2005. The Journal of Arachnology 33:390±397

SIZE DEPENDENT INTRAGUILD PREDATION AND CANNIBALISM IN COEXISTING WOLF (ARANEAE, LYCOSIDAE)

Ann L. Rypstra: Department of Zoology, Miami University, 1601 Peck Blvd. Hamilton, Ohio 45011 USA. E-mail: [email protected] Ferenc Samu: Department of Zoology, Plant Protection Institute, Hungarian Academy of Sciences, PO Box 102, Budapest, H-1525 Hungary

ABSTRACT. Two species of wolf , Hogna helluo (Walckenaer 1837) and Pardosa milvina Hentz 1844 dominate the predatory community on the soil surface of agroecosystems in eastern North America. Although as adults they are very different in size, differences in phenology ensure that they overlap in size at various times during the year. In a laboratory experiment, we explored the propensity of each species to attack and kill the other species (intraguild predation), conspeci®cs (cannibalism) or crickets (ordinary predation). Both spiders attacked and killed a broader size range of crickets more quickly than they approached other spiders. We found no differences in Hogna foraging on conspeci®cs or Pardosa, but Pardosa attacked and killed Hogna more readily than conspeci®cs. Because Hogna was so slow in attacking other spiders, their impact as an intraguild predator may be quite small, especially if their approach to crickets is an indication of their predatory tendencies with insects. On the other hand, Pardosa attacked and killed small Hogna as readily as crickets, which suggests they may have an in¯uence on Hogna populations if Hogna young emerge coincident with large juvenile or adult Pardosa. Keywords: Cannibalism, intraguild predation, agrobiont spiders, predator-prey

Cannibalism and intraguild predation (IGP) tory tendencies of two species of wolf spider are important to spider communities and have (Araneae, Lycosidae) that coexist on the soil the potential to affect population sizes and/or surface in agricultural ®elds across the eastern species diversity of spiders as well as that of portion of North America. Because the species potential insect prey (Wagner & Wise 1996; differ in size, activity, and phenology, we Hodge 1999; Samu et al. 1999; Finke & Den- wanted to characterize the circumstances un- no 2002; Matasumura et al. 2004; Denno et der which these spiders engaged in cannibal- al. 2004). Predation is a dynamic process, the ism or intraguild predation and compare those outcome of which depends on the relative siz- predatory interactions to attacks on insect es of the predator and prey, their physiological prey. Under controlled laboratory conditions, state, attack strategy and inherent aggressive- we paired a wide size range of individuals ness (Walker et al. 1999; Persons et al. 2001; with conspeci®cs, the other species of spider, Buddle 2002; Balfour et al. 2003; Buddle et or crickets and documented the outcome and al. 2003; Mayntz et al. 2005). Many of these timing of predation. In this way, we hoped to gain a better understanding of the speci®c factors will shift over time both with age and predatory strategy of each of the spider spe- recent experience and thus the relative impor- cies and the relative in¯uence that these spe- tance of cannibalism and/or IGP to foraging cies have on their insect prey, which would individuals, population structure and commu- help us to gain insight into the nature of their nity composition will shift as well (Wagner & co-existence. Wise 1996; Balfour et al. 2003; Buddle et al. 2003). For spiders that coexist, an understand- METHODS ing of the situations under which cannibalism Study species.ÐHogna helluo (Walckenaer and IGP occur is critical to understanding how 1837) and Pardosa milvina Hentz 1844 co- and when they can persist in the same habitat. exist on the soil surface in disturbed riparian In the present study we explore the preda- habitats and agroecosystems throughout the

390 RYPSTRA & SAMUÐPREDATION & CANNIBALISM IN WOLF SPIDERS 391

sity Ecology Research Center (Oxford, Butler County, Ohio, USA) and held in the labora- tory or reared from collected at that site. When not involved in experimentation, spiders were housed individually in translu- cent plastic cylindrical containers 8 cm in di- ameter with 5 cm walls with 1±2 cm of damp peat moss covering the bottom. Spiders were watered and fed once or twice weekly on a diet of crickets (Acheta domesticus), fruit ¯ies (Drosophila spp.) and or meal worms (Tene- brio spp.). Containers with spiders were held in an environmental chamber between 23Ð25 ЊC on a 12:12 L:D cycle at 60±75% humidity. Experimental protocol.ÐSpiders were Figure 1.ÐMean prey to predator mass ratio randomly selected from the laboratory popu- (PPR Ϯ S.E.) for captured prey vs. the time (min lation and brought to standard hunger levels Ϯ S.E.) it took the prey to be captured. Trials where by feeding them ad libitum with Drosophila Hogna was predator are indicated by solid squares melanogaster for 2 days. Spiders were then and those where Pardosa was predator are indicated held for 7 days before testing to ensure that by open squares. Speci®c prey types are listed with they were similarly hungry. Spiders were ran- an arrow pointing to the data for that treatment. domly assigned to be paired with conspeci®cs (to monitor cannibalism), heterospeci®cs (to eastern portion of North America (Dondale & monitor intraguild predation) or crickets (to Redner 1990; Marshall & Rypstra 1999; Mar- monitor ordinary predation). Those assigned shall et al. 2002). Pardosa is small (20 mg), to be paired with conspeci®cs were marked active, and can be found at high densities (10± with a drop of acrylic paint on the abdomen 15 per m2) whereas Hogna is large (800 mg), or cephalothorax so that we could identify in- less active, and found at relatively low den- dividuals. All spiders and crickets were sities (1±2 per m2) in soybean ®elds in the weighed and then introduced into a testing midwestern section of the United States (Mar- arena simultaneously. The arenas consisted of shall & Rypstra 1999; Walker et al. 1999; 14 cm diameter Petri dishes with a base of Marshall et al. 2002). Pardosa is an annual dampened plaster of Paris (as in Samu et al. species with a mid-July population peak. Ex- 1999). Animals were allowed to interact in the cept for a relatively short period during which arena for 24 h during which time we recorded the adults and spiderlings co-occur, the size if and when predation occurred. Experiments distribution of Pardosa individuals active in were run in groups that included representa- the ®elds at any given time is fairly narrow tives of all treatments between July 1998 and (Marshall et al. 2002). On the other hand, July 2001. Hogna seems to have a two-year life cycle Statistical analysis.ÐIn order to determine with more stages occurring in the ®elds at the how similar the spiders and insects used in same time (Marshall et al. 2002). Although each treatment were, we compared the mass Hogna are usually larger than Pardosa, be- of predators and prey across all treatments in cause of the variability in their life cycle, it is ANOVAs. In addition, we calculated prey/ possible for large subadult or adult Pardosa predator mass ratio (PPR) by dividing the to coexist with early stages of Hogna. Previ- mass of the prey by the mass of the predator. ous studies have revealed that each species In cases where there was no predation, we readily consumes smaller individuals of the randomly assigned one of the spiders as prey other in the laboratory (Persons et al. 2001; and the other as predator using a coin toss Balfour et al. 2003). Here we explore those algorithm. In order to ensure pairings were predatory interactions systematically across a similar across treatments, PPRs were also broad range of size ratios. compared in an ANOVA. The effects of pred- Both species of spiders were collected from ator species, prey type, and PPR on the fre- corn and soybean ®elds at the Miami Univer- quency of predation were compared using a 392 THE JOURNAL OF ARACHNOLOGY

Figure 2.ÐFifty percent lethal mass ratio (LR50) (Ϯ 95% con®dence interval) for Hogna (on the left) and Pardosa (on the right). Prey type is on the X-axis. Where treatments are indicated with the same small letter in the middle of each ®gure, the overlap of the 95% con®dence ranges suggests that there was no signi®cant difference.

logistic regression analysis. From the logistic between prey types. We de®ned PPRcritical as regression, we determined the PPR at which the maximum PPR value where the differenc- there was a 50% likelihood of a predatory es between predation on two prey types were event (LR50). Differences in LR50s across no longer signi®cantly different (P ϭ 0.05). treatments were evaluated by comparison of In order to ®nd the PPRcritical, we started by the 95% con®dence intervals. The effects of removing the sample with the highest PPR the same factors (predator species, prey type, and rerunning the statistical test, if it was still and PPR) on the time until predation were signi®cant, we removed the sample with the evaluated using a parametric survival analysis next highest PPR, and ran the test again. We using the Proportional Hazards model. In this continued this process until the P-value as- case pairwise comparisons were made using sociated with any difference was equal to the Bonferroni test with an overall P-value of 0.05. 0.05. Both the logistic regression and survival models were run initially with all interactions RESULTS included. The non-signi®cant interactions Overall there were no differences in the were removed after the ®rst run and the mod- mass of the Hogna, Pardosa, or crickets used els were run again. in our treatments (Predator mass F ϭ 1.81, df We also wanted to determine if any of the ϭ 5, 401, P ϭ 0.11; Prey mass F ϭ 1.02, df observed preferences were due to size or if ϭ 5, 401, P ϭ 0.40) (Table 1). In addition, they had to be attributed to some other quality animals were paired so that the PPR values of the prey (e.g. nutrition, taste). We hypoth- were similar across treatments (F ϭ 1.71, df esized that, if the preference was size related, ϭ 5, 401, P ϭ 0.13) (Table 1). Even though then there would be a size ratio (PPRcritical) be- not signi®cantly different, the PPR for Hogna low which predators would not discriminate on crickets was somewhat higher than the oth- RYPSTRA & SAMUÐPREDATION & CANNIBALISM IN WOLF SPIDERS 393

killed larger crickets than they killed other spiders (higher PPR) (Fig. 1). However at

PPRs less than 0.54 for Pardosa (PPRcritical, ␹2 ϭ ϭ 97 3.82, P 0.05) and 0.57 for Hogna ␹2 ϭ ϭ (PPRcritical, 74 3.46, P 0.05) there were no differences between spider vs. cricket prey. The process by which we generated the

PPRcritical inevitably resulted in a loss of sam- ple size, and therefore power, however, we re- gard the remaining case numbers still large ϭ ϭ enough (NPardosa 98, NHogna 75) to draw valid conclusions on PPRcritical values. The PPR at which there was a 50% chance of a predatory event (LR50) was signi®cantly higher for crickets than for spider prey (Fig. 2). Likewise, overall crickets were killed more quickly than other spiders (Figs. 1, 3). Comparing the predation strategy of Hogna and Pardosa.ÐThe two predators dif- fered in their responses to the prey types test- ed (Table 2). Hogna consistently took larger prey than Pardosa from every prey category (Figs. 1, 2). On the other hand, Pardosa was consistently faster than Hogna in taking every prey type (Figs. 1, 3). Pardosa was more like- ␹2 ϭ ly to capture Hogna than conspeci®cs ( 139 5.53, P ϭ 0.018, Table 1) but there was no difference in the capture rate of Hogna on ei- ␹2 ϭ ther spider species ( 131 0.27, NS; Table 1). Likewise, the LR50 was larger for Pardosa preying on Hogna than it was for Pardosa cannibalism (Fig. 2) but there was no differ- Figure 3.ÐCapture success over time of Hogna ence in the LR50 for Hogna preying on het- and Pardosa on the three different prey types. erospeci®cs or conspeci®cs (Fig. 2). Similarly Those indicated with the same letter (at the right of the line) were not signi®cantly different using Bon- Pardosa captured Hogna more quickly than it ferroni comparisons with a critical value of 0.05. captured other Pardosa but there were no dif- ferences in the Hogna's predatory speed on conspeci®cs or heterospeci®c spiders (Figs. 1, ers. To some degree, this variation was inten- 3). tional as we attempted to observe interactions across the complete prey size range that each DISCUSSION spider would take. Note that even though the Clearly these two spider species, Hogna PPR is close to one, meaning that the prey helluo and Pardosa milvina, differ in their for- were the same size as the predator, the capture aging behavior across the various sizes of the rate is still very high (94.3%) as compared to different prey types tested here. Hogna is gen- other treatments (Table 1). Predator species, erally slower to attack and kill a potential prey prey species and the PPR all affected the oc- but generally take prey in larger size classes currence and the timing of predation in com- than Pardosa. Although Hogna differentiated plex ways (Table 2). between crickets and spiders, they did not Predation on crickets vs. spider prey.Ð seem to differentiate between conspeci®cs and Both Hogna and Pardosa had higher capture a common coexisting intraguild predator, Par- success on crickets than on spiders (Pardosa: dosa. On the other hand, Pardosa reacted dif- ␹2 ϭ ϭ ␹2 ϭ 195 10.19, P 0.001; Hogna: 209 ferently to all three prey types; killing larger 54.54, P Ͻ 0.0001; Table 1). Both species crickets faster than they killed Hogna and kill- 394 THE JOURNAL OF ARACHNOLOGY

Table 1.ÐSummary of the sample size, capture frequency, predator mass (ϮS.E.), prey mass (ϮS.E.), and prey to predator mass ratio (PPR Ϯ S.E.) for each treatment.

Number Predator Treatments n captured (%) mass (mg) Prey mass (mg) PPR Hogna on crickets 70 66 (94.3%) 14.4 Ϯ 1.2 14.6 Ϯ 1.9 0.97 Ϯ 0.11 Hogna on Hogna 78 34 (43.6%) 19.2 Ϯ 1.2 11.7 Ϯ 1.7 0.84 Ϯ 0.07 Hogna on Pardosa 54 28 (48.1%) 19.5 Ϯ 1.4 9.9 Ϯ 2.1 0.63 Ϯ 0.05 Pardosa on crickets 64 50 (78.1%) 19.6 Ϯ 1.3 11.1 Ϯ 1.8 0.75 Ϯ 0.07 Pardosa on Hogna 74 44 (59.5%) 17.6 Ϯ 1.2 9.2 Ϯ 1.8 0.64 Ϯ 0.06 Pardosa on Pardosa 66 38 (42.4%) 19.1 Ϯ 1.3 12.2 Ϯ 1.9 0.68 Ϯ 0.06 All Groups 406 260 (64.1%) 18.4 Ϯ 1.6 11.4 Ϯ 2.1 0.76 Ϯ 0.03

ing larger Hogna faster than they killed con- trients in proportions that are more closely speci®cs. aligned with the predator's nutritional needs Predation on crickets vs. spider prey.Ð (Polis 1981; Wildy et al. 1998; Fagan et al. Both cannibalism and IGP have been exten- 2002), however several studies have demon- sively documented in wolf spiders (Wagner & strated that growth and survival of wolf spi- Wise 1996; Samu et al. 1999; Balfour et al. ders is lower when maintained on spider diets 2003; Buddle et al. 2003). Because these in- than when provided with insect prey (Toft & teractions carry with them an increased risk of Wise 1999; Oelbermann & Scheu 2002; Mat- injury and/or reciprocal predation, we expect- sumura et al. 2004). Another reason not to eat ed a different, and perhaps more cautious, closely related species is that they may carry predatory approach to other spiders when pathogens that can invade more easily when compared to crickets. We reasoned that the consumed by a conspeci®c or phylogenetical- relative size of the prey to its predator (PPR) ly close host (Pfennig, et al. 1998; Pfennig would be one measure of risk and we found 2000; MacNeil et al. 2003). Thus, selection that the PPR at which there was a 50% chance may favor preferences for non-spider prey. of a predatory event was much higher for Of course wolf spiders behave differently crickets than spider prey (Fig. 2). However from crickets, which may have reduced their further exploration of the data reveals that, for susceptibility to capture. We attempted to con- both spider species, there was a PPRcritical be- trol the circumstances of the interaction so low which there were no differences in the that the predator had access to the same kind rate of predation on spiders as compared to of sensory information in a con®ned space, crickets. Thus, the signi®cant differences in which should minimize the small differences predatory strategy that we observed were due in capture and escape tactics. Nevertheless, it to behavioral shifts that occurred when the is impossible to totally uncouple the prey pref- prey were large relative to the predator. These erences and ease of capture from the speci®c results con®rm that relative size was more im- signals by which the predator detects and portant for spider on spider contests than for identi®es prey items (Uetz 2000; Uetz & Rob- attacks of crickets and suggests that both spi- erts 2002). Thus a further exploration of the der species were sensitive to the risk that a role of speci®c sensory modalities in the pred- large predatory prey item might pose. This ator interactions of these species is warranted. connection may be particularly true for Hog- Comparing the predation strategy of na, which easily subdued large crickets but Hogna and Pardosa.ÐA variety of differenc- were much slower to take smaller individuals es between the foraging strategies of Pardosa of either spider species (Fig. 2). and Hogna have been documented (Walker et Even though risk may be important to the al. 1999; Walker & Rypstra 2002) and this observed differences in predation frequency, study clari®es some additional aspects of there may be other reasons for spiders to pre- those differences. In particular, although Hog- fer insect over spider prey. It has been argued na was the most effective predator on crickets, that organisms feeding on the same trophic Pardosa distinguished between the three prey level, and especially conspeci®cs, provide nu- types in the proportion (Table 1), size (Figs. RYPSTRA & SAMUÐPREDATION & CANNIBALISM IN WOLF SPIDERS 395

Table 2.ÐResults of logistic regression to predict context of the options offered here, prefers prey capture and the results of the proportional haz- large harmless prey. On the other hand, Par- ards survival model to predict the time it took the dosa was generally faster to attack and dis- spiders to capture prey. Both models used predator criminated more ®nely between the three prey species, prey species and prey to predator mass ra- tio (PPR) as predictors. options we included in this study. Implications for species co-existence in Source df Chi squared P the ®eld.ÐThese results may be especially important for agrobiont spiders, such as Hog- Logistic regression model for outcome na and Pardosa, as they may be important Ͻ Whole model 7 299.782 0.0001 agents of biological control. The fact that Predator species 1 9.333 0.0023 crickets were more susceptible to predation Prey species 2 66.809 Ͻ0.0001 PPR 1 82.225 Ͻ0.0001 across a much larger size range than spider PPR * predator 1 20.895 Ͻ0.0001 prey suggests that the in¯uence that two wolf PPR * prey 2 21.807 Ͻ0.0001 spider species have on one another may not Survival model for time until capture be exceedingly strong when alternative insect prey are abundant. To fully assess the ®eld Whole model 9 401.4444 Ͻ0.0001 Predator species 1 8.534 0.0035 importance of these interactions, our ®ndings Prey species 2 189.060 Ͻ0.0001 need to be interpreted in the context of the life PPR 1 302.410 Ͻ0.0001 history of natural populations. Unfortunately, PPR * predator 1 36.730 Ͻ0.0001 in spite of existing ®eld surveys (Marshall & PPR * prey 2 85.380 Ͻ0.0001 Rypstra 1999; Marshall et al. 2002), the life Predator * prey 2 14.433 0.0007 histories of the two species seem to be highly variable and, as a result, are not well enough understood to be predictable. Nevertheless, 1, 2) and timing (Figs. 1, 3) of predation. Par- the available data suggest that Hogna typical- dosa is a much more active species than Hog- ly has a two or three-year life cycle with sev- na (Walker et al. 1999; Walker & Rypstra eral juvenile stages coexisting and Pardosa is 2002), which might have caused us to predict an annual species with a narrower size dis- that they would be more susceptible to pre- tribution at any given time in the season. dation by other wolf spiders which use motion Thus, all life stages of Pardosa have the po- to detect prey (Persons & Uetz 1997). How- tential of coexisting with larger Hogna where- ever, there is no evidence here to demonstrate as only when Hogna spiderlings emerge at that activity made Pardosa any more suscep- tible to the sit and wait predator, Hogna.In times of the year when large juveniles and fact, it appears here that activity translated adult Pardosa are around, do Hogna face pre- into effective search behavior that increased dation risk from Pardosa. Although the trials Pardosa's ability to detect and attack more suggest that Hogna exert modest predatory sluggish prey such as Hogna. pressure on both conspeci®cs and Pardosa, Although not signi®cantly different, the their attacks were very slow (Figs. 1, 3). As PPRs for Hogna paired with crickets were a consequence, Hogna may not exert much somewhat higher than the other pairings be- predation pressure on a quick wolf spider like cause of our desire to cover the full size range Pardosa that in an open ®eld situation could of prey that each spider would attack. Thus run away. On the other hand, Pardosa appear we considered whether the longer capture to prey on small Hogna as quickly as on crick- times observed for Hogna on crickets (Fig. 1) ets, so Hogna that emerge and attempt to go might be due primarily to the fact that they through the ®rst few instars during the early were tested with larger prey items. However, summer, when Pardosa are adults, maybe se- if we compare the mean capture time for verely impacted by Pardosa predation. Clear- crickets larger than the Hogna (PPR Ͼ 1.0; n ly further explorations of the interactions be- ϭ 19) with the capture times for those prey tween these two species in more natural smaller than the Hogna (PPR Ͻ 1.0; n ϭ 51), situations are required to fully quantify their there was no difference (t ϭ 2.0, P ϭ 0.24). in¯uence on one another, the nature of their This fact furthers the characterization of Hog- coexistence and their potential role in the eco- na as a slow selective predator that, in the system. 396 THE JOURNAL OF ARACHNOLOGY

ACKNOWLEDGMENTS tion: implications for prey suppression. Ecology We would like to thank M. Brueseke, E. 83:643±652. Hodge, M.A. 1999. The implications of intraguild Henley, C. Burkett and C.M. Buddle for as- predation for the role of spiders in biological sistance with execution of experiments. We control. Journal of Arachnology 27:351±362. are grateful to S. Marshall, M. Persons, E Langelotto, G.A. & R.F. Denno. 2004. Response of Channell, R. Balfour, S.E. Walker, B. Reif, C. invertebrate natural enemies to complex-struc- Weig, and A. Tolin for ensuring that we had tured habitats: a meta-analytical synthesis. Oec- spiders to work with when they were needed. ologia 139:1±10. S. Wilder, J. Riem, J. Schmidt and other mem- MacNeil, C., J.T.A. Dick, M.J. Hatcher, N.J. Field- bers of the Miami University spider lab pro- ing, K.D. Hume & A.M. Dunn. 2003. Parasite vided useful suggestions on early drafts of this transmission and cannibalism in an amphipod manuscript. F. Samu was Bolyai Fellow of the (Crustacea). International Journal for Parasitolo- gy 33:795±798. Hungarian Academy of Sciences whilst this Marshall, S. D., D. M. Pavuk & A. L. Rypstra. work was conducted. Funding was provided 2002. A comparative study of phenology and by the following sources: OTKA (Hungary) daily activity patterns in the wolf spiders Par- Grants No. T048434 & F030264, NKFP dosa milvina and Hogna helluo in soybean (Hungary) project No. 6/00013/2005; NSF agroecosystems in southwestern Ohio (Araneae, (USA) grants DEB 9527710, DBI 0216776 & Lycosidae). Journal of Arachnology 30:503±510. DBI 0216947; Miami University's Philip and Marshall, S. D. & A. L. Rypstra. 1999. Patterns in Elaina Hampton Fund for Faculty Internation- the distribution of two wolf spiders (Araneae: al Initiatives & International Visiting Scholar Lycosidae) in two soybean agroecosytems. En- Fund. vironmental Entomology 28:1052±1059. Matsumura, M., G.M. Trafelet-Smith, C. Gratton, LITERATURE CITED D.L. Finke, W.F. Fagan & R.F. Denno. 2004. Balfour, R. A., C. M. Buddle, A. L. Rypstra, S. E. Does intraguild predation enhance predator per- Walker & S. D. Marshall. 2003. Ontogenetic formance? a stoichiometric perspective. Ecology shifts in competitive interactions and intra-guild 85:2601±2615. predation between two wolf spider species. Eco- Mayntz, D., D. Raubenheimer, M. Salomon, S. Toft, logical Entomology 28:25±30. & S.J. Simpson. 2005. Nutrient-speci®c foraging Buddle, C.M. 2002. Interactions among young stag- in invertebrate predators. Science 207:111±113. es of the wolf spiders Pardosa moesta and P. Oelbermann, K. & S. Scheu. 2002. Effects of prey mackenziana (Araneae, Lycosidae). Oikos 96: type and mixed diets on survival, growth and 130±136. development of a generalist predator, Pardosa lu- Buddle, C.M., S.E. Walker & A.L. Rypstra. 2003. gubris (Araneae: Lycosidae). Basic and Applied Cannibalism and density-dependent mortality in Ecology 3:285±291. the wolf spider Pardosa milvina (Araneae: Ly- Persons, M.H. & G.W. Uetz. 1997. The effect of cosidae). Canadian Journal of Zoology 81:1293± prey movement on attack behavior and patch res- 1297. idence decision rules of wolf spiders (Araneae: Denno, R.F., M.S. Mitter, G.A. Langellotto, C. Grat- Lycosidae). Journal of Insect Behavior 10:737± ton & D.L. Finke. 2004. Interactions between a 752. hunting spider and a web-builder: consequences Persons, M.H., S.E. Walker, A.L. Rypstra & S.D. of intraguild predation and cannibalism for prey Marshall. 2001. Wolf spider predator avoidance suppression. Ecological Entomology 29:566± tactics and survival in the presence of diet-as- 577. sociated predator cues (Araneae: Lycosidae). An- Dondale, C. D. & J. H. Redner. 1990. The Insects imal Behaviour 61:43±51. and of Canada and Alaska. Part 17. Pfennig, D.W. 2000. Effect of predator-prey phy- The Wolf Spiders, Nurseryweb spiders, and Lynx logenetic similarity on the ®tness consequences spiders of Canada and Alaska (Araneae: Lycos- of predation: A trade-off between nutrition and idae, Pisauridae and Oxyopidae). Biosystematics disease? American Naturalist 115:335±345. Research Centre, Ottawa, Canada. Pfennig, D.W., S.G. Ho & E.A. Hoffman. 1998. Fagan, W.F., E. Seimann, C. Mitter, R.F. Denno, Pathogen transmission as a selective force A.F. Hubery, A.H. Woods & J.J. Elser. 2002. Ni- against cannibalism. Behaviour 55: trogen in insects: Implications for trophic com- 1255±1261. plexity and species diversi®cation. American Polis, G.A. 1981. The evolution and dynamics of Naturalist 160:784±802. intraspeci®c predation. Annual Review of Ecol- Finke, D.L. & R.F. Denno. 2002. Intraguild preda- ogy and Systematics 12:225±251. tion diminished in complex-structured vegeta- Samu, F., S. Toft & B. Kiss. 1999. Factors in¯u- RYPSTRA & SAMUÐPREDATION & CANNIBALISM IN WOLF SPIDERS 397

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