Selectivity underlies the dissociation between seasonal prey availability and prey consumption in a generalist predator

Generalist predators are capable of selective foraging, but are predicted to feed in close proportion to prey availability to maximize energetic intake especially when overall prey availability is low. By extension, they are also expected to feed in a more frequency‐dependent manner during winter compared to the more favourable foraging conditions during spring, summer and fall seasons. For 18 months, we observed the foraging patterns of forest‐dwelling wolf spiders from the genus Schizocosa (Araneae: Lycosidae) using PCR‐based gut‐content analysis and simultaneously monitored the activity densities of two common prey: springtails (Collembola) and flies (Diptera). Rates of prey detection within spider guts relative to rates of prey collected in traps were estimated using Roualdes’ c_st model and compared using various linear contrasts to make inferences pertaining to seasonal prey selectivity. Results indicated spiders foraged selectively over the course of the study, contrary to predictions derived from optimal foraging theory. Even during winter, with overall low prey densities, the relative rates of predation compared to available prey differed significantly over time and by prey group. Moreover, these spiders appeared to diversify their diets; the least abundant prey group was consistently overrepresented in the diet within a given season. We suggest that foraging in generalist predators is not necessarily restricted to frequency dependency during winter. In fact, foraging motives other than energy maximization, such as a more nutrient‐focused strategy, may also be optimal for generalist predators during prey‐scarce winters.     

Habitat-dependent foraging in a classic predator–prey system: a fable from snowshoe hares

Current research contrasting prey habitat use has documented, with virtual unanimity, habitat differences in predation risk. Relatively few studies have considered, either in theory or in practice, simultaneous patterns in prey density. Linear predator–prey models predict that prey habitat preferences should switch toward the safer habitat with increasing prey and predator densities. The density‐dependent preference can be revealed by regression of prey density in safe habitat versus that in the riskier one (the isodar). But at this scale, the predation risk can be revealed only with simultaneous estimates of the number of predators, or with their experimental removal. Theories of optimal foraging demonstrate that we can measure predation risk by giving‐up densities of resource in foraging patches. The foraging theory cannot yet predict the expected pattern as predator and prey populations covary. Both problems are solved by measuring isodars and giving‐up densities in the same predator–prey system. I applied the two approaches to the classic predator–prey dynamics of snowshoe hares in northwestern Ontario, Canada. Hares occupied regenerating cutovers and adjacent mature‐forest habitat equally, and in a manner consistent with density‐dependent habitat selection. Independent measures of predation risk based on experimental, as well as natural, giving‐up densities agreed generally with the equal preference between habitats revealed by the isodar. There was no apparent difference in predation risk between habitats despite obvious differences in physical structure. Complementary studies contrasting a pair of habitats with more extreme differences confirmed that hares do alter their giving‐up densities when one habitat is clearly superior to another. The results are thereby consistent with theories of adaptive behaviour. But the results also demonstrate, when evaluating differences in habitat, that it is crucial to let the organisms we study define their own habitat preference.     

Numerical and functional responses of Common Buzzards Buteo buteo to prey abundance on a Scottish grouse moor

Predators will often respond to reductions in preferred prey by switching to alternative prey resources. However, this may not apply to all alternative prey groups in patchy landscapes. We investigated the demographic and aggregative numerical and functional responses of Common Buzzards Buteo buteo in relation to variations in prey abundance on a moor managed for Red Grouse Lagopus lagopus scotica in south‐west Scotland over three consecutive breeding and non‐breeding seasons. We predicted that predation of Red Grouse by Buzzards would increase when abundance of their preferred Field Vole Microtus agrestis prey declined. As vole abundance fluctuated, Buzzards responded functionally by eating voles in relation to their abundance, but they did not respond demographically in terms of either breeding success or density. During a vole crash year, Buzzards selected a wider range of prey typical of enclosed farmland habitats found on the moorland edge but fewer Grouse from the heather moorland. During a vole peak year, prey remains suggested a linear relationship between Grouse density and the number of Grouse eaten (a Type 1 functional response), which was not evident in either intermediate or vole crash years. Buzzard foraging intensity varied between years as vole abundance fluctuated, and foraging intensity declined with increasing heather cover. Our findings did not support the prediction that predation of Red Grouse would increase when vole abundance was low. Instead, they suggest that Buzzards predated Grouse incidentally while hunting for voles, which may increase when vole abundances are high through promoting foraging in heather moorland habitats where Grouse are more numerous. Our results suggest that declines in their main prey may not result in increased predation of all alternative prey groups when predators inhabit patchy landscapes. We suggest that when investigating predator diet and impacts on prey, knowledge of all resources and habitats that are available to predators is important.     

Variation in foraging success among predators and its implications for population dynamics

The effects of the expected predation rate on population dynamics have been studied intensively, but little is known about the effects of predation rate variability (i.e., predator individuals having variable foraging success) on population dynamics. In this study, variation in foraging success among predators was quantified by observing the predation of the wolf spider Pardosa pseudoannulata on the cricket Gryllus bimaculatus in the laboratory. A population model was then developed, and the effect of foraging variability on predator–prey dynamics was examined by incorporating levels of variation comparable to those quantified in the experiment. The variability in the foraging success among spiders was greater than would be expected by chance (i.e., the random allocation of prey to predators). The foraging variation was density‐dependent; it became higher as the predator density increased. A population model that incorporates foraging variation shows that the variation influences population dynamics by affecting the numerical response of predators. In particular, the variation induces negative density‐dependent effects among predators and stabilizes predator–prey dynamics.     

Does a top predator reduce the predatory impact of an invasive mesopredator on an endangered rodent?

The mesopredator release hypothesis (MRH) predicts that reduced abundance of top‐order predators results in an increase in the abundance of smaller predators (mesopredators) due to a reduction in intra‐guild predation and competition. The irruption of mesopredators that follows the removal of top‐order predators can have detrimental impacts on the prey of the mesopredators. Here we investigated the mechanisms via which the presence of a top‐order predator can benefit prey species. We tested predictions made according to the MRH and foraging theory by contrasting the abundances of an invasive mesopredator (red fox Vulpes vulpes) and an endangered prey species (dusky hopping mouse Notomys fuscus), predator diets, and N. fuscus foraging behaviour in the presence and absence of a top‐predator (dingo Canis lupus dingo). As predicted by the MRH, foxes were more abundant where dingoes were absent. Dietary overlap between sympatric dingoes and foxes was extensive, and fox was recorded in 1 dingo scat possibly indicating intra‐guild predation. Notomys fuscus were more likely to occur in fox scats than dingo scats and as predicted by the MRH N. fuscus were less abundant in the absence of dingoes. The population increase of N. fuscus following rainfall was dampened in the absence of dingoes suggesting that mesopredator release can attenuate bottom‐up effects, although it remains conceivable that differences in grazing regimes associated with dingo exclusion could have also influenced N. fuscus abundance. Notomys fuscus exhibited lower giving‐up densities in the presence of dingoes, consistent with the prediction that their perceived risk of predation would be lower and foraging efficiency greater in the presence of a top‐predator. Our results suggest that mesopredator suppression by a top predator can create a safer environment for prey species where the frequency of fatal encounters between predators and prey is reduced and the non‐consumptive effects of predators are lower.     

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.     

Effects of Structural Refuge and Density on Foraging Behaviour and Mortality of Hungry Tadpoles Subject to Predation Risk

Theoretical models of prey behaviour predict that food‐limited prey engage in risk‐prone foraging and thereby succumb to increased mortality from predation. However, predation risk also may be influenced by factors including prey density and structural cover, such that the presumed role of prey hunger on predation risk may be obfuscated in many complex predator–prey systems. Using a tadpole (prey) – dragonfly larva (predator) system, we determined relative risk posed to hungry vs. sated prey when both density and structural cover were varied experimentally. Overall, prey response to perceived predation risk was primarily restricted to increased cover use, and hungry prey did not exhibit risk‐prone foraging. Surprisingly, hungry prey showed lower activity than sated prey when exposed to predation risk, perhaps indicating increased effort in search of refuge or spatial avoidance of predator cues among sated animals. An interaction between hunger level and predation risk treatments indicated that prey state affected sensitivity to perceived risk. We also examined the lethal implications of prey hunger by allowing predators to select directly between hungry and sated prey. Although predators qualitatively favoured hungry prey when density was elevated and structural cover was sparse, the overall low observed variation in mortality risk between hunger treatments suggests that preferential selection of hungry prey was weak. This implies that hunger effects on prey mortality risk may not be readily observed in complex landscapes with additional factors influencing risk. Thus, current starvation‐predation trade‐off theory may need to be broadened to account for other mechanisms through which undernourished prey may cope with predation risk.     

Nest and foraging‐site selection in Yellowhammers Emberiza citrinella: implications for chick provisioning

Capsule Vegetation structure and invertebrate abundance interact to influence both foraging sites and nestling provisioning rate; when invertebrate availability is low, adults may take greater risks to provide food for their young.

Aims To investigate nesting and foraging ecology in a declining farmland bird whose fledging success is influenced by the availability of invertebrate prey suitable for feeding to offspring, and where perceived predation risk during foraging can be mediated by vegetation structure.

Methods Provisioning rates of adult Yellowhammers feeding nestlings were measured at nests on arable farmland. Foraging sites were compared with control sites of both the same and different microhabitats; provisioning rate was related to habitat features of foraging‐sites.

Results Foraging sites had low vegetation density, probably enhancing detection of predators, or high invertebrate abundance at high vegetation density. Parental provisioning rate decreased with increasing vegetation cover at foraging sites with high invertebrate abundance; conversely, where invertebrate abundance was low, provisioning rate increased with increasing vegetation cover.

Conclusions Vegetation structure at foraging sites suggests that a trade‐off between predator detection and prey availability influences foraging site selection in Yellowhammers. Associations between parental provisioning rate and vegetation variables suggest that where invertebrate abundance is high birds increase time spent scanning for predators at higher vegetation densities; however, when prey are scarce, adults may take more risks to provide food for their young.     

Associations between Non‐Lethal Injury,Body Size,and Foraging Ecology in an Amphibian Intraguild Predator

Theoretical treatments of intraguild predation and its effects on behavioral interactions regard the phenomenon as a size‐structured binary response wherein predation among competitors is completely successful or completely unsuccessful. However, intermediate outcomes occur when individuals escape intraguild (IG) interactions with non‐lethal injuries. While the effects of wounds for prey include compromised mobility and increased predation risk, the consequences of similar injuries among top predators are not well understood, despite the implications for species interactions. Using an amphibian IG predator, Ambystoma opacum (Caudata: Ambystomatidae), we examined associations between non‐lethal injuries and predator body size, foraging strategy, microhabitat selection, and intraspecific agonistic interactions. Wounds were common among IG predators, generally increasing in frequency throughout larval ontogeny. Non‐lethal injuries were associated with differences in predator body size and behavior, with injured predators exhibiting smaller body sizes, increased use of benthic microhabitats, reduced agonistic displays, and increased risk of intraspecific aggression. While such effects were not ultimately associated with reduced foraging success, non‐lethal injury could contribute to niche partitioning between injured and healthy predators via habitat selection, but injured predators likely continue to exert predatory pressure on IG and basal prey populations. Our results indicate that studies of top‐down population regulation should incorporate injury‐related modifications to both prey and predator behavior and size structure.     

Do predators limit the abundance of alternative prey? Experiments with vole‐eating avian and mammalian predators

Predation has been invoked as a factor synchronizing the population oscillations of sympatric prey species, either because predators kill prey unselectively (the Shared Predation Hypothesis; hereafter SPH), or because predators switch to alternative prey after a density decline in their main prey (the Alternative Prey Hypothesis; APH). A basic assumption of the APH is that the impact of predators on alternative prey depends more on the density of main prey than on the predator/alternative prey ratio. Both SPH and APH assume that the impact of predators on alternative prey is at least periodically strong enough to depress prey populations. To examine these assumptions, we utilized data from replicated field experiments in large areas where we reduced the breeding densities of avian predators during three years and the numbers of least weasels (Mustela nivalis) in two years when vole populations declined. In addition, we reduced the breeding densities of avian predators in two years when vole populations were high. The reduction of least weasels increased the abundance of their alternative prey, small birds breeding on the ground, but did not affect the abundance of common shrews (Sorex araneus). In years when vole populations declined, the reduction of avian predators increased the abundance of their alternative prey, common shrews and small birds. Therefore, vole‐eating predators do at least periodically depress the abundance of their alternative prey. At high vole densities, the reduction of avian predators did not increase the abundance of common shrews, although the ratio of avian predators to alternative prey was similar to years when vole populations declined, which supported APH. In contrast, the abundance of small birds increased after the reduction of avian predators also at high vole densities, which supported SPH. The manipulations had no obvious effect on the number of game birds, which are only occasionally killed by these small‐sized predators. We conclude that in communities where most predators are small or specialize on a single prey type, the synchronizing impact of predation is restricted to a few similar‐sized species.     

The effect of predation pressure and predator adaptive foraging on the relative importance of consumptive and non‐consumptive predator net effects in a freshwater model system

An important challenge in community ecology is identifying the functional characteristics capable of predicting the nature and strength of predator effects on food webs. We developed an individual‐based model, based on a shallow lake model system, to evaluate the total, consumptive, and non‐consumptive indirect effect that predators have on basal resources when the predators differ in their foraging types (active adaptive foraging or sedentary foraging). Overall, both predator types caused similar total indirect effects on lower trophic levels. However, the nature net effects of predators diverged between predator foraging types. Active predators caused larger non‐consumptive effects, relative to the total indirect effect, irrespective of predation pressure levels. On the other hand, sedentary predators caused larger non‐consumptive effects for lower predation pressure levels, but consumptive effects became more important as predation pressure increased. Our simulations showed that the reliance on a particular mechanism driving consumer–resource interactions is altered by predator foraging behavior and highlight the importance of both prey and predator foraging behaviors to predict the causes and consequences of cascading effects observed in food webs.     

Occupation of submerged aquatic vegetation by fishes: testing the roles of food and refuge

Summary We conducted a series of field experiments to examine the roles of refuge and food availability in explaining the distribution and abundance of fish in tidal freshwater marsh creeks. Two hypotheses were tested: (1) relative predation pressure is less in SAV than in unvegetated areas and (2) fish food availability is greater in SAV than in nearby unvegetated areas. Tethering experiments using mummichogs (Fundulus heteroclitus) in vegetated and unvegetated areas revealed that relative predation pressure was significantly less in areas with SAV. Banded killifish (Fundulus diaphanus) maintained in vegetated enclosures consumed prey associated with SAV, whereas those held in unvegetated pens had empty stomachs. No differences were found in the number of prey eaten by bluespotted sunfish (Enneacanthus gloriosus) or mummichogs when confined in vegetated or unvegetated enclosures. However, larger prey were consumed by bluespotted sunfish and mummichogs maintained in vegetated enclosures. These data suggest that foraging profitability is significantly enhanced by feeding in the SAV. Submerged plant beds in tidal freshwater marsh creeks not only afford protection from predators, but also provide a rich foraging habitat. By foraging in SAV, fish consume larger prey and may have higher growth rates, lower mortality, and higher fecundity.     

Predator cannibalism can shift prey community composition toward dominance by small prey species

Cannibalism among predators is a key intraspecific interaction affecting their density and foraging behavior, eventually modifying the strength of predation on heterospecific prey. Interestingly, previous studies showed that cannibalism among predators can increase or reduce predation on heterospecific prey; however, we know less about the factors that lead to these outcomes. Using a simple pond community consisting of Hynobius retardatus salamander larvae and their associated prey, I report empirical evidence that cannibalism among predators can increase predation on large heterospecific prey but reduce that on small heterospecific prey. In a field‐enclosure experiment in which I manipulated the occurrence of salamander cannibalism, I found that salamander cannibalism increased predation on frog tadpoles but reduced that on aquatic insects simultaneously. The contrasting effects are most likely to be explained by prey body size. In the study system, frog tadpoles were too large for non‐cannibal salamanders to consume, while aquatic insects were within the non‐cannibals’ consumable prey size range. However, when cannibalism occurred, a few individuals that succeeded in cannibalizing reached large enough size to consume frog tadpoles. Consequently, although cannibalism among salamanders reduced their density, salamander cannibalism increased predation on large prey frog tadpoles. Meanwhile, salamander cannibalism reduced predation on small prey aquatic insects probably because of a density reduction of non‐cannibals primarily consuming aquatic insects. Body size is often correlated with various ecological traits, for instance, diet width, consumption, and excretion rates, and is thus considered a good indicator of species’ effects on ecosystem function. All this considered, cannibalism among predators could eventually affect ecosystem function by shifting the size composition of the prey community.     

Weeds mediate the level of intraguild predation in arthropod food webs

Intraguild predation, which is common for generalist predators, is a specific form of omnivory that may suppress the biological control of a pest. The dietary flexibility of a given organism depends on the choice of the C3 (banana crop) and the C4 (weeds) pathways they use and on the trophic level on which they feed. Understanding the conditions in which intraguild predation decreases biological control is a major issue in agroecosystems. We tested whether the contribution of different primary producer pathways in diets of generalist predators mediates the level of intraguild predation. We studied 10 agroecosystems in which banana plants (C3 metabolism) were diversely associated with weeds (C4 metabolism). Diversity in litter macrofauna was relatively low, with a mean between three and eight species per trap. Measurement of stable isotopes showed a significant decrease in the δ¹⁵N values of generalist predators when the C4 pathway contributed more than the C3 pathway to their diet. We rejected hypotheses that an increase in the abundance of prey and that a decrease in prey's δ¹⁵N values occur when the C4 pathway contributes more than the C3 pathway to their diet. The results are consistent with the diet modification hypothesis, that is, intraguild predation is lower when the C4 (weeds) pathway is preferred to the C3 pathway. Our results suggest that when the C4 pathway of weeds is more exploited by herbivores (or detritivores), generalist predators tend to consume these herbivores and thus neglect the intraguild prey. The diverse C4 plant community probably supports a diverse herbivore community that provides alternative prey. Our results provide evidence that increasing plant diversity in agroecosystems should decrease intraguild predation of generalist predators and should therefore improve pest regulation. In an applied perspective, plant diversity could be increased by establishing a more diverse cover‐crop community.     

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.     

Selection of aphid prey by a generalist predator: do prey chemical defences matter?

1. For predators, prey selection should maximise nutrition and minimise fitness costs. In the present study, it was investigated whether a generalist predator [Chrysoperla carnea (Stephens) lacewing larvae] rejected harmful, chemically‐defended prey [Brevicoryne brassicae (Linnaeus) aphids] when non‐defended prey [Myzus persicae (Sulzer) aphids] were available. 2. It was tested: (i) whether consuming different prey species affects predator mortality; (ii) whether naïve predators reject chemically‐defended prey while foraging when non‐defended prey are available; (iii) whether the relative abundance of each prey affects the predator's prey choice; and (iv) whether predators learn to avoid consuming chemically‐defended prey after exposure to both prey species. 3. Consumption of B. brassicae yielded greater C. carnea mortality than M. persicae consumption, but naïve C. carnea did not reject B. brassicae in favour of M. persicae during foraging. When presented at unequal abundances, naïve predators generally consumed each aphid species according to their initial relative abundance, although, predation of non‐defended prey was less than expected when defended prey were initially more abundant, indicating a high consumption of B. brassicae impeded M. persicae consumption. With experience, C. carnea maintained predation of both aphid species but consumed more M. persicae than B. brassicae, indicating a change in behaviour. 4. Although prey choice by C. carnea may change with experience of available prey, prey chemical defences do not appear to influence prey choice by naïve predators. This inability to avoid harmful prey could facilitate wider, indirect interactions. Myzus persicae may benefit where high consumption of B. brassicae hinders predators in the short term, and in the long term, increases predator mortality.     

Predation risk in tadpole populations shapes behavioural responses of prey but not strength of trait‐mediated indirect interactions

Prey animals often respond to predators by reducing activity levels. This can produce a trait‐mediated indirect interaction (TMII) between predators and prey resources, whereby reduced foraging by prey in the presence of a predator causes an increase in prey resources. TMIIs play important roles in structuring communities, and it is important to understand factors that determine their strength. One such influence may be behavioural variation in the prey species, with indirect effects of predators being stronger within populations that are more responsive to the presence of a predator. We tested 1) whether the behavioural responsiveness of populations of wood frog tadpoles to predator cues was related to the predation risk in their native ponds, and 2) whether more responsive tadpoles yielded stronger TMIIs. To do this, we 1) measured the activity of tadpoles from 18 populations in mesocosms with and without caged predators, and 2) measured changes in the biomass of periphyton (the tadpoles’ diet) between predator treatments for each population. We found that tadpoles from higher predation risk ponds reduced their time outside refuges more in the presence of predators and tended to move less when visible, suggesting possible local adaptation to predation regimes. Though the presence of predators generally resulted in higher periphyton biomass – a TMII – there was no evidence that the strength of this TMII was affected by variation in tadpole behaviour. Foraging activity and general activity may be decoupled to some extent, enabling high predation risk‐adapted tadpoles to limit the fitness costs of reduced foraging when predators are present.     

Mimicry between unequally defended prey can be parasitic: evidence for quasi-Batesian mimicry

The nature of signal mimicry between defended prey (known as Müllerian mimicry) is controversial. Some authors assert that it is always mutualistic and beneficial, whilst others speculate that less well defended prey may be parasitic and degrade the protection of their better defended co-mimics (quasi-Batesian mimicry). Using great tits (Parus major) as predators of artificial prey, we show that mimicry between unequally defended co-mimics is not mutualistic, and can be parasitic and quasi-Batesian. We presented a fixed abundance of a highly defended model and a moderately defended dimorphic (mimic and distinct non-mimetic) species, and varied the relative frequency of the two forms of the moderately defended prey. As the mimic form increased in abundance, per capita predation on the model-mimic pair increased. Furthermore, when mimics were rare they gained protection from predation but imposed no co-evolutionary pressure on models. We found that the feeding decisions of the birds were affected by their individual toxic burdens, consistent with the idea that predators make foraging decisions which trade-off toxicity and nutrition. This result suggests that many prey species that are currently assumed to be in a simple mutualistic mimetic relationship with their co-mimic species may actually be engaged in an antagonistic co-evolutionary process.     

Local prey vulnerability increases with multiple attacks by a predator

Theory and empirical evidence suggest that predator activity makes prey more wary and less vulnerable to predation. However if at least some prey in the population are energetically or spatially constrained, then predators may eventually increase local prey vulnerability because of the cumulative costs of anti‐predation behaviour. We tested whether repeated attacks by a predator might increase prey vulnerability in a system where redshanks on a saltmarsh are attacked regularly by sparrowhawks from adjacent woodland. Cumulative attack number led to a reduction in redshank numbers and flock size (but had no effect on how close redshanks fed to predator‐concealing cover) because some redshanks moved to safer but less profitable habitats, leaving smaller flocks on the saltmarsh. This effect held even though numbers of redshank on the saltmarsh increased with time of day. As a result of the change in flock size, predicted attack‐success increased up to 1.6‐fold for the sparrowhawk, while individual risk of capture for the redshank increased up to 4.5‐fold among those individuals remaining on the saltmarsh. The effect did not arise simply because hawks were more likely to attack smaller flocks because attack rate was not dependent on flock size or abundance. Our data demonstrate that when some individual prey are constrained in their ability to feed on alternative, safer foraging sites, their vulnerability to predation increases as predator attacks accumulate, although those, presumably better quality individuals that leave the immediate risky area will have lower vulnerability, so that the mean vulnerability across the entire population may not have changed substantially. This suggests that the selective benefits of multiple low‐cost attacks by predators on prey could potentially lead to 1) locally heightened trait‐mediated interactions, 2) locally reduced interference among competing predators, and 3) the evolution of active prey manipulation by predators.     

Remembering the good and the bad: memory-based mediation of the food–safety trade-off in dynamic landscapes

Predator–prey interactions are central to fitness as animals simultaneously avoid death and consume resources to ensure growth and reproduction. Along with direct effects, predators can also exert strong non-consumptive effects. For example, prey shift habitat use in the presence of predators, a potentially learned behavior. The impact of cognition on movement and predator interactions is largely unexplored despite evidence of learned responses to predation threat. We explore how learning and spatial memory influence predator–prey dynamics by introducing predators into a memory-driven movement modeling framework. To model various aspects of risk, we vary predator behavior: their persistence and spatial correlation with the prey’s resources. Memory outperforms simpler movement processes most in patchy environments with more predictable predators that are more easily avoided once learned. In these cases, memory aids foragers in managing the food–safety trade-off. For example, particular parameterizations of the predation memory reduce encounters while maintaining consumption. We found that non-consumptive effects are highest in landscapes of concentrated, patchy resources. These effects are intensified when predators are highly correlated with the forager’s resources. Smooth landscapes provide more opportunities for foragers to simultaneously consume resources and avoid predators. Predators are able to effectively guard all resources in very patchy landscapes. These non-consumptive effects are also seen with the shift away from the best quality habitat compared to foraging in a predator-free environment.