Estimating the prevalence and strength of non-independent predator effects

Understanding whether multiple predator species have independent effects on shared prey is critical for understanding community dynamics. We describe the prevalence and strength of non-independence between predators by quantifying the prey’s risk of predation and the degree to which it deviates from the risk predicted from a null model of independent predator effects. Specifically, we document how frequently non-independent effects occur among ten different multiple predator combinations with mayfly larvae as prey. These predator combinations vary both predator density and predator species richness. Overall, the predator effects were non-independent and translated to an average of 27% fewer prey being consumed compared to independent predator effects. Non-independence of this magnitude is likely to have population level consequences for the prey and influence the distribution or prey preference of predators. Closer inspection shows that much of the risk reduction in this system is weak, to the point of being indistinguishable from independent predator effects, while few effects are strong. This pattern of many weak interactions and few strong ones parallels the pattern of interaction strengths documented previously in intertidal communities. Consequently, understanding strong interactors in multiple predator systems may help us understand the importance of a species.     

Partitioning mechanisms of Predator Interference in different Habitats

Prey are often consumed by multiple predator species. Predation rates on shared prey species measured in isolation often do not combine additively due to interference or facilitation among the predator species. Furthermore, the strength of predator interactions and resulting prey mortality may change with habitat type. We experimentally examined predation on amphipods in rock and algal habitats by two species of intertidal crabs, Hemigrapsus sanguineus (top predators) and Carcinus maenas (intermediate predators). Algae provided a safer habitat for amphipods when they were exposed to only a single predator species. When both predator species were present, mortality of amphipods was less than additive in both habitats. However, amphipod mortality was reduced more in rock than algal habitat because intermediate predators were less protected in rock habitat and were increasingly targeted by omnivorous top predators. We found that prey mortality in general was reduced by (1) altered foraging behavior of intermediate predators in the presence of top predators, (2) top predators switching to foraging on intermediate predators rather than shared prey, and (3) density reduction of intermediate predators. The relative importance of these three mechanisms was the same in both habitats; however, the magnitude of each was greater in rock habitat. Our study demonstrates that the strength of specific mechanisms of interference between top and intermediate predators can be quantified but cautions that these results may be habitat specific. An erratum to this article can be found at     

Influence of predator density on nonindependent effects of multiple predator species

Interactions between multiple predator species are frequent in natural communities and can have important implications for shared prey survival. Predator density may be an important component of these interactions between predator species, as the frequency of interactions between species is largely determined by species density. Here we experimentally examine the importance of predator density for interactions between predator species and subsequent impacts on prey. We show that aggressive interactions between the predatory shore crabs Carcinus maenas and Hemigrapsus sanguineus increased with predator density, yet did not increase as fast as negative interactions between conspecifics. At low density, interactions between conspecific and heterospecific predators had similar inhibitory impacts on predator function, whereas conspecific interference was greater than interference from heterospecifics at high predator density. Thus the impact of conspecific interference at high predator density was sufficient in itself that interactions with a second predator species had no additional impact on per capita predation. Spatial and temporal variability in predator density is a ubiquitous characteristic of natural systems that should be considered in studies of multiple predator species.     

Nonconsumptive effects in a multiple predator system reduce the foraging efficiency of a keystone predator

Many studies have demonstrated that the nonconsumptive effect (NCE) of predators on prey traits can alter prey demographics in ways that are just as strong as the consumptive effect (CE) of predators. Less well studied, however, is how the CE and NCE of multiple predator species can interact to influence the combined effect of multiple predators on prey mortality. We examined the extent to which the NCE of one predator altered the CE of another predator on a shared prey and evaluated whether we can better predict the combined impact of multiple predators on prey when accounting for this influence. We conducted a set of experiments with larval dragonflies, adult newts (a known keystone predator), and their tadpole prey. We quantified the CE and NCE of each predator, the extent to which NCEs from one predator alters the CE of the second predator, and the combined effect of both predators on prey mortality. We then compared the combined effect of both predators on prey mortality to four predictive models. Dragonflies caused more tadpoles to hide under leaf litter (a NCE), where newts spend less time foraging, which reduced the foraging success (CE) of newts. Newts altered tadpole behavior but not in a way that altered the foraging success of dragonflies. Our study suggests that we can better predict the combined effect of multiple predators on prey when we incorporate the influence of interactions between the CE and NCE of multiple predators into a predictive model. In our case, the threat of predation to prey by one predator reduced the foraging efficiency of a keystone predator. Consequently, the ability of a predator to fill a keystone role could be compromised by the presence of other predators.     

Non-interactive multiple predator effects on tadpole survival

Interactions among and within three species of predators were estimated in terms of their effects on prey survival using short-term predation experiments. The prey were tadpoles (Rana temporaria), and the predators were dragonfly larvae (Anax imperator), newts (Triturus alpestris), and backswimmers (Notonecta glauca). Mortality rate per predator imposed by Triturus and Notonecta did not decline with predator density, whereas the predation rate of Anax was strongly reduced when the number of predator individuals increased. Impacts of all three predators were not altered by the presence of other species in pairwise combinations. This system is therefore characterized by interference between individual dragonflies but relatively independent effects of predator species. These results were largely predictable based on the natural history of the predators and are encouraging for attempts to model communities as assemblages of interacting species.     

Does aggregation benefit bark beetles by diluting predation? Links between a group‐colonisation strategy and the absence of emergent multiple predator effects

Abstract. 1. Aggregation in bark beetles (Coleoptera: Scolytidae) aids in mate attraction and resource procurement when colonising well‐defended plants; however, some species colonise primarily poorly defended plants, and intraspecific competition increases mortality. The hypothesis that decreased risk of predation was a potential benefit to aggregation in such circumstances was tested, using the pine engraver, Ips pini (Say) and its two major predators Thanasimus dubius (F.) (Coleoptera: Cleridae) and Platysoma cylindrica (Paykull) (Coleoptera: Histeridae). Both single‐ and multiple‐predator effects, across a range of prey densities, were tested. 2. Both male and female colonisation events increased with herbivore density, in an asymptotic fashion. 3. Predators decreased the number of colonisers in a density‐dependent manner, consistent with a type II functional response. 4. The proportional impact of predators decreased with increased herbivore colonisation densities. These findings indicate that predator dilution may be a viable benefit to aggregation. 5. Total emergence of the herbivore also increased with density, although the net replacement rate during one generation was independent of initial arrival density. This was likely due to larval predation, which negates potential relationships between per capita reproductive success and establishment density. 6. Each predator species decreased I. pini's net replacement rate by approximately 42%, and their combined effect was approximately 70%. 7. Overall, these predators modified their prey's establishment and adult mortality relationships in additive manners. This is somewhat surprising, given the potential for emergent effects due to interactions between multiple predators foraging within a common habitat. The persistence of additivity, rather than risk reduction or enhancement to the prey, may increase the predator‐swamping benefit to aggregation for this herbivore. 8. The effects of these predators are substitutable, and likely exert equivalent selective pressures to mask signals at the whole‐plant level.     

Multiple predator effects result in risk reduction for prey across multiple prey densities

Investigating how prey density influences a prey’s combined predation risk from multiple predator species is critical for understanding the widespread importance of multiple predator effects. We conducted experiments that crossed six treatments consisting of zero, one, or two predator species (hellgrammites, greenside darters, and creek chubs) with three treatments in which we varied the density of mayfly prey. None of the multiple predator effects in our system were independent, and instead, the presence of multiple predator species resulted in risk reduction for the prey across both multiple predator combinations and all three levels of prey density. Risk reduction is likely to have population-level consequences for the prey, resulting in larger prey populations than would be predicted if the effects of multiple predator species were independent. For one of the two multiple predator combinations, the magnitude of risk reduction marginally increased with prey density. As a result, models predicting the combined risk from multiple predator species in this system will sometimes need to account for prey density as a factor influencing per-capita prey death rates.     

Positive indirect effect of aquatic insects on terrestrial prey is not offset by increased predator density

1. Changes in one prey species' density can indirectly affect the abundance of another prey species if a shared predator eats both species. Sometimes, indirect effects occur when prey straddle habitats, including when riparian predator populations grow in response to emergent aquatic insects and increase predation on terrestrial prey. However, predators may largely switch to aquatic insects or become satiated, reducing predation on terrestrial prey. 2. To determine the net indirect effect of aquatic insects on terrestrial arthropods via generalist spider predators, a field experiment was conducted mimicking midge influx and a wolf spider numerical response inside enclosures near an Icelandic lake. Lab mesocosms were also used to assess per capita rates of spider predation u nder differing levels of midge abundance. 3. Midges always decreased sentinel prey predation, but this effect increased with predator density. When midges were absent, predation increased 30% at a high spider density, but predation was equal between spider treatments when midges were present. In situ arthropods showed no effect of midge or spider treatments, although non‐significant abundance patterns were observed congruent with sentinel prey results. 4. In lab mesocosms, prey survivorship increased ≥50% where midges were present and rapidly saturated; the addition of 5, 20, 50, and 100 midges equivalently reduced spider predation, supporting predator distraction rather than satiation as the root cause. 5. The present results demonstrate a strong positive indirect effect of midges and broadly support the concept that predator responses to alternative prey are a major influence on the magnitude and direction of predator‐mediated indirect effects.     

Additive multiple predator effects can reduce mosquito populations

1. Multiple predator interactions may profoundly alter ecological community dynamics and can complicate predictions of simpler pairwise predator–prey interaction strengths. In particular, multiple predator effects may lessen or enhance prey risk, with implications for community-level stability. Such emergent effects may modulate natural enemy efficacy towards target organisms. 2. In the present study, a functional response approach was used to quantify emergent multiple predator effects among natural enemies towards the disease vector mosquito complex, Culex pipiens. Conspecific multiple predator–predator interactions of the cyclopoid copepod Macrocyclops albidus (intermediate predator) were quantified by comparing multiple predator consumption simulations, based on individual consumption rates, with multiple predator consumption rates that were experimentally observed. Further, the study examined the influence of the presence of a predator at a higher trophic level, Chaoborus flavicans, on copepod group predation. 3. Both predators displayed type II functional responses, with C. flavicans consuming significantly more prey than M. albidus individually. Overall consumption levels of mosquitoes increased with greater predator density and richness. Antagonistic or synergistic emergent multiple predator effects between conspecifics of M. albidus were not detected, and the higher-level predator did not reduce effects of the intermediate predator. Accordingly, evidence for additive multiple predator interactions was found. 4. The lack of predator–predator interference between cyclopoid copepods and larval chaoborid midges provides strong support for their combined application in mosquito biocontrol. It is proposed that there should be increased examination of multiple predator effects in assessments of natural enemy efficacies to better understand overall predatory effects within communities and utilities in vector control.     

Prey and predator density‐dependent interactions under different water volumes

Predation is a critical ecological process that directly and indirectly mediates population stabilities, as well as ecosystem structure and function. The strength of interactions between predators and prey may be mediated by multiple density dependences concerning numbers of predators and prey. In temporary wetland ecosystems in particular, fluctuating water volumes may alter predation rates through differing search space and prey encounter rates. Using a functional response approach, we examined the influence of predator and prey densities on interaction strengths of the temporary pond specialist copepod Lovenula raynerae preying on cladoceran prey, Daphnia pulex, under contrasting water volumes. Further, using a population dynamic modeling approach, we quantified multiple predator effects across differences in prey density and water volume. Predators exhibited type II functional responses under both water volumes, with significant antagonistic multiple predator effects (i.e., antagonisms) exhibited overall. The strengths of antagonistic interactions were, however, enhanced under reduced water volumes and at intermediate prey densities. These findings indicate important biotic and abiotic contexts that mediate predator–prey dynamics, whereby multiple predator effects are contingent on both prey density and search area characteristics. In particular, reduced search areas (i.e., water volumes) under intermediate prey densities could enhance antagonisms by heightening predator–predator interference effects.     

Density-dependent effects of multiple predators sharing a common prey in an endophytic habitat

Multiple predator species feeding on a common prey can lead to higher or lower predation than would be expected by simply combining their individual effects. Such emergent multiple predator effects may be especially prevalent if predators share feeding habitat. Despite the prevalence of endophagous insects, no studies have examined how multiple predators sharing an endophytic habitat affect prey or predator reproduction. We investigated density-dependent predation of Thanasimus dubius (Coleoptera: Cleridae) and Platysoma cylindrica (Coleoptera: Histeridae) on a bark beetle prey, Ips pini (Coleoptera: Scolytidae), in a laboratory assay. I. pini utilize aggregation pheromones to group-colonize and reproduce within the stems of conifers. T. dubius and P. cylindrica exploit these aggregation pheromones to arrive simultaneously with the herbivore. Adult T. dubius prey exophytically, while P. cylindrica adults enter and prey within the bark beetle galleries. Larvae of both predators prey endophytically. We used a multiple regression analysis, which avoids confounding predator composition with density, to examine the effects of varying predator densities alone and in combination on herbivore establishment, herbivore reproduction, and predator reproduction. Predators reduced colonization success by both sexes, and decreased I. pini reproduction on a per male and per female basis. The combined effects of these predators did not enhance or reduce prey establishment or reproduction in unexpected manners, and these predators were entirely substitutable. The herbivores net replacement rate was never reduced significantly below one at prey and predator densities emulating field conditions. Similar numbers of each predator species emerged from the logs, but predator reproduction suffered from high intraspecific interference. The net replacement rate of P. cylindrica was not affected by conspecifics or T. dubius. In contrast, the net replacement rate of T. dubius decreased with the presence of conspecifics or P. cylindrica. Combinations of both predators led to an emergent effect, a slightly increased net replacement rate of T. dubius. This may have been due to predation by larval T. dubius on pupal P. cylindrica, as P. cylindrica develops more rapidly than T. dubius within this shared habitat.     

Keystone nonconsumptive effects within a diverse predator community

The number of prey killed by diverse predator communities is determined by complementarity and interference among predators, and by traits of particular predator species. However, it is less clear how predators' nonconsumptive effects (NCEs) scale with increasing predator biodiversity. We examined NCEs exerted on Culex mosquitoes by a diverse community of aquatic predators. In the field, mosquito larvae co‐occurred with differing densities and species compositions of mesopredator insects; top predator dragonfly naiads were present in roughly half of surveyed water bodies. We reproduced these predator community features in artificial ponds, exposing mosquito larvae to predator cues and measuring resulting effects on mosquito traits throughout development. Nonconsumptive effects of various combinations of mesopredator species reduced the survival of mosquito larvae to pupation, and reduced the size and longevity of adult mosquitoes that later emerged from the water. Intriguingly, adding single dragonfly naiads to ponds restored survivorship of larval mosquitoes to levels seen in the absence of predators, and further decreased adult mosquito longevity compared with mosquitoes emerging from mesopredator treatments. Behavioral observations revealed that mosquito larvae regularly deployed “diving” escape behavior in the presence of the mesopredators, but not when a dragonfly naiad was also present. This suggests that dragonflies may have relaxed NCEs of the mesopredators by causing mosquitoes to abandon energetically costly diving. Our study demonstrates that adding one individual of a functionally unique species can substantially alter community‐wide NCEs of predators on prey. For pathogen vectors like mosquitoes, this could in turn influence disease dynamics.     

Female blue tits with brighter yellow chests transfer more carotenoids to their eggs after an immune challenge

Predicting the consequences of predator biodiversity loss on prey requires an understanding of multiple predator interactions. Predators are often assumed to have independent and additive effects on shared prey survival; however, multiple predator effects can be non-additive if predators foraging together reduce prey survival (risk enhancement) or increase prey survival through interference (risk reduction). In marine communities, juvenile reef fish experience very high mortality from two predator guilds with very different hunting modes and foraging domains—benthic and pelagic predator guilds. The few previous predator manipulation studies have found or assumed that mortality is independent and additive. We tested whether interacting predator guilds result in non-additive prey mortality and whether the detection of such effects change over time as prey are depleted. To do so, we examined the roles of benthic and pelagic predators on the survival of a juvenile shoaling zooplanktivorous temperate reef fish, Trachinops caudimaculatus, on artificial patch reefs over 2 months in Port Phillip Bay, Australia. We observed risk enhancement in the first 7 days, as shoaling behaviour placed prey between predator foraging domains with no effective refuge. At day 14 we observed additive mortality, and risk enhancement was no longer detectable. By days 28 and 62, pelagic predators were no longer significant sources of mortality and additivity was trivial. We hypothesize that declines in prey density led to reduced shoaling behaviour that brought prey more often into the domain of benthic predators, resulting in limited mortality from pelagic predators. Furthermore, pelagic predators may have spent less time patrolling reefs in response to declines in prey numbers. Our observation of the changing interaction between predators and prey has important implications for assessing the role of predation in regulating populations in complex communities.     

水生生态环境中捕食信息素的生态学效应

捕食信息素是捕食者释放的,能够引发猎物反捕食反应的化学信号。在水生生态系统中,捕食信息素在捕食者和猎物之间信息传递及协同进化过程中发挥着重要的作用,其生态学效应在国际上受到广泛关注。捕食信息素的来源有多种形式,研究中常使用养殖过捕食者的水溶液作为捕食信息素的来源。捕食信息素的作用效果受到捕食者和猎物的种类、信息素的浓度、观察的指标等多方面因素的影响。捕食信息素可以对水生生物的行为、形态和生活史特征等方面造成影响。水生生物通过感知捕食信息素来提前预知潜在的被捕食风险,并作出适应性调整,以降低被捕食的风险。在某些情况下,捕食信息素可以与污染物产生交互作用,从而干扰污染物对水生生物的毒性。对水生环境中捕食信息素的研究现状做了综述,介绍了当前对捕食信息素来源和理化性质等本质问题的认识,总结捕食信息素对水生生物行为、形态和生活史特征的影响,以及捕食信息素对污染物毒性的干扰,并分析了这一研究领域尚存在的困难和今后的研究方向。加强对捕食信息素的研究,将为解析水生环境中捕食者和猎物的生态关系提供新依据。     

Complementary predation on metamorphosing species promotes stability in predator–prey systems

Functionally redundant predation and functionally complementary predation are both widespread phenomena in nature. Functional complementary predation can be found, for example, when predators feed on different life stages of their prey, while functional redundant predation occurs when different predators feed on all life stages of a shared prey. Both phenomena are common in nature, and the extent of differential life-stage predation depends mostly on prey life history; complementary predation is expected to be more common on metamorphosing prey species, while redundant predation is thought to be higher on non-metamorphosing species. We used an ordinary differential equation model to explore the effect of varying degree of complementary and redundant predation on the dynamic properties of a system with two predators that feed on an age-structured prey. Our main finding was that predation on one stage (adult or juvenile) resulted in a more stable system (i.e., it is stable for a wider range of parameters) compared to when the two predators mix the two prey developmental stages in their diet. Our results demonstrate that predator–prey dynamics depends strongly on predators' functionality when predator species richness is fixed. Results also suggest that systems with metamorphosing prey are expected to be more diverse compared to systems with non-metamorphosing prey.     

Predator diversity and density affect levels of predation upon strongly interactive species in temperate rocky reefs

Indirect effects of predators in the classic trophic cascade theory involve the effects of basal species (e.g. primary producers) mediated by predation upon strongly interactive consumers (e.g. grazers). The diversity and density of predators, and the way in which they interact, determine whether and how the effects of different predators on prey combine. Intraguild predation, for instance, was observed to dampen the effects of predators on prey in many ecosystems. In marine systems, species at high trophic levels are particularly susceptible to extinction (at least functionally). The loss of such species, which is mainly attributed to human activities (mostly fishing), is presently decreasing the diversity of marine predators in many areas of the world. Experimental studies that manipulate predator diversity and investigate the effects of this on strongly interactive consumers (i.e. those potentially capable of causing community-wide effects) in marine systems are scant, especially in the rocky sublittoral. I established an experiment that utilised cage enclosures to test whether the diversity and density of fish predators (two sea breams and two wrasses) would affect predation upon juvenile and adult sea urchins, the most important grazers in Mediterranean sublittoral rocky reefs. Changes in species identity (with sea breams producing major effects) and density of predators affected predation upon sea urchins more than changes in species richness per se. Predation upon adult sea urchins decreased in the presence of multiple predators, probably due to interference competition between sea breams and wrasses. This study suggests that factors that influence both fish predator diversity and density in Mediterranean rocky reefs (e.g. fishing and climate change) may have the potential to affect the predators' ability to control sea urchin population density, with possible repercussions for the whole benthic community structure.     

Quantifying predator dependence in the functional response of generalist predators

A long‐standing debate concerns how functional responses are best described. Theory suggests that ratio dependence is consistent with many food web patterns left unexplained by the simplest prey‐dependent models. However, for logistical reasons, ratio dependence and predator dependence more generally have seen infrequent empirical evaluation and then only so in specialist predators, which are rare in nature. Here we develop an approach to simultaneously estimate the prey‐specific attack rates and predator‐specific interference (facilitation) rates of predators interacting with arbitrary numbers of prey and predator species in the field. We apply the approach to surveys and experiments involving two intertidal whelks and their full suite of potential prey. Our study provides strong evidence for predator dependence that is poorly described by the ratio dependent model over manipulated and natural ranges of species abundances. It also indicates how, for generalist predators, even the qualitative nature of predator dependence can be prey‐specific.     

Is prey predation risk influenced more by increasing predator density or predator species richness in stream enclosures?

The direct lethal impacts and the indirect effects predators have on prey characteristics, such as behavior, have fitness consequences for the prey. Whether the level of predation risk that prey face in the presence of multiple predator species can be predicted from a null model that sums the risk from each predator species in isolation is unclear. In field enclosures, we tested whether the predation risk experienced by Stenonema mayfly larvae from a dragonfly larva (Boyeria vinosa) and a hellgrammite (Corydalus cornutus) together matched the predictions of the multiplicative risk model. We then compared whether any deviations from the models predictions were larger in the presence of two predator species than in the presence of an equivalent density of individuals from either predator species alone, to determine if unique effects arise for the prey in the presence of multiple predator species. We also determined if prey moved preferentially into predator-free refuge spaces or decreased their movement in the presence of predators. Stenonemas risk of predation was reduced compared to the models prediction, but no unique multiple predator species effects were present because this risk reduction was comparable in magnitude to the level exhibited in the presence of each predator species alone. The prey did not move into predator-free refuge spaces in the presence of predators in the field enclosures. Thus, these predators appear to interfere interspecifically and intraspecifically, which may facilitate the coexistence of the predators and the prey.     

Chemical defense of toad tadpoles under risk by four predator species

Many organisms use inducible defenses as protection against predators. In animals, inducible defenses may manifest as changes in behavior, morphology, physiology, or life history, and prey species can adjust their defensive responses based on the dangerousness of predators. Analogously, prey may also change the composition and quantity of defensive chemicals when they coexist with different predators, but such predator‐induced plasticity in chemical defenses remains elusive in vertebrates. In this study, we investigated whether tadpoles of the common toad (Bufo bufo) adjust their chemical defenses to predation risk in general and specifically to the presence of different predator species; furthermore, we assessed the adaptive value of the induced defense. We reared tadpoles in the presence or absence of one of four caged predator species in a mesocosm experiment, analyzed the composition and quantity of their bufadienolide toxins, and exposed them to free‐ranging predators. We found that toad tadpoles did not respond to predation risk by upregulating their bufadienolide synthesis. Fishes and newts consumed only a small percentage of toad tadpoles, suggesting that bufadienolides provided protection against vertebrate predators, irrespective of the rearing environment. Backswimmers consumed toad tadpoles regardless of treatment. Dragonfly larvae were the most voracious predators and consumed more predator‐naïve toad tadpoles than tadpoles raised in the presence of dragonfly cues. These results suggest that tadpoles in our experiment had high enough toxin levels for an effective defense against vertebrate predators even in the absence of predator cues. The lack of predator‐induced phenotypic plasticity in bufadienolide synthesis may be due to local adaptation for constantly high chemical defense against fishes in the study population and/or due to the high density of conspecifics.     

Differences in predator composition alter the direction of structure‐mediated predation risk in macrophyte communities

Structural complexity strongly influences the outcome of predator–prey interactions in benthic marine communities affecting both prey concealment and predator hunting efficacy. How habitat structure interacts with species‐specific differences in predatory style and antipredatory strategies may therefore be critical in determining higher trophic functions. We examined the role of structural complexity in mediating predator–prey interactions across several macrophyte habitats along a gradient of structural complexity in three different bioregions: western Mediterranean Sea (WMS), eastern Indian Ocean (EIO) and northern Gulf of Mexico (NGM). Using sea urchins as model prey, we measured survival rates of small (juveniles) and medium (young adults) size classes in different habitat zones: within the macrophyte habitat, along the edge and in bare sandy spaces. At each site we also measured structural variables and predator abundance. Generalised linear models identified biomass and predatory fish abundance as the main determinants of predation intensity but the efficiency of predation was also influenced by urchin size class. Interestingly though, the direction of structure‐mediated effects on predation risk was markedly different between habitats and bioregions. In WMS and NGM, where predation by roving fish was relatively high, structure served as a critical prey refuge, particularly for juvenile urchins. In contrast, in EIO, where roving fish predation was low, predation was generally higher inside structurally complex environments where sea stars were responsible for much of the predation. Larger prey were generally less affected by predation in all habitats, probably due to the absence of large predators. Overall, our results indicate that, while the structural complexity of habitats is critical in mediating predator–prey interactions, the direction of this mediation is strongly influenced by differences in predator composition. Whether the regional pool of predators is dominated by visual roving species or chemotactic benthic predators may determine if structure dampens or enhances the influence of top–down control in marine macrophyte communities.