Reversibility of predator‐induced plasticity and its effect at a life‐history switch point

In natural systems, organisms are frequently exposed to spatial and temporal variation in predation risk. Prey organisms are known to develop a wide array of plastic defences to avoid being eaten. If inducible plastic defences are costly, prey living under fluctuating predation risk should be strongly selected to develop reversible plastic traits and adjust their defences to the current predation risk. Here, we studied the induction and reversibility of antipredator defences in common frog Rana temporaria tadpoles when confronted with a temporal switch in predation risk by dragonfly larvae. We examined the behaviour and morphology of tadpoles in experimental treatments where predators were added or withdrawn at mid larval development, and compared these to treatments with constant absence or presence of predators. As previous studies have overlooked the effects that developing reversible anti‐predator responses could have later in life (e.g. at life history switch points), we also estimated the impact that changes in antipredator responses had on the timing of and size at metamorphosis. In the presence of predators, tadpoles reduced their activity and developed wider bodies, and shorter and wider tails. When predators were removed tadpoles switched their behaviour within one hour to match that found in the constant environments. The morphology matched that in the constant environments in one week after treatment reversal. All these responses were highly symmetrical. Short time lags and symmetrical responses for the induction/reversal of defences suggest that a strategy with fast switches between phenotypes could be favoured in order to maximise growth opportunities even at the potential cost of phenotypic mismatches. We found no costs of developing reversible responses to predators in terms of life‐history traits, but a general cost of the induction of the defences for all the individuals experiencing predation risk during some part of the larval development (delayed metamorphosis). More studies examining the reversibility of plastic defences, including other type of costs (e.g. physiological), are needed to better understand the adaptive value of these flexible strategies.     

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.     

Escape behaviour and ultimate causes of specific induced defences in an anuran tadpole

Induced defences, such as the predator avoidance morphologies in amphibians, result from spatial or temporal variability in predation risk. One important component of this variability should be the difference in hunting strategies between predators. However, little is known about how specific and effective induced defences are to different types of predators. We analysed the impact of both pursuing (fish, Gasterosteus aculeatus) and sit-and-wait (dragonfly, Aeshna cyanea) predators on tadpole (Rana dalmatina) morphology and performance (viz locomotive performance and growth rate). We also investigated the potential benefits of the predator-induced phenotype in the presence of fish predators. Both predators induced deeper tail fins in tadpoles exposed to threat of predation, and stickleback presence also induced longer tails and deeper tail muscles. Morphological and behavioural differences resulted in better escape ability of stickleback-induced tadpoles, leading to improved survival in the face of stickleback predation. These results clearly indicate that specific morphological responses to different types of predators have evolved in R. dalmatina. The specific morphologies suggest low correlations between the traits involved in the defence. Independence of traits allows prey species to fine-tune their response according to current predation risk, so that the benefit of the defence can be maximal.     

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.     

Predation risk causes oxidative damage in prey

While there is increasing interest in non-consumptive effects of predators on prey, physiological effects are understudied. While physiological stress responses play a crucial role in preparing escape responses, the increased metabolic rates and shunting of energy away from other body functions, including antioxidant defence, may generate costs in terms of increased oxidative stress. Here, we test whether predation risk increases oxidative damage in Enallagma cyathigerum damselfly larvae. Under predation risk, larvae showed higher lipid peroxidation, which was associated with lower levels of superoxide dismutase, a major antioxidant enzyme in insects, and higher superoxide anion concentrations, a potent reactive oxygen species. The mechanisms underlying oxidative damage are likely to be due to the shunting of energy away from antioxidant defence and to an increased metabolic rate, suggesting that the observed increased oxidative damage under predation risk may be widespread. Given the potentially severe fitness consequences of oxidative damage, this largely overlooked non-consumptive effect of predators may be contributing significantly to prey population dynamics.     

Feedback between size balance and consumption strongly affects the consequences of hatching phenology in size‐dependent predator–prey interactions

In many size‐dependent predator–prey systems, hatching phenology strongly affects predator–prey interaction outcomes. Early‐hatched predators can easily consume prey when they first interact because they encounter smaller prey. However, this process by itself may be insufficient to explain all predator–prey interaction outcomes over the whole interaction period because the predator–prey size balance changes dynamically throughout their ontogeny. We hypothesized that hatching phenology influences predator–prey interactions via a feedback mechanism between the predator–prey size balance and prey consumption by predators. We experimentally tested this hypothesis in an amphibian predator–prey model system. Frog tadpoles Rana pirica were exposed to a predatory salamander larva Hynobius retardatus that had hatched 5, 12, 19 or 26 days after the frog tadpoles hatched. We investigated how the salamander hatch timing affected the dynamics of prey mortality, size changes of both predator and prey, and their subsequent life history (larval period and size at metamorphosis). The predator–prey size balance favoured earlier hatched salamanders, which just after hatching could successfully consume more frog tadpoles than later hatched salamanders. The early‐hatched salamanders grew rapidly and their accelerated growth enabled them to maintain the predator‐superior size balance; thus, they continued to exert strong predation pressure on the frog tadpoles in the subsequent period. Furthermore, frog tadpoles exposed to the early‐hatched salamanders were larger at metamorphosis and had a longer larval period than other frog tadpoles. These results suggest that feedback between the predator‐superior size balance and prey consumption is a critical mechanism that strongly affects the impacts of early hatching of predators in the short‐term population dynamics and life history of the prey. Because consumption of large nutrient‐rich prey items supports the growth of predators, a similar feedback mechanism may be common and have strong impacts on phenological shifts in size‐dependent trophic relationships.     

Maternally derived chemical defences are an effective deterrent against some predators of poison frog tadpoles (Oophaga pumilio)

Parents defend their young in many ways, including provisioning chemical defences. Recent work in a poison frog system offers the first example of an animal that provisions its young with alkaloids after hatching or birth rather than before. But it is not yet known whether maternally derived alkaloids are an effective defence against offspring predators. We identified the predators of Oophaga pumilio tadpoles and conducted laboratory and field choice tests to determine whether predators are deterred by alkaloids in tadpoles. We found that snakes, spiders and beetle larvae are common predators of O. pumilio tadpoles. Snakes were not deterred by alkaloids in tadpoles. However, spiders were less likely to consume mother-fed O. pumilio tadpoles than either alkaloid-free tadpoles of the red-eyed treefrog, Agalychnis callidryas, or alkaloid-free O. pumilio tadpoles that had been hand-fed with A. callidryas eggs. Thus, maternally derived alkaloids reduce the risk of predation for tadpoles, but only against some predators.     

Having the guts to compete: how intestinal plasticity explains costs of inducible defences

Predators commonly induce phenotypic changes that make prey better at surviving predation at the cost of reduced growth. While we have a good understanding of how trait changes affect predation risk, we lack a mechanistic understanding of why predator‐induced phenotypes differ in growth. Using two mesocosm experiments, we combined phenotypic plasticity theory with predictions from optimal digestion theory to demonstrate that intra‐ and interspecific competition induced relatively long guts while predators induced relatively short guts. The longer guts induced by competition appear to be an adaptive response that allows more efficient digestion and more rapid growth whereas the shorter guts induced by predators appear to result from a tradeoff of building larger tails in predator environments at the cost of smaller bodies. By combining these two bodies of theory, we now have a much better understanding of the mechanisms that cause the phenotypic trade‐offs that select for inducible defences.     

Interactive effects of predation risk and conspecific density on the nutrient stoichiometry of prey

The mere presence of predators (i.e., predation risk) can alter consumer physiology by restricting food intake and inducing stress, which can ultimately affect prey‐mediated ecosystem processes such as nutrient cycling. However, many environmental factors, including conspecific density, can mediate the perception of risk by prey. Prey conspecific density has been defined as a fundamental feature that modulates perceived risk. In this study, we tested the effects of predation risk on prey nutrient stoichiometry (body and excretion). Using a constant predation risk, we also tested the effects of varying conspecific densities on prey responses to predation risk. To answer these questions, we conducted a mesocosm experiment using caged predators (Belostoma sp.), and small bullfrog tadpoles (Lithobates catesbeianus) as prey. We found that L. catesbeianus tadpoles adjust their body nutrient stoichiometry in response to predation risk, which is affected by conspecific density. We also found that the prey exhibited strong morphological responses to predation risk (i.e., an increase in tail muscle mass), which were positively correlated to body nitrogen content. Thus, we pose the notion that in risky situations, adaptive phenotypic responses rather than behavioral ones might partially explain why prey might have a higher nitrogen content under predation risk. In addition, the interactive roles of conspecific density and predation risk, which might result in reduced perceived risk and physiological restrictions in prey, also affected how prey stoichiometry responded to the fear of predation.     

Functional mechanisms of an inducible defence in tadpoles: morphology and behaviour influence mortality risk from predation

In many amphibian larvae a suite of morphological and behavioural characters varies together in an induced defence against predators, but it remains unclear which features are functionally related to defence. We independently manipulated behaviour and morphology in tadpoles of Hyla versicolor and assessed their consequences for swimming performance and predator escape. Data on burst swimming showed that tadpoles which accelerated rapidly were elongate, with shallow bodies and tails. Predator escape was measured by exposing tadpoles to predators (larval Anax dragonflies or larval Ambystoma salamanders) and recording time until death. Tadpoles were first reared for 30 days in ponds containing either caged Anax or no predators; individuals responded to predators by developing large brightly coloured tails and short bodies. We placed tadpoles of both morphological phenotypes into plastic tubs, and manipulated their behaviour using food and chemical cues from predators. Mortality risk experienced by the predator‐induced phenotype was about half that of the no‐predator phenotype, and risk increased with time spent swimming. An interaction between morphology and behaviour arose because increasing activity caused higher risk for tadpoles with deep tail fins but not shallow tail fins.     

Risk‐taking and the evolution of mechanisms for rapid escape from predators

Flight initiation distance (FID) is the distance at which an individual animal takes flight when approached by a human. This behavioural measure of risk‐taking reflects the risk of being captured by real predators, and it correlates with a range of life history traits, as expected if flight distance optimizes risk of predation. Given that FID provides information on risk of predation, we should expect that physiological and morphological mechanisms that facilitate flight and escape predict interspecific variation in flight distance. Haematocrit is a measure of packed red blood cell volume and as such indicates the oxygen transport ability and hence the flight muscle contracting reaction of an individual. Therefore, we predicted that species with short flight distances, that allow close proximity between a potential prey individual and a predator, would have high haematocrit. Furthermore, we predicted that species with large wing areas and hence relatively low costs of flight and species with large aspect ratios and hence high manoeuvrability would have evolved long flight speed. Consistent with these predictions, we found in a sample of 63 species of birds that species with long flight distances for their body size had low levels of haematocrit and large wing areas and aspect ratios. These findings provide evidence consistent with the evolution of risk‐taking behaviour being underpinned by physiological and morphological mechanisms that facilitate escape from predators and add to our understanding of predator–prey coevolution.     

Measuring responses to simulated predation threat using behavioral and physiological metrics: the role of aquatic vegetation

An organism's daily activities are affected by predation and predation risk that have behavioral and physiological costs, which translate into long-term population and community consequences. We tested the hypothesis that the perception of predation risk from sand seatrout, Cynoscion arenarius, affects the behavior, and immediate and intermediate physiological responses of longnose killifish, Fundulus majalis. We further hypothesized that prey responses change if prey are buffered by artificial submerged aquatic vegetation (SAV), a potential refuge from predators. Experiments were conducted to quantitatively estimate the behavior, plasma cortisol (PC) concentration, mass-specific oxygen consumption, and short-term growth rate changes relative to full, partial, and no visual exposure to the predator. The partial visual exposure treatment involved the use of artificial SAV. Our results indicate that there are significant behavior and physiological responses of longnose killifish to predation threat. Longnose killifish in the full visual and partial exposure treatments displayed different behaviors than the control treatments by shifting towards the rear of the aquaria. In addition, longnose killifish in the full visual exposure compared to the partial exposure and the control treatments responded by exhibiting an elevation of PC and mass-specific oxygen consumption rate, and through decreased short-term growth. These responses were less intense in the partial exposure, when artificial SAV was present. The significance of this study is that it examines a suite of responses from cellular to the whole-organism level as they are affected by predation threat and modified by the presence or absence of artificial SAV.     

Costs and limits of dosage response to predation risk: to what extent can tadpoles invest in anti-predator morphology?

Inducible defences have long been considered as a polyphenism opposing defended and undefended morphs. However, in nature, preys are exposed to various levels of predation risk and scale their investment in defence to actual predation risk. Still, among the traits that are involved in the defence, some are specific to one predator type while others act as a more generalised defence. The existence of defence costs could prevent an individual investing in all these traits simultaneously. In this study, we investigate the impact of an increasing level of predator density (stickleback, Gasterosteus aculeatus) on the expression of morphological inducible defences in tadpoles of Rana dalmatina. In this species, investment in tail length and tail muscle is a stickleback-specific response while increased tail fin depth is a more general defence. As expected, we found a relationship between investment in defence and level of risk through the responses of tail fin depth and tail length. We also found an exponential increase of defence cost, notably expressed by convex decrease of growth and developmental rates. We found a relative independence of investment in the different traits that compose the defence, revealing a high potential for fine tuning the expression of defended phenotypes with respect to local ecological conditions.     

Predation risk shapes thermal physiology of a predaceous damselfly

Predation risk has strong effects on organismal physiology that can cascade to impact ecosystem structure and function. Physiological processes in general are sensitive to temperature. Thus, the temperature at which predators and prey interact may shape physiological response to predation risk. We measured and evaluated how temperature and predation risk affected growth rates of predaceous damselfly nymphs (Enallagma vesperum, Odonata: Coenagrionidae). First, we conducted growth trials at five temperatures crossed with two levels of predation risk (fish predator present versus absent) and measured growth rates, consumption rates, assimilation efficiencies, and production efficiencies of 107 individual damselflies. Second, we used a model to evaluate if and how component physiological responses to predation risk affected growth rates across temperatures. In the absence of mortality threat, growth rates of damselflies increased with warming until about 23.5 °C and then began to decline, a typical unimodal response to changes in temperature. Under predation risk, growth rates were lower and the shape of the thermal response was less apparent. Higher metabolic and survival costs induced by predation risk were only partially offset by changes in consumption rates and assimilation efficiencies and the magnitude of non-consumptive effects varied as a function of temperature. Furthermore, we documented that thermal physiology was mediated by predation risk, a known driver of organismal physiology that occurs in the context of species interactions. A general understanding of climatic impacts on ectothermic populations requires consideration of the community context of thermal physiology, including non-consumptive effects of predators.     

Chemical cues from multiple predator-prey interactions induce changes in behavior and growth of anuran larvae

Chemical signals are used as information by prey to assess predation risk in their environment. To evaluate the effects of multiple predators on prey growth, mediated by a change in prey activity, I exposed small and large bullfrog (Rana catesbeiana) larvae (tadpoles) to chemical cues from different combinations of bluegill sunfish (Lepomis macrochirus) and larval dragonfly (Anax junius) predators. Water was regularly transferred from predation trials (outdoor experiment) to aquaria (indoor experiment) in which activity and growth of tadpoles was measured. The highest predation mortality of small bullfrog larvae in the outdoor experiment was due to Anax, and it was slightly lower in the presence of both predators, probably resulting from interactions between predators. There was almost no mortality of prey with bluegill. The activity and growth of small bullfrog larvae was highest in the absence of predators and lowest in the presence of Anax. In the presence of bluegill only, or with both predators, the activity and growth of small bullfrog tadpoles was intermediate. Predators did not affect large tadpole activity and growth. Regressing mortality of small bullfrog tadpoles against activity and growth of bullfrog tadpoles revealed a significant effect for small bullfrog larvae but a non-significant effect for large bullfrog larvae. This shows that the response of bullfrog tadpoles to predators is related to their own body size. The experiment demonstrates that chemical cues are released both as predator odor and as alarm substances and both have the potential to strongly alter the activity and growth of prey. Different mechanisms by which chemical cues may be transmitted to species interactions in the food web are discussed. Received: 28 June 1999 / Accepted: 15 November 1999     

Density is more important than predation risk for predicting growth and developmental outcomes in tadpoles of spotted tree frog,Litoria spenceri (Dubois 1984)

The diverse benefits of group living include protection against predators through dilution effects and greater group vigilance. However, intraspecific aggregation can decrease developmental rates and survival in prey species. We investigated the impact on tadpole development and behaviour of the interaction between population density and predation risk. Spotted tree frog (Litoria spenceri: Hylidae, Dubois 1984) tadpoles were kept at one of three different densities (two tadpoles per litre, five tadpoles per litre or 10 tadpoles per litre) until metamorphosis in the presence or absence of predatory cues. We aimed to determine the influence of population density, predation and the interaction of both factors in determining growth rates in tadpoles. Tadpoles were measured weekly to assess growth and development and filmed to quantify differences in activity and feeding frequency between groups. Generally, tadpoles housed without predators had longer developmental periods when housed with a predator, but there was no effect on tail length or total length. There was no effect of either predation cues or density on percentage of individuals feeding or moving. Although the effects of the presence of predators alone may appear to be less than the effects of the presence of competitors, the prioritisation of competitiveness over predator avoidance may increase vulnerability of tadpoles to the lethal threat of predators. This is particularly important in species such as L. spenceri, which is at risk from introduced fish predators.     

Assessing the risk effects of native predators on the exotic American bullfrog (Lithobates catesbeianus) and their indirect consequences to ecosystem function

Understanding the factors and mechanisms that affect the impacts of invasive species in invaded environments has been widely debated among researchers. However, few studies about invasive species have explored the effects of predation risks by native predators on exotic prey. Herein, the traditional invasive predator-native prey framework was reversed. We tested if tadpoles, of the worldwide invasive American Bullfrog Lithobates catesbeianus, were affected by the predation risk imposed by native predators. We used two different species of belostomatid predators and tested whether and how predation-induced phenotypic plasticity on L. catesbeianus reverberated in morphological, physiological, and ecosystem-level processes. Individuals of L. catesbeianus modified their morphological (tail muscle width), behavioral (activity and foraging), and physiological (growth and growth efficiency) traits in the presence of predation risk. Based on the observed morphological changes, our results suggest that prey may recognize predator-specific cues. In addition, we observed that L. catesbeianus' responses to predation risk can affect ecosystem-level properties, by inducing trophic cascades and reducing animal-mediated nutrient recycling rates. In summary, our study supports that exotic prey species who are subjected to native predators may display anti-predator responses, with implications for their development, as well as possible ecosystem-level effects.     

Risk of predation and behavioural response in three anuran species: influence of tadpole size and predator type

Many species alter their activity, microhabitat use, morphology and life history in response to predators. Predation risk is related to predator size and palatability of prey among others factors. We analyzed the predation risk of three species of tadpoles that occur in norwestern Patagonia, Argentina: Pleurodema thaul, Pleurodema bufoninum and Rhinella spinulosa. We sampled aquatic insect predators in 18 ponds to determine predator–tadpole assemblage in the study area. In laboratory conditions, we analysed the predation rate imposed by each predator on each tadpole species at different tadpole sizes. Finally, we tested whether tadpoles alter their activity in the presence of chemical and visual cues from predators. Small P. thaul and P. bufoninum tadpoles were the most vulnerable prey species, while small R. spinulosa tadpoles were only consumed by water bugs. Dragonflies and water bugs were the most dangerous tadpole predators. Small P. thaul tadpoles reduced their activity when they were exposed to all predators, while large tadpoles only reduced the activity in the presence of large predators (dragonfly larvae and water bugs). Small P. bufoninum tadpoles reduced the activity when they were exposed to beetle larvae and dragonfly larvae, while large tadpoles only reduced activity when they were exposed to larger predators (water bugs and dragonfly larvae). R. spinulosa tadpoles were the less sensitive to presence of predators, only larger tadpoles responded significantly to dragonfly larvae by reducing their activity. We conclude that behavioural responses of these anuran species were predator-specific and related to the risk imposed by each predator.     

Manipulating Individual Decisions and Environmental Conditions Reveal Individual Quality in Decision-Making and Non-Lethal Costs of Predation Risk

Habitat selection is a crucial decision for any organism. Selecting a high quality site will positively impact survival and reproductive output. Predation risk is an important component of habitat quality that is known to impact reproductive success and individual condition. However, separating the breeding consequences of decision-making of wild animals from individual quality is difficult. Individuals face reproductive decisions that often vary with quality such that low quality individuals invest less. This reduced reproductive performance could appear a cost of increased risk but may simply reflect lower quality. Thus, teasing apart the effects of individual quality and the effect of predation risk is vital to understand the physiological and reproductive costs of predation risk alone on breeding animals. In this study we alter the actual territory location decisions of pied flycatchers by moving active nests relative to breeding sparrowhawks, the main predators of adult flycatchers. We experimentally measure the non-lethal effects of predation on adults and offspring while controlling for effects of parental quality, individual territory choice and initiation of breeding. We found that chicks from high predation risk nests (<50 m of hawk) were significantly smaller than chicks from low risk nests (>200 m from hawk). However, in contrast to correlative results, females in manipulated high risk nests did not suffer decreased body condition or increased stress response (HSP60 and HSP70). Our results suggest that territory location decisions relative to breeding avian predators cause spatial gradients in individual quality. Small adjustments in territory location decisions have crucial consequences and our results confirm non-lethal costs of predation risk that were expressed in terms of smaller offspring produced. However, females did not show costs in physiological condition which suggests that part of the costs incurred by adults exposed to predation risk are quality determined.     

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.