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Letters, (2005) 8: 1048–1056 doi: 10.1111/j.1461-0248.2005.00808.x LETTER Cascading effects of predator diversity and omnivory in a marine web

Abstract John F. Bruno1* and Mary Over-harvesting, loss and exotic invasions have altered predator diversity and I. O’Connor2 composition in a variety of communities which is predicted to affect other trophic levels 1Department of Marine and functioning. We tested this hypothesis by manipulating predator identity Sciences, CB 3300, The University and diversity in outdoor mesocosms that contained five of macroalgae and a of North Carolina at Chapel Hill, macroinvertebrate assemblage dominated by amphipods and isopods. We used Chapel Hill, NC 27599, USA five common predators including four (, , blennies and killifish) 2Curriculum in Ecology, CB 3275, and one (pinfish). Three carnivorous predators each induced a strong trophic The University of North Carolina cascade by reducing herbivore and increasing algal and diversity. at Chapel Hill, Chapel Hill, NC 27599, USA Surprisingly, increasing predator diversity reversed these effects on macroalgae and *Correspondence: E-mail: altered algal composition, largely due to the inclusion and performance of omnivorous [email protected] fish in diverse predator assemblages. Changes in predator diversity can cascade to lower trophic levels; the exact effects, however, will be difficult to predict due to the many complex interactions that occur in diverse food webs.

Keywords , ecosystem functioning, , macroalgae, omnivory, predator, , .

Ecology Letters (2005) 8: 1048–1056

considered or biodiversity in a food web INTRODUCTION context (Ives et al. 2005) (but see Wardle et al. 2003; Despite decades of research on the importance of , Aquilino et al. 2005; Duffy et al. 2005). Thus, little is known the role of predator diversity in regulating food web about the -wide effects of diversity at any single dynamics, community , and ecosystem functioning (Paine 2002; Schmitz 2003; & Duffy is largely unknown. Speculation about the role of predator 2003; Petchey et al. 2004). diversity and its potential importance in ecosystem man- Predators can indirectly increase plant biomass by agement and restoration is common (e.g. Jackson et al. 2001; suppressing herbivore (Silliman & Bertness Duffy 2003) and descriptive data has frequently been used 2002). Such trophic cascades are common (Schmitz et al. to estimate predator diversity effects (Sinclair et al. 2003). 2000; Shurin et al. 2002) and could become stronger if But because there have been few experimental investigations increasing predator diversity further reduces the intensity of the effects of predator diversity (Cardinale et al. 2003; of herbivory (Sih et al. 1998; Ives et al. 2005). Food web Worm & Duffy 2003; Finke & Denno 2004; Ives et al. ecology and biodiversity–ecosystem functioning research 2005), and none on the importance of vertebrate predator support multiple predictions about the strength and diversity or in the , we cannot predict and may not direction of predator diversity effects on other trophic even recognize the cascading effects of predator . levels. For example, dietary complementarity or inter- Recent research indicates that in many , plant specific facilitation among predators could increase diversity can substantially increase primary production and herbivore suppression (Sih et al. 1998; Duffy 2002). can influence several other ecosystem properties (Hooper Diverse predator assemblages are more likely to include et al. 2005). However, work on the effects of predators with traits that enhance prey consumption such diversity is rare (Duffy 2002; Petchey et al. 2004) (but see as a keystone predator or facilitator (Ives et al. 2005). Naeem & Li 1998; Duffy et al. 2003) and few studies have The probability of selecting such species in either natural

2005 Blackwell Publishing Ltd/CNRS Cascading effects of predator diversity 1049 or experimental communities increases with species acting effects, the magnitude and direction of the influence diversity. on community and ecosystem properties is difficult to The net effects of multispecies predator assemblages are predict. sometimes referred to as the nonlinear or ‘emergent’ effects Here we report an experimental test of the importance of of multiple predators (Sih et al. 1998; Schmitz & Sokol- predator identity and diversity in controlling the structure Hessner 2002). For instance, high levels of predator of lower trophic levels using coastal marine food webs of diversity could induce indirect interactions that modify 18–23 species (Fig. 1). We reconstructed a well-studied predator behaviour and decrease herbivore consumption estuarine food web in outdoor mesocosms that mimicked (Cardinale et al. 2003; Finke & Denno 2004; Siddon & natural field conditions and included most common species Witman 2004). Predators can indirectly affect by of macroalgae, macroinvertebrate , and vertebrate altering herbivore density, a density-mediated indirect and predators. We measured the effects of interaction (DMII), and by altering herbivore predator and polycultures of three and five behaviour, a trait-mediated indirect interaction (TMII) species on herbivore populations and macroalgal biomass, (Trussell et al. 2002). Both trait-mediated and density- composition and diversity. mediated interactions are important determinants of the strength and direction of cascading predator effects (Trussell et al. 2002, 2003; Schmitz et al. 2004) and changes METHODS in predator diversity could influence the of DMIIs Experimental design and TMIIs. and could increase with predator diversity, also reducing the suppres- The experiment was performed at The University of North sion of herbivore populations (Sih et al. 1998; Finke & Carolina at Chapel Hill’s Institute of Marine Science (IMS) Denno 2004). Additionally, by consuming plants, omnivor- in Morehead , NC. The experiment began on 25 ous predators could weaken or reverse the generally September 2004, ran for 22 days, and included eight predicted positive relationship between predator diversity treatments (n ¼ 11): no predators, monocultures of each and plant biomass (Petchey et al. 2004) (Fig. 1). Because of the five predators, a mixture of three predators (the predator diversity could potentially have multiple counter- composition of each replicate was chosen randomly from the five-) and five predator polycultures (i.e. the high diversity treatment). The initial richness, composition Predators and biomass of herbivores and were the same in all 88 mesocosms. We used a replacement design, manipulating Shrimp predator identity and richness while holding predator Blennies density constant at five individuals per mesocosm (c. 16 g _ predator wet biomass/mesocosm), which is comparable Crabs with natural predator densities in North Carolina subtidal Pinfish communities (Nelson 1979a; Powers 2005). One limitation of a replacement design is that species-specific densities are + lower in mixture than in treatments, potentially Herbivores dampening important intraspecific interactions. However, Isopods this likely reflects natural conditions in the field, where _ Amphipods negative interspecific interactions (including intraguild pre- dation, and antagonistic behaviour) likely _ reduce densities when predator diversity is high. Plants Additionally, the replacement design provides the clearest test of predator diversity effects, unlike the additive design Green algae in which predator diversity and density are confounded as in Red algae most multiple predator effect experiments (Sih et al. 1998) and most recent attempts to test the predator diversity hypothesis (e.g. Finke & Denno 2004). Figure 1 Interaction web of the experimental ecosystem illustra- ting the positive and negative effects of direct (solid lines) and indirect (dashed lines) species interactions on the production, Study and experimental biomass and diversity of lower trophic levels. Interactions among species within a given trophic level are not included. Pinfish are Hard-substratum communities in the South Atlantic Bight omnivorous predators. are composed of a diverse assemblage of macroalgae,

2005 Blackwell Publishing Ltd/CNRS 1050 J. F. Bruno and M. I. O’Connor and fishes, including both temperate and three types of predators but possibly more than three species tropical species ( & Sutherland 1988). Macroalgae are (i.e. some of those that included two shrimp or blennies). The the main primary producers in these ecosystems and are five predators are similar in size, ranging in wet mass from an grazed intensely by macroinvertebrate herbivores (e.g. average of 1.8 to 6.1 g and from 3.4 to 7.2 cm in length (or amphipods and isopods), urchins and some fishes, although maximum width for crabs) (Table S1). The mean mass of the palatability is highly alga- and herbivore-specific (Miller & three fish predators only varied slightly (1.8–3.6 g) and the wet Hay 1996; Duffy & Hay 2000). The macroinvertebrate mass of the heaviest predators, swimming crabs, was likely herbivore assemblage is diverse (Nelson 1979a) and is exaggerated by water retention beneath the carapace. We consumed by vertebrate and invertebrate predators including specifically collected predators within this size range and shrimp, crabs and fishes (Nelson 1979b). Experimental attempted to minimize variability in predator biomass among organisms were collected at nearby sites just prior to the replicate mesocosms. beginning of the experiment and cultured and/or maintained The algal, herbivore and predator species were all chosen at IMS. The five macroalgal species (browns: because they are dominant organisms in this habitat in the filipendula and Dictyota menstrualis, greens: Enteromorpha linza summer and fall (Hay & Sutherland 1988). The 23 and the exotic Codium fragile, and red: Hypnea musciformis) experimental species are all common and generally co-occur typically dominate hard-substratum environments in North in North Carolina benthic estuarine ecosystems (Hay & Carolina (Hay & Sutherland 1988; Powers 2005; Sutherland 1988; Powers 2005), which the mesocosm Bruno et al., in press). Algae were attached to 25 · 25 cm communities were designed to mimic. Additionally, all species Vexar mesh screens secured to the bottom of the mesocosms except killifish co-occur on shallow, subtidal macroalgal (6 g per species per mesocosm). We included at least two above or within the algal (Powers 2005) (JFB thalli of each species on each screen, positions on screens and MIO, personal observations and unpublished data). were determined haphazardly, and the algae floated upward Killifish can be found in this environment and on other hard- in a natural orientation. Initial total macroalgal biomass was substrate habitats, such as reefs, but occur more 30 g which was less than the typical field biomass (Bruno regularly in soft-substrate habitats (which are usually in close et al., in press) to allow the algae for growth. Algal wet proximity to hard-substrate, macroalgal habitats) such as mass was measured after removing excess water using a salad grass beds and sand flats adjacent to salt . spinner (60 revolutions). An equal volume of herbivores was added to each Mesocosms mesocosm before the addition of predators and again during the first and second week of the experiment to mimic Experimental communities were maintained in outdoor, natural (totalling c. 100 amphipods and 100 flow-through mesocosms in 30 L clear, plastic isopods). The assemblage of 13 macroinvertebrate herbivore aquaria. Seawater was drawn from the adjacent Bogue species (Supplementary material) was dominated by the Sound and gravel filtered. The mesocosms were situated in isopod Paracerceis caudata and the amphipods Hyale spp., shallow water tables (12/table) that collected outflow and Elasmopus levi and Corophium spp. Herbivores were also helped to maintain a constant . , tempera- stocked at densities below conditions and increased ture and salinity in the mesocosms were very similar to field naturally during the experiment via local conditions (J. F. Bruno, unpublished data). Dump buckets (Duffy et al. 2003, 2005). Amphipod and isopod herbiv- that gradually filled with seawater and periodically emptied ores are extremely common in temperate benthic marine into the mesocosms below maintained the turbulence and habitats including macroalgal communities in North Caro- aeration typical of marine shallow-water, hard-substrate lina estuaries, with densities often exceeding several thou- habitats. The mesocosms were originally randomly assigned sand per square meter (Nelson 1979a; Duffy 1989; Brawley to tables and positions, and were then rearranged with 1992). respect to table number and position within the table every Predator species included: Portunus spinimanus (swimming 5 days to minimize location artefacts. crabs), Penaeus setiferus and P. aztecus (shrimp), Hypleurochilus geminatus and Hypsoblennius hentzi (blennies), Fundulus heteroclitus Response variables (killifish) and Lagodon rhomboides (pinfish). We chose predators with different strategies and we included multiple At the end of the experiment we quantified treatment species of blenny and shrimp because they are morpholog- effects on herbivore abundances, macroalgal biomass and ically and functionally much more similar to each other than to microalgal a concentration. All herbivores were other predators and are very difficult to distinguish. Thus, the removed from each mesocosm and stored in 70% EtOH shrimp and blenny ‘monocultures’ potentially included two until they could be sorted, identified to species, and counted species, and some of the three predator polycultures included under a dissecting . Predators can also influence

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plant diversity and relative abundances (Schmitz 2003), and a 500 these properties may therefore be more important measures 400 Isopods of the strength of trophic cascades than total plant biomass 300 (Schmitz 2003; Duffy et al. 2005). We measured treatment Amphipods effects on macroalgal diversity by calculating three indices 200 based on the final presence and wet mass of the macroalgae: Abundance 100 richness (S, the total number of species), Shannon–Weiner 0 diversity (H ¢), and evenness (Simpson’s 1 ) k). Microalgae including the early-succession alga Cladophora sp. and the b 800 All herbivores chain-forming Tabellaria sp. naturally colonized the 600 mesocosms. We quantified the final biomass of the entire microalgal community by measuring the chlorophyll a 400

concentration (see Supplementary Material). Abundance 200

0 Statistical analysis c 90 One-factor ANOVA (performed in the Fit Model platform Macroalgae of JMP) was used to test predator treatment effects on 60 final herbivore abundance, marcoalgal biomass, and micro- algal chlorophyll a concentration and derived responses 30 such as macroalgal diversity (data were log transformed to Wet mass (g) meet the assumptions of ANOVA). Least Squares Means 0 (LSM) planned contrasts were performed to compare the

effects of specific treatments, e.g. comparing the effects of Pinfish Killifish Shrimp the five predator polyculture treatment with all monocul- Blennie 3 species 5 species

ture treatments is a test of the effect of predator diversity No predators Generalist on each response variable. MANOVA was used to test the carnivores effect of the predator treatments on macroalgal community structure. Figure 2 Effects of predator identity and diversity treatments on: (a) isopod and amphipod abundance (no./mesocosm), (b) total herbivore abundance and (c) macroalgal biomass. RESULTS Predator presence and identity had significant effects on the winter and spring when predator abundances are greatly herbivore abundance (one-factor ANOVA: F7,78 ¼ 58.5, reduced, and is lowest during summer months (Nelson P < 0.0001) and composition (Fig. 2a,b). Herbivores were 1979a; Duffy 1989). Likewise, we found that at the end of ) 12–64 more abundant in the absence of predators the experiment mean grazer density was 6460 m 2 (or ) than in four of the five predator monocultures and in the 219 g 1 algal wet mass) in the no predator treatments three- and five-species polycultures. Four of the five (which is similar to field values when predators are ) ) predators apparently are generalists, consuming and effect- essentially absent in January) and 487 m 2 (or 9 g 1)in ively regulating nearly all species of small invertebrate the predator mixtures. herbivores (Fig. 2) (see Table S4). Crabs, however, only The predator treatments significantly affected total final reduced isopod abundances resulting in significantly higher macroalgal biomass (F7,78 ¼ 8.8, P < 0.0001) and predator total herbivore abundance than in any of the other predator identity and diversity influenced the strength of the trophic monoculture treatments (P ¼ 0.0001; LSM planned com- cascade measured as the degree of increase in total parison). This is likely because amphipods are more cryptic macroalgal biomass in comparison with predator-free and difficult to catch within structurally heterogeneous controls. The three generalist carnivores (shrimp, blennies macroalgal communities (in supplemental feeding trials and killifish) each induced a strong trophic cascade, crabs readily ate amphipods when prey were unable to hide doubling macro- and microalgal biomass (Fig. 2c and or escape, Table S2). Macroinvertebrate grazer abundance Fig. S1; both P < 0.0001; based on LSM planned contrasts in nearby sea grass communities ranges from 128 to within a one-factor ANOVA). Macroalgal biomass was ) 6461 individuals m 2 (Nelson 1979a) and from 10 to significantly lower in the crab (P ¼ 0.01), pinfish (P ¼ ) 145 g 1 algal wet mass in Sargassum filipendula (Duffy 0.0001) and high diversity (P ¼ 0.001) treatments than in 1989). Macroinvertebrate density in the field peaks during the generalist monocultures (Fig. 2c). Additionally,

2005 Blackwell Publishing Ltd/CNRS 1052 J. F. Bruno and M. I. O’Connor macroalgal biomass did not differ between the no predator and 1 ) k). Crabs and pinfish in contrast had only and pinfish treatments (P ¼ 0.18); this was almost certainly marginal effects on richness (both P > 0.05) and slightly due to the consumption of Hypnea and Enteromorpha by increased diversity (H ¢) and evenness (1 ) k) (all pinfish since herbivores were nearly absent in the pinfish P < 0.05), although not to the same degree as the monocultures (Fig. 2b). generalist carnivores (all P < 0.0001; contrast of crab and Macroalgal composition varied significantly among the pinfish effects with the other three predators). Their predator treatments (MANOVA: Wilks’ Lambda, F ¼ 4.45, inclusion in predator mixtures reduced the overall effect of P < 0.0001, Fig. 3a) and between the five-species polycul- the generalist carnivores on macroalgal diversity, especially ture and monoculture treatments (P ¼ 0.004; post-MANOVA in the five-species polycultures in which macroalgal LSM contrast). The presence of each of the generalist diversity (H ¢) was significantly lower than in the average carnivore species significantly increased each of the three predator monoculture (P ¼ 0.009), indicative of a negative, measures of diversity (all P < 0.0001 based on univariate non-additive diversity effect. The final algal richness and ANOVA contrasts with the no predator treatment for S,H ¢ biomass in the experiment closely mimicked natural field conditions. For example, at the scale of our experimental algal communities average natural macroalgal is 3–4 and wet biomass averages 87 g at four field sites close to IMS (based on surveys in May and June 2004) (Bruno et al., in press).

DISCUSSION Although trophic cascades were once thought to be restricted to simple aquatic communities (Strong 1992), more recent research and meta-analyses indicate that they are common in all types of terrestrial, freshwater and marine ecosystems (Schmitz et al. 2000; Shurin et al. 2002). The enhancement of primary production and plant biomass accumulation by predators can be dramatic (Schmitz et al. 2000), often transforming the physical framework of whole communities by altering the density and abundance of habitat-forming plants. For example, strong trophic cas- cades can facilitate the presence of and salt marshes by controlling the abundance of keystone herbiv- ores (Estes & Palmisano 1974; Silliman & Bertness 2002). However, most demonstrations of trophic cascades only consider a single predator and usually one that was already known or suspected to be a strong interactor. Thus, we do not know how general trophic cascades are across predator species or what role predator diversity plays in controlling cascade strength. We found that predator identity and diversity had striking Figure 3 Effects of predator identity and diversity on the relative effects on the strength of the trophic cascade. The presence abundances and composition of macroalgae. (a) First and second of a generalist carnivore (shrimp, blennies or killifish) canonical scores from MANOVA on the final wet mass of all five doubled macroalgal biomass, presumably by limiting algal species. Generalist carnivores (shrimp, blennies and killifish) herbivore abundance and . The trophic cascade are pooled together for clarity. The two replicate three predator was present but weaker in the crab treatment, likely because polycultures without pinfish are labelled with arrows. Macroalgal the crabs were less effective in regulating herbivores composition varied significantly between the generalist carnivore (Fig. 2a,b). Although the omnivorous pinfish effectively and pinfish treatments (P ¼ 0.0001; post-MANOVA LSM contrast). (b) Treatment effects on the final biomass of edible green suppressed herbivores (Fig. 2b), their direct consumption of (Enteromorpha), red (Hypnea), and brown (Sargassum and Dictyota) algae completely short-circuited the trophic cascade. Total macroalgae (none of the herbivores or the pinfish consumed macroalgal biomass was marginally lower in the high Codium). Initial wet mass of each alga was 6 g in all treatments. predator diversity mesocosms, which all included omni- Values are mean + 1 SE. vores, than in the average monoculture (macroalgal biomass,

2005 Blackwell Publishing Ltd/CNRS Cascading effects of predator diversity 1053

P ¼ 0.06; microalgal chlorophyll concentration, P ¼ 0.05). a 5.5 Thus, three of the five predators we tested induced a strong Richness trophic cascade but increasing predator diversity decreased 5 macroalgal biomass. 4.5 The magnitude and direction of the effects of predator extinctions and diversity on lower trophic levels are 4 Number species (S) probably highly context dependent and influenced by 3.5 predator traits, especially their dietary preferences. We b found that average macroalgal biomass was 2.3 times 1.7 higher (133 g; n ¼ 2; P ¼ 0.04, Kruskal–Wallis test) in the 1.6 Diversity three-species polycultures that did not include pinfish (i.e. 1.5 the carnivore-only mixtures) than in the average predator 1.4 monoculture. Algal composition also appeared to differ 1.3 between polycultures with and without pinfish (Fig. 3a), 1.2 largely due to the drastic difference in Entermorpha biomass. Shannon–Weiner (H') 1.1 This suggests that in the absence of an omnivore, multiple c 0.85 Evenness carnivores have complementary effects, maximizing algal )

λ 0.8 biomass. Herbivore abundances were not further reduced in the carnivore-only species mixtures, and the enhanced algal 0.75 biomass could have been caused by a change in herbivore 0.7

behaviour (e.g. a TMII that reduced activity to limit Simpson's (1– 0.65 predation risk) that decreased algal consumption. Whatever 0.6 the mechanism of this potential positive effect of carnivore diversity on algal biomass, the negative effect of selecting an Crab Pinfish Killifish Shrimp Blennie

omnivore was clearly much stronger. 3 species 5 species Generalist

Although it is not widely appreciated, predators can No predators carnivores increase plant diversity by limiting overall grazing pressure or by concentrating grazing on competitively dominant plants Figure 4 Effects of predator identity and diversity treatments on (Schmitz 2003). In our experiment, all three generalist three indices of algal community diversity. Values are mean + 1 carnivores substantially increased macroalgal diversity by SE. increasing both richness and evenness (Fig. 4). However, our results also indicate that the aggregate effect of predators on algal diversity can be substantially dampened when predator tures, and did not eat brown algae or Codium (Fig. 3b and diversity is relatively high. Pinfish and crabs also increased Table S4). macroalgal diversity, but to a lesser degree, thereby reducing Recent meta-analyses and experiments indicate that the net positive effect on diversity in the three- and five- increasing prey diversity can reduce consumer control of species polycultures. Predator identity and diversity also plant and herbivore populations (Schmitz et al. 2000; Shurin strongly influenced the relative abundances and composition et al. 2002; Hillebrand & Cardinale 2004; Duffy et al. 2005). A of macroalgae via selective consumption by predators and probable mechanism driving these results is that increasing herbivores. Algal communities in the predator polycultures prey diversity improves the likelihood of including unpalat- were compositionally intermediate between those in the able prey species (Worm & Duffy 2003; Duffy et al. 2005). pinfish and generalist carnivore treatments, while the carni- Consumer diversity effects based on dietary complementar- vore-only polyculture replicates were the most divergent from ity are essentially governed by the ratio of predator to prey the pinfish monocultures (Fig. 3a). Amphipod and isopod diversity (although absolute diversity could also be relevant). herbivores prefer brown and filamentous green algae such as Therefore, increasing predator diversity relative to herbivore Enteromorpha and in general do not consume Hypnea and diversity should strengthen the trophic cascade (Schmitz Codium (Duffy & Hay 2000) (Table S5), which dominated the et al. 2000). Yet except for work in an no predator treatment (Fig. 3b). Brown algae dominated all (Cardinale et al. 2003) there is currently little published predator combinations except the crab monocultures, in evidence supporting this prediction. In fact, our experiment which selective predation on isopods apparently indirectly indicates that the net effects of predator diversity on plant produced a Hypnea-dominated algal community. Pinfish biomass and diversity can be negative. This result and other completely consumed Enteromorpha, driving it locally extinct recent investigations of the effects of diversity using a in all of the pinfish monocultures and five-species polycul- single prey species (Schmitz & Sokol-Hessner 2002; Finke &

2005 Blackwell Publishing Ltd/CNRS 1054 J. F. Bruno and M. I. O’Connor

Denno 2004) contradict prevailing assumptions about the terrestrial food webs (Duffy 2003; Petchey et al. 2004). role of predator diversity. However, in many freshwater habitats the addition of exotic Our experiment also suggests that omnivory may play an fish has probably increased predator diversity (Gido & important role in mediating biodiversity–ecosystem func- Brown 1999; Sax & Gaines 2003). On oceanic exotic tioning relationships, although have received diversity is directly related to the number of native little attention in this context (Petchey et al. 2004). Trophic avian extinctions (Blackburn et al. 2004), suggesting a direct cascades are thought to be less common in complex food link between changes in predator and prey diversity. The webs due to the inclusion of indirect interactions and shrimp used in this experiment are directly targeted and omnivory (Polis & Strong 1996; Finke & Denno 2004). The several other species are incidentally killed as by-catch. abundance and importance of omnivores in natural However, the current abundance and diversity of small communities has been controversial (Polis & Strong predators in this and many other estuarine ecosystems may 1996), but a recent analysis of nine well-studied food webs actually be higher than historical levels due to the indicated that 12–43% of consumers are omnivores widespread depletion of larger fourth and fifth trophic level (Williams & Martinez 2004). The chance of having at least vertebrate predators (Botsford et al. 1997; Jackson et al. one functional omnivore in a community should increase 2001). Decreases and increases in predator diversity could with predator diversity and omnivores are likely to be both have dramatic cascading effects on lower trophic present in all diverse food webs (Polis & Strong 1996). In levels. Our results indicate that increasing predator diversity most estuarine habitats in North Carolina omnivores are can induce surprising changes in community structure and ubiquitous and pinfish are a common and important food web dynamics. The consequences of changes in consumer (Duffy & Hay 2000). predator diversity are difficult to predict due to the many Our results demonstrate that increasing the diversity of complex interactions that occur in diverse food webs and and invertebrate predators can substan- are likely highly context dependent. Understanding the tially dampen the positive effects that individual predators effects of predator diversity on community and ecosystem have on algal biomass and diversity. Predator identity and properties can help address practical concerns such as the diversity also had strong effects on relative algal abun- ecological implications of the decline of top predator species dances and community composition. This outcome was and the conservation value of biodiversity. due in part to the selection of an omnivore. However, macroalgal biomass and diversity in the five-predator ACKNOWLEDGEMENTS polycultures was significantly lower than in the average monoculture, indicating that the observed diversity effects We thank J.E. Duffy, K. France, L. Hazen, S. Lee, C. Lewis, were greater than expected based on the monoculture Z. Long, C. Peterson, E. Selig, C. Shields, J. Wall, the editors values (i.e. the effects are non-additive). Pinfish apparently and three anonymous referees and the staff of the consumed algae at a greater per capita rate in mixture UNC-CH’s Institute of Marine Science. This research was than in monoculture, potentially due to interspecific funded in part by the National Science Foundation (#2002- interactions among predators or the reduction of intra- 23 and 2002-52 to J.F.B. and a G.R.F. to M.I.O.) and the specific competition. 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2005 Blackwell Publishing Ltd/CNRS 1056 J. F. Bruno and M. I. O’Connor

SUPPLEMENTARY MATERIAL Table S4 Per cent of herbivores and macroalgae consumed after 24 h. The following supplementary material is available for this Table S5 Per cent of algae consumed after 72 h. article from http://www.Blackwell-Synergy.com: Figure S1 Effects of predator identity and diversity treat- ments on microalgal biomass production. Table S1 Sizes of predators used in the predator diversity Editor, Oswald Schmitz experiment. Manuscript received 5 May 2005 Table S2 Species used in feeding experiment. First decision made 31 May 2005 Table S3 Two-way ANOVA results for single species feeding Manuscript accepted 17 June 2005 trials.

2005 Blackwell Publishing Ltd/CNRS