings, which was not feasible. Because estimating predator numbers would have required removing and sifting the litter, we did not estimate densities of natural enemies in the plots during the experiment. Evaluation of the effectiveness of the predator removal was restricted to examining predator densities across all treatments at the end of the experiment, when all plots were sampled destructively.
Estimating response variables: Schizocosa ocreata densities, S. ocreata growth rate, and densities of prey. – Densities of S. ocreata were estimated in three ways during the experiment. First, we obtained an index of initial abundance/activity by trapping, counting, and releasing spiderlings within the enclosures daily on 3-5 August (represented by Day 6, the middle of the sampling period). Second, at the end of the experiment (Day 84) all the plots were destructively sampled over 7 d to capture all remaining S. ocreata and natural enemies. Third, we also monitored changes in S. ocreata densities in the surrounding natural habitat. Estimates of activity/abundance in open areas based on pitfall trapping are not comparable to those obtained in the fenced enclosures because the sizes of the areas effectively sampled are different. Therefore, we estimated S. ocreata densities directly from 0.05-[m.sup.2] litter grabs. Approximately 10 sample sites were chosen haphazardly in the open areas surrounding each block. At each site, a metal frame (25 cm in diameter) was rapidly placed on the forest floor and all the litter was removed and sifted to capture S. ocreata. Density estimates for the open areas were determined 1-3 d after each 3-d trapping period.
To determine treatment effects on spider growth, we sampled the plots by live pitfall trapping at four periods during the experiment (designated as Days 23, 46, 69, and 84). Pitfall traps were opened for 3 d at each sampling period. Trapped spiders were weighed and their cepbalothorax width was measured without their being anesthetized. Spiders were housed in the laboratory and not returned to the plot until the end of the 3-d trapping period.
Field studies utilizing labeling techniques to examine energy flow have shown that Collembola are a major prey item for cursorial spiders in forest-floor food webs (Moulder and Reichle 1972, Manley et al. 1976). We monitored collembolan densities as an indicator of prey availability for S. ocreata. Litter samples (0.05-[m.sup.2]) were taken from each plot and Collembola were extracted using a modified Kempson-Macfadyen extractor (Kempson et al. 1963, Schauermann 1982). Initial Collembola samples were taken 10 d prior to Day 0, and subsequent samples were collected on Days 12, 26, and 66.
We used ANOVA (SAS Institute 1990) to examine the effects of spiderling density, predator removal, and litter sifting on spiderling growth (based on mean cephalothorax width and change in mean mass), and spiderling survival (measured as the percentage of the number introduced that were collected at the end of the experiment). Since initial densities of Collembola differed significantly between some treatments prior to the introduction of spiderlings, treatment effects on Collembola densities were analyzed by ANCOVA using initial Collembola density as the covariate.
Laboratory rates of cannibalism
The two laboratory experiments were conducted the year following the field experiment in order to: (1) exclude all mortality from natural enemies and thereby directly measure rates of cannibalism among S. ocreata spiderlings of similar age/size class; and (2) evaluate whether cannibalism among S. ocreata spiderlings occurs at a magnitude strong enough to explain the density decline observed in the field study.
The basic experimental arena was a clear plastic storage box (50 x 76.25 x 15 cm; inside floor area = 0.3 [m.sup.2]) open on the top. The bottom contained 5 cm of plaster-of-Paris, which was kept moist to maintain high humidity. Though spiderlings were generally unable to climb the smooth walls of the arena, horizontal Plexiglas strips (7.5 cm wide) riveted along the top edge of the arena acted as an additional barrier to escape.
Attempts to manipulate rates of cannibalism were made by altering prey availability and/or habitat complexity (sensu Anholt 1990, Johansson 1992). Prey was absent or present and habitat complexity was either simple or complex. We expected that increasing habitat complexity would decrease rates of cannibalism due to lower rates of encounter with conspecifics and an increase in potential refuges.
Simple habitats (“Simple”) consisted of bare plaster-of-Paris; complex habitats (“Complex”) contained 80 g (dry mass) of leaf litter on top of plaster-of-Paris. Litter depth (2.5 cm) was comparable to that in the field. The litter was a beech-oak-tulip poplar-sweet gum mixture from the site where the parental spiders were collected. The litter was processed and dried in a modified Kempson-McFadyen extractor (Kempson et al. 1963, Schauermann 1982) to remove any potential predators and prey.
Prey were Collembola (Tomocerus sp.) obtained from laboratory cultures started with individuals collected from the Patuxent Wildlife Research Center (PWRC), and first-instar crickets (Acheta domestica) obtained from a supplier. Based on data from the field study, we determined that 300 collembolans per laboratory arena were comparable to field densities. Introduced Collembola were not counted directly; instead, their numbers were estimated by live mass (0.3 g of collembolans yielded [approximately equal to]300 individuals, 0.5-3 mm in size). Crickets were added to increase the diversity of the spider’s diet and substituted for alternative field prey.
The spiderlings used to stock the laboratory arenas were the progeny of adult females with egg sacs (n = 146) collected on 3-4 July 1993 from our field site at PWRC. The protocol of the field experiment was used to isolate spiderlings and assign them to treatments.
Experiments were conducted at [approximately equal to]25 [degrees] C and a 14:10 L:D photocycle. Relative humidity within the arenas was [approximately equal to]65%, equivalent to that within the leaf litter at the field site. Arenas were misted approximately every other day to maintain humidity and introduce a source of drinking water for the spiders. After 14 d all surviving spiders were removed, counted, and weighed.
Since emigration was prevented and natural enemies were absent, missing spiders were considered to have been cannibalized (mortality by starvation was minimal; see Results: Mortality by starvation, below). Wolf spiders use chelicerae to masticate their prey during feeding. Since prey carcasses from wolf spiders are typically an unrecognizable mass (Foelix 1982), the six intact spiderling carcasses found were not considered to have resulted from cannibalism, and were added to the number of spiderlings alive at the end of the experiment to estimate rates of cannibalism.
Laboratory Experiment 1: measuring rates of cannibalism. – This experiment determined cannibalism rates under a range of environmental conditions with the goal of comparing them to the rates of disappearance observed in the field. The design was a 2 x 2 factorial (habitat complexity x prey level) with six replicates per treatment. A humidity/temperature gradient from ceiling to floor in the experimental room prompted us to employ a randomized block design with treatment blocked by shelf height.
Each arena was stocked with 60 newly dispersed spiderlings, a number equivalent to the High density treatment (180 spiderlings/[m.sup.2]) in the field experiment. Spiderlings from 36 broods were divided equally among arenas. Average spiderling mass per arena was 0.755 [+ or -] 0.02 mg. Each arena in the Prey treatment received 300 Collembola and 50 pinhead crickets at the beginning, with 50 additional crickets added on Day 7.
Laboratory Experiment 2: density-dependent cannibalism. – To examine the density-dependent component of cannibalism, we stocked the arenas with spiderling densities equivalent to either the Low or High densities used in the field experiment. We used a 2 x 2 factorial design (spiderling density x prey level) replicated two times for each treatment (n = 2). Habitat was Complex for all treatments. We had only enough spiderlings of similar age and size to stock two replicates for each treatment. Consequently we did not block treatment by shelf, but instead used a completely randomized design.
The High-density treatment had 60 spiders/arena and the Low-density treatment had 15 spiders/arena. Some of the introduced spiders were newly dispersed spiderlings, but most had been used in Laboratory Experiment 1. Spiderlings alive at the end of the first experiment were ranked by mass and only those between 0.7 and 3.0 mg were used. This experiment used spiderlings with a large range in mass. Since a large size differential enhances the potential for cannibalism (Fox 1975b, Polis 1981, Van Buskirk 1989, Orr et al. 1990), we distributed spiderlings proportionally across treatments so as to maintain equivalent size distributions. There was no significant difference in initial masses between spider-density treatments (High: 1.74 [+ or -] 0.039 mg; Low: 1.72 [+ or -] 0.077 mg).
This experiment examined the effects of spider density on cannibalism rates when prey are in excess. In the Prey treatment, 300 Collembola (0.3 mg) were added initially and on Day 7; 50 crickets were added initially and every 3 d thereafter. As in Experiment 1, the only prey available in the No Prey treatment were other spiders.
In both laboratory experiments ANOVA was used to analyze percentage survival (arcsine transformed) and final spider masses. Multiple comparisons of treatment means within an analysis were made using a Fisher’s least-significant-difference test. All values reported are means [+ or -] 1 standard error.
Evaluating mortality by starvation
Though adult spiders can withstand starvation for several weeks (Wise 1993), young spiderlings may lack the stored energy resources needed to withstand starvation for long periods. We assessed the possibility that starvation was creating the increase in mortality when spider densities were high in both the field and laboratory experiments.
During the field experiment, a separate laboratory study measured mortality in newly dispersed spiderlings assigned to one of three treatments: (1) starved, (2) fed a collembolan once a week, and (3) fed Collembola in excess. Spiderlings were housed individually in plastic jars (4.7 cm wide and 6.5 cm deep) with plaster-of-Paris bases to maintain high humidity. Broods from three different females were used to stock the 20 jars for each treatment. A moistened piece of foam stopper was included in each jar as a source of water. Spiderlings fed once a week were given a 1-2 mm collembolan. Each spider was hand-fed to ensure it captured the prey. Spiderlings in the excess-prey treatment were housed with [approximately equal to]10 collembolans, which were replaced as they were eaten. Percentage survival was determined at the end of 14 d.
During the laboratory experiments, rates of mortality in the No-Prey arenas were compared to rates for starved spiderlings housed alone, where cannibalism was prevented. Spiderlings in the Simple habitat treatments of Laboratory Experiment 1 could be seen easily and counted on the bare plaster-of-Paris. This allowed spiderling survival to be evaluated at Day 7 as well as at the end of the experiment (Day 14). Comparing survival rates of starved spiderlings housed individually or in groups allowed us to separate the contributions of cannibalism and starvation in overall spiderling mortality.
The index of abundance/activity determined from pitfall trapping on Day 6 indicated the desired four-fold difference in spiderling density was established within the plots. In the High-density treatment, numbers of 50.0 [+ or -] 5.35 spiderlings per day were trapped and 8.17 [+ or -] 2.29 spiderlings per day were trapped in the Low-density treatments (mean [+ or -] 1 SE). Spiderling density at Day 9 for the open, unfenced areas was 35.4 [+ or -] 9 spiderlings/[m.sup.2] (mean [+ or -] 1 SE), indicating our Low density of 45 individuals/[m.sup.2] was similar to the density of spiderlings in the open areas.
In the first 3 wk of the experiment, nine additional adult female Schizocosa ocreata were removed from the 32 plots. Four of the females were from three different Low-density plots, three were from two High-density plots and two were removed from the same Zero-density plot. None of these adult females was carrying spiderlings. Only one of the nine adults made an egg sac in the laboratory within a few days of capture, suggesting that spiderlings had already dispersed from the remaining eight females. The lip barrier was successful in preventing significant spiderling movement since a total of only 24 spiderlings was removed from 7 of the 12 Zero plots during the experiment. At the end of the study the Zero plots contained an average of only 0.50 [+ or -] 0.2 spiderling/[m.sup.2].
Before the S. ocreata spiderlings were added, 6.32 [+ or -] 1.0 predators/[m.sup.2] (other spiders, beetles, and centipedes) were removed from the Predator-removal plots. During the experiment we removed an additional 3.7 [+ or -] 0.65 predators/[m.sup.2] from the Zero, 6.2 [+ or -] 1.4 predators/[m.sup.2] from the Low, and 7.6 [+ or -] 1.8 predators/[m.sup.2] from the High spider density Predator-removal plots. By the end of the experiment (Day 84), predator densities had declined by 80% and did not differ between the Predator-removal, Sift, and No Sift plots (0.95 [+ or -] 0.2 predator/[m.sub.2] and 1.0 [+ or -] 0.3 predators/[m.sup.2], 1.0 [+ or -] 0.4 predators/[m.sup.2], respectively; spider density treatments combined).
Collembola densities. – To examine the impact of fencing on the numbers of a major prey type, Collembola densities within the Low-No Sift and Low-Sift plots were compared to densities in open, unmanipulated areas on Days – 10, 12, 26, and 66. We analyzed Low-density plots because their densities of S. ocreata were similar to densities in the open areas. Fencing resulted in a significant increase in numbers of Collembola early in the experiment (Day 12), but this difference disappeared by Day 26 [ILLUSTRATION FOR FIGURE 1 OMITTED]. Collembolan densities were not significantly affected by sifting the litter or by removing and excluding natural enemies. Increasing densities of S. ocreata had no consistent effect on collembolan densities [ILLUSTRATION FOR FIGURE 2 OMITTED].
Spiderling growth. – Increasing spiderling density had a negative effect on growth, but the reduction was minor ([ILLUSTRATION FOR FIGURE 3 OMITTED]; Table 1). Spiders in the High-density plots were 2% smaller in cephalothorax width, and weighed 9% less, than spiderlings in the Low-density plots. Spiderlings in the fenced enclosures, regardless of density treatment, were larger than those spiderlings collected in the surrounding open areas [ILLUSTRATION FOR FIGURE 3 OMITTED].
Spiderling survival. – Overall spiderling survival was low; 81% of all the spiderlings introduced into the plots had disappeared by the end of the experiment. Removing and excluding predators had no significant effect on spiderling survival ([ILLUSTRATION FOR FIGURE 4 OMITTED]; Table 2). In contrast, spider density significantly affected spiderling survival – the final percentage surviving in the Low-density plots (29.1%) was twice that of the High treatment (15.7%).
TABLE 1. Analysis of growth rates of Schizocosa ocreata spiderlings
in the field experiment. (A) Mass: ANOVA on the index of change in
spider mass(*) at the end of the experiment. (B) Cephalothorax
width: ANOVA on the final mean cephalothorax width.
TABLE 2. ANOVA table of final percentage survival (arc-sine [square
root of proportion)] of Schizocosa ocreata in the field experiment
TABLE 6. ANOVA table of percentage mortality (arcsine [square root
of proportion)] of Schizocosa ocreata at the end of Laboratory
Cannibalism among S. ocreata spiderlings can also reduce the number of potential competitors, thereby diminishing possible negative effects from exploitative and/or interference competition. Although exploitative competition for prey appears to be uncommon in web-building spiders, it may occur more frequently among cursorial spiders (reviewed in Wise 1993). Our field studies have shown that increasing densities of S. ocreata spiderlings results in a small reduction in spiderling growth, evidence that S. ocreata spiderlings are experiencing minor levels of exploitative and/or interference competition (results are similar to those reported in Wise and Wagner 1992). Absence of evidence for strong exploitative competition agrees with the lack of a consistent effect of S. ocreata density on Collembola numbers. High rates of cannibalism among S. ocreata spiderlings may reduce their effect on lower trophic levels.
Our experiments have revealed some counterintuitive attributes of cannibalism among young instars of S. ocreata. Cannibalism typically occurs between different-sized individuals, with relative rather than absolute size as a determining factor (Polls and McCormick 1987, Van Buskirk 1989, Orr et al. 1990). However, we found rates of cannibalism were high among S. ocreata spiderlings of similar age and size. Although increasing habitat complexity is generally considered to decrease rates of predation and cannibalism (Crowley et al. 1987, Thompson 1987, Convey 1988), we found no effect of habitat complexity on rates of cannibalism when prey were absent, and an increase in the rate of cannibalism when prey were present.
To maintain similar rates of cannibalism with an increase in habitat complexity requires that: (a) overall spider foraging activity also increases and/or (b) the rate of success for an individual cannibalistic attack increases. The similar low rates of growth within the Complex and Simple + No-Prey habitats suggest intensities of cannibalism were similar because hunger levels were equivalent. Wolf spiders use both visual and vibrational cues to detect potential prey (Foelix 1982, Lizotte and Rovner 1988) and can avoid cannibalistic attacks through leg-waving displays (Aspey 1974, Koomans et al. 1974, Stratton 1984) or acoustical signals (Rovner 1967). However, an increase in habitat complexity may obstruct an individual’s ability to detect a conspecific predator and thereby increase the probability of an ambush. Although rates of encounter may decrease with increasing habitat complexity, we propose that the rate of successful cannibalistic attacks increases, thereby preventing the expected reduction in rates of cannibalism. The paradoxically higher rate of cannibalism in the Complex habitat + Prey treatment compared to the Simple habitat + Prey treatment may reflect the difference in hunger level between treatments. Growth in the Complex habitat + Prey arenas was significantly less than that in the Simple habitat + Prey arenas, indicating that increasing habitat complexity indirectly increased rates of cannibalism by reducing the spiders’ access to prey. Hunger level is often directly correlated with rates of cannibalism (Fox 1975b, Orr et al. 1990, Agarwala and Dixon 1992, Johansson 1992, 1993) and a spider’s tolerance of con-specifics (Rypstra 1983).
Models of cannibalism as a population-regulating factor have produced conflicting predictions (Landahl and Hansen 1975, Gabriel 1985, Gabriel and Lambert 1985, Dickmann et al. 1986, Bosch et al. 1988, Cushing 1991, Hastings and Costantino 1991). Population oscillations are predicted by those models that rely on cannibalism between different age classes (adults cannibalizing eggs or young age classes [Diekmann et al. 1986, Hastings and Costantino 1991, Frauenthal 1983, Cushing 1991]) and such oscillations have been obtained in some empirical studies (Tribolium, Mertz 1969; Notonecta, Ocr et al. 1990). However, cannibalism has been shown to reduce population oscillations if it acts equally across all age groups (Landahl and Hansen 1975, Polis 1980, Crowley and Hopper 1994). At our study site S. ocreata is an annual species in which the majority of the females reproduce within a 3-4 wk period. When spiderlings disperse, their density is high (60-90 individuals/[m.sup.2]) and adult females are scarce ([less than] 1.5 spiders/[m.sup.2]). Cannibalism by adult female wolf spiders on conspecific spiderlings is inhibited when females are carrying an egg sac or young (Higashi and Rovner 1975, Wagner 1995). These facts together suggest that the majority of cannibalism occurs between young S. ocreata of similar size and age.
Our field and laboratory experiments indicate that cannibalism among young S. ocreata is a strong DD mortality factor that intensifies with decreasing prey availability. Schizocosa ocreata spiderlings exhibited significant growth from energy obtained solely from cannibalism. Since cannibalism readily occurs among newly dispersed spiderlings, it has the potential to be a dominant mortality factor at a time when spider densities are at their highest. The rapid decline in spiderling densities observed in open, unfenced areas of this field study and a previous study (Wise and Wagner 1992) are similar to those observed in the experimental fenced plots. Within the fenced enclosures, cannibalism appears to be the dominant mortality factor. Consequently, we propose that cannibalism is also a dominant mortality factor for open, unfenced populations of S. ocreata spiderlings, and that cannibalism contributes significantly to the regulation of population densities of this abundant wolf spider.
We would like to thank the Patuxent Wildlife Research Center, U.S. Fish and Wildlife Service, for allowing use of its beech-oak forest, and the TENAX Corporation, Jessup, Maryland, for donating the C-80045 netting material. We would also like to thank Gall Langellotto for her excellent research assistance in the field experiment. The field study was supported in part by a Special Research Initiative Support award and a Graduate Research Assistantship award from the University of Maryland Graduate School, Baltimore, to D. Wise. Support for the laboratory experiments, which were conducted at the University of Kentucky, was provided by NSF grant DEB-9306692 (awarded to D. Wise). We wish to thank Randy Hunt, David Wooster, and Ken Yeargan for their constructive criticisms on the various manifestations of this manuscript. We would also like to thank Frank Messina, David Spiller, and two anonymous reviewers; their collective comments greatly improved the manuscript. The senior author would also like to thank Etelvina Fonseca-Wagner for helping to keep things in perspective. These experiments were conducted by the senior author for partial fulfillment of a Ph.D. degree at the University of Kentucky. This is manuscript number 94-7-126 from the Kentucky Agricultural Experimental Station.
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