Predation risk makes bees reject rewarding flowers and reduce foraging activity

In the absence of predators, pollinators can often maximize their foraging success by visiting the most rewarding flowers. However, if predators use those highly rewarding flowers to locate their prey, pollinators may benefit from changing their foraging preferences to accept less rewarding flowers. Previous studies have shown that some predators, such as crab spiders, indeed hunt preferentially on the most pollinator-attractive flowers. In order to determine whether predation risk can alter pollinator preferences, we conducted laboratory experiments on the foraging behavior of bumble bees (Bombus impatiens) when predation risk was associated with a particular reward level (measured here as sugar concentration). Bees foraged in arenas containing a choice of a high-reward and a low-reward artificial flower. On a bee’s first foraging trip, it was either lightly squeezed with forceps, to simulate a crab spider attack, or was allowed to forage safely. The foragers’ subsequent visits were recorded for between 1 and 4 h without any further simulated attacks. Compared to bees that foraged safely, bees that experienced a simulated attack on a low-reward artificial flower had reduced foraging activity. However, bees attacked on a high-reward artificial flower were more likely to visit low-reward artificial flowers on subsequent foraging trips. Forager body size, which is thought to affect vulnerability to capture by predators, did not have an effect on response to an attack. Predation risk can thus alter pollinator foraging behavior in ways that influence the number and reward level of flowers that are visited.     

A review of predator diet effects on prey defensive responses

Numerous studies have examined how predator diets influence prey responses to predation risk, but the role predator diet plays in modulating prey responses remains equivocal. We reviewed 405 predator–prey studies in 109 published articles that investigated changes in prey responses when predators consumed different prey items. In 54 % of reviewed studies, prey responses were influenced by predator diet. The value of responding based on a predator’s recent diet increased when predators specialized more strongly on particular prey species, which may create patterns in diet cue use among prey depending upon whether they are preyed upon by generalist or specialist predators. Further, prey can alleviate costs or accrue greater benefits using diet cues as secondary sources of information to fine tune responses to predators and to learn novel risk cues from exotic predators or alarm cues from sympatric prey species. However, the ability to draw broad conclusions regarding use of predator diet cues by prey was limited by a lack of research identifying molecular structures of the chemicals that mediate these interactions. Conclusions are also limited by a narrow research focus. Seventy percent of reviewed studies were performed in freshwater systems, with a limited range of model predator–prey systems, and 98 % of reviewed studies were performed in laboratory settings. Besides identifying the molecules prey use to detect predators, future studies should strive to manipulate different aspects of prey responses to predator diet across a broader range of predator–prey species, particularly in marine and terrestrial systems, and to expand studies into the field.     

Subhabitat selection by foraging threespine stickleback (<Emphasis Type="Italic">Gasterosteus aculeatus</Emphasis>): previous experience and social conformity

Any mechanism that allows animals to increase their foraging efficiency is likely to be selected for, including the ability to learn to recognise and subsequently discriminate between habitat types based on their profitability. In a series of laboratory studies, we manipulated prey densities across two different experimental subhabitats and demonstrated that threespine stickleback (Gasterosteus aculeatus) can develop foraging preferences for subhabitats that have previously yielded prey. Fish were not recalling the spatial location of prey patches; rather, they were discriminating between subhabitats based on foraging experience there and allocating foraging effort accordingly. Foraging preferences took around 14 days to develop, and once established, they persisted independently of experimental prey density, suggesting that fish were using experience rather than real-time sampling to select foraging grounds. When we presented focal fish with social information cues, we found that they preferentially used local enhancement and current public information cues when they conflicted with previous experience, but that they did not use prior public information. This suggests that in the presence of conspecifics, individuals prioritise social conformity over the use of private information. We discuss our results in the context of optimal foraging and the trade-offs associated with balancing conflicting private and social information.     

Habitat use and foraging behavior of tiger sharks (<Emphasis Type="Italic">Galeocerdo cuvier</Emphasis>) in a seagrass ecosystem

Understanding the foraging behavior and spatial distribution of top predators is crucial to gaining a complete understanding of communities. However, studies of top predators are often logistically difficult and it is important to develop appropriate methods for identifying factors influencing their spatial distribution. Sharks are top predators in many marine communities, yet no studies have quantified the habitat use of large predatory sharks or determined the factors that might influence shark spatial distributions. We used acoustic telemetry and animal-borne video cameras ("Crittercam") to test the hypothesis that tiger shark (Galeocerdo cuvier) habitat use is determined by the availability of their prey. We also used Crittercam to conduct the first investigation of foraging behavior of tiger sharks. To test for habitat preferences of sharks, the observed proportion of time in each habitat for each individual was compared to the predicted values for that individual based on correlated random walk and track randomization methods. Although there was individual variation in habitat use, tiger sharks preferred shallow seagrass habitats, where their prey is most abundant. Despite multiple encounters with potential prey, sharks rarely engaged in prolonged high-speed chases, and did not attack prey that were vigilant. We propose that the tiger sharks' foraging tactic is one of stealth, and sharks rely upon close approaches to prey in order to be successful. This study shows that using appropriate analysis techniques and a variety of field methods it is possible to elucidate the factors influencing habitat use and gain insights into the foraging behavior of elusive top predators.     

Chemosensory responses and foraging behavior of the seastar<Emphasis Type="Italic"> Pycnopodia helianthoides</Emphasis>

Chemical cues released by damaged or dead organisms can affect how and where benthic organisms feed. These cues may cause predators to act as opportunistic scavengers in lieu of their normal predatory role. A scavenger, as defined in this study, is an organism that consumes damaged and/or dead organisms. In-situ experiments were performed to determine how the seastar Pycnopodia helianthoides (Brandt) reacts in the presence of chemical cues from one of its prey species, the butter clam Saxidomus giganteus (Deshayes), using both intact and damaged individuals. The results of these experiments suggest that P. helianthoides use their chemosensory abilities to locate damaged/dead prey. The role of current in propagating chemical cues was paramount in this foraging activity. P. helianthoides chose damaged prey over live prey even when live prey was encountered en route to the damaged individual. This study suggests that chemical cues emitted from damaged or dead individuals may cause significant changes in foraging tactics of key predators, thus altering food-web dynamics.Communicated by J.P. Grassle, New Brunswick     

Behavioral response of a generalist predator to chemotactile cues of two taxonomically distinct prey species

Chemotactile cues unintentionally left by animals can play a major role in predator–prey interactions. Specialized predators can use them to find their prey, while prey individuals can assess predation risk. However, little is known to date about the importance of chemotactile cues for generalist predators such as ants. Here, we investigated the response of a generalized predatory ant, Formica polyctena, to cues of two taxonomically distinct prey: a spider (Pisaura mirabilis) and a cricket (Nemobius sylvestris). In analogy, we studied whether crickets and spiders showed antipredator behavior in response to ant cues. When confronted with cues of the two prey species, Formica polyctena workers showed increased residence time and reduced movement speed, which suggests success-motivated searching behavior and thus increased foraging effort. The ants’ response did not differ between cues of the two prey species, coinciding with similar aggression and consumption rates of dead prey. However, the cuticular hydrocarbons, which likely resemble part of the potential cues, differed strongly between the species, with only few methyl-branched alkanes in common. This suggests that ants respond to multiple compounds left by other organisms with prey-search behavior. The two prey species, in turn, showed no detectable antipredator behavior in response to ant cues. Our study shows that ants can detect and respond to chemotactile cues of taxonomically and ecologically distinct prey species, probably to raise their foraging success. Using such chemotactile cues for prey detection may drastically increase their foraging efficiency and thus contribute to the high ecological success of ants.     

Ontogenetic differences in chemical alarm cue production determine antipredator responses and learned predator recognition

How individuals assess, respond and subsequently learn from alarm cues is crucial to their survival and future fitness. Yet this information is not constant through time; many individuals are exposed to different predators throughout their life as they outgrow some predators or move to habitats containing different predators. To maximise overall fitness, individuals should discriminate between different cues and respond and learn from only those that are relevant to their current ontogenetic stage. We tested whether juvenile spiny chromis, Acanthochromis polyacanthus, could distinguish between chemical alarm cues from conspecific donors of different ontogenetic stages and whether the cue ontogenetic stage of the cue donor affected the efficacy of learning about predators. Juveniles displayed a significant antipredator response when conditioned with juvenile chemical alarm cues paired with predator odour but failed to respond when conditioned with predator odour paired with either adult alarm cues or with saltwater. Subsequently, individuals only recognised the predator odour alone as a threat when conditioned with juvenile alarm cues. This demonstrates that prey may be highly specific in how they use information from conspecific alarm cues, selectively responding to and learning from only those cues that are relevant to their developmental stage.     

Predator hunting mode and habitat domain alter nonconsumptive effects in predator-prey interactions

Predators can affect prey populations through changes in traits that reduce predation risk. These trait changes (nonconsumptive effects, NCEs) can be energetically costly and cause reduced prey activity, growth, fecundity, and survival. The strength of nonconsumptive effects may vary with two functional characteristics of predators: hunting mode (actively hunting, sit-and-pursue, sit-and-wait) and habitat domain (the ability to pursue prey via relocation in space; can be narrow or broad). Specifically, cues from fairly stationary sit-and-wait and sit-and-pursue predators should be more indicative of imminent predation risk, and thereby evoke stronger NCEs, compared to cues from widely ranging actively hunting predators. Using a meta-analysis of 193 published papers, we found that cues from sit-and-pursue predators evoked stronger NCEs than cues from actively hunting predators. Predator habitat domain was less indicative of NCE strength, perhaps because habitat domain provides less reliable information regarding imminent risk to prey than does predator hunting mode. Given the importance of NCEs in determining the dynamics of prey communities, our findings suggest that predator characteristics may be used to predict how changing predator communities translate into changes in prey. Such knowledge may prove particularly useful given rates of local predator change due to habitat fragmentation and the introduction of novel predators.     

Threat-sensitive learning of predators by larval mosquitoes <Emphasis Type="Italic">Culex restuans</Emphasis>

A prerequisite for prey to show adaptive behavioural responses to predators is that the prey has the ability to recognise predators as threats. While predator recognition can be innate in many situations, learning is often essential. For many aquatic species, one common way to learn about predators is through the pairing of a novel predator odour with alarm cues released from injured conspecifics. One study with fish demonstrated that this mode of learning not only allows the prey to recognise the predatory cues as a threat, but also mediates the level of threat associated with the predator cues (i.e. threat-sensitive learning). When the prey is exposed to the novel predator with a high concentration of alarm cues, they subsequently show a high intensity of antipredator response to the predator cues alone. When exposed to the predator with a low concentration of alarm cues, they subsequently show a low-intensity response to the predator cues. Here, we investigated whether larval mosquitoes Culex restuans have the ability to learn to recognise salamanders as a threat through a single pairing of alarm cues and salamander odour and also whether they would learn to respond to salamander cues in a threat-sensitive manner. We conditioned individual mosquitoes with water or a low, medium or high concentration of crushed conspecific cues (alarm cues) paired with salamander odour. Mosquitoes exposed to salamander odour paired with alarm cues and subsequently exposed to salamander odour alone responded to the salamander as a threat. Moreover, the intensity of antipredator response displayed during the conditioning phase matched the response intensity during the testing phase. This is the first demonstration of threat-sensitive learning in an aquatic invertebrate.     

Linking marine predator diving behavior to local prey fields in contrasting habitats in a subarctic glacial fjord

Foraging theory predicts that animals will adjust their foraging behavior in order to maximize net energy intake and that trade-offs may exist that can influence their behavior. Although substantial advances have been made with respect to the foraging ecology of large marine predators, there is still a limited understanding of how predators respond to temporal and spatial variability in prey resources, primarily due to a lack of empirical studies that quantify foraging and diving behavior concurrently with characteristics of prey fields. Such information is important because changes in prey availability can influence the foraging success and ultimately fitness of marine predators. We assessed the diving behavior of juvenile female harbor seals (Phoca vitulina richardii) and prey fields near glacial ice and terrestrial haulout sites in Glacier Bay (58°40′N, ?136°05′W), Alaska. Harbor seals captured at glacial ice sites dived deeper, had longer dive durations, lower percent bottom time, and generally traveled further to forage. The increased diving effort for seals from the glacial ice site corresponded to lower prey densities and prey at deeper depths at the glacial ice site. In contrast, seals captured at terrestrial sites dived shallower, had shorter dive durations, higher percent bottom time, and traveled shorter distances to access foraging areas with much higher prey densities at shallower depths. The increased diving effort for seals from glacial ice sites suggests that the lower relative availability of prey may be offset by other factors, such as the stability of the glacial ice as a resting platform and as a refuge from predation. We provide evidence of differences in prey accessibility for seals associated with glacial ice and terrestrial habitats and suggest that seals may balance trade-offs between the costs and benefits of using these habitats.     

Does risk of intraspecific interactions induce shifts in prey-size preference in aquatic predators?

Interactions between foragers may seriously affect individual foraging efficiency. In a laboratory study of handling time, prey value and prey-size preference in northern pike and signal crayfish, we show that risk of intraspecific interactions between predators does not affect handling time or value of prey. However, the presence of agonistic intraspecific interactors shifts prey-size preference in these predators. Neither northern pike nor signal crayfish foraging alone show a prey-size preference, while pike foraging among conspecifics prefer small prey, and crayfish foraging in groups prefer large prey. We ascribe the different outcomes in prey preference to differences in susceptibility to interactions: northern pike under risk avoid large prey to avoid long handling times and the associated risk of interactions, while signal crayfish foraging among conspecifics may defend themselves and their prey during handling, and thus select prey to maximise investment. In addition, the value of pike prey (roach) is low for very small prey, maximises for small prey, and then decreases monotonically for larger prey, while crayfish prey (pond snail) value is low for very small prey, has a maximum at small prey, but does not decrease as much for larger prey. Therefore, a large and easily detected snail prey provides a crayfish with as much value as a small prey. We conclude that interaction risk and predator density affect prey-size preference differently in these aquatic predators, and therefore has different potential effects on prey-size structure and population and community dynamics. Received: 4 October 1999 / Revised: 20 March 2000 / Accepted: 27 May 2000     

Risk level of chemical cues determines retention of recognition of new predators in Iberian green frog tadpoles

In aquatic environments, many prey rely on chemosensory information from injured (alarm cues) or stressed conspecifics (disturbance cues) to assess predation risk. Alarm cues are considered as a sign of higher risk than disturbance cues. These cues could be used by prey to learn potential new predators. In this study, we tested whether Iberian green frog tadpoles (Pelophylax perezi) exhibited antipredator responses to alarm and disturbance cues of conspecifics and whether tadpoles could associate new predators with alarm or disturbance cues. Tadpoles reduced their activity in the presence of disturbance cues, but only weakly when compared with their response to alarm cues. Also, tadpoles learned to recognize new predators from association with alarm or disturbance cues. However, the period of retention of the learned association was shorter for disturbance than alarm cues. Our results indicate that tadpoles are able to modify their antipredatory behavior according to (1) the degree of risk implied by the experimental cues (2) their previous experience of chemical cues of the predator.     

Consumer-resource body-size relationships in natural food webs

It has been suggested that differences in body size between consumer and resource species may have important implications for interaction strengths, population dynamics, and eventually food web structure, function, and evolution. Still, the general distribution of consumer-'resource body-size ratios in real ecosystems, and whether they vary systematically among habitats or broad taxonomic groups, is poorly understood. Using a unique global database on consumer and resource body sizes, we show that the mean body-size ratios of aquatic herbivorous and detritivorous consumers are several orders of magnitude larger than those of carnivorous predators. Carnivorous predator-prey body-size ratios vary across different habitats and predator and prey types (invertebrates, ectotherm, and endotherm vertebrates). Predator-prey body-size ratios are on average significantly higher (1) in freshwater habitats than in marine or terrestrial habitats, (2) for vertebrate than for invertebrate predators, and (3) for invertebrate than for ectotherm vertebrate prey. If recent studies that relate body-size ratios to interaction strengths are general, our results suggest that mean consumer-resource interaction strengths may vary systematically across different habitat categories and consumer types.     

Seeding phenology influences wood mouse seed choices: the overlooked role of timing in the foraging decisions by seed-dispersing rodents

Scatter-hoarding rodents influence the population dynamics of plants by acting as seed predators and dispersers. Therefore, rodent foraging preferences for certain seed traits (species, size, condition) have been extensively studied. However, to what extent these preferences are fixed or they track the temporal changes on seed characteristics due to phenological differences has been seldom explored. We studied the temporal variability in seed preferences by wood mouse (Apodemus sylvaticus), according to phenological changes in seed characteristics of two co-occurring oaks (Quercus ilex and Quercus pubescens). The phenology of acorn abundance and the acorn predation/dispersal patterns by rodents were monitored over an entire seeding season. Results revealed temporal changes in rodent preferences for acorns of the two oaks, matching their different seeding phenology (earlier in Q. pubescens and later in Q. ilex). On the other hand, whatever the species considered, rodents preferred larger and sound acorns along the entire season, although the dispersal of infested ones increased slightly during the peaks of acorn drop. The observed influence of seeding phenology on seed choices by rodents warns about inferring definite conclusions regarding their foraging behavior when arising from short-term experiments. Indeed, this study reveals that foraging preferences may be highly dynamic and context-dependent for some seed traits (e.g., species and condition), rather than fixed behavioral patterns. Plasticity in rodent foraging choices may allow them to successfully exploit different oaks with uncoupled seeding phenologies, while potentially favoring their coexistence.     

The influence of subsurface thermal structure on the diving behavior of northern fur seals (<Emphasis Type="Italic">Callorhinus ursinus</Emphasis>) during the breeding season

In the heterogeneous marine environment, predators can increase foraging success by targeting physical oceanographic features, which often aggregate prey. For northern fur seals (Callorhinus ursinus), two prevalent oceanographic features characterize foraging areas during summer in the Bering Sea: a stable thermocline and a subsurface “cold pool”. The objective of this study was to examine the influence of these features on foraging behavior by equipping fur seals from St. Paul Island (Alaska, USA) with time-depth recorders that also measured water temperature. Foraging bout variables (e.g., mean dive depth and percent time diving in a bout) were compared with respect to subsurface thermal characteristics (thermocline presence and strength and cold pool presence). Over 74% of bouts occurred in association with strong thermoclines (temperature change > 5°C). Few differences were found for dive behavior in relation to the presence of a thermocline and the cold pool, but for epipelagic bouts, a strong thermocline resulted in increased bottom times, number of dive wiggles, and percent time diving when compared to moderate thermoclines. There was also a positive relationship between mean dive depth and thermocline depth. The combination of increasing foraging effort in areas with strong thermoclines and diving to depths closely related to the thermocline indicates this feature is important foraging habitat for northern fur seals and may act to concentrate prey and increase foraging success. By recognizing the environmental features northern fur seals use to find prey, managers will be better equipped to identify and protect foraging habitat that is important to northern fur seals, and possibly other marine predators in the Bering Sea.     

Ecological and hormonal correlates of antipredator behavior in adult Belding’s ground squirrels (Spermophilus beldingi)

Predator–prey relationships provide an excellent opportunity to study coevolved adaptations. Decades of theoretical and empirical research have illuminated the various behavioral adaptations exhibited by prey animals to avoid detection and capture, and recent work has begun to characterize physiological adaptations, such as immune reactions, metabolic changes, and hormonal responses to predators or their cues. A 2-year study quantified the activity budgets and antipredator responses of adult Belding’s ground squirrels (Spermophilus beldingi) living in three different California habitats and likely experiencing different predation pressures. At one of these sites, which is visually closed and predators and escape burrows are difficult to see, animals responding to alarm calls remain alert longer and show more exaggerated responses than adults living in two populations that likely experience less intense predation pressure. They also spend more time alert and less time foraging than adults at the other two sites. A 4-year study using noninvasive fecal sampling of cortisol metabolites revealed that S. beldingi living in the closed site also have lower corticoid levels than adults at the other two sites. The lower corticoids likely reflect that predation risk at this closed site is predictable, and might allow animals to mount large acute cortisol responses, facilitating escape from predators and enhanced vigilance while also promoting glucose storage for the approaching hibernation. Collectively, these data demonstrate that local environments and perceived predation risk influence not only foraging, vigilance, and antipredator behaviors, but adrenal functioning as well, which may be especially important for obligate hibernators that face competing demands on glucose storage and mobilization.     

Taste-rejection by predators and the evolution of unpalatability in prey

Aposematic species advertise their unpalatability to potential predators using conspicuous warning colouration. The initial evolution of aposematism is thought to occur by warningly coloured mutants emerging in an already unpalatable cryptic species. However, possessing defence chemicals is often costly, and it is difficult to understand what the selective benefits might be for a mutation causing its bearer to be defended in a population of otherwise palatable cryptic prey. One solution to this problem is that chemically defended individuals are tasted and rejected by predators, and are, therefore, more likely to survive predatory attacks than undefended individuals. Using naïve domestic chicks Gallus gallus domesticus as predators and cryptic green chick crumbs as prey, we asked whether the accuracy with which birds discriminated between palatable and unpalatable prey was affected by the palatability of the unpalatable prey (moderately or highly defended), or their frequency in the population (10 or 25%). Birds could discriminate between green prey on the basis of their defences, and showed better discrimination between palatable and unpalatable prey when defended crumbs were highly unpalatable, compared to when they were moderately unpalatable. Although there was no detectable effect of the frequency of unpalatable prey in the population on predator taste-rejection behaviour in our main analysis, frequency did appear to affect the strategies that birds used in their foraging decisions when prey were only moderately unpalatable. How birds used taste to reject prey also suggests that birds may be able to monitor and regulate their chemical intake according to the frequency and defence levels of the unpalatable prey. Taken together, these results show that avian predators can generate selection for unpalatability in cryptic prey by sampling and taste-rejecting prey, but that a relatively large chemical difference between palatable and unpalatable prey may be necessary before unpalatable prey can enjoy a selective advantage. The exact nature of this evolutionary dynamic will depend on other environmental factors, such as defence costs and prey availability, but it provides a mechanism by which defences can evolve in a cryptic population.     

Predator choice in the field; grouping guppies, Poecilia reticulata, receive more attacks

Capture success of many predator species has been shown to decrease with increasing prey group size and it is therefore suggested that predators should choose to attack stragglers and/or small groups. Predator choice in the laboratory has shown mixed results with some species preferentially attacking large groups and others preferring to attack stragglers over groups. Such predator choices have not been tested in the field. In our study we presented a binary choice between a shoal of guppies and a single guppy to predators in pools in the Arima river, Trinidad. We observed attacks in 11 different pools from a total of 53 predators (20 acara cichlids, Aequidens pulcher, 32 pike cichlids, Crenicichla frenata, and one wolf-fish, Hoplias malabaricus) and found that all predators showed a strong preference for the shoal of guppies in terms of both first choice and total number of attacks. We discuss the implications of these preferences with regards to predator–prey interactions.     

Search behavior of the checker-throated antwren foraging in aerial leaf litter

Summary Checker-throated antwrens (Formicariidae: Myrmotherula fulviventris) live in lowland neotropical forests and forage from dead curled leaves in the understory. Because they search each leaf individually they provide an opportunity to study the use of potential visual cues by an insectivorous bird. Long and highly curled leaves contain the most arthropods and checker-throated antwrens were more successful when foraging at those leaves. Yet, they used leaves at random with respect to these potential cues. Antwrens spent longer searching for arthropods in each highly curled leaf than in less curled leaves. Because of this additional search time, prey capture success per unit foraging time was only slightly greater for highly curled leaves than at the average dead leaf in the aerial leaf litter. Thus, the cues that antwrens could use to locate richer leaves are those features that obscure the prey from avian predators. Unlike other foraging systems, the antwrens appear to have no reliable cues indicating more profitable foraging sites.Address for correspondence     

Tidal regime dictates the cascading consumptive and nonconsumptive effects of multiple predators on a marsh plant

Prey perception of predators can dictate how prey behaviorally balance the need to avoid being eaten with the need to consume resources, and this perception and consequent behavior can be strongly influenced by physical processes. Physical factors, however, can also alter the density and diversity of predators that pursue prey. Thus, it remains uncertain to what extent variable risk perception and antipredator behavior vs. variation in predator consumption of prey underlie prey-resource dynamics and give rise to large-scale patterns in natural systems. In an experimental food web where tidal inundation of marsh controls which predators access prey, crab and conch (predators) influenced the survivorship and antipredator behavior of snails (prey) irrespective of whether tidal inundation occurred on a diurnal or mixed semidiurnal schedule. Specifically, cues of either predator caused snails to ascend marsh leaves; snail survivorship was reduced more by unrestrained crabs than by unrestrained conchs; and snail survivorship was lowest with multiple predators than with any single predator despite interference. In contrast to these tidally consistent direct consumptive and nonconsumptive effects, indirect predator effects differed with tidal regime: snail grazing of marsh leaves in the presence of predators increased in the diurnal tide but decreased in the mixed semidiurnal tidal schedule, overwhelming the differences in snail density that resulted from direct predation. In addition, results suggest that snails may increase their foraging to compensate for stress-induced metabolic demand in the presence of predator cues. Patterns from natural marshes spanning a tidal inundation gradient (from diurnal to mixed semidiurnal tides) across 400 km of coastline were consistent with experimental results: despite minimal spatial variation in densities of predators, snails, abiotic stressors, and marsh productivity, snail grazing on marsh plants increased and plant biomass decreased on shorelines exposed to a diurnal tide. Because both the field and experimental results can be explained by tidal-induced variation in risk perception and snail behavior rather than by changes in snail density, this study reinforces the importance of nonconsumptive predator effects in complex natural systems and at large spatial scales.