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The very presence of predators can strongly influence flexible prey traits such as behavior, morphology, life history, and physiology. In a rapidly growing body of literature representing diverse ecological systems, these trait (or “fear”) responses have been shown to influence prey fitness components and density, and to have indirect effects on other species. However, this broad and exciting literature is burdened with inconsistent terminology that is likely hindering the development of inclusive frameworks and general advances in ecology. We examine the diverse terminology used in the literature, and discuss pros and cons of the many terms used. Common problems include the same term being used for different processes, and many different terms being used for the same process. To mitigate terminological barriers, we developed a conceptual framework that explicitly distinguishes the multiple predation-risk effects studied. These multiple effects, along with suggested standardized terminology, are risk-induced trait responses (i.e., effects on prey traits), interaction modifications (i.e., effects on prey–other-species interactions), nonconsumptive effects (i.e., effects on the fitness and density of the prey), and trait-mediated indirect effects (i.e., the effects on the fitness and density of other species). We apply the framework to three well studied systems to highlight how it can illuminate commonalities and differences among study systems. By clarifying and elucidating conceptually similar processes, the framework and standardized terminology can facilitate communication of insights and methodologies across systems and foster cross-disciplinary perspectives

We tested the effect of the native container-dwelling predator Toxorhynchites rutilus on the codominant container-dwelling mosquitoes: native Aedes triseriatus and invasive Aedes japonicus. We established two predator treatments (predator, no predator) by removing T. rutilus from all containers, and stocking T. rutilus larvae (1/3.5 liters) in the predator treatment. Removal and stocking was repeated every 3 d and established significantly different predator abundances in both large and small containers. Repeated-measures analysis of variance (ANOVA) on standard samples showed larvae+pupae/liter of A. japonicus was greater without versus with predation, and this difference increased across samples. In contrast, repeated-measures ANOVA showed larvae+pupae/liter of A. triseriatus was statistically indistinguishable for predation treatments and was greater in small versus large containers.Thus, predation reduced invasive A. japonicus while having no detectable effect on A. triseriatus larvae and pupae. A final destructive census of pupae showed that predation reduced pupae/liter of both species, but this effect was greater and more consistent across container sizes for A. japonicus. Predator effects on abundances were not products of the nonlethal effect of predator avoidance by ovipositing females, as T. rutilus presence did not lead to reduced egg inputs by either Aedes, nor by Aedes spp. as a group. Effects of predation thus are best explained by differential success of developing larvae due to the greater lethal effect of T. rutilus on A. japonicus than on A. triseriatus.Thus, this system is consistent with the hypothesis that native predators can limit success and potential impacts of invasive mosquitoes.

Most ungulate populations are regulated by hunting, and harvest rate is regulated through quotas and hunting season duration. Hunting is well known to affect behaviour of ungulates, but how annual variation in quotas and hunting season duration affects individual behaviour remains uncertain. Harvest rates reach their highest level when aiming to limit outbreaks of infectious diseases. In Norway, marked changes to hunting regulations of wild reindeer Rangifer tarandus were implemented as part of chronic wasting disease (CWD) management (2019‒2022) in the Hardangervidda population, Norway. Here, we quantify the movements (step length) of 135 female GPS-marked alpine reindeer during years (2001‒2022) with largely variable levels of quotas, harvest rates (0‒33%) and hunting season duration (31‒58 days). A strong predictor of movement was season with a decline from mid- August to early October, and longer and more variable movement distances during daytime compared to the night. Reindeer moved more in years with higher harvest rates, but mainly in late September and not in the beginning of the hunting season. Movements were higher during weekends, only in early September. Our study shows that responses to hunting of an alpine ungulate living in open habitat appear to differ from those of forest living ungulates. The high level of sociality of reindeer may further increase disturbance effects relative to less social and forest-dwelling species. Hunting season occurs at a critical time before the winter season in northern Europe, and further studies are needed to investigate whether increased movements during hunting affect body condition

Predator–prey relationships are not limited to predation. A predator may release chemical cues that can directly affect the physiology and behavior of prey by fear and intimidation. We aimed to test the effects of predator (dragonfly larvae) body size on larval development time and mortality, and on survival time, wing asymmetry, and adult size, of Aedes aegypti. We simulated predation risk by dragonflies (Odonata, Libelulidae) on Ae. aegypti using a microcosm approach. Microcosms consisted of 40 larvae of Ae. aegypti under the indirect effect of predators of different body sizes (four treatments of different body size classes, n = 5 per treatment) and predator absence (control treatment). Predator presence was found to have a strong effect on mosquito life history traits, whereas predator size had a weak effect. Predator presence increased prey larval mortality and wings size asymmetry, and accelerated development time, while it reduced adult body size and survival time. The results support predator presence as being responsible for changes in traits of Ae. aegypti during larval and adult stages, but with a weak effect of predator size.

While the impact of predator-induced stress on prey has received considerable attention, there has been far less research into the effect of competitors. Cues from aggressive competitors should be particularly likely to evoke behavioral and/or physiological responses, since they may be indicative of both direct (interference) and indirect (exploitative) threats. The danger posed by such competitors, and the \"fear\" they evoke, should be reduced at lower competitor densities and by the presence of individual conspecifics specialized for defense. We assessed how Reticulitermes flavipes termite workers and soldiers were affected by cues from conspecific nestmates, conspecific non-nestmates, and the heterospecific competitor R. virginicus. Competitor cues altered flavipes worker and soldier behavior, decreasing worker growth and increasing their mortality. The presence of flavipes soldiers largely ameliorated these negative impacts: adding even a single soldier (5% of flavipes individuals) decreased worker mortality by 50–80%. Although worker mortality increased with competitor density, increased soldier densities did not increase the benefit to workers. The small number of soldiers required to substantially alter cue-mediated interactions suggests that this caste, in addition to providing direct defense, also occupies a \"keystone role\" by providing homeostatic feedback to workers functioning in stressful environments.

Experiments have shown that predation‐risk effects on prey fitness can be highly contingent on environmental conditions, suggesting a potential difficulty in generalizing risk effects on prey abundance in natural settings. Rather than study the influence of a particular controlled factor, we examine the problem with a novel approach. We examined the influence of risk effects in multiple experiments performed under similar study conditions. Any differences in the experiments would typically be deemed incidental, that is, they would not be given attention in methodology, nor be presented as factors affecting results or inferences. Therefore, any differences in the magnitude and direction of risk effects among experiments would indicate that risk effects on prey population abundance are strongly influenced by context in natural communities. The multiple experiments were conducted under similar conditions, objectives, measurables and implementation, and captured much of the complexity of natural systems (e.g., they were performed with diverse prey assemblages (≥ 11 taxa) over multiple prey generations). Our results highlight the potentially profound context dependence of risk effects: risk effects on the density of some zooplankton species varied between a significant negative effect in one experiment to a significant positive effect in another, whereas other species showed significant negative or positive effects in one experiment and no effect in another. We review mechanisms that could underlie risk effects having opposite effects on the same prey. Our findings illustrate that risk effects observed in one study may not hold, even for the same species in the same system. Experiments in simple systems have shown that predation‐risk effects on prey fitness can be highly contingent on environmental conditions, suggesting a potential difficulty in generalizing risk effects on prey abundance in natural settings. Here we show that the response of zooplankton communities, and even the density responses of component species, to the presence of fish predators can vary from positive, to no effect, to negative across experiments that have identical set ups. Our findings highlight the need to understand the mechanisms linking trait responses to fitness and, ultimately, to abundance of prey, to understand risk effects across studies and systems.

Prolonged physiological stress response may lead to an excessive production of reactive oxygen species (ROS) and ultimately to oxidative stress and severe fitness costs. We investigated whether natural variation in predation risk, induced by pygmy owls (Glaucidium passerinum), modifies the oxidative status of two free-living food-supplemented passerine bird species—the great tit (Parus major) and the willow tit (Poecile montanus)—in March 2012 and 2013. Predation risk significantly affected antioxidant enzyme activities of willow tits. Antioxidant enzyme activities (principal component factor 2 [PC2] representing glutathione-S-transferase and superoxide dismutase activities) were higher in high predation risk areas in 2013 than in low predation risk areas in the same year. Higher enzyme activities may suggest higher ROS production in birds living under high predation risk. In addition, antioxidant enzyme activities (PC2) were also higher in high predation risk areas in 2013 than in high predation risk areas in the previous year, 2012. This may represent variation in the risk represented by pygmy owls, which is probably inversely related to the natural fluctuations in the densities of their main prey, voles. In willow tits, PC1 (representing catalase, total glutathione, the ratio of reduced to oxidized glutathione, and protein carbonylation) was not affected by perceived predation risk, nor were antioxidant levels or enzyme activities in great tits. Higher enzyme activities observed in willow tits suggest that predator presence can modify the antioxidant status of avian prey, but the response also seem to be influenced by other environmental characteristics, like harsh winter conditions.

We conducted a study to determine the contribution of lethal and nonlethal effects to a predator's net effect on a prey's population growth rate in a natural setting. We focused on the effects of an invasive invertebrate predator, Bythotrephes longimanus, on zooplankton prey populations in Lakes Michigan and Erie. Field data taken at multiple dates and locations in both systems indicated that the prey species Daphnia mendotae, Daphnia retrocurva, and Bosmina longirostris inhabited deeper portions of the water column as Bythotrephes biomass increased, possibly as an avoidance response to predation. This induced migration reduces predation risk but also can reduce birth rate due to exposure to cooler temperatures. We estimated the nonlethal (i.e., resulting from reduced birth rate) and lethal (i.e., consumptive) effects of Bythotrephes on D. mendotae and Bosmina longirostris. These estimates used diel field survey data of the vertical gradient of zooplankton prey density, Bythotrephes density, light intensity, and temperature with growth and predation rate models derived from laboratory studies. Results indicate that nonlethal effects played a substantial role in the net effect of Bythotrephes on several prey population growth rates in the field, with nonlethal effects on the same order of magnitude as or greater (up to 10-fold) than lethal effects. Our results further indicate that invasive species can have strong nonlethal, behaviorally based effects, despite short evolutionary coexistence with prey species.

Defensive modifications in prey traits that reduce predation risk can also have negative effects on prey fitness. Such nonconsumptive effects (NCEs) of predators are common, often quite strong, and can even dominate the net effect of predators. We develop an intuitive graphical model to identify and explore the conditions promoting strong NCEs. The model illustrates two conditions necessary and sufficient for large NCEs: (1) trait change has a large cost, and (2) the benefit of reduced predation outweighs the costs, such as reduced growth rate. A corollary condition is that potential predation in the absence of trait change must be large. In fact, the sum total of the consumptive effects (CEs) and NCEs may be any value bounded by the magnitude of the predation rate in the absence of the trait change. The model further illustrates how, depending on the effect of increased trait change on resulting costs and benefits, any combination of strong and weak NCEs and CEs is possible. The model can also be used to examine how changes in environmental factors (e.g., refuge safety) or variation among predator—prey systems (e.g., different benefits of a prey trait change) affect NCEs. Results indicate that simple rules of thumb may not apply; factors that increase the cost of trait change or that increase the degree to which an animal changes a trait, can actually cause smaller (rather than larger) NCEs. We provide examples of how this graphical model can provide important insights for empirical studies from two natural systems. Implementation of this approach will improve our understanding of how and when NCEs are expected to dominate the total effect of predators. Further, application of the models will likely promote a better linkage between experimental and theoretical studies of NCEs, and foster synthesis across systems.

1. How group size affects predator attack and success rate, and so prey vulnerability, is important in determining the nonlethal consequences of predation risk on animal populations and communities. Theory predicts that both predator attack success rate and the dilution effect decline exponentially with group size and that selection generates optimal group sizes at a ‘risk threshold' above which antipredation benefits are outweighed by costs, such as those owing to higher attack rates. 2. We examined whether flock size risk thresholds for attack rate, success rate or dilution differed, and therefore whether the strength of selection for group size differed for these three factors, using a system of redshank Tringa totanus flocks being hunted by Eurasian sparrowhawks Accipiter nisus. We also asked which of the three thresholds, on their own or in combination, predicted the most commonly observed group size. 3. Mean redshank flock size increased with a very gradual quadratic function (i.e. approximately linearly) with population size, although at a rate half that possible; when population size was not limiting, individuals almost always avoided flocks of less than 30 and birds were frequently in flocks up to at least 80. Sparrowhawk attack rate showed a quadratic relationship with flock size and peaked at 55 redshanks. Sparrowhawk attack success rate, however, declined exponentially, becoming less steep at flock sizes of about 40 and remaining uniformly low (a 95% decrease) by 70. Combined with dilution, individual risk of death per attack decreased by 95% when group size reached 30 (20 for the dilution effect alone). 4. Redshanks most commonly formed group sizes that gained the maximum individual predation risk reduction. They also commonly formed group sizes far above any further substantial advantages from the dilution effect or from reducing attack rate, but that continued to reduce predation risk by lowering attack success rate. Individuals did not always form the largest groups possible which we suggest is because individual variation in risk-taking subdivides the population. This places a constraint on the ability of individuals to compensate for predation risk and will have a variety of important effects on animal populations.