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1. Predator-induced stress has been used to exemplify the concept of stress for close to a century because almost everyone can imagine the terror of fleeing for one's life from a lion or a tiger. Yet, because it has been assumed to be acute and transitory, predator-induced stress has not been much studied by either comparative physiologists or population ecologists, until relatively recently. 2. The focus in biomedical research has always been on chronic stress in humans, which most comparative physiologists would agree results from 'sustained psychological stress — linked to mere thoughts' rather than 'acute physical crises' (like surviving a predator attack) or 'chronic physical challenges' (such as a shortage of food). Population ecologists have traditionally focused solely on the acute physical crisis of surviving a direct predator attack rather than whether the risk of such an attack may have a sustained effect on other demographic processes (e.g. the birth rate). 3. Demographic experiments have now demonstrated that exposure to predators or predator cues can have sustained effects that extend to affecting birth and survival in free-living animals, and a subset of these have documented associated physiological stress effects. These and similar results have prompted some authors to speak of an 'ecology of fear', but others object that 'the cognitive and emotional aspects of avoiding predation remain unknown'. 4. Recent biomedical studies on animals in the laboratory have demonstrated that exposure to predators or predator cues can induce 'sustained psychological stress' that is directly comparable to chronic stress in humans, and this has now in fact become one of the most common stressors used in studies of the animal model of post-traumatic stress disorder (PTSD). 5. We review these recent findings and suggest ways the laboratory techniques developed to measure the 'neural circuitry of fear' could be adapted for use on free-living animals in the field, in order to: (i) test whether predator risk induces 'sustained psychological stress' in wild animals, comparable to chronic stress in humans and (ii) directly investigate 'the cognitive and emotional aspects of avoiding predation' and hence the 'ecology of fear'.

Non‐consumptive effects of predation can strongly impact reproduction and demography of prey species. Still, the underlying mechanisms that drive non‐consumptive effects are not fully understood, and the circumstances under which chronic physiological stress may mediate these effects remain unclear. Benefiting from over 23 years of environmental, physiological and demographic data, we tested the hypothesis that predation risk may impair reproduction of mountain goats through chronic elevation of physiological stress. We conducted path analyses to assess the relationships between predation risk, faecal glucocorticoid metabolites and hair cortisol concentration, and reproduction, while taking into account the potential effects of age class, sex, body mass, season and within individual variation in glucocorticoid concentration. Predation risk had a direct positive effect on the average annual faecal glucocorticoid concentration in the population, which, in turn, negatively affected the proportion of reproductive females. The same pattern was observed with hair cortisol concentration, but these results were inconclusive potentially due to methodological challenges in estimating annual average of hair cortisol at the population level. Our study presents one of the first robust evidence that stress‐mediated breeding suppression can occur in a wild ungulate following increased predation risk, thereby providing a major insight on the mechanisms underlying non‐consumptive effects of predation in wild mammals. A free Plain Language Summary can be found within the Supporting Information of this article. A free Plain Language Summary can be found within the Supporting Information of this article.

Predators affect prey by killing them directly (lethal effects) and by inducing costly antipredator behaviours in living prey (risk effects). Risk effects can strongly influence prey populations and cascade through trophic systems. A prerequisite for assessing risk effects is characterizing the spatiotemporal variation in predation risk. Risk effects research has experienced rapid growth in the last several decades. However, preliminary assessments of the resultant literature suggest that researchers characterize predation risk using a variety of techniques. The implications of this methodological variation for inference and comparability among studies have not been well recognized or formally synthesized. We couple a literature survey with a hierarchical framework, developed from established theory, to quantify the methodological variation in characterizing risk using carnivore-ungulate systems as a case study. Via this process, we documented 244 metrics of risk from 141 studies falling into at least 13 distinct subcategories within three broader categories. Both empirical and theoretical work suggest risk and its effects on prey constitute a complex, multi-dimensional process with expressions varying by spatiotemporal scale. Our survey suggests this multi-scale complexity is reflected in the literature as a whole but often underappreciated in any given study, which complicates comparability among studies and leads to an overemphasis on documenting the presence of risk effects rather than their mechanisms or scale of influence. We suggest risk metrics be placed in a more concrete conceptual framework to clarify inference surrounding risk effects and their cascading effects throughout ecosystems. We recommend studies (i) take a multi-scale approach to characterizing risk; (ii) explicitly consider 'true' predation risk (probability of predation per unit time); and (iii) use risk metrics that facilitate comparison among studies and the evaluation of multiple competing hypotheses. Addressing the pressing questions in risk effects research, including how, to what extent and on what scale they occur, requires leveraging the advantages of the many methods available to characterize risk while minimizing the confusion caused by variability in their application.

In recent years, it has been argued that the effect of predator fear exacts a greater demographic toll on prey populations than the direct killing of prey. However, efforts to quantify the effects of fear have primarily relied on experiments that replace predators with predator cues. Interpretation of these experiments must consider two important caveats: (1) the magnitude of experimenter-induced predator cues may not be realistically comparable to those of the prey’s natural sensory environment, and (2) given functional predators are removed from the treatments, the fear effect is measured in the absence of any consumptive effects, a situation which never occurs in nature. We contend that demographic consequences of fear in natural populations may have been overestimated because the intensity of predator cues applied by experimenters in the majority of studies has been unnaturally high, in some instances rarely occurring in nature without consumption. Furthermore, the removal of consumption from the treatments creates the potential situation that individual prey in poor condition (those most likely to contribute strongly to the observed fear effects via starvation or reduced reproductive output) may have been consumed by predators in nature prior to the expression of fear effects, thus confounding consumptive and fear effects. Here, we describe an alternative treatment design that does not utilize predator cues, and in so doing, better quantifies the demographic effect of fear on wild populations. This treatment substitutes the traditional cue experiment where consumptive effects are eliminated and fear is simulated with a design where fear is removed and consumptive effects are simulated through the experimental removal of prey. Comparison to a natural population would give a more robust estimate of the effect of fear in the presence of consumption on the demographic variable of interest. This approach represents a critical advance in quantifying the mechanistic pathways through which predation structures ecological communities. Discussing the merits of both treatments will motivate researchers to go beyond simply describing the existence of fear effects and focus on testing their true magnitude in wild populations and natural communities.

Community ecology was traditionally an integrative science devoted to studying interactions between species and their abiotic environments in order to predict species' geographic distributions and abundances. Yet for philosophical and methodological reasons, it has become divided into two enterprises: one devoted to local experimentation on species interactions to predict community dynamics; the other devoted to statistical analyses of abiotic and biotic information to describe geographic distribution. Our goal here is to instigate thinking about ways to reconnect the two enterprises and thereby return to a tradition to do integrative science. We focus specifically on the community ecology of predators and prey, which is ripe for integration. This is because there is active, simultaneous interest in experimentally resolving the nature and strength of predator–prey interactions as well as explaining patterns across landscapes and seascapes. We begin by describing a conceptual theory rooted in classical analyses of non-spatial food web modules used to predict species interactions. We show how such modules can be extended to consideration of spatial context using the concept of habitat domain. Habitat domain describes the spatial extent of habitat space that predators and prey use while foraging, which differs from home range, the spatial extent used by an animal to meet all of its daily needs. This conceptual theory can be used to predict how different spatial relations of predators and prey could lead to different emergent multiple predator–prey interactions such as whether predator consumptive or non-consumptive effects should dominate, and whether intraguild predation, predator interference or predator complementarity are expected. We then review the literature on studies of large predator–prey interactions that make conclusions about the nature of multiple predator–prey interactions. This analysis reveals that while many studies provide sufficient information about predator or prey spatial locations, and thus meet necessary conditions of the habitat domain conceptual theory for drawing conclusions about the nature of the predator–prey interactions, several studies do not. We therefore elaborate how modern technology and statistical approaches for animal movement analysis could be used to test the conceptual theory, using experimental or quasi-experimental analyses at landscape scales.

Top predators can dramatically suppress populations of smaller predators, with cascading effects throughout communities, and this pressure is often unquestioningly accepted as a constraint on mesopredator populations. In this study, we reassess whether African lions suppress populations of cheetahs and African wild dogs and examine possible mechanisms for coexistence between these species. Using long‐term records from Serengeti National Park, we tested 30 years of population data for evidence of mesopredator suppression, and we examined six years of concurrent radio‐telemetry data for evidence of large‐scale spatial displacement. The Serengeti lion population nearly tripled between 1966 and 1998; during this time, wild dogs declined but cheetah numbers remained largely unchanged. Prior to their local extinction, wild dogs primarily occupied low lion density areas and apparently abandoned the long‐term study area as the lion population ‘saturated’ the region. In contrast, cheetahs mostly utilized areas of high lion density, and the stability of the cheetah population indicates that neither high levels of lion‐inflicted mortality nor behavioural avoidance inflict sufficient demographic consequences to translate into population‐level effects. Population data from fenced reserves in southern Africa revealed a similar contrast between wild dogs and cheetahs in their ability to coexist with lions. These findings demonstrate differential responses of subordinate species within the same guild and challenge a widespread perception that lions undermine cheetah conservation efforts. Paired with several recent studies that document fine‐scale lion‐avoidance by cheetahs, this study further highlights fine‐scale spatial avoidance as a possible mechanism for mitigating mesopredator suppression.

Predators and prey engage in games where each player must counter the moves of the other, and these games include multiple phases operating at different spatiotemporal scales. Recent work has highlighted potential issues related to scale-sensitive inferences in predator–prey interactions, and there is growing appreciation that these may exhibit pronounced but predictable dynamics. Motivated by previous assertions about effects arising from foraging games between white-tailed deer and canid predators (coyotes and wolves), we used a large and year-round network of trail cameras to characterize deer and predator foraging games, with a particular focus on clarifying its temporal scale and seasonal variation. Linear features were strongly associated with predator detection rates, suggesting these play a central role in canid foraging tactics by expediting movement. Consistent with expectations for prey contending with highly mobile predators, deer responses were more sensitive to proximal risk metrics at finer spatiotemporal scales, suggesting that coarser but more commonly used scales of analysis may miss useful insights into prey risk-response. Time allocation appears to be a key tactic for deer risk management and was more strongly moderated by factors associated with forage or evasion heterogeneity (forest cover, snow and plant phenology) than factors associated with the likelihood of predator encounter (linear features). Trade-offs between food and safety appeared to vary as much seasonally as spatially, with snow and vegetation phenology giving rise to a “phenology of fear.” Deer appear free to counter predators during milder times of year, but a combination of poor foraging state, reduced forage availability, greater movements costs, and reproductive state dampen responsiveness during winter. Pronounced intra-annual variation in predator–prey interactions may be common in seasonal environments.

Predators can exert strong direct and indirect effects on ecological communities by intimidating their prey. The nature of predation risk effects is often context dependent, but in some ecosystems these contingencies are often overlooked. Risk effects are often not uniform across landscapes or among species. Indeed, they can vary widely across gradients of habitat complexity and with different prey escape tactics. These context dependencies may be especially important for ecosystems such as coral reefs that vary widely in habitat complexity and have species‐rich predator and prey communities. With field experiments using predator decoys of the black grouper (Mycteroperca bonaci), we investigated how reef complexity interacts with predation risk to affect the foraging behaviour and herbivory rates of large herbivorous fishes (e.g. parrotfishes and surgeonfishes) across four coral reefs in the Florida Keys (USA). In both high and low complexity areas of the reef, we measured how herbivory changed with increasing distance from the predator decoy to examine how herbivorous fishes reconcile the conflicting demands of avoiding predation vs. foraging within a reefscape context. We show that with increasing risk, herbivorous fishes consumed dramatically less food (ca. 90%) but fed at a faster rate when they did feed (ca. 26%). Furthermore, we show that fishes foraging closest to the predator decoy were 40% smaller than those that foraged at further distances. Thus, smaller individuals showed muted response to predation risk compared to their larger counterparts, potentially due to their decreased risk to predation or lower reproductive value (i.e. the asset protection principle). Habitat heterogeneity mediated risk effects differently for different species of herbivores, with predation risk more strongly suppressing herbivore feeding in more complex areas and for individuals at higher risk of predation. Predators appear to create a reefscape of fear that changes the size structure of herbivores towards smaller individuals, increases individual feeding rates, but suppresses overall amounts of primary producers consumed, potentially altering patterns of herbivory, an ecosystem process critical for healthy coral reefs.

Running the gauntlet of predators consumes critical time and energy resources, as all species are vulnerable to one or, typically, more predators at some life stage. Prey employ a vast array of mechanisms to avoid predation, and predators, likewise, come in a bewildering variety. Thus, defensive adaptations are rarely one size fits all. Considerable work has addressed multi-predator consumptive effects, but we now know that non-consumptive effects of predators can dramatically impact individuals, (meta)populations, and (meta)communities. However, little is known regarding the community-wide dynamics of non-consumptive effects generated by multiple predators. Predator avoidance by choosing a patch that is free of a particular predator or predators can be the most effective strategy if conditions at colonization are a reliable predictor of absence, which is often true for fish in freshwater systems. We experimentally manipulated composition of the predator assemblage in aquatic mesocosms in a substitutive design, with zero, one, two, or three caged predatory fish species (one benthic, one pelagic, and one surface fish) at constant density and biomass, and assayed responses of naturally colonizing aquatic insects. We addressed three related questions; first, how do members of a diverse assemblage of colonizing aquatic insects respond to this variation in species and species combinations, second, do individual species (and higher taxa), respond differently to single vs. multiple predator species (species richness), and third how do any responses to fish species and species combinations, and effects on species richness, translate into community-wide changes in the composition of colonists. Prey had varied responses to specific predators or combinations of predators, resulting in distinct community composition across treatments and higher β-diversity with predators. Prey showed emergent multi-predator effects, where certain species only responded to predator species combinations, but not to any individual predator, and stronger effects of multiple predator vs. single-predator treatments, despite strong responses to individual predators in many taxa. Habitat selection effects can range from the individual to the metacommunity, and the dynamics of habitat selection in response to predators is a complex function of predator identity, density, richness, species composition, and patch spatial context.

Outcomes of management efforts to recover or restore populations of harvested species can be highly dependent on environmental and community context. Predator–prey interactions can alter recovery trajectories, and the timing of management actions within multi-trophic level harvest scenarios may influence the dynamics of recovery and lead to management trade-offs. Recent work using a generalist predator–prey model suggests that management promoting synchronized recovery of predators and prey leads to faster and less variable recovery trajectories than sequential recovery (predator or prey first). However, more complex communities may require different management actions to minimize recovery time and variability. Here, we use a tri-trophic level rocky reef community dynamics model with size-structure and fisheries at multiple trophic levels to investigate the importance of three ecological processes to recovery of fished communities: (1) size-structured predation, (2) non-consumptive effects of predators on prey behavior, and (3) varying levels of recruitment. We also test the effects of initiating recovery from community states associated with varying degrees of fishery-induced degradation and develop a simulation in which the basal resource (kelp) is harvested. In this system, a predator-first closure generally leads to the least volatile and quickest recovery, whether from a kelp forest, urchin barren, or intermediate community state. The benefits gained by selecting this strategy are magnified when recovering from the degraded community, the urchin barren, because initial conditions in the degraded state lead to lengthy recovery times. However, the shape of the size-structured predation relationship can strongly affect recovery volatility, where the differences between alternate management strategies are negated with size-independent predation. External recruitment reduces return times by bolstering the predatory lobster population. These results show that in a tightly linked tri-trophic level food web with top-down control, a predator-first fishery closure can be the most effective strategy to reduce volatility and shorten recovery, particularly when the system is starting from the degraded community state. Given the ubiquity of top predator loss across many ecosystems, we highlight the value of incorporating insights from community ecology into ecosystem management.