Interactions among forest-floor guild members in structurally simple microhabitats.
|Abstract:||Intraguild predation in structurally complex habitats is thought to weaken trophic cascades and increase food web stability. However, many predators commonly found in leaf litter become restricted to simple microhabitat beneath rocks and logs during periods between rains. It is within this structurally simple microhabitat that some predators defend rich prey resources and are likely to interact strongly as the surrounding forest becomes too dry to forage broadly in space. We conducted a 4-y press experiment where we removed focal predators from unfenced field plots. To evaluate the effects of predators on one another we removed either salamanders or centipedes from beneath artificially placed cover objects and compared abundances of these and other intraguild predators to those in non-removal controls. We predicted that salamanders and centipedes would have strong negative effects on each other and on carabid beetles and spiders. We removed a total of 1988 salamanders and 1056 centipedes over 98 sampling dates. In salamander removal plots spider abundance increased by 34%, and carabid beetles decreased by 15% relative to the control. In centipede removal plots salamanders increased by 18% and carabid beetles increased by 29%, but spider abundance decreased by 15%. Interaction strengths were strongest in the drier summer months when territorial predators were confined in spatially fixed microhabitats. It is during these periods that predators may strongly regulate the abundances of guild members. In territorial species that defend areas beneath natural cover, the effect of intraguild predators may be an important mechanism that regulates distribution and abundance of forest floor predators.|
Food chains (Ecology) (Research)
Animal communication (Research)
Hickerson, Cari-Ann M.
Anthony, Carl D.
Walton, B. Michael
|Publication:||Name: The American Midland Naturalist Publisher: University of Notre Dame, Department of Biological Sciences Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Earth sciences Copyright: COPYRIGHT 2012 University of Notre Dame, Department of Biological Sciences ISSN: 0003-0031|
|Issue:||Date: July, 2012 Source Volume: 168 Source Issue: 1|
|Topic:||Event Code: 310 Science & research|
|Geographic:||Geographic Scope: United States Geographic Code: 1USA United States|
Early studies of community dynamics used simple models to predict distribution and abundance patterns of species in various systems. Hairston and Hairston (1993) noted that simple models operated under a number of assumptions. For example, it was assumed that links in food chains were equal in value, and early models may have ignored the multitude of interactions (e.g., intraguild predation (IGP) and competition) within trophic levels that are likely to affect the remainder of the web. More recently ecologists have begun to focus on the kinds of biotic interactions that influence the ecology of organisms in communities. Ecologists now understand that IGP and omnivory are widespread (Polls and Holt, 1992; Polls, 1998; Wise and Chen, 1999; Moya-Larano and Wise, 2007) and that it may be insufficient to look at linear chains if we are to understand the variables that affect community structure, stability, and ecosystem function (Polls, 1991, 1994; Bascompte, 2009). A more effective approach to understanding the consequences of perturbations on communities is to apply network theory which emphasizes connections within a web rather than focusing on pair-wise interactions between species (Bascompte, 2009). Research investigating behavior among generalist predators in food webs is important because the strength of non-trophic interactions can influence organisms at other positions within the web. For example, IGP is thought to weaken trophic cascades in terrestrial habitats (Holt and Polis, 1997) and increase web stability (McCann et al., 1998) because predation within trophic compartments should reduce pressure on more basal trophic levels. One way to further our understanding of the consequences of competition and/or IGP at the community level is to study the numerical response of potentially interacting guild members to changing densities of single predator groups in complex terrestrial webs (Moya-Lareno and Wise, 2007).
Our research was conducted in a temperate deciduous forest-floor system. Such systems are thought to have characteristics (e.g., high species diversity, structural habitat complexity, IGP, and omnivory) that should attenuate the effects of predators at more basal trophic levels (Finke and Denno, 2004). Terrestrial salamanders, spiders, carabid beetles, and centipedes are among the most abundant predators living in the litter on the forest-floor at our field site. Previous laboratory studies on local populations of ecologically similar predators have shown that competition may be an important mechanism influencing the distribution of secondary and tertiary consumers in the litter of temperate forests in northeast Ohio (Gall et al., 2003, carabid beetles; Hickerson et al., 2004, centipedes; Anthony et al., 2007, centipedes; Figura, 2007, spiders), and a growing body of evidence suggests that terrestrial salamanders in the family Plethodontidae are important regulators of invertebrate communities and of decomposition of organic material on temperate forest-floors (Burton and Likens, 1975; Hairston, 1987; Wyman, 1998; Rooney et al., 2000; Walton, 2005; Walton and Steckler, 2005). These salamanders can be extremely abundant (Test and Bingham, 1948; Jaeger, 1980a; Mathis, 1991), and their biomass has been estimated in one locality to be greater than that of birds and equal to small mammals (Burton and Likens, 1975). The impact of this abundant group of salamanders may exceed that estimated for forest-floor spiders (Moulder and Reichle, 1972). Therefore, plethodontid salamanders are expected to be influential in determining community structure in the detrital web of temperate forests. The dominant plethodontid at our field site is the Eastern Red-backed Salamander, Plethodon cinereus, a territorial and completely terrestrial species (Jaeger, 1984; Mathis et al., 1995; Petranka, 1998; Anthony et al., 2008). Natural cover objects (e.g., rocks, bark, and logs) serve as territories for P. cinereus during dry periods, and it is beneath such cover objects that we expect red-backed salamanders to interact with large invertebrate predators most strongly. The microhabitats beneath cover are generally simple in structure as they are formed by the interface of relatively impervious cover and the organic soil. Simple habitats generally do not support a high diversity of predators (reviewed in Langellotto and Denno, 2004), but in forest systems that experience periodic drying, these small spaces serve as aggregation points for IG predators and for their desiccation intolerant prey.
One commonly held assumption is that predation occurs within guilds whenever adults of species "A" are large enough to eat juveniles of species "B" and in an ontogenetic reversal (Polis et al., 1989) when adults of species "B" prey on juveniles of species "A." It is further assumed that the likelihood of ontogenetic reversal of predation is high because both groups are generalist predators that experience large changes in size through ontogeny. Although many IG predators likely share prey resources (Wise, 1993; Foelix, 1996; Werner and Raffa, 2003), recent studies investigating predation by adult invertebrates on juveniles of Plethodon cinereus report no evidence of predation (Anthony et al., 2007; Figura, 2007). The full spectrum of prey preferences of most large invertebrates are unknown due to difficulties in identifying stomach contents, (but see Wise and Chen, 1999 for centipede predation rates on juvenile spiders of the genus Schizocosa), but the diet of P. cinereus is well documented (Jaeger, 1990; Maglia, 1996; Adams and Rohlf, 2000; Anthony et al., 2008) and guild members are rarely eaten by red-backed salamanders. These examples indicate that it may be incorrect to assume that IGP occurs based on size asymmetries alone.
Although previous studies suggest that the relationships between Plethodon cinereus and large arthropod predators is competitive (Gall et al., 2003; Hickerson et al., 2004; Anthony et al., 2007; Figura, 2007) rather than predatory, the ways in which the interactions function within the intricate forest-floor food web are unclear. Because terrestrial salamanders are important regulators of leaf litter arthropods, and because other large invertebrate predators appear to be ecologically similar, understanding both how and when these predators interact with one another seems a logical part of determining which mechanisms are most influential in structuring the community. For example, Peckarsky et al. (2008) illustrate how inclusion of data on non-predatory effects in classic textbook predator/prey data sets (e.g., lynx-snowshoe hair cycles) can alter our original understanding of population and community dynamics.
Despite the growing body of literature that suggests terrestrial salamanders are important regulators of invertebrate abundance and leaf litter decomposition rates, there are few long-term, unenclosed field experiments on temperate forest-floor systems that examine types and strengths of interactions among predators that occur in specific microhabitats beneath cover objects. Between precipitation events the soil (humus) beneath rocks and logs retains moisture longer than the surrounding leaf litter. Therefore, microhabitats under rocks and logs become refugia from desiccation for terrestrial amphibians and many arthropods (Jaeger, 1980b). During such periods, these cover objects also become defendable territories for red-backed salamanders (Jaeger, 1972; Jaeger et al., 1982). If large invertebrate predators, like centipedes, enter salamander territories in search of prey, they likely interact directly with one another. Furthermore, given the structurally simple microhabitat beneath cover, the potential for strong interactions that affect the distribution and abundance of these guild members seems inevitable.
We conducted a 4-y press experiment in unrestricted open field plots to evaluate how relative predator abundances affect one another. A press experiment is a repeated alteration of species densities (e.g., removals) maintained over time to examine densities of unperturbed species. We focused our efforts on removing centipedes and salamanders because these two taxa are ecologically similar and are numerically abundant. We hypothesized that the removal of salamanders or centipedes from beneath artificially placed cover objects in the field should cause changes in the abundances of other guild members. More specifically we predicted that salamanders and centipedes would have negative effects on each other and on carabid beetles and spiders. These predictions are based upon the expectation that interactions among organisms in structurally simple habitats (i.e., beneath cover objects) should be strong relative to those in complex habitats (i.e., leaf litter).
MATERIALS AND METHODS
On 12 and 13 Apr. 2004 we placed 216 artificial cover objects (ACOs) on the forest-floor in the Cuyahoga Valley National Park (CVNP), Summit County, Ohio (41[degrees]13'46.62"N, 81[degrees]31'7.77"W). The field site is mixed deciduous forest that is dominated by Acer saccharum (Sugar Maple), Fagus grandifolia (American Beech), Liriodendron tulipifera (Tulip Poplar), and Quercus rubra (Red Oak) and lies on a north/northeast facing slope (elevational range 260-271 m). We used dull white ceramic floor tiles measuring 30 x 30 cm as ACOs, and we removed the leaf litter from directly beneath each tile before placing the ACOs in position. This put the tile in direct contact with the soil substrate. The ACOs were arranged in 24 plots; each was separated by 2 m and covered a 5 x 5 [m.sup.2] area. Each of the 24 plots consisted of a cluster of nine ACOs separated by 1 m, all of which received the same treatment application. Each plot was randomly assigned to eight blocks, each of which contained three treatments. The design was employed with two contstraints; (1) adjacent plots did not receive the same treatment application, and (2) each block was comprised of one of each treatment. The three treatments were; (1) no removal/control (NR), (2) salamander removal (SR), and (3) centipede removal (CR). Additional predator removal combinations would have been interesting but would have necessitated loss of replication. The combinations chosen were based upon previous work that suggests individuals of the salamander, Plethodon cinereus behave aggressively toward intraspecific intruders (reviewed in Jaeger and Forester, 1993; Mathis et al., 1995) and centipede intruders (Hickerson et al., 2004).
Data collection began on 23 Apr. 2004 and continued through 23 May 2008. The field site was visited every 2 wk, except for winter months, through the end of 2005, and weekly beginning in spring 2006 through the duration of the study (total of 98 visits). Red-backed salamanders retreat to subsurface hibernacula as deep as 1 m during Dec.-Feb. when temperatures and precipitation are low (Grizzell, 1949; Caldwell and Jones, 1973). During each visit we hand-turned ACOs, counted and identified macrofauna from beneath each, and removed predators from the appropriate treatments. All 24 plots were visited in random order on each sampling day to remove any hourly temporal bias in sampling. Macropredators recorded at our field site were centipedes [Lithobiomorpha (Bothropolys and Lithobius), Scolopendromorpha (Scolopocryptops sexspinosus) and Geophilomorpha], spiders [the largest and most abundant were amaurobiids (Wadotes)], beetles (Carabidae; the most abundant were Pterostichus stygicus which made up over 40% of the total each year). Invertebrates under ACOs were not identified to finer taxonomic resolution because identification had to be done in the field. Invertebrates from removal treatments were hand caught and preserved in 70% ethanol. At our field site the salanaander, Plethodon cinereus, made up 99.5% of the total number of observed salamanders. Only rarely did we see other salamander species (P. glutinosus, Notophthalmus viridescens and Eurycea bislineata). Salamanders in removal plots were relocated across barriers (streams or roads) at a distance of at least 0.5 km so that they were unable to move back into the experimental plots (Marsh et al., 2007).
During every other visit we measured and recorded soil temperature and percent relative humidity at the center ACO of each plot (24 points evenly spaced throughout the site) to determine if there was variation in the abiotic environment at the microhabitat scale that might influence the distribution and abundance of the occupants of the forest-floor detrital web. We used an Oakton digital max./min, thermohygrometer to measure percent relative humidity.
The combined effects of treatment and season on predator abundance were examined using multivariate analysis of covariance (MANCOVA). Treatment and season were fixed factors (independent variables), numbers of salamanders, centipedes, spiders, and carabid beetles were the dependent variables, and soil temperature and relative humidity were covariates in our model. We were unable to use repeated measures MANOVA to analyze our data because the test assumes that conditions do not vary from sampling date to sampling date (McCall and Appelbaum, 1973). At out our field site, conditions vary dally and seasonally and these conditions have strong effects on surface activity of predators. To determine if initial abundances of predators differed among treatments, we ran a MANOVA on the data from the first sampling day. To assess our effectiveness in removing the centipedes and salamanders, we compared the numbers in the control plots (NR) to the appropriate removal plots (SR, CR). We explored differences among treatments with Scheffe's post-hoc test which corrects alpha for all pairwise comparisons and is, therefore, a conservative approach.
Interaction strengths were calculated to examine the effects of each predator taxon on the other guild members. We used the raw difference index [i.e., (N- D)/Y) where N = the number of intraguild prey in the control, D = the number of intraguild prey in the predator removal treatment, and Y = the abundance of predators in the removal treatment (Berlow et al., 1999)]. For example, the effect of salamanders on spiders was calculated by subtracting the mean numbers of spiders (D) in the salamander removal treatment from the mean number of spiders (N) in the control and dividing that result by the number of salamanders (Y) in treatments where they were removed. This index is appropriate for long term experiments that examine both positive and negative effects of predators (Berlow et al., 1999). The mean interaction strengths were examined over the 98 sampling days with a sign test to determine if there were significant departures from zero. Abiotic measurements were compared among all treatments over time. All data were log-transformed, [log.sub.10](x + 1) to improve adherence to normality. To reduce the probability of making a Type I error we applied Bonferroni adjustments and considered a significant alpha to be P < 0.02.
EFFECTIVENESS OF TREATMENT APPLICATIONS
We removed a total of 1288 salamanders from SR and 1056 centipedes from CR plots over the 4 y study (spanning 98 sampling dates). Initial densities of focal predators did not differ among treatments (P = 0.106, F = 1.82), so we conclude that our statistical findings are a direct result of our treatment manipulations rather than resulting from differences in initial densities in plots at our field site. We rarely missed predators under cover objects, so our removal rate (based upon visual inspection) approached 100% for all observed predator groups that were encountered beneath ACOs. Despite the open plot design of our experiment, reductions in salamander numbers persisted well after each collection date (Fig. 1a and Fig. 2a). We reduced total salamander abundance by 28% and adult salamander abundance by 47% in salamander removal (SR) plots relative to control (NR) plots. Centipedes re-invaded cover objects from the surrounding litter matrix most rapidly and by the following collection date had reached 93% of their original numbers (Fig. lb). We were only able to reduce centipede numbers effectively during the spring season (Fig. 2b).
INTRAGUILD PREDATOR INTERACTIONS
We found a significant effect of treatment (P < 0.0001, F = 17.47) and season (P < 0.0001, F = 56.63) on the abundance of the focal predators in our plots. We also found a weak treatment x season interaction (P = 0.049, F = 1.65) that was only evident for salamanders after the addition of soil temperature and relative humidity as covariates in our model (Table 1). This interaction is a result of a weak treatment effect during the summer months when salamanders retreat from the surface (Fig. 2a). We found a strong treatment effect on salamander, spider, and carabid beetle abundances but only a weak treatment effect on centipede abundance.
[FIGURE 1 OMITTED]
Centipedes most strongly affected salamander abundance in spring and fall (Fig. 2a). We observed more salamanders in the centipede removal treatment (mean = 1.76 per plot) compared to controls (mean = 1.36 per plot). Salamanders had no effect on centipede abundance (P = 0.160, F = 3.62), and we were only able to reduce centipede abundance in the spring (Fig. 2b). The significant reduction of salamanders in the SR treatment had a very strong positive effect on spider abundance. There were significantly more spiders in SR plots (P < 0.0001, F = 48.03, mean = 2.76/plot), and significantly fewer in CR plots (P = 0.001, F = 48.03, mean = 1.82/plot) compared to control (NR) plots (mean = 1.90/plot, Fig. 2c). The removal of salamanders had no significant effect on carabid beetle abundance (P = 0.295, F = 9.08), but there was a significant increase in carabid beeries in the centipede removal treatment (P < 0.0001, F = 9.08, mean = 1.35/plot) compared to controls (mean = 0.915/plot, Fig. 2d).
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Our estimates of interaction strengths indicate significant positive and negative interactions among predators and the direction of these interactions was consistent over the duration of the experiment (Figs. 3a, b). Centipedes exerted significant positive effects on spiders (sign test, P = 0.0004, z = 3.52) and negative effects on beetles (sign test, P = 0.0003, z = -3.65). Salamanders had significant negative effects on spiders (sign test, P < 0.0001, z = -5.67) and significant positive effects on beetles (sign test, P = 0.019, z = 2.33). The strength of these interactions, however, varied by season. Effects of salamanders on both beetles and on spiders were most evident in the summer months (Fig. 3b).
Our data add to a growing body of evidence suggesting that distantly related predators may engage in competitive interactions that potentially limit population sizes of more basal consumers (Mokany and Shine, 2003; Huntzinger and Karban, 2008; Jennings et al., 2010). These studies do not imply that competition between related species is unimportant, but they do suggest that competition among unrelated species may be much more widespread than previously thought. Clearly, unrelated pairs of species are more common in any ecosystem than are related species, thus undetected interactions among these species may have important roles in the determination of community and food web structure. In our study, the removal of either salamanders or centipedes resulted in significant changes in the abundance of other invertebrate predators (i.e., spiders and carabid beetles) within the forest-floor food web. We detected complex interactions that often involve multiple groups such that removals positively affect some taxa but negatively affect others. Our ability to test more specific hypotheses is limited by the types of removals conducted, but in light of previous pair-wise studies of salamanders and large predatory invertebrates (Gall et al., 2003; Hickerson et al., 2004; Anthony et al., 2007; Figura, 2007), we posit that interference competition rather than predator-prey interactions (IGP) may be the best explanation for our findings. Our data indicate that both direct (2 predator), and indirect (3 predator) effects are operating in this system. For example, the negative relationship between salamander abundance and spider abundance may be the result of a direct interaction between salamanders and spiders. Alternatively, the removal of centipedes may have had a direct positive effect on salamander abundance and an indirect negative effect on spider abundance. Dunham (2008) found that spiders were 2.3 times more abundant in bird and mammal exclosures compared to control exclosures, a result that is similar to ours. We agree with Dunham who points out the difficulty in discerning whether the observed increase in spider abundance in her predator exclosures was the result of reduced predation by birds and mammals (a direct link), or reduced interspecific competition in the absence of predators for macro-invertebrate prey.
We found a significant increase in the number of salamanders and carabid beetles in centipede removal (CR) plots compared to controls. Although the effectiveness of centipede removal was not detectable by the next sampling date, the immediate and short term response by guild members to each removal must have been sufficient to drive the significant differences that we observed. It is impossible, however, to say whether the changes in abundance were the result of direct interactions, in which the removal of centipedes led to increases in beetle abundance, or of indirect interactions, in which the removal of centipedes led to increased numbers of salamanders, and in turn increased numbers of beetles. The existing evidence on beetle--salamander interactions indicates that the salamander, Plethodon cinereus, and the carabid beetle, Platynus tenuicollis, are mutually territorial (Gall et al., 2003); and, therefore, it is unlikely that salamanders and carabid beetles would be positively associated with each other. It is possible that in our CR treatment the temporary reduction of centipedes between sampling dates was enough to elicit an increase in carabid beetle abundance. However, we do not know whether centipedes prey upon adult carabid beetles or whether they experienced competitive release in the CR treatment. We currently are conducting laboratory trials to determine which of these interactions is most likely responsible for the observed numerical response. Preliminary analyses of co-occurrence of carabid beetles and centipedes from the control treatment ACOs indicate that these predators rarely occupy the same ACOs at the same time.
Data on gut contents of predatory invertebrates like centipedes, spiders, and beetles are few. However, data on stomach contents of Plethodon cinereus reveal that spiders, centipedes, and carabid beetles do not make up a significant proportion of the diet (Jaeger, 1990; Maglia, 1996; Adams and Rohlf, 2000; Anthony et al., 2008). For example, Maglia (1996) reported that spiders made up only 1.6% of the total invertebrates by number in the diet of 172 P. cinereus from Tennessee, and Stuczka (2011) found that centipedes, spiders, and beetles combined made up only 3.2% of 4629 prey items taken from 258 salamanders at our field site. Studies on asymmetrical IGP and predator diets, like those described above, provide further support for competition, rather than direct consumption, as the mechanism driving changes in the abundance of predators in this forest-floor guild.
Recently there has been discussion about food web stability occurring through fast and slow energy channels that are linked by predators. Mobile predators couple strong and weak interaction chains by switching between chains based on prey density (Rooney et al., 2006). In forest ecosystems that experience alternating periods of moisture and drying, species that have strict moisture requirements (amphibians and arthropods) may move from one energy channel to another during wet periods but may be restricted from doing so during dry periods between rains. Plethodon cinereus, a terrestrial lungless salamander, can become confined to moist, relatively simple environments and so the localized effects of this predator should be most pronounced during dry periods when prey are trapped within territories under cover objects. We found that, in summer, interaction strengths between salamanders and IG predators were of greater magnitude than in the spring and fall (Fig. 3b, points farthest from the zero line) suggesting that competitive effects are strongest when salamanders are confined to microhabitat beneath cover objects. During wetter periods, in the spring and fall, when salamanders forage widely in the complex microhabitat of the surrounding leaf litter and vegetation, their role may be more like that described by Rooney et al. (2006) where predators regulate prey in fast chains before moving from those depleted chains to the chains that have experienced some degree of recovery. One interesting avenue of further investigation would be to assess the direct effects of microhabitat heterogeneity and moisture on top-down trophic cascades in the field by examining relative abundances of these predator groups in structurally complex versus simple habitats along a gradient from moist to dry.
A recent review of the literature discussing population regulation and intraguild interactions in plethodontid salamanders concludes that although we have learned a great deal in the past few decades concerning interactions within guilds of salamanders, much remains to be explored. Especially important in adding to our understanding of population and community regulation are long-term, time series studies in unenclosed field sites (Bruce, 2008). We examined interactions among top predators within a forest-floor food web in a natural and unenclosed field experiment focused on structurally simple microhabitats that may serve as territories for forest-floor predators. We suggest that trophic cascades may be localized in space and in time and we predict that territorial defenders are more likely to generate strong top-down effects than are widely foraging species.
Acknowledgments.--CMH was supported by a National Science Foundation Doctoral Dissertation Improvement Grant (DEB-0608239). CDA was supported by a John Carroll University Summer Faculty Fellowship and by a George Grauel Faculty Fellowship. We thank J. Keiper and The Cleveland Museum of Natural History for funding. This manuscript was improved by the constructive comments of two anonymous reviewers. The Cuyahoga Valley National Park granted permission to conduct fieldwork and provided access to the Woodlake Environmental Field Station. Field work was conducted under National Park Service scientific research permit number CUVA-2004-SCI-0010 and IACUC protocol numbers JCU503 (JCU) and 2604-WAL-AS (CSU).
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SUBMITTED 18 AUGUST 2011
ACCEPTED 20 JANUARY 2012
CARI-ANN M. HICKERSON (1) AND CARL D. ANTHONY
Department of Biology, John Carroll University, University Heights, Ohio 44118
B. MICHAEL WALTON
Department of Biological, Geological and Environmental Sciences, Cleveland State University, Cleveland, Ohio 44115
(1) Corresponding author: Telephone: (216) 397-4489; FAX: (216) 397-4482; e-mail: chickerson@ jcu.edu
TABLE 1.--Multivariate analysis of covariance (MANCOVA) results for between-subjects effects comparing specific predator abundances among treatments (CR, SR, and controls) and seasons (spring, summer, and fall). Overall MANOVA results are presented in the text. Soil temperature and relative humidity were covariates in the model Treatment Season Intraguild predators F P F P Salamanders 26.53 <0.0001# 141.90 <0.0001# Spiders 48.03 <0.0001# 71.67 <0.0001# Carabid beetles 9.08 <0.0001# 32.320 <0.0001# Centipedes 3.62 0.027 4.90 0.008 Soil temperature Relative humidity Intraguild predators F P F P Salamanders 14.37 <0.0001# 0.186 0.666 Spiders 5.86 0.016# 5.74 0.017# Carabid beetles 11.03 0.001# 0.316 0.574 Centipedes 3.05 0.081# 1.33 0.250 Treatment*Season Intraguild predators F P Salamanders 2.71 0.029 Spiders 0.986 0.414 Carabid beetles 1.03 0.389 Centipedes 1.55 0.185 Statistically significant results (P < 0.02) are shown in bold; marginally significant results (0.05 > P > 0.02) are underlined Note: Statistically significant results (P < 0.02) are shown in bold is indicated with #; marginally significant results (0.05 > P > 0.02) are underlined
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