Fleeing towards death – leech‐induced behavioural defences increase freshwater snail susceptibility to predatory fish

Prey species are often exposed to multiple predators, which presents several difficulties to prey species. This is especially true when the response to one predator influences the prey’s susceptibility to other predators. Predator‐induced defences have evolved in a wide range of prey species, and experiments involving predators with different hunting strategies allow researchers to evaluate how prey respond to multiple threats. Freshwater snails are known to respond to a variety of predators with both morphological and behavioural defences. Here we studied how freshwater snails Radix balthica responded behaviourally to fish and leech predators, both separately and together. Our aim was to explore whether conflicting predator‐induced responses existed and, if so, what effect they had on snail survival when both predatory fish and leeches were present. We found that although R. balthica increased refuge use when exposed to predatory fish, they decreased refuge use when exposed to predatory leeches. When both predators were present, snails showed a stronger response towards leech than fish and responded by leaving the refuge. This response made the snails more susceptible to fish predation, which increased snail mortality when exposed to both fish and leech compared to fish only. We show that predators that have a relatively low predation rate can substantially increase mortality rates by indirect effects. By forcing snails out of refuges such as rock and macrophyte habitats, leeches can indirectly increase predation from molluscivorous fish and may thus affect snail densities.     

Relative strengths of trait-mediated and density-mediated indirect effects of a predator vary with resource levels in a freshwater food chain

Predators can affect the density and traits (e.g. morphology, behavior) of their prey, and either change may influence how prey interact with their resources. Thus, predators can interact indirectly with resource species (i.e. two trophic levels below) through two separate mechanisms. The relative strengths of these two kinds of indirect effects have rarely been compared directly, and how their relative importance varies across environmental gradients is virtually unknown. We investigated the relative strength of trait- and density-mediated indirect effects of the predatory insect Belostoma flumineum on algal communities through predation on the pond snail, Physa gyrina , across a gradient of basal resource abundance. Because prey balance the benefits of foraging against the increased risk of predation while foraging, the availability of the prey's resource should influence the strength of anti-predator behavioral responses and hence the strength of trait-mediated indirect interactions. Belostoma presence had positive indirect effects on resources as expected and total predator effects were constant across the basal resource gradient. At low initial resource levels, trait-mediated indirect effects on algal biomass exceeded density-mediated indirect effects, while at high initial resources the reverse was true. Snails showed similar habitat use across the resource gradient suggesting that the anti-predator response was most likely a depression of activity levels.     

Trophic cascades in rocky shore tide pools: distinguishing lethal and nonlethal effects

The effects of predators on the density of their prey can have positive indirect effects on the abundance of the preys resource via a trophic cascade. This concept has strongly influenced contemporary views of how communities are structured. However, predators also can transmit indirect effects by inducing changes in prey traits. We show that the mere presence of predator risk cues can initiate a trophic cascade in rocky shore tide pools. In large (mean surface area =9 m²), natural tide pools, we manipulated crab density and their foraging ability to examine the relative importance of lethal (density-mediated) and non-lethal (trait-mediated) predator effects to algal community development. We found that perceived predation risk reduced snail density as much as the direct predation treatment, showing that green crab predation was not an important factor regulating local snail density. Instead, snail emigration away from resident crabs appears to be the most important factor regulating local snail density. As a result, the abundance of ephemeral green algae was similar in the predation risk and direct predation treatments, suggesting that the consumption of snails by crabs plays a minimal role in mediating the trophic cascade. Increased attention to trait-mediated effects that are transmitted by predator-induced changes in prey behavior may change our view of how predators exert their strong influence on community structure.     

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.     

Implications of shared predation for space use in two sympatric leporids

Spatial variation in habitat riskiness has a major influence on the predator–prey space race. However, the outcome of this race can be modulated if prey shares enemies with fellow prey (i.e., another prey species). Sharing of natural enemies may result in apparent competition, and its implications for prey space use remain poorly studied. Our objective was to test how prey species spend time among habitats that differ in riskiness, and how shared predation modulates the space use by prey species. We studied a one‐predator, two‐prey system in a coastal dune landscape in the Netherlands with the European hare (Lepus europaeus) and European rabbit (Oryctolagus cuniculus) as sympatric prey species and red fox (Vulpes vulpes) as their main predator. The fine‐scale space use by each species was quantified using camera traps. We quantified residence time as an index of space use. Hares and rabbits spent time differently among habitats that differ in riskiness. Space use by predators and habitat riskiness affected space use by hares more strongly than space use by rabbits. Residence time of hare was shorter in habitats in which the predator was efficient in searching or capturing prey species. However, hares spent more time in edge habitat when foxes were present, even though foxes are considered ambush predators. Shared predation affected the predator–prey space race for hares positively, and more strongly than the predator–prey space race for rabbits, which were not affected. Shared predation reversed the predator–prey space race between foxes and hares, whereas shared predation possibly also released a negative association and promoted a positive association between our two sympatric prey species. Habitat riskiness, species presence, and prey species’ escape mode and foraging mode (i.e., central‐place vs. noncentral‐place forager) affected the prey space race under shared predation.     

Direct and indirect effects of predation on mosquitofish behavior and survival

Predation can have strong direct and indirect effects on the behavior of prey. We investigated whether predation by chain pickerel (Esox niger) caused adult eastern mosquitofish (Gambusia holbrooki) to alter their habitat use and whether pickerel predation influenced survival of adult and neonate mosquitofish. The number of adult mosquitofish using the riskier of three habitats was lowest when two predators occupied the risky habitat, intermediate in the treatment with one predator, and highest when no predators occurred there. More mosquitofish neonates survived high predation treatments than treatments lacking pickerel. We conclude that pickerel predation causes adult mosquitofish to shift to refuge habitats. The pattern of neonate survival suggests that adult habitat use may create a refuge from cannibalism for neonate mosquitofish, resulting in higher neonate survival in treatments with more pickerel. Hence, pickerel predation has a direct effect on adult mosquitofish behavior and a strong indirect effect on neonate survival. Both interspecific and intraspecific predation can effect prey populations and can interact to produce important indirect effects.     

Salamander evolution across a latitudinal cline in gape-limited predation risk

General predictions of community dynamics require that insights derived from local habitats can be scaled up to explain phenomena across geographic scales. Across these larger spatial extents, adaptation can play an increasing role in determining the outcome of species interactions. If local adaptation is common, then our ability to generalize measures of species interaction strength across communities will be limited without an additional understanding of the genetic variation underlying interaction traits. In the context of predator–prey interactions, prey individuals commonly are expected to reduce risky foraging behaviors and subsequent growth under predation threat. However, rapid growth into a large body size can defend against gape-limited predators, creating a tradeoff between increased predation risk due to elevated foraging activity and decreased predation risk due to large size. Here I combine field observations, natural selection experiments, and common garden assays to understand potential adaptations of spotted salamander Ambystoma maculatum larvae to gape-limited and gape-unconstrained predators. Field observations and natural selection trials suggested antagonistic selection on prey body size among ponds dominated by gape-limited predator salamanders A. opacum and gape-unconstrained beetle larvae Dytiscus . In common garden experiments, prey from sites with high gape-limited predation risk grew larger than those from other sites, suggesting the evolution of rapid growth into a prey size refuge. Larvae from all sites grew to a large size when exposed to the gape-limited N. viridescens predator's kairomones. Hence, induced rapid growth into a size refuge may be an adaptive response to gape-limited predation risk. Results point to an important role for cross-community generalizations based on functional classifications of predators by their gape constraints and inter-site genetic variation in prey growth rates and behaviors.     

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.     

Susceptibility to predation affects trait-mediated indirect interactions by reversing interspecific competition

Numerous studies indicate that the behavioral responses of prey to the presence of predators can have an important role in structuring assemblages through trait-mediated indirect interactions. Few studies, however, have addressed how relative susceptibility to predation influences such interactions. Here we examine the effect of chemical cues from the common shore crab Carcinus maenas on the foraging behavior of two common intertidal gastropod molluscs. Of the two model consumers studied, Littorina littorea is morphologically more vulnerable to crab predation than Gibbula umbilicalis, and it exhibited greater competitive ability in the absence of predation threat. However, Littorina demonstrated a greater anti-predator response when experimentally exposed to predation cues, resulting in a lower level of foraging. This reversed the competitive interaction, allowing Gibbula substantially increased access to shared resources. Our results demonstrate that the susceptibility of consumers to predation can influence species interactions, and suggest that inter-specific differences in trait-mediated indirect interactions are another mechanism through which non-consumptive predator effects may influence trophic interactions.     

The influence of temporally variable predation risk on indirect interactions in an aquatic food chain

We know little about how temporally variable predation risk influences prey behavior. The risk allocation hypothesis predicts that prey facing more frequent risk should show weak anti-predator responses, and should be particularly active foragers during rare periods of safety, compared to prey facing infrequent risk. Several studies offer support for the risk allocation hypothesis, but how these responses might propagate through the larger ecological community remains largely unknown. We experimentally investigated the relative strength of trait- and density-mediated indirect effects of a predator on its prey’s resource across predation treatments that varied the lethality (caged or free-swimming predators) and temporal variability (always, often, or sometimes present) of predation. We performed this experiment in pond mesocosms using a giant water bug predator (Belostoma lutarium), an herbivorous pond snail (Physa gyrina), and algae as the basal resource. Snails greatly reduced the abundance of their algal resource when in the absence of predation. Lethal predation at low and medium intensities had significant positive indirect effects on the abundance of algae, mostly by reducing snail density. Snails responded behaviorally to high levels of deadly predation by foraging more and hiding less than in other situations, as predicted by the risk allocation hypothesis, and thus ameliorated the density-mediated indirect effects of predators on algae. Behavioral responses to caged predators, and the subsequent trait-mediated indirect effects, were negligible regardless of predation intensity. Our previous work has demonstrated that trait-mediated indirect effects are weak when resources are abundant, as they were in this experiment. This work demonstrates that temporal variation in predation intensity plays a key role in determining the relative strength of TMIIs and DMIIs in an aquatic food chain.     

Effect of temperature extremes on the spatial dynamics of predator–prey interactions: A case study with dragonfly nymphs and newt larvae

Theory predicts that predators are more vulnerable to increasing temperature than prey. Despite huge variations in the magnitude and duration of thermally-extreme episodes in nature, most empirical studies on predator–prey interactions consider conditions induced by a climatic shift in mean temperature. We asked whether the increased vulnerability of predators holds under daily thermal extremes occurring during heat waves, using dragonfly nymphs and newt larvae as the predator–prey model system. Direct exposure of predator to prey in heated and non-heated aquaria under semi-natural conditions revealed that predator movements increased with rising temperature, whereas prey activity decreased. In contrast to the theory of predator–prey space use, the spatial co-occurrence of predator and prey individuals increased with temperature, while predation rates diminished. We conclude that daily thermal extremes affect trophic interactions in the same way, i.e. through the increased vulnerability of predators, as do long-term shifts in mean environmental temperature. Our results highlight the importance of behavioral studies for understanding mechanisms mediating the effect of extreme thermal events on species interactions.     

Of mice and mallards: positive indirect effects of coexisting prey on waterfowl nest success

Coexisting prey species interact indirectly via their shared predators when one prey type influences predation rates of the second prey type. In a temperate system where the predominant shared predator is a generalist, I studied the indirect effects of rodent populations on waterfowl nest success, both within the nesting season among sites and among years. Among six to ten upland fields (14 to 27 ha), mallard ( Anas platyrhynchos ) nest success was positively correlated with rodent abundance in all three years of the study. After removing year effects, mallard nest success remained positively correlated with the relative abundance of rodents. Of the rodent species present, California voles ( Microtus californicus ) were the most important coexisting prey type influencing nest success. Among years, mallard nest success was positively correlated with vole abundance; the asymptotic relationship suggests a threshold response to vole abundance, beyond which predators become satiated and additional voles do little to affect nest success. I tested and rejected three alternative explanations for the observed positive correlation between mallard nest success and rodent abundance that do not involve an indirect effect of coexisting prey populations. The influences of dense nesting cover, nesting density, and predator activity did not explain the observed patterns of nest success. These results suggest that rodent populations buffer predation on waterfowl nests, both within and among years, via the behavioral responses of shared predators to coexisting prey.     

Spatial refuge from intraguild predation: implications for prey suppression and trophic cascades

The ability of predators to elicit a trophic cascade with positive impacts on primary productivity may depend on the complexity of the habitat where the players interact. In structurally-simple habitats, trophic interactions among predators, such as intraguild predation, can diminish the cascading effects of a predator community on herbivore suppression and plant biomass. However, complex habitats may provide a spatial refuge for predators from intraguild predation, enhance the collective ability of multiple predator species to limit herbivore populations, and thus increase the overall strength of a trophic cascade on plant productivity. Using the community of terrestrial arthropods inhabiting Atlantic coastal salt marshes, this study examined the impact of predation by an assemblage of predators containing Pardosa wolf spiders, Grammonota web-building spiders, and Tytthus mirid bugs on herbivore populations (Prokelisia planthoppers) and on the biomass of Spartina cordgrass in simple (thatch-free) and complex (thatch-rich) vegetation. We found that complex-structured habitats enhanced planthopper suppression by the predator assemblage because habitats with thatch provided a refuge for predators from intraguild predation including cannibalism. The ultimate result of reduced antagonistic interactions among predator species and increased prey suppression was enhanced conductance of predator effects through the food web to positively impact primary producers. Behavioral observations in the laboratory confirmed that intraguild predation occurred in the simple, thatch-free habitat, and that the encounter and capture rates of intraguild prey by intraguild predators was diminished in the presence of thatch. On the other hand, there was no effect of thatch on the encounter and capture rates of herbivores by predators. The differential impact of thatch on the susceptibility of intraguild and herbivorous prey resulted in enhanced top-down effects in the thatch-rich habitat. Therefore, changes in habitat complexity can enhance trophic cascades by predator communities and positively impact productivity by moderating negative interactions among predators.     

Modeling the impacts of global warming on predation and biotic resistance: mosquitoes, damselflies and avian malaria in Hawaii

Biotic resistance from native predators can play an important role in regulating or limiting exotic prey. We investigate how global warming potentially alters the strength and spatial extent of these predator–prey interactions in aquatic insect ecosystems. As a simple model system, we use rock pools in streams of rainforests of Hawaii, which contain the beautiful Hawaiian damselfly Megalagrion calliphya as predator and the invasive southern house mosquito Culex quinquefasciatus as prey. This abundant mosquito is the major vector of avian malaria transmission to native forest birds. We use mathematical modeling to evaluate the potential impacts of damselfly predation and temperature on mosquito population dynamics. We model this predator–prey system along an elevational gradient (749-1952 m elevation) and assess the effect of 1°C and 2°C climate warming scenarios as well as the effects of El Niño and La Niña oscillations, on predator–prey dynamics. Our results indicate that the strength of biotic resistance of native predators on invasive prey may decrease with increasing temperature because demographic rates of predator and prey are differentially affected by temperature. Future warming could therefore increase the abundance of invasive species by releasing them from predation pressure. If the invasive species is a disease vector, these shifts could increase the impact of disease on both humans and wildlife.     

Piscivore efficiency and refuging prey: the importance of predator search mode

In predator-prey interactions, the efficiency of the predator is dependent on characteristics of both the predator and the prey, as well as the structure of the environment. In a field enclosure experiment, we tested the effects of a prey refuge on predator search mode, predator efficiency and prey behaviour. Replicated enclosures containing young of the year (0+) and 1-year-old (1+) perch were stocked with 3 differentially sized individuals of either of 2 piscivorous species, perch (Perca fluviatilis), pike (Esox lucius) or no piscivorous predators. Each enclosure contained an open predator area with three small vegetation patches, and a vegetated absolute refuge for the prey. We quantified the behaviour of the predators and the prey simultaneously, and at the end of the experiment the growth of the predators and the mortality and habitat use of the prey were estimated. The activity mode of both predator species was stationary. Perch stayed in pairs in the vegetation patches whereas pike remained solitary and occupied the corners of the enclosure. The largest pike individuals stayed closest to the prey refuge whereas the smallest individuals stayed farthest away from the prey refuge, indicating size-dependent interference among pike. Both size classes of prey showed stronger behavioural responses to pike than to perch with respect to refuge use, distance from refuge and distance to the nearest predator. Prey mortality was higher in the presence of pike than in the presence of perch. Predators decreased in body mass in all treatments, and perch showed a relatively stronger decrease in body mass than pike during the experiment. Growth differences of perch and pike, and mortality differences of prey caused by predation, can be explained by predator morphology, predator attack efficiency and social versus interference behaviour of the predators. These considerations suggest that pike are more efficient piscivores around prey refuges such as the littoral zones of lakes, whereas perch have previously been observed to be more efficient in open areas, such as in the pelagic zones of lakes.     

Elephants facilitate impact of large predators on small ungulate prey species

Predators and megaherbivores have profound impacts on ecosystem structure and functioning. Following the reintroduction of apex predators (lion and spotted hyaena) into the Main Camp Section (Main Camp) of the Addo Elephant National Park (Addo – Eastern Cape, South Africa) populations of small (5–50 kg) prey species declined. Following the recent reintroduction of apex predators into the neighbouring Colchester Section, a similar decline in small prey species might have occurred. However, we predict that the dense nature of Thicket vegetation in Colchester has provided the small prey species a refuge from predation from the reintroduced apex predators. Using camera trap data collected over three years following the apex predator reintroduction into Colchester, we show that declines in small ungulate prey species have not taken place. The primary difference between these two sections at the time of the apex predator reintroduction was the state of the Thicket (dense vegetation type characteristic of both sections). Main Camp is characterized by fragmented Thicket that has been altered as a result of high elephant densities, whereas Colchester has intact Thicket following a long history of elephant absence. The fragmented Thicket in Main Camp allowed access to the Thicket habitats (as indicated by GPS collar data on lions), in which these small prey species reside, potentially increasing the predation on these species in Main Camp. The intact Thicket in Colchester, however, may provide a refuge away from the apex predators (and possibly meso-predators) for the small prey species. We suggest that the impact of predators on this prey community is conditional on the long history of ecosystem transformation by ecosystem engineers such as elephants.     

You can’t run but you can hide: refuge use in frog tadpoles elicits density-dependent predation by dragonfly larvae

The potential role of prey refuges in stabilizing predator–prey interactions is of longstanding interest to ecologists, but mechanisms underlying a sigmoidal predator functional response remain to be fully elucidated. Authors have disagreed on whether the stabilizing effect of prey refuges is driven by prey- versus predator-centric mechanisms, but to date few studies have married predator and prey behavioural observations to distinguish between these possibilities. We used a dragonfly nymph–tadpole system to study the effect of a structural refuge (leaf litter) on the predator’s functional response, and paired this with behavioural observations of both predator and prey. Our study confirmed that hyperbolic (type II) functional responses were characteristic of foraging predators when structural cover was low or absent, whereas the functional response was sigmoidal (type III) when prey were provided with sufficient refuge. Prey activity and refuge use were density independent across cover treatments, thereby eliminating a prey-centric mechanism as being the genesis for density-dependent predation. In contrast, the predator’s pursuit length, capture success, and handling time were altered by the amount of structure implying that observed shifts in density-dependent predation likely were related to predator hunting efficiency. Our study advances current theory by revealing that despite fixed-proportion refuge use by prey, presence of a prey refuge can induce density-dependent predation through its effect on predator hunting strategy. Ultimately, responses of predator foraging decisions in response to changes in prey availability and search efficiency may be more important in producing density-dependent predation than the form of prey refuge use.     

Prey refuges as predator hotspots: ocelot (Leopardus pardalis) attraction to agouti (Dasyprocta punctata) dens

We tested the hypothesis that prey refuges attract predators, leading to elevated predator activity in the vicinity of refuges. We used camera traps to determine whether the spatial activity of a predator, the ocelot (Leopardus pardalis), was biased toward refuge locations of its principal prey, the agouti (Dasyprocta punctata). We radio-tracked agoutis at night to locate active refuges and compared the activity of ocelots between these refuges and surrounding control grid locations. We found that ocelots visited the area near agouti refuges significantly more often and for longer periods of time than control locations, and that they actively investigated the refuge entrances. Both occupied and unoccupied refuges were visited, but the duration of inspection was longer at occupied refuges. As the ocelots could probably not see the agoutis within the refuges, olfaction likely cued foraging ocelots. Two refuges were repeatedly visited by the same ocelots on different days, suggesting spatial memory. Overall, our results suggest that predators can be attracted to prey refuges or refuging prey. The benefits to prey of staying nearby a refuge would thus be counterbalanced by higher likelihoods of predator encounter. This should stimulate prey to use multiple refuges alternatingly and to not enter or exit refuges at times of high predator activity.     

Interactions between the information content of different chemical cues affect induced defences in tadpoles

Animals often alter their behaviour, morphology and physiology in the presence of predators. These induced defences can be fine‐tuned by a variety of environmental factors such as predator species, acute predation risk or food availability. It has, however, remained unclear what cues influence the extent and quality of induced defences and how the information content of these cues interact to determine the development of antipredator defences. We performed an experiment to study the significance of direct chemical cues, originating from the predators themselves, and indirect cues, released by attacked or consumed prey, for phenotypic responses in Rana dalmatina tadpoles. We reared tadpoles in the presence of caged predators (Triturus vulgaris, Aeshna cyanea) fed either one or three tadpoles every other day outside the tadpole‐rearing tanks. Fifteen hours after food provisioning, predators were put back into the tanks containing focal tadpoles either after washing (direct + digestion‐released cues) or with the water containing remnants of the prey (direct + all types of indirect cues). Our results suggest that direct cues together with digestion‐released cues can be sufficient to induce strong antipredator responses. Induced defences depended on both direct cues, affecting predator‐specific responses, and the quantity of indirect cues, resulting in graded responses to differences in predation threat. Moreover, direct and indirect cues interacted in behaviour, resulting in predator‐specific graded responses. We also observed a decrease in the extent of predator‐induced responses in large tadpoles as compared to small ones. Our results, thus, suggest that prey integrate multiple cues about predators to optimize induced defences and that this process changes during ontogeny.     

Foraging and refuge use by a pond snail: Effects of physiological state,predators, and resources

The costs and benefits of anti-predator behavioral responses should be functions of the actual risk of predation, the availability of the prey's resources, and the physiological state of the prey. For example, a food-stressed individual risks starvation when hiding from predators, while a well-fed organism can better afford to hide (and pay the cost of not foraging). Similarly, the benefits of resource acquisition are probably highest for the prey in the poorest state, while there may be diminishing returns for prey nearing satiation. Empirical studies of state-dependent behavior are only beginning, however, and few studies have investigated interactions between all three potentially important factors. Here I present the results of a laboratory experiment where I manipulated the physiological state of pond snails (Physa gyrina), the abundance of algal resources, and predation cues (Belostoma flumineum waterbugs consuming snails) in a full factorial design to assess their direct effects on snail behavior and indirect effects on algal biomass. On average, snails foraged more when resources were abundant, and when predators were absent. Snails also foraged more when previously exposed to physiological stress. Snails spent more time at the water's surface (a refuging behavior) in the presence of predation cues on average, but predation, resource levels, and prey state had interactive effects on refuge use. There was a consistent positive trait-mediated indirect effect of predators on algal biomass, across all resource levels and prey states.