Non‐consumptive effects of a top‐predator decrease the strength of the trophic cascade in a four‐level terrestrial food web

The fear of predators can strongly impact food web dynamics and ecosystem functioning through effects on herbivores morphology, physiology or behaviour. While non‐consumptive predator effects have been mostly studied in three‐level food chains, we lack evidence for the propagation of non‐consumptive indirect effects of apex predators in four level food‐webs, notably in terrestrial ecosystems. In experimental mesocosms, we manipulated a four‐level food chain including top‐predator cues (snakes), mesopredators (lizards), herbivores (crickets), and primary producers (plants). The strength of the trophic cascade induced by mesopredators through the consumption of herbivores decreased in the presence of top‐predator cues. Specifically, primary production was higher in mesocosms where mesopredators were present relative to mesocosms with herbivores only, and this difference was reduced in presence of top‐predator cues, probably through a trait‐mediated effect on lizard foraging. Our study demonstrates that non‐consumptive effects of predation risk can cascade down to affect both herbivores and plants in a four‐level terrestrial food chain and emphasises the need to quantify the importance of such indirect effects in natural communities.     

Interspecific differences in antipredator strategies determine the strength of non‐consumptive predator effects on stream detritivores

Animal species differ considerably in their response to predation risks. Interspecific variability in prey behaviour and morphology can alter cascading effects of predators on ecosystem structure and functioning. We tested whether species‐specific morphological defenses may affect responses of leaf litter consuming invertebrate prey to sit‐and‐wait predators, the odonate Cordulegaster boltonii larvae, in aquatic food webs. Partly or completely blocking the predator mouthparts (mandibles and/or extensible labium), thus eliminating consumptive (i.e. lethal) predator effects, we created a gradient of predator‐prey interaction intensities (no predator < predator – no attack < predator – non‐lethal attacks < lethal predator). A field experiment was first used to assess both consumptive and non‐consumptive predator effects on leaf litter decomposition and prey abundances. Laboratory microcosms were then used to examine behavioural responses of armored and non‐armored prey to predation risk and their consequences on litter decomposition. Results show that armored and non‐armored prey responded to both acute (predator – non‐lethal attacks) and chronic (predator – no attack) predation risks. Acute predation risk had stronger effects on litter decomposition, prey feeding rate and prey habitat use than predator presence alone (chronic predation risk). Predator presence induced a reduction in feeding activity (i.e. resource consumption) of both prey types but a shift to predator‐free habitat patches in non‐armored detritivores only. Non‐consumptive predator effects on prey subsequently decreased litter decomposition rate. Species‐specific prey morphological defenses and behaviour should thus be considered when studying non‐consumptive predator effects on prey community structure and ecosystem functioning.     

Plant resistance attenuates the consumptive and non‐consumptive impacts of predators on prey

Plant resistance and predation have strong independent and interacting effects on herbivore survival, behavior, and patterns of herbivory. Historically, research has emphasized variation in the consumption of herbivores by enemies. Recent work, however, demonstrates that predators also elicit important changes in the traits of their prey, but we do not know how this is influenced by plant quality. In this study, we quantify how the consumptive and non‐consumptive effects of predators vary along a gradient of plant resistance using tomato plants (Solanum lycopersicum), tobacco hornworms (Manduca sexta), and predaceous stinkbugs (Podisus maculiventris). We manipulated resource quality using three tomato lines that vary in the expression of the jasmonate pathway, a phytohormonal pathway that is central in mediating resistance to insects. Resistant plants had higher levels of defensive proteins and glandular trichomes than low resistance plants. The consumptive and non‐consumptive effects of predators were quantified on the three tomato lines by comparing the impact of ‘lethal’ predators that both kill and scare prey with ‘risk’ predators whose mouthparts were surgically impaired to prevent killing. Across several field experiments, the total cascading effect of predators on plant damage was 80.4% lower on jasmonate‐overex‐pressing (highly resistant) plants compared to that on wild‐type or jasmonate‐insensitive (low resistance) plants. This dramatic attenuation of predator effects was due to a 66% reduction in consumption on high resistance plants, and also because of a 65% decline in non‐consumptive effects. Numerous studies in natural and agricultural habitats have documented that predator effects tend to be weaker on well‐defended plants; our results provide novel mechanistic insight into this pattern by demonstrating that plant resistance substantially weakens both the consumptive and non‐consumptive impacts of predators.     

The context dependence of non‐consumptive predator effects

Non‐consumptive predator effects (NCEs) are now widely recognised for their capacity to shape ecosystem structure and function. Yet, forecasting the propagation of these predator‐induced trait changes through particular communities remains a challenge. Accordingly, focusing on plasticity in prey anti‐predator behaviours, we conceptualise the multi‐stage process by which predators trigger direct and indirect NCEs, review and distil potential drivers of contingencies into three key categories (properties of the prey, predator and setting), and then provide a general framework for predicting both the nature and strength of direct NCEs. Our review underscores the myriad factors that can generate NCE contingencies while guiding how research might better anticipate and account for them. Moreover, our synthesis highlights the value of mapping both habitat domains and prey‐specific patterns of evasion success (‘evasion landscapes’) as the basis for predicting how direct NCEs are likely to manifest in any particular community. Looking ahead, we highlight two key knowledge gaps that continue to impede a comprehensive understanding of non‐consumptive predator–prey interactions and their ecosystem consequences; namely, insufficient empirical exploration of (1) context‐dependent indirect NCEs and (2) the ways in which direct and indirect NCEs are shaped interactively by multiple drivers of context dependence.     

Linking the green and brown worlds through nonconsumptive predator effects

Predators can indirectly affect lower trophic levels by either consuming their prey (consumptive effect, CE) or by changing the physiology or behavior of their prey (nonconsumptive effect, NCE). Cascading effects of predators on primary producers are common, and can be propagated by CEs, NCEs, or a combination of both mechanisms. Predator impacts in detrital food webs (the ‘brown world’) have received considerably less attention than their effects on systems with primary producers at the base (the ‘green world’), and only recently have we begun to appreciate the importance of above‐ground predators indirectly impacting below‐ground processes. Numerous studies reveal the total impact (CEs and NCEs) of predators in brown food webs, but our understanding of the role of isolated NCEs is limited. Many habitats and major taxa have not been studied, and patterns are difficult to distinguish due to frequent reporting of mixed effects. Predators play an important role as connectors between brown and green worlds when they feed from both food webs (multichannel feeding). We are only beginning to understand how NCEs influence detrital food webs, and it is unknown whether multichannel fear is an essential component of predator–prey ecology that regulates ecosystem function. Synthesis Predators have been shown to impact ecosystems through both consumptive and nonconsumptive effects on their prey Historically, herbivory‐based ‘green’ systems have been the venue for documenting these predator effects, while detritus‐based ‘brown’ systems received considerably less attention. However, similar mechanisms exist in green and brown worlds, suggesting strong parallels. We review and synthesize predator effects in detrital systems, highlighting important shortcomings in current understanding. Furthermore, we build upon the idea of multichannel feeding (i.e. consumption of prey from both green and brown food webs) to propose the existence of ‘multichannel fear’. We provide a framework for documenting multichannel fear to facilitate continued exploration of how predators link seemingly disparate systems.     

Predator–prey naïveté, antipredator behavior,and the ecology of predator invasions

We present a framework for explaining variation in predator invasion success and predator impacts on native prey that integrates information about predator–prey naïveté, predator and prey behavioral responses to each other, consumptive and non‐consumptive effects of predators on prey, and interacting effects of multiple species interactions. We begin with the ‘naïve prey’ hypothesis that posits that naïve, native prey that lack evolutionary history with non‐native predators suffer heavy predation because they exhibit ineffective antipredator responses to novel predators. Not all naïve prey, however, show ineffective antipredator responses to novel predators. To explain variation in prey response to novel predators, we focus on the interaction between prey use of general versus specific cues and responses, and the functional similarity of non‐native and native predators. Effective antipredator responses reduce predation rates (reduce consumptive effects of predators, CEs), but often also carry costs that result in non‐consumptive effects (NCEs) of predators. We contrast expected CEs versus NCEs for non‐native versus native predators, and discuss how differences in the relative magnitudes of CEs and NCEs might influence invasion dynamics. Going beyond the effects of naïve prey, we discuss how the ‘naïve prey’, ‘enemy release’ and ‘evolution of increased competitive ability’ (EICA) hypotheses are inter‐related, and how the importance of all three might be mediated by prey and predator naïveté. These ideas hinge on the notion that non‐native predators enjoy a ‘novelty advantage’ associated with the naïveté of native prey and top predators. However, non‐native predators could instead suffer from a novelty disadvantage because they are also naïve to their new prey and potential predators. We hypothesize that patterns of community similarity and evolution might explain the variation in novelty advantage that can underlie variation in invasion outcomes. Finally, we discuss management implications of our framework, including suggestions for managing invasive predators, predator reintroductions and biological control.     

Rainfall and hydrological stability alter the impact of top predators on food web structure and function

Climate change will alter the distribution of rainfall, with potential consequences for the hydrological dynamics of aquatic habitats. Hydrological stability can be an important determinant of diversity in temporary aquatic habitats, affecting species persistence and the importance of predation on community dynamics. As such, prey are not only affected by drought‐induced mortality but also the risk of predation [a non‐consumptive effect (NCE)] and actual consumption by predators [a consumptive effect (CE)]. Climate‐induced changes in rainfall may directly, or via altered hydrological stability, affect predator–prey interactions and their cascading effects on the food web, but this has rarely been explored, especially in natural food webs. To address this question, we performed a field experiment using tank bromeliads and their aquatic food web, composed of predatory damselfly larvae, macroinvertebrate prey and bacteria. We manipulated the presence and consumption ability of damselfly larvae under three rainfall scenarios (ambient, few large rainfall events and several small rainfall events), recorded the hydrological dynamics within bromeliads and examined the effects on macroinvertebrate colonization, nutrient cycling and bacterial biomass and turnover. Despite our large perturbations of rainfall, rainfall scenario had no effect on the hydrological dynamics of bromeliads. As a result, macroinvertebrate colonization and nutrient cycling depended on the hydrological stability of bromeliads, with no direct effect of rainfall or predation. In contrast, rainfall scenario determined the direction of the indirect effects of predators on bacteria, driven by both predator CEs and NCEs. These results suggest that rainfall and the hydrological stability of bromeliads had indirect effects on the food web through changes in the CEs and NCEs of predators. We suggest that future studies should consider the importance of the variability in hydrological dynamics among habitats as well as the biological mechanisms underlying the ecological responses to climate change.     

Mesopredator suppression by an apex predator alleviates the risk of predation perceived by small prey

Predators can impact their prey via consumptive effects that occur through direct killing, and via non-consumptive effects that arise when the behaviour and phenotypes of prey shift in response to the risk of predation. Although predators'' consumptive effects can have cascading population-level effects on species at lower trophic levels there is less evidence that predators'' non-consumptive effects propagate through ecosystems. Here we provide evidence that suppression of abundance and activity of a mesopredator (the feral cat) by an apex predator (the dingo) has positive effects on both abundance and foraging efficiency of a desert rodent. Then by manipulating predators'' access to food patches we further the idea that apex predators provide small prey with refuge from predation by showing that rodents increased their habitat breadth and use of ‘risky′ food patches where an apex predator was common but mesopredators rare. Our study suggests that apex predators'' suppressive effects on mesopredators extend to alleviate both mesopredators'' consumptive and non-consumptive effects on prey.     

Independent and combined effects of multiple predators across ontogeny of a dominant grazer

Ecosystems host multiple coexisting predator species whose interactions may strengthen or weaken top–down control of grazers. Grazer populations often exhibit size‐structure, but the nature of multiple predator effects on suppression of size‐structured prey has seldom been explicitly considered. In a southeastern US salt‐marsh, we used both field (additive design) and mesocosm (additive‐substitutive design) experiments to test the independent and combined effects of two species of predatory crab on the survival and predator‐avoidance behavior (i.e. a non‐consumptive effect) of both juveniles and adults of a dominant grazing snail. Results showed: 1) juvenile snails were more vulnerable to predation; 2) consumptive impacts of predators were hierarchically nested, i.e. the larger predator consumed both juvenile and adult snails, while the smaller‐bodied predator consumed only juvenile snails; 3) there were no emergent multiple predator effects on snail consumption; and 4) non‐consumptive effects differed from consumptive effects, with only the large predator inducing predator‐avoidance behavior of individuals within either snail ontogenetic class. The smaller predator therefore played a functionally redundant trophic role across the prey classes considered, augmenting and potentially stabilizing trophic regulation of juvenile snails. Meanwhile, the larger predator played a complementary and functionally unique role by both expanding the size‐spectrum of prey trophic regulation and non‐consumptively altering prey behavior. While our study suggests that nestedness of consumptive interactions determined by predator and prey body sizes may allow prediction of the functional redundancy of particular predator species, it also shows that traits beyond predator body size (e.g. habitat domain) may be required to predict potentially cascading non‐consumptive effects. Future studies of multiple predators (and predator biodiversity) should continue to strive towards greater realism by incorporating not only size‐structured prey, but also other aspects of resource and environmental heterogeneity typical of natural ecosystems.     

An island-wide predator manipulation reveals immediate and long-lasting matching of risk by prey

Anti-predator behaviour affects prey population dynamics, mediates cascading effects in food webs and influences the likelihood of rapid extinctions. Predator manipulations in natural settings provide a rare opportunity to understand how prey anti-predator behaviour is affected by large-scale changes in predators. Here, we couple a long-term, island-wide manipulation of an important rodent predator, the island fox (Urocyon littoralis), with nearly 6 years of measurements on foraging by deer mice (Peromyscus maniculatus) to provide unequivocal evidence that prey closely match their foraging behaviour to the number of fox predators present on the island. Peromyscus maniculatus foraging among exposed and sheltered microhabitats (a measure of aversion to predation risk) closely tracked fox density, but the nature of this effect depended upon nightly environmental conditions known to affect rodent susceptibility to predators. These effects could not be explained by changes in density of deer mice over time. Our work reveals that prey in natural settings are cognizant of the dynamic nature of their predators over timescales that span many years, and that predator removals spanning many generations of prey do not result in a loss of anti-predator behaviour.     

Indirect interactions in a rice ecosystem: density dependence and the interplay between consumptive and non‐consumptive effects of predators

1. Density‐ and trait‐mediated indirect interactions (DMIIs and TMIIs, respectively) in food chains play crucial roles in community structure and processes. However, factors affecting the relative strength of these interactions are poorly understood, including in widespread and important freshwater rice ecosystems. 2. We studied the strength of DMIIs and TMIIs in a food chain involving a predator (the Reeve’s turtle Chinemys reevesii), its herbivorous prey (the apple snail Pomacea canaliculata) and a plant (rice Oryza sativa) in outdoor containers simulating rice fields. We also evaluated consumptive and non‐consumptive effects of the predator on the snail. We removed a fixed proportion of snails every 2 days to simulate prey consumption and introduced a caged turtle that was fed daily with snails to simulate non‐consumptive effects. 3. Direct consumptive effects increased growth of the remaining snails and their per capita feeding rate. Moreover, consumptive and non‐consumptive effects, and their interaction, affected the proportion of snails buried in the soil. This interaction was presumably because increasing food availability per snail induced their self‐burying behaviour. 4. Both DMIIs and TMIIs affected the number of rice plants remaining, whereas their interaction term was not significant. 5. In summary, density dependence and interactions between consumptive and non‐consumptive effects influenced snail growth and behaviour, respectively. However, no cascading effects of these complicated interactions on rice plants were detected.     

Parasites as prey in aquatic food webs: implications for predator infection and parasite transmission

While the recent inclusion of parasites into food‐web studies has highlighted the role of parasites as consumers, there is accumulating evidence that parasites can also serve as prey for predators. Here we investigated empirical patterns of predation on parasites and their relationships with parasite transmission in eight topological food webs representing marine and freshwater ecosystems. Within each food web, we examined links in the typical predator–prey sub web as well as the predator–parasite sub web, i.e. the quadrant of the food web indicating which predators eat parasites. Most predator– parasite links represented ‘concomitant predation’ (consumption and death of a parasite along with the prey/host; 58–72%), followed by ‘trophic transmission’ (predator feeds on infected prey and becomes infected; 8–32%) and predation on free‐living parasite life‐cycle stages (4–30%). Parasite life‐cycle stages had, on average, between 4.2 and 14.2 predators. Among the food webs, as predator richness increased, the number of links exploited by trophically transmitted parasites increased at about the same rate as did the number of links where these stages serve as prey. On the whole, our analyses suggest that predation on parasites has important consequences for both predators and parasites, and food web structure. Because our analysis is solely based on topological webs, determining the strength of these interactions is a promising avenue for future research.     

Sensory cues of a top‐predator indirectly control a reef fish mesopredator

Behavioural trophic cascades highlight the importance of indirect/risk effects in the maintenance of healthy trophic‐level links in complex ecosystems. However, there is limited understanding on how the loss of indirect top–down control can cascade through the food‐web to modify lower level predator–prey interactions. Using a reef fish food‐web, our study examines behavioural interactions among predators to assess how fear elicited by top‐predator cues (visual and chemical stimuli) can alter mesopredator behaviour and modify their interaction with resource prey. Under experimental conditions, the presence of any cue (visual, chemical, or both) from the top‐predator (coral trout Plectropomus leopardus) strongly restricted the distance swum, area explored and foraging activity of the mesopredator (dottyback Pseudochromis fuscus), while indirectly triggering a behavioural release of the resource prey (recruits of the damselfish Pomacentrus chrysurus). Interestingly, the presence of a large non‐predator species (thicklip wrasse Hemigymnus melapterus) also mediated the impact of the mesopredator on prey, as it provoked mesopredators to engage in an ‘inspection’ behaviour, while significantly reducing their feeding activity. Our study describes for the first time a three‐level behavioural cascade of coral reef fish and stresses the importance of indirect interactions in marine food‐webs.     

Differential vulnerability of prey to an invading top predator: integrating field surveys and laboratory experiments

Abstract 1. A new top predator, the dragonfly Cordulegaster boltonii, invaded Broadstone Stream (U.K.) in the mid‐1990s. This provided a rare opportunity to assess the impact of a new, large carnivore on a community that has been studied since the 1970s and has one of the most detailed food webs yet published. The vulnerability of the resident species to the invader was assessed by integrating experiments, which examined discrete stages in the predation sequence, with empirical survey data. 2. Although the new predator preyed on nearly every macro‐invertebrate in the food web, vulnerability varied considerably among prey species. Size‐related handling constraints initially set the predator's diet, resulting in strong ontogenetic shifts, with progressively larger prey being added while small prey were retained in the diet, as predators grew. Within the size range of vulnerable prey, encounter rate limited the strength of predation, with mobile, epibenthic species being most at risk. Contrary to most studies of interactions between freshwater predators (usually stoneflies) and prey (usually mayflies), the new predator did not elicit avoidance responses from its prey, probably because it combined a highly cryptic feeding posture with an extremely rapid attack response. 3. The invader exploited its prey heavily in experiments, even at prey densities orders of magnitude above ambient. In the field, electivity reflected prey availability, as determined by mobility and microhabitat use, rather than prey abundance or active predator choice. Consequently, the invader had skewed effects within the prey assemblage, with sedentary, interstitial species being far less vulnerable than more active, epibenthic species, some of which, including a previous top predator, have declined markedly since the invasion. 4. By examining the predation sequence in detail and integrating surveys with experiments, species traits and system characteristics that determine the strength of trophic interactions may be identified, and their potential importance in natural food webs assessed. In so doing, greater insight can be gained into which species (and systems) will be most vulnerable to invading or exotic predators, an imperative in both pure and applied ecology.     

Dying from the lesser of three evils: facilitation and non‐consumptive effects emerge in a model with multiple predators

Prey modify their behaviour to avoid predation, but dilemmas arise when predators vary in hunting style. Behaviours that successfully evade one predator sometimes facilitate exposure to another predator, forcing the prey to choose the lesser of two evils. In such cases, we need to quantify behavioural strategies in a mix of predators. We model optimal behaviour of Atlantic cod Gadus morhua larvae in a water column, and find the minimal vulnerability from three common predator groups with different hunting modes; 1) ambush predators that sit‐and‐wait for approaching fish larvae; 2) cruising invertebrates that eat larvae in their path; and 3) fish which are visually hunting predators. We use a state‐dependent model to find optimal behaviours (vertical position and swimming speed over a diel light cycle) under any given exposure to the three distinct modes of predation. We then vary abundance of each predator and quantify direct and indirect effects of predation. The nature and strength of direct and indirect effects varied with predator type and abundance. Larvae escaped about half the mortality from fish by swimming deeper to avoid light, but their activity level and cumulative predation from ambush predators increased. When ambush invertebrates dominated, it was optimal to be less active but in more lit habitats, and predation from fish increased. Against cruising predators, there was no remedy. In all cases, the shift in behaviour allowed growth to remain almost the same, while total predation were cut by one third. In early life stages with high and size‐dependent mortality rates, growth rate can be a poor measure of the importance of behavioural strategies.     

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.     

Fear and lethality in snowshoe hares: the deadly effects of non‐consumptive predation risk

Predators play a critical, top–down role in shaping ecosystems, driving prey population and community dynamics. Traditionally, studies of predator‐prey interactions have focused on direct effects of predators, namely the killing of prey. More recently, the non‐consumptive effects of predation risk are being appreciated; e.g. the ‘ecology of fear’. Prey responses to predation risk can be morphological, behavioural, and physiological, and are assumed to come at a cost to prey fitness. However, few studies have examined the relationship between predation risk and survival in wild animals. We tested the hypothesis that predation risk itself could reduce survival in wild‐caught snowshoe hares. We exposed female snowshoe hares to a simulated predator (a trained dog) during gestation only, and measured adult survival and, in surviving females, their ability to successfully wean offspring. We show for the first time in a wild mammal that the risk of predation can itself be lethal. Predation risk reduced adult female survival by 30%, and had trans‐generational effects, reducing offspring survival to weaning by over 85% – even though the period of risk ended at birth. As a consequence of these effects the predator‐exposed group experienced a decrease in number, while the control group substantially increased. Challenges remain in determining the importance of risk‐induced mortality in natural field settings; however, our findings show that non‐lethal predator encounters can influence survival of both adults and offspring. Future work is needed to test these effects in free‐living animals.     

Fish predation can induce mesohabitat-specific differences in food web structures in small stream ecosystems

Diverse benthic communities in streams include a wide variety of predators with different habitat preferences, e.g. for pools or riffles. We hypothesised that these preferences result in mesohabitat-specific predator community structures with quantitative differences concerning predation intensity by vertebrate and invertebrate predators, importance of intraguild predation, or top–down pressure. This hypothesis was evaluated for a small submontane stream by means of mesohabitat-specific quantification of prey consumption by two benthivorous fish species (Gobio gobio and Barbatula barbatula) and several invertebrate predators. The estimation was based on daily food rations and diet composition of predators and mesohabitat-specific predator biomass. We found clear differences between the two mesohabitat types. Predator food webs were less complex in pools than in riffles. Fish predation was more important than invertebrate predation in pools, and intraguild predation had a higher relative importance in these mesohabitats. These differences were probably caused by the mesohabitat use of G. gobio, the largest top predator, which preferred pools. Consequently, the predator food webs were more similar between the mesohabitats when fish were absent. Top–down pressure on primary consumers by all predators together was lowest in pools without fish, but the effect was not significant. Omnivory (including cannibalism) was intense, but its potentially destabilising effects were probably counterbalanced by mesohabitat connectivity. From the results of our experimental study, we conclude that even in small stream ecosystems, food web structures and predation pathways can differ between mesohabitats and that a mesohabitat-specific consideration will help to explain the variety of top–down effects on benthic communities.     

Non-lethal effects of predation in birds

Predators can affect individual fitness and population and community processes through lethal effects (direct consumption or ‘density’ effects), where prey is consumed, or through non‐lethal effects (trait‐mediated effects or interactions), where behavioural compensation to predation risk occurs, such as animals avoiding areas of high predation risk. Studies of invertebrates, fish and amphibians have shown that non‐lethal effects may be larger than lethal effects in determining the behaviour, condition, density and distribution of animals over a range of trophic levels. Although non‐lethal effects have been well described in the behavioural ecology of birds (and also mammals) within the context of anti‐predation behaviour, their role relative to lethal effects is probably underestimated. Birds show many behavioural and physiological changes to reduce direct mortality from predation and these are likely to have negative effects on other aspects of their fitness and population dynamics, as well as affecting the ecology of their own prey and their predators. As a consequence, the effects of predation in birds are best measured by trade‐offs between maximizing instantaneous survival in the presence of predators and acquiring or maintaining resources for long‐term survival or reproduction. Because avoiding predation imposes foraging costs, and foraging behaviour is relatively easy to measure in birds, the foraging–predation risk trade‐off is probably an effective framework for understanding the importance of non‐lethal effects, and so the population and community effects of predation risk in birds and other animals. Using a trade‐off approach allows us to predict better how changes in predator density will impact on population and community dynamics, and how animals perceive and respond to predation risk, when non‐lethal effects decouple the relationship between predator density and direct mortality rate. The trade‐off approach also allows us to identify where predation risk is structuring communities because of avoidance of predators, even when this results in no observable direct mortality rate.     

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.