The impact of a coastal invasive predator on infaunal communities: Assessing the roles of density and a native counterpart

Journal of Sea Research 66 (2011) 181–186

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The impact of a coastal invasive predator on infaunal communities: Assessing the roles of density and a native counterpart Garry J. Gregory, Pedro A. Quijón ⁎ Department of Biology, University of Prince Edward Island, 550 University Avenue, Charlottetown, PE, Canada C1A 4P3

a r t i c l e

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Article history: Received 21 January 2011 Received in revised form 9 May 2011 Accepted 12 May 2011 Available online 24 May 2011 Keywords: Green crab Rock crab Coastal invasions Predation Infaunal community

a b s t r a c t Our understanding of the influence of many predatory invaders in areas of increasing overlap with native counterparts remains elusive. In Atlantic Canada, the European green crab, Carcinus maenas, overlaps with the native rock crab, Cancer irroratus, but surprisingly little is known about their effects in the light of their potential interactions. In this study we used short-term cage inclusion experiments to assess the impact of low and high green crab densities upon infaunal communities. Then, we re-assessed this impact by running identical manipulations combining green crabs with rock crabs of comparable size and at similar overall predator densities. Our results indicate that both low and high green crab densities accounted for severe declines in infaunal organisms with respect to ambient cages. Polychaetes, the group best represented in this trial, accounted for most of such decline with densities at least 50% lower in the green crab inclusions. A similar (~ 50%) decline in infaunal density was observed when green crabs and rock crabs were combined at low predator densities. However, at high predator densities their impact on polychaetes, molluscs and total infauna was less severe and non-significant with respect to ambient cages. Our results indicate that betweenpredator interactions have serious indirect effects on benthic prey and contrast previous results on the role played by these crab species. We propose a re-assessment of the role played by native counterparts while searching for management alternatives to minimize the impact of invasive predators in areas heavily invaded. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Invasions, like other large-scale historical phenomena affecting the marine realm (eg. overfishing) are having a profound effect on the organization of coastal ecosystems. Marine invaders are best known for threatening shellfish populations (Dare and Edwards, 1976; Glude, 1955) but their effects frequently involve more subtle (and less studied) changes at the community-structure level (Carlton and Cohen, 2003). The arrival of invasive predatory species, for instance, will almost certainly have cascading effects upon the lower trophic levels that support benthic assemblages. With the rapid spread of these invaders and the increasing spatial overlap with native counterparts (i.e. comparable predators that belong to a same guild) direct and indirect interactions are likely to occur (Iribarne et al., 2003). In this changing ecological scenario, a critical question is whether some of these native predators could potentially interfere with or mitigate the community effects of the invasive predators. Answering this question has broad connotations as most literature suggests that these invasions, and their impacts, are occurring at increasing rates in North America and worldwide (e.g. Brickman and Smith, 2007; Drake et al., 2007; Fofonoff et al., 2003).

⁎ Corresponding author. Tel.: + 1 902 566 6059; fax: + 1 902 566 0740. E-mail address: [email protected] (P.A. Quijón). 1385-1101/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.seares.2011.05.009

Atlantic Canada appears to be particularly vulnerable to invasions, as nine conspicuous invaders have been introduced into the southern Gulf of St. Lawrence in the last decade alone (Klassen and Locke, 2007). One of these species, the European green crab (Carcinus maenas) has spread and become a key predatory species in rocky and sedimentary systems (Audet et al., 2003; Klassen and Locke, 2007). Green crabs are responsible for the decline of several shellfish species (Congleton et al., 2005; Floyd and Williams, 2004; Lindsay and Savage, 1978) but are also well suited to alter community assembly by exploiting almost every food resource. The literature shows that native counterparts play contrasting roles while facing the arrival of this invader. Some studies have shown that native crabs, shrimps and juvenile lobsters are displaced by green crabs while foraging (Eriksson et al., 1975; Jensen et al., 2002; Rossong et al., 2006; Williams et al., 2006) or seeking shelter (McDonald et al., 2001). Others suggest that native counterparts such as Cancer productus and C. antennarius in the Pacific coast of North America (Hunt and Yamada, 2003; Jensen et al., 2007), and Callinectes sapidus in the Atlantic shoreline (DeRivera et al., 2005) interact and are able to displace green crabs. The rock crab (Cancer irroratus) overlaps a substantial part of the green crab's depth, habitat, and diet (Moody and Steneck, 1993; Stehlik, 1993), and has been identified as a key infaunal predator in Atlantic Canada (Quijón and Snelgrove, 2005). In light of limited prey availability, the interactions between these two species would likely have implications for benthic invertebrate assemblages.

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In the eastern shore of Prince Edward Island (hereafter PEI), growing green crab populations coexist with populations of rock crabs and provide an opportunity to study the indirect effects of their interactions. We used caging experiments to test two null hypotheses: i) predation by green crabs at low and high density levels has a similar (null) impact on community structure, and, ii) irrespective of predator density, green crabs in conspecific or heterospecific combinations (with rock crabs) exhibit similar impacts on community structure. In order to test the first hypothesis we measured the effects of low and high densities of green crabs upon the abundance of two representative size fractions of the macrobenthic infauna: organisms between 2 and 5 mm and organisms N5 mm in size. In order to test the second hypothesis we used identical manipulations and the same overall crab densities and sizes to assess whether native counterparts (rock crabs in a heterospecific trial) would alter the outcome of these experiments. Based on the literature available, we expected to find strong density-dependent effects in the conspecific trial. Less predictable was the outcome of the heterospecific trial, as this could provide evidence of either additive effects (worsening the impacts) or counter-effects (attenuation of predation effects). To our knowledge only two studies have tested related hypotheses. Bélair and Miron (2009) measured these species' feeding rates over blue mussels whereas Breen and Metaxas (2008) compared the effects of the juvenile stage of both crab species on the same prey. Both studies suggested that these species have the ability to co-exist without harm and did not identify any particular indirect effect on benthic prey. However, the outcome of experiments like ours, that involve invertebrate communities rather than individual prey, may not necessarily correspond to theirs and until now, remained unexplored.

Gulf of St. Lawrence PEI Canada 45°N

US 61°W

Souris River Estuary

Souris River estuary

Study Exp. area Area

*

2. Materials and methods 750 m

2.1. Study area, crab collection, and set-up of inclusion cages Two experimental trials were conducted during spring tides of July and August of 2007 (see Table 1 for details) in the lower intertidal of Souris River estuary, PEI (46˚21′5″ N, 62˚16′54″ W; Fig. 1). The experimental area was characterized by sandy sediments (primarily “medium” and “fine sands” according to the Wentworth Scale). No prominent eelgrass and clam beds are located in this area but they are common in other areas of the estuary where muddy sediments are more prominent. No preliminary information on infaunal composition and abundance in the study area was available before the beginning of our experiments. The estuary is a microtidal system that in low tide conditions expose the area where the cages were placed between 1 and 1,5 h. During the trials weather conditions were stable without occurrence of unusual storms or drastic changes due to elevated freshwater input. Consequently, water temperature and salinity did fluctuate but only in response to the usual tide variations (~ 15–20 °C; ~ 15–29 ppt).

Table 1 Trial dates, low tide levels, crab characteristics, mortality levels and densities. Crab sizes are reported as ranges, crab mortalities refer to high crab density cages only, and actual crab densities take into consideration crab escape and mortality. NA: Not applicable. Experiment details

Conspecific trial

Heterospecific trial

Dates in 2007 (spring tides) Lowest LT level (m) Green crab size (CW in mm) Rock crab size (CW in mm) Green crab mortality (total #) Rock crab mortality (total #) Actual crab densities after day 1 (#)

11–17 July 0.21 53.2–59.7 NA 2 NA 2 (low) and 4–5 (high)

09–15 August 0.13 54.3–59.5 56.0-60.6 1 2 2 (low) and 4–5 (high)

Colville Bay

Fig. 1. Map of Prince Edward Island (PEI) in the Southern Gulf of St. Lawrence with the approximate location of Souries River estuary (insert) and the experimental area. The asterisk indicates the area wherre crabs used for experimental trials were collected.

Green crabs and rock crabs were collected from an area immediately adjacent within the same estuary (see Fig. 1) using rectangular 46 × 23 × 23 cm wire traps baited with soft-shell clams or frozen mackerel. Although baited traps are not necessarily the best quantitative method to estimate crab densities, they have been extensively used to characterize crab populations in PEI (P Quijón, Pers. Comm). Rock crab densities in the study area were similar to most other comparable areas in PEI (typically 3–4 rock crabs per trap per day) except in one deployment when ~10 rock crabs per trap per day were collected. Instead, the density of green crabs in this and other estuaries in the eastern part of the island are the highest in PEI. Only short trap deployments (1–4 h) were required in order to collect large number of green crabs (30–40, ~ 3 full traps). Those numbers are consistent with the ones obtained from 24 deployments in other areas of the island such as Charlottetown (Pickering and Quijón, in press). To minimize potential biases, only undamaged male crabs with similar carapace width (Table 1) were kept and used in the experimental trials. We looked for signs of molting as this process is known to affect the outcome of feeding trials and is not uncommon during the months that our experiments were conducted (Audet et al., 2008). However, careful examination of the crabs before and after their use in the trials suggested that none of them undergo a molting process during the experiments. Before their inclusion in the cages, all

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crabs were deprived of food for 48 h to standardize hunger levels (Mascaro and Seed, 2000). Eighteen cages with dimensions 95 × 95 × 30 cm were built with 0.5″ wire mesh (~1.25 cm mesh size) and placed along the low intertidal level (~0.3 m above LT; daily exposure time ~1.0–1.5 h). These cages had open bottoms and were inserted into the sediment to a depth of ~ 5–8 cm. The cages were haphazardly assigned to one of three treatment levels (n = 6 replicates per treatment level): ambient cages (no crabs), low crab density cages (2 per cage) and high crab density cages (6 crabs per cage). The latter treatment level was deliberately high to account for potential green crab escape, as preliminary runs showed that typically 1–2 green crab escaped from high density cages during the first day of confinement. That density also accounted for the occasional occurrence of crab mortality (see Table 1) for which no evidence of attraction of scavenger species was detected. Considering crab escape and crab mortality, the actual high crab densities fluctuated between 4 and 5 crabs per cage (2–3 green crabs and 2–3 rock crabs in every cage). Both densities (low and high) were comparable with the ones used before in the region (e.g. Floyd and Williams, 2004) and consistent with information on crab densities in the study area (P. Quijón, unpublished). Our experiments aimed to assess predation effects in the short term (6 days), so they lasted long enough to detect potential feeding activity (usually evident after 2–3 days; cf. Quijón and Snelgrove, 2005) and short enough to prevent the cage artifacts occurring in manipulations lasting from several weeks to months (cf. Hurlberg and Oliver, 1980). During both trials cages were checked at low tide on a daily basis for evidence of fine sediment accumulation and/or occasional stranding of eelgrass or seaweeds.

error term. All the analyses were conducted after ensuring that the assumptions of the ANOVA were satisfied. In those cases where this did not occur, data were log (x+ 1) or square root (x) transformed. In those cases where significant differences were detected, Tukey HSD post-hoc comparisons were subsequently applied in order to identify significant differences among individual treatment levels. 3. Results 3.1. Conspecific predator trial At the end of the trial, infaunal densities in ambient cages averaged 27.5 individuals per quadrat (0.0625 m 2) in the size range 2–5 mm, and 12.8 individuals per quadrat in the case of organisms larger than 5 mm (Fig. 2). At both size fractions, polychaetes were better represented than molluscs (at least twice as many). In comparison to ambient cages, sediments exposed to low and high green crab densities exhibited substantially lower densities (at least a 50% decline with respect to ambient cages; p = b0.001 to 0.009; Table 2; Fig. 2). Polychaetes, particularly nereids, accounted for most of such declines in density. The density of small (2–5 mm) molluscs followed a different pattern: this particular group did not show significant differences among treatment levels (p = 0.209; Table 2). Post-hoc

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In order to document infaunal densities and their potential change as a result of crab predation, we used a 25× 25 cm (0.0625 m2) metal quadrat inserted up to 10 cm into the sediment. This quadrat allowed us to collect fairly large sediment samples from each treatment level at the end of each trial, immediately after the removal of the crabs (one sample per cage; n = 6 replicates per treatment level). Sediment samples were then processed through 5 mm and 2 mm sieves to distinguish between two fractions of relatively large invertebrates (N5 mm and 2–5 mm infauna). These fractions were expected to be the most relevant in the diet of adult-size crabs of these species (P. Quijón, unpublished). All the invertebrates collected were preserved using 70% ethanol with rose Bengal. In the laboratory, samples were sorted, identified and compiled in three distinctive groups: total infauna, polychaetes and bivalves. Overall, polychaetes were numerically dominated by nereids, maldanids and spionids whereas bivalves were dominated by soft shell clams (Mya arenaria) and gem clams (Gemma gemma). Other groups such as nemertines and peracarids were also present in the samples but in numbers substantially lower. The first experimental trial (conspecific crabs) tested the influence of low and high densities of green crabs with respect to ambient cages (without crabs). The second trial tested the influence of green crabs and rock crabs combined (heterospecific trial) using identical overall predator densities: 1 green crab and 1 rock crab for the low density level and 2–3 green crabs and 2–3 rock crabs for the high density level. Since the trials were conducted separately (approximately three weeks apart) data from each trial was also assessed separately: We considered inappropriate to conduct direct comparisons between treatment levels from conspecific and heterospecific trials, as some of the differences could be related to temporal variation rather than the treatment itself. Independent comparisons among treatment levels were therefore conducted with a one-way analysis of variance. Its model can be described as follows: γ = μ + t + ε, where ‘γ’ stands for the dependent variable (infaunal density), ‘μ’ is a constant, ‘t’ represents treatment level (ambient versus low versus high crab density cages) and ε stands for the

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Fig. 2. Average (+1SE) infaunal densities per quadrat in two size categories (2–5 mm and N 5 mm) measured in the conspecific trial (green crabs only). Infaunal groups associated to ambient cages (A) as well as crab inclusion cages with low (L) and high (H) crab densities are represented by open, gray and black bars, respectively. Different letters on top of those bars indicate significant differences among the corresponding treatment levels, as identified by post-hoc comparisons.

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Table 2 Results of one-way ANOVAs assessing the influence of treatment (ambient versus low versus high crab density) on the density of infauna (separate ANOVAs for the two size categories and for total infauna, polychaetes and molluscs). All the analyses were conducted with 2 and 15 degrees of freedom (n = 6 per treatment). Results of post-hoc comparisons among treatment levels are summarized in Fig. 2 (conspecific) and Fig. 3 (heterospecific trial). Predator trials

Dependent variable

Source of variation

Conspecific: (green crab)

Total infauna

Treatment Error Treatment Error Treatment Error

231.17 25.58 398.72 14.51 24.89 14.31

Treatment Error Treatment Error Treatment Error

624.39 49.26 129.5 16.73 341.06 22.23

Polychaetes Molluscs

Heterospecific: (green + rock crab)

Total infauna Polychaetes Molluscs

comparisons also confirmed that for every comparison done in this trial, infaunal densities in low and high density green crab levels (L versus H in Fig. 2) were not significantly different. 3.2. Heterospecific predator trial The average infaunal density in the ambient cages of this trial was higher than in the former trial for the 2–5 mm fraction (39.3 individuals per quadrat) but slightly lower for the N5 mm fraction (10.2 individuals per quadrat) (Fig. 3). Polychaetes were more abundant in the N5 mm fraction but not in the 2–5 mm fraction (Fig. 3). In cages with low predator density (1 green crab and 1 rock crab combined) the density of the infauna was significantly lower than in ambient cages (~50% less individuals in the 2–5 mm size range and ~75% less individuals in the N5 mm size range; p b 0.05; Fig. 3). However, no significant differences were detected between infaunal densities measured in ambient cages and high predator density cages for any of the 6 comparisons conducted (Table 2; Fig. 3). Post-hoc comparisons confirmed that infaunal densities in low and high predator density cages (L versus H) were significantly different with one exception: polychaetes in the 2–5 mm size fraction (Fig. 3).

ANOVA terms (2–5 mm infauna)

ANOVA terms (N5 mm infauna)

MS

MS

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185.72 12.07 130.67 9.78 5.06 0.78

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b0.001

densities with respect to cages without crabs. However, we were not expecting to find an almost identical outcome from cages with sharply different green crab densities. This apparent lack of densitydependence on green crab effects supported part of our first null hypothesis and contradicts several studies conducted elsewhere. Those studies have shown that a density increase in green crabs (Carcinus maenas; e.g., Smallegange et al., 2006) or Mediterranean

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Green crabs and rock crabs share a variety of prey items and have been previously identified as potential competitors (e.g. Miron et al., 2005). However, no studies so far had provided experimental evidence indicating a lowered predation impact as a result of their interactions. On the contrary, the evidence so far has shown co-existence without apparent interference among adults (Bélair and Miron, 2009) and among juvenile crabs (Breen and Metaxas, 2008). Our results suggest otherwise but do not necessarily contradict those studies. While those authors focused on predation upon individual types of prey (such as mussels), our study focused on full infaunal communities dominated by both polychaetes and molluscs (primarily clams). Thus, even when we are mindful of the spatial and temporal constraints of our manipulations, we suggest that interactions between predators may have subtle consequences that may be better reflected in community than in population responses. Below, we briefly discuss the implications of our results and propose a few working hypotheses to promote further research in this and similar systems elsewhere. 4.1. Predator density and counterpart interactions Based on previous trials (Quijón and Snelgrove, 2008) and the literature available (e.g. Klassen and Locke, 2007; Ropes, 1968) we were expecting green crabs to consume and therefore lower infaunal

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Fig. 3. Average (+1SE) infaunal densities per quadrat in two size categories (2–5 mm and N5 mm) measured in the heterospecific trial (green crabs combined with rock crabs). Other details as in caption of Fig. 2.

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shore crabs (Carcinus aestuarii; Mistri, 2003) promotes intraspecific interactions that translate into weaker foraging upon benthic prey. Although the spatial and temporal scope of our experiments is admittedly limited, these results are in our opinion meaningful when weighed against the parallel experiments conducted with a combination of green crabs and rock crabs. Despite being similar in every other way, high density heterospecific trials produced clear evidence of a reduction on predation levels, and led us to reject our second null hypothesis. Separated, both the green crab (this study; Pickering and Quijón, in press) and the rock crab (Quijón and Snelgrove, 2005; Stehlik, 1993) have been shown to have a strong influence on benthic prey (“severe” impacts sensu Ólafsson et al., 1994). However, as our results suggest, together and at relatively high densities their impact on benthic prey can be severely attenuated. We do not intend to scaleup (sensu Schneider et al., 1997) these results to the full extent of a summer season or an entire estuarine system here or elsewhere. Instead, we hypothesize that in areas of spatial overlap and aggregation in relatively large numbers (as in our study area), the role played by predation by these species is simply less prominent than in areas or periods of time where such overlap does not occur. The milder effects resulting from the combination of several green crabs and rock crabs can be explained by more than one mechanism. First, the high probability of green crab-rock crab encounters likely triggers density-dependent agonistic responses that ultimately could reduce their foraging upon infauna. We were unable to systematically monitor the behavior of the crabs, but in the few cases when aggressive behavior was detected it occurred on cages with high density of both species. This explanation is also consistent with the evidence provided by Jensen et al. (2007) who demonstrated that in sites where green crabs and cancrid crabs coexist, green crabs exhibit an unusually high frequency of limb damage and loss. Alternatively, the perception of the presence of rock crabs may have promoted green crab escape, burying or hiding, and therefore longer periods of inactivity as a measure of caution or avoidance (Barshaw and Able, 1990; Nye, 1974). To our surprise, the same type of interactions did not occur among high densities of green crabs, and even if they occurred and were undetected, they did not deter the green crabs' ability to reduce infaunal numbers. Given the lack of evidence to separate the actual contribution of these two mechanisms (agonism versus avoidance) we propose them as alternative working hypotheses and suggest two simple, yet explicit experiments to separate their potential role. First, our results should be compared against those obtained from the inclusion of similar densities of green crabs and rock crabs separated by a fence or mesh that precludes their physical contact. Second, given that both species are to some extent chemo-tactile (and not only visual) predators (cf. Elner and Hughes, 1978; Zhou and Rebach, 1999), we suggest to use a laboratory setting to explore the role played by the diffusion and perception of speciesspecific odors on the predation rates exhibited by both crab species separately. Most literature describes green crabs as primarily mollusc predators with a strong preference for clams such as Mya arenaria (MacPhail et al., 1955; Pickering and Quijón, in press; Ropes, 1968). However, during the conspecific trial the fraction of small molluscs (comprised primarily of clam species) was ironically the only group that was not affected by green crab predation. This suggests that adult green crabs were not able to effectively handle and consume 2–5 mm clams, or most likely, that they choose to focus on polychaetes (cf. Le Calvez, 1984), a prey at least twice more abundant in that particular trial. The complete lack of preliminary information on infaunal species composition and abundance from the area precludes any speculations regarding a priori prey preferences, so we must arbitrarily assume that green crabs will consume the most abundant prey. At high predator densities, when green crabs and rock crabs became less active or less focused on foraging, prey mobility, particularly in the case of polychaetes, may have also contributed to reduce overall infaunal consumption (Frid, 1989; Thrush, 1999). The

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evidence gathered in this study leads us to hypothesize that prey size and mobility play a significant role on green crab prey preferences, and to suggest the use laboratory experiments to further address this. Incidentally, we noticed that a few ambient cages acted as temporal refuges for juvenile (b30 mm CW) green crabs escaping adult cannibalism (Moksnes, 2004). These small crabs likely consumed infaunal preys in the size ranges studied here (cf. Grosholz and Olin, 2000) but their effects, if any, were irrelevant in comparison to the ones inflicted by adult green crabs. We stress the need to study the role of cannibalism in green crabs, a role that until now has been primarily identified as a population self-regulation mechanism (Moksnes, 2004), but that in a community context, may be much more complex albeit influential than expected. 4.2. Potential biases and limitations Although the confinement of crabs within cages is always a concern for the interpretation of experimental results (Hall et al., 1991) they represent one of the best practical approaches for the study of the role of this type of predators. The crab densities used here (2 and 4–5 per cage) reflect those measured in crab aggregations in the study area (P Quijón, unpublished) and in nearby areas in Prince Edward Island (Pickering and Quijón, in press). They also fall within the range previously used in similar field experiments by, for example, Floyd and Williams (2004). These authors used 5 green crabs per cage in inclusion cages that were slightly smaller than ours. Crab mortality due to agonistic interactions had the potential to attract scavengers or alter to some degree crab behavior (Hall et al., 1991). However, the number of dead crabs in our experiments was marginal (5 out of 96 crabs used) and these specimens were removed as soon as they were detected (within 12–15 h). No aggregation of scavengers was observed during the daily monitoring of the cages or later on during the careful sorting of the samples, so we consider a severe bias like this to be highly unlikely. Cage artifacts, a factor that can be minimized but never entirely removed from manipulations in soft-bottoms should be addressed as well (cf. Hurlberg and Oliver, 1980). We did not add partial-cage treatments to control for this factor as we considered them unnecessary given the brief duration of our manipulations and the characteristics of the substrate (sandy sediments). Indeed, daily observations offered no indication of the issues typically associated to longer-term caging (e.g. trapping and building up of muddy sediments; see review by Ólafsson et al., 1994). Furthermore, several infaunal samples collected in adjacent un-caged ambient sediments at the beginning of the second trial, had average densities that were very similar to those measured in the ambient cages at the end of the trial. Lastly, even if an undetected cage artifact introduced a partial bias in the comparison between ambient and crab cages, the comparison between low and high crab density cages remains valid and meaningful. Thus, without disregarding the potential role of any of these factors, we remain confident that our results are causally related to crab predation and their variation, indirectly related to crab interactions. To better understand the causes of the changes in infaunal abundance we suggest that future studies use additional laboratory and field trials to separate the contribution of crab feeding, crab physical disturbance and local prey escape due to both of those factors (cf. Frid, 1989). As indicated above, we are well aware that the temporal and spatial scales of our manipulations are small, but remain confident that our results are meaningful and would promote further research. 4.3. Conclusions Due to the intensity of their impacts, green crabs require effective management strategies in the regions they invade. Much research has been devoted to explore means of reducing green crab abundances, including the use of parasites (Hoeg and Lutzen, 1985; Lafferty and Kuris, 1996; Thresher et al., 2000), selective harvest (Walton, 2000), and chemical control using pesticides and poisons (Hanks, 1961). All these methods have been shown to exhibit varying degrees of success and

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severe drawbacks. Based on the results presented here, in areas where rock crab populations are abundant, their use as a partial but safer alternative control method should be further explored. Priority should be given to research efforts addressing the role of these predator interactions across different habitat types. If done at a meaningful spatial scale (Schneider et al., 1997), such research will shed light on the potential application of these interactions in the mitigation of green crabs impacts. Acknowledgments We thank A. Malyshev, V. Lutz-Collins, L. Rosenberg, J. Saffary, and J. Gallant for their help during the field trials. We also thank G. Miron (U. Moncton), D. Guignion and M. Silva (UPEI), and two anonymous reviewers for their valuable comments on earlier versions of the ms. Funding for this project was provided by a NSERC Discovery Grant (PAQ) and a NSERC-USRA (GG). References Audet, D., Miron, G., Moriyasu, M., 2008. 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