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Defensive traits are hypothesized to benefit prey by reducing predation risk from a focal predator but come at a cost to the fitness of the prey. Variation in the expression of defensive traits is seen among individuals within the same population, and in the same individual in response to changes in the environment (i.e., phenotypically plastic responses). It is the relative magnitude of the cost and benefit of the defensive trait that underlies the defensive trait expression and its consequences to the community. However, whereas the cost has received much attention in ecological research, the benefit is seldom examined. Even in a defensive trait as extensively studied as vigilance, there are few studies of the purported benefit of the behavior, namely that vigilance enhances survival. We examined whether prey vigilance increased survival and quantified that benefit in a natural system, with white‐tailed deer (Odocoileus virginianus) experiencing unmanipulated levels of predation risk from Florida panther (Puma concolor coryi). Deer that spent more time vigilant (as measured by head position using camera trap data) had a higher probability of survival. Indeed, an individual deer that was vigilant 75% of the time was more than three times as likely to be killed by panthers over the course of a year than a deer that was vigilant 95% of the time. Our results therefore show that within‐population variation in the expression of a defensive trait has profound consequences for the benefit it confers. Our results provide empirical evidence supporting a long‐held but seldom‐tested hypothesis, that vigilance is a behavior that reduces the probability of predation and quantifies the benefit of this defensive trait. Our work furthers an understanding of the net effects of a trait on prey fitness and predator–prey interactions, within‐population variation in traits, and predation risk effects. [ABSTRACT FROM AUTHOR]

Human activities catalyse risk avoidance behaviours in wildlife across taxa and systems. However, the broader ecological significance of human‐induced risk perception remains unclear, with a limited understanding of how phenotypic responses scale up to affect population or community dynamics. We present a framework informed by predator–prey ecology to predict the occurrence of non‐consumptive effects (NCE) and trait‐mediated indirect effects (TMIE) of anthropogenic disturbances. We report evidence from a comprehensive review of the different types of human‐induced behavioural and physiological phenotypic changes and their influence on vital rates and population parameters in wildlife. Evidence for human‐induced NCEs and TMIEs is mixed, with half of published studies finding a relationship between human activities, phenotypic change and population outcomes. The net effects of anthropogenic NCEs and TMIEs depend on the mismatch between the phenotypic response and the lethality of human activity. However, strong research biases in taxa, systems, human disturbance types and demographic measures prevent unified inference about the prevalence of population responses to human activities. Coexistence with and conservation of wildlife requires additional research linking human‐induced phenotypic change to population and community outcomes. [ABSTRACT FROM AUTHOR]

Predators can directly affect prey populations both through predation (consumption of prey) and risk‐induced trait responses (RITRs) that reduce predation risk but are often associated with a fitness cost. Thousands of studies make clear that RITRs (also termed anti‐predator or defensive traits) including changes in behavior, morphology, and life history, are employed by numerous taxa across diverse ecological systems, and there is large variation in their magnitude. A natural goal is to elucidate the species and circumstances for which and to what magnitude RITRs are expected. A candidate hypothesis is that prey species that experience higher mortality from a predator will exhibit a higher RITR. This hypothesis is an intuitive extension of the fact that invulnerable animals are not expected to exhibit an RITR, while vulnerable species are. We present an example that clarifies why this relationship is not always expected and when it is expected. Other factors may influence the level of the RITR leading to the possibility that a positive relationship is not expected. We elucidate this problem using a mesocosm experiment with a fish–cladoceran system in which there is large variability in the predation rate on different cladoceran species. Results not only did not show a positive relationship, but rather a negative trend between predation rate and the RITR. In fact, highly preyed upon taxa did not respond, while the least preyed upon taxa had the largest responses. These results clarify how the level of predation risk interacts with many factors to determine the RITR of prey. [ABSTRACT FROM AUTHOR]

We evaluated predation by the invasive invertebrate planktivore Bythotrephes longimanus on a Lake Michigan prey assemblage as a function of light intensity. Daphnia mendotae was the only prey type significantly reduced and light intensity strongly affected this reduction. Specifically, Bythotrephes consumption of D. mendotae was not detected under low light intensity (100 [micro]mol-[m.sup.-2.][s.sup-1]) at which point Bythotrephes ingestion rate was 2.3 Daphnia per hour. These results indicate that Bythotrephes predation is more sensitive to light than previously thought, a discrepancy that can be explained after considering the ability of D. mendotae to detect Bythotrephes' hydromechanic disturbance. The observed effect of light intensity on Bythotrephes predation is more like that of planktivorous fish than that of other previously studied invertebrate planktivores. Our findings elucidate the role that Bythotrephes plays in the food web and provide a novel explanation for its tendency to invade lakes of high water clarity. The importance of light-dependent predation found here may extend to other visually oriented predatory cladocerans. Nous evaluons la predation par l'invertebre envahissant planctonophage Bythotrephes longimanus sur un ensemble de proies du lac Michigan en fonction de l'intensite lumineuse. Daphnia mendotae est le seul type de proie a subir une reduction significative et l'intensite lumineuse affecte fortement cette reduction. En particulier, on ne decele pas de consommation de D. mendotae par Bythotrephes aux faibles intensites lumineuses (100 mmol.[m.sup.-2.][s.sup-1]) et a ce point, le taux d'ingestion de Bythotrephes est de 2,3 Daphnia par heure. Ces resultats indiquent que la predation de Bythotrephes est plus sensible a la lumiere qu'on ne le croyait anterieurement, un ecart qui peut s'expliquer si l'on tient compte de la capacite de D. mendotae a detecter la perturbation hydraulique causee par Bythotrephes. L'effet de l'intensite lumineuse observe sur la predation chez Bythotrephes ressemble plus a celui qui se produit chez les poissons planctonophages que chez les autres invertebres planctonophages etudies. Nos observations mettent en lumiere le role joue par Bythotrephes dans le reseau alimentaire et apportent une explication inedite a la tendance qu'a Bythotrephes a envahir les lacs a eau tres claire. L'importance de la predation dependante de la lumiere s'applique peut-etre aussi a d'autres cladoce res predateurs a orientation visuelle. [Traduit par la Redaction], Introduction Light can strongly influence the composition and dynamics of communities in aquatic systems (Interlandi and Kilham 2001; Wissel et al. 2003; Sornes and Aksnes 2006). One mechanism underlying this [...]

A narrative in ecology is that prey modify traits to reduce predation risk, and the trait modification has costs large enough to cause ensuing demographic, trophic and ecosystem consequences, with implications for conservation, management and agriculture. But ecology has a long history of emphasising that quantifying the importance of an ecological process ultimately requires evidence linking a process to unmanipulated field patterns. We suspected that such process‐linked‐to‐pattern (PLP) studies were poorly represented in the predation risk literature, which conflicts with the confidence often given to the importance of risk effects. We reviewed 29 years of the ecological literature which revealed that there are well over 4000 articles on risk effects. Of those, 349 studies examined risk effects on prey fitness measures or abundance (i.e., non‐consumptive effects) of which only 26 were PLP studies, while 275 studies examined effects on other interacting species (i.e., trait‐mediated indirect effects) of which only 35 were PLP studies. PLP studies were narrowly focused taxonomically and included only three that examined unmanipulated patterns of prey abundance. Before concluding a widespread and influential role of predation‐risk effects, more attention must be given to linking the process of risk effects to unmanipulated patterns observed across diverse ecosystems. [ABSTRACT FROM AUTHOR]

In ecological meta‐analyses, nonindependence among observed effect sizes from the same source paper is common. If not accounted for, nonindependence can seriously undermine inferences. We compared the performance of four meta‐analysis methods that attempt to address such nonindependence and the standard random‐effect model that ignores nonindependence. We simulated data with various types of within‐paper nonindependence, and assessed the standard deviation of the estimated mean effect size and Type I error rate of each method. Although all four methods performed substantially better than the standard random‐effects model that assumes independence, there were differences in performance among the methods. A two‐step method that first summarizes the multiple observed effect sizes per paper using a weighted mean and then analyzes the reduced data in a standard random‐effects model, and a robust variance estimation method performed consistently well. A hierarchical model with both random paper and study effects gave precise estimates but had a higher Type I error rates, possibly reflecting limitations of currently available meta‐analysis software. Overall, we advocate the use of the two‐step method with a weighted paper mean and the robust variance estimation method as reliable ways to handle within‐paper nonindependence in ecological meta‐analyses. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

There is a large and growing interest in non‐consumptive effects (NCEs) of predators. Diverse and extensive evidence shows that predation risk directly influences prey traits, such as behaviour, morphology and physiology, which in turn, may cause a reduction in prey fitness components (i.e. growth rate, survival and reproduction). An intuitive expectation is that NCEs that reduce prey fitness will extend to alter population growth rate and therefore population size.However, our intensive literature search yielded only 10 studies that examined how predator‐induced changes in prey traits translate to changes in prey population size. Further, the scant evidence for risk‐induced changes on prey population size have been generated from studies that were performed in very controlled systems (mesocosm and laboratory), which do not have the complexity and feedbacks of natural settings. Thus, although likely that predation risk alone can alter prey population size, there is little direct empirical evidence that demonstrates that it does. There are also clear reasons that risk effects on population size may be much smaller than the responses on phenotype and fitness components that are typically measured, magnifying the need to show, rather than infer, effects on population size.Herein we break down the process of how predation risk influences prey population size into a chain of events (predation risk affects prey traits, which affect prey fitness components and population growth rate, which affect prey population size), and highlight the complexity of each transition. We illustrate how the outcomes of these transitions are not straightforward, and how environmental context strongly dictates the direction and magnitude of effects. Indeed, the high variance in prey responses is reflected in the variance of results reported in the few studies that have empirically quantified risk effects on population size. It is therefore a major challenge to predict population effects given the complexity of how environmental context interacts with predation risk and prey responses.We highlight the critical need to appreciate risk effects at each level in the chain of events, and that changes at one level cannot be assumed to translate into changes in the next because of the interplay between risk, prey responses, and the environment. The gaps in knowledge we illuminate underscore the need for more evidence to substantiate the claim that predation risk effects extend to prey population size. The lacunae we identify should inspire future studies on the impact of predation risk on population‐level responses in free‐living animals. [ABSTRACT FROM AUTHOR]

Predators can strongly affect prey communities, but their influence may be difficult to distinguish from bottom‐up and other environmental effects. The problem of assessing predator impact is especially difficult in large systems that do not allow for comparisons across multiple units (e.g., small lakes) that have varying predator density. For instance, the invasion of the predatory zooplankter, Bythotrephes longimanus, into the Laurentian Great Lakes contributed to the nearly complete disappearance of several zooplankton species, but current effects on extant zooplankton are not well understood. We used generalized additive models (GAMs) applied to long‐term data time series (1994–2012) to examine B. longimanus effects on zooplankton species in Lake Michigan. Because B. longimanus abundance varied over time, our approach allowed assessment of predator effects from field data while accounting for other factors, including food resources, temperature, and seasonality. Results suggest that B. longimanus substantially reduces some zooplankton population growth rates, with the largest effects on species that B. longimanus affected more strongly in experiments. For example, at maximum B. longimanus abundance, Daphnia mendotae, Bosmina longirostris, and Diacyclops thomasi population growth rates were estimated to be reduced by 17%, 30%, and 21%, respectively, compared to no effect on calanoid copepods. Results further indicated positive temperature effects on population growth that differed by species. Our study thus provides field‐based evidence for ongoing impacts of invasive species and temperature on zooplankton production and composition, with potential consequences for planktivorous fish, and exemplifies how GAMs can be used to determine predator effects from time series data. [ABSTRACT FROM AUTHOR]

Predators can induce changes in prey phenotype that reduce predation risk but also incur costs (so-called 'non-consumptive effects' or NCEs)., Few studies, however, have examined NCEs over time scales sufficient to link individual- and demographic-level processes to changes in prey abundance., Using a laboratory experiment, we investigated how NCEs of the invasive planktivorous cladoceran Bythotrephes longimanus on the condition of native zooplankton from Lake Michigan affect population abundance., Presence of chemical cues (i.e. kairomones) from Bythotrephes led to a greater than 13-fold reduction in abundance of diaptomid copepods in the absence of any direct predation. This dramatic reduction was a result of reduced fecundity and likely reduced survival. The costly anti-predator response underlying the demographic effects was reduced food intake and/or assimilation as evidenced by reduced gut fullness, which led to poorer condition, as evidenced by lower lipid accumulation., These predator effects may remain undetected in studies of animal populations that fail to include sufficient efforts to search for them, and illustrate the far-reaching potential of predator non-consumptive effects ('fear effects') to influence prey populations. [ABSTRACT FROM AUTHOR]

For example, the containment method for degrees of freedom gives the exact degrees of freedom when the design is balanced, i.e., all random effects in the model are nested and sample sizes within each group defined by the random effects are equal. More simply, using the so-called containment method for degrees of freedom also reduced the type I error rate. Recently, Nakagawa et al. (2021) provided a timely and insightful comment to our paper on statistical methods for non-independent data in ecological meta-analyses (Song et al. 2020). [Extracted from the article]

Abstract: Interaction terms from statistical tests are often used to make inferences about biological processes. For interaction terms to be biologically meaningful, it is critical that the statistical method used tests a model that corresponds to a realistic null hypothesis. A commonly used data analysis method, the analysis of variance F-test (ANOVA), is limited when examining interactions because there are a limited number of statistical models that it can test. This is further complicated by the fact that data transformations, which affect the model being tested, are sometimes required to meet ANOVA''s assumptions. Thus, when using ANOVA, it can be difficult to determine whether interactions that are found in data were produced by biological mechanisms or are statistical artifacts due to an unrealistic model. A survey of the literature indicated that these shortcomings are often not recognized despite ANOVA''s widespread use. In this paper, we evaluate the suitability of an alternate method, permutation tests, compared to ANOVA. We compare the range of potential statistical models that each method can test and the power of each method to detect interactions when using an appropriate model. We provide two simulated experiments on species interactions that show that ANOVA and permutation tests have similar power when testing an appropriate statistical model, but that permutation tests provide an advantage over ANOVA in their ability to test a wider range of models. We conclude that permutation tests can be used to make inferences, potentially impossible with ANOVA, concerning biological interactions. [Copyright &y& Elsevier]

1. Antipredator behaviour by the facultative planktivorous fish species roach ( Rutilus rutilus), perch ( Perca fluviatilis) and rudd ( Scardinius erythrophthalmus) was studied in a multi-year whole-lake experiment to evaluate species-specific behavioural and numerical responses to the stocking of pikeperch ( Sander lucioperca), a predator with different foraging behaviour than the resident predators large perch ( P. fluviatilis) and pike ( Esox lucius). 2. Behavioural responses to pikeperch varied greatly during the night, ranging from reduced activity (roach and small perch) and a shift in habitat (roach), to no change in the habitat use and activity of rudd. The differing responses of the different planktivorous prey species highlight the potential variation in behavioural response to predation risk from species of similar vulnerability. 3. These differences had profound effects on fitness; the density of species that exhibited an antipredator response declined only slightly (roach) or even increased (small perch), whereas the density of the species that did not exhibit an antipredator response (rudd) decreased dramatically (by more than 80%). 4. The maladaptive behaviour of rudd can be explained by a ‘behavioural syndrome’, i.e. the interdependence of behaviours expressed in different contexts (feeding activity, antipredator) across different situations (different densities of predators). 5. Our study extends previous studies, that have typically been limited to more controlled situations, by illustrating the variability in intensity of phenotypic responses to predators, and the consequences for population density, in a large whole-lake setting. [ABSTRACT FROM AUTHOR]

Food webs are abstract models that represent who eats whom relationships in ecosystems. Classical food web representations do not typically include phenotypic plasticity, in which one species responds to changes in density of other species by modifying traits such as behavior and morphology. Such changes, which are presumably adaptive, will affect the magnitude of both direct and indirect effects on species fitness. Empirical evidence suggests that phenotypic plasticity is likely to have large impacts on the structure and dynamics of ecological communities. Whereas theoretical studies support this, there is much that we do not understand that may require new theoretical approaches. We have constructed a computational system, Digital Organisms in a Virtual Ecosystem (DOVE), to address this problem. Its features include an individual-based approach, in which a type of genetic algorithm is used to evolve animal behavior in a dynamic environment. Here we present an overview of the ecological problems motivating the creation of DOVE and its basic structure and operation. We also discuss the kinds of decisions and tradeoffs that were considered to make DOVE as simple as possible but still rich enough to allow us to address our fundamental questions. We then use DOVE to examine optimal foraging strategies of prey in the presence of fluctuating predator risk, and show that activity levels are highly dependent on competitor density in a manner that would be difficult or impossible to explore with traditional techniques. This, and other pilot studies of DOVE, suggests that it can be used to gain insight into the origin and consequences of phenotypic plasticity and other properties of ecological communities. [Copyright &y& Elsevier]

Abstract: The origin of variation in animal growth rate and body size is not well understood but central to ecological and evolutionary processes. We develop a relationship that predicts the change in relative body size variation within a cohort will be approximately equal to the relative change in mean per unit size growth rate, when only size-dependent factors affect growth. When modeling cohort growth, relative size variation decreased, remained unchanged, or increased, as a function of growth rate-size scaling relationships, in a predictable manner. We use the approximation to predict how environmental factors (e.g., resource level) affect body size variation, and verified these predictions numerically for a flexible growth model using a wide range of parameter values. We also explore and discuss the assumptions underlying the approximation. We find that factors that similarly affect mean growth rate may differently affect size variation, and competition may increase body size variation without changing size-independent relationships. We discuss implications of our results to the choice of growth equations used in models where body size variation is an important variable or output. [Copyright &y& Elsevier]

Understanding species invasion is a central problem in ecology because invasions of exotic species severely impact ecosystems, and because invasions underlie fundamental ecological processes. However, the influence on invasions of phenotypic plasticity, a key component of many species interactions, is unknown. We present a model in which phenotypic plasticity of a resident species increases its ability to oppose invaders, and plasticity of an invader increases its ability to displace residents. Whereas these effects are expected due to increased fitness associated with phenotypic plasticity, the model additionally reveals a new and unforeseen mechanism by which plasticity affects invasions: phenotypic plasticity increases the steepness of the fitness surface, thereby making invasion more difficult, even by phenotypically plastic invaders. Our results should apply to phenotypically plastic responses to any fluctuating environmental factors including predation risk, and to other factors that affect the fitness surface such as the generalism of predators. We extend the results to competition, and argue that phenotypic plasticity's effect on the fitness surface will destabilize coexistence at local scales, but stabilize coexistence at regional scales. Our study emphasizes the need to incorporate variable interaction strengths due to phenotypic plasticity into invasion biology and ecological theory on competition and coexistence in fragmented landscapes. [ABSTRACT FROM AUTHOR]

1. We evaluated the antipredator behaviour of Daphnia mendotae to the invasive invertebrate predator, Bythotrephes longimanus, and the consequent effect of the predator on prey growth rate (referred to as a non-lethal effect of the predator). 2. In a laboratory experiment, Daphnia in the absence of Bythotrephes kairomones remained in the top, warmer regions of experimental columns, whereas in the presence of Bythotrephes kairomones, Daphnia migrated vertically, occupying a middle region by night and a low, cold region during the day. Over a 4-day experiment, the vertical migration induced by Bythotrephes caused a 36% reduction in the somatic growth rate of Daphnia, a level that is sufficient to have an effect on prey population growth rate. 3. A second laboratory experiment indicated that concentrations of Bythotrephes kairomones in water taken directly from the field (Lake Michigan) were high enough to induce behavioural shifts that led to these large reductions in somatic growth rate. 4. Our results identify a means by which Bythotrephes has substantial effects on native prey populations other than through direct consumption. [ABSTRACT FROM AUTHOR]

Predators can have a large influence on their prey through induced changes in prey phenotype. Such "nonlethal" predator effects have been abundantly demonstrated empirically in both terrestrial and aquatic systems. But the extent to which changes in species traits alter short-term responses such as growth rate or probability of survival is not clear. Here we develop models to examine the nonlethal effects of predators on prey growth. Our analyses illustrate how the nonlethal effects of predators on individual prey growth depend on environmental context; e.g., factors such as focal species density, competitor density, resource dynamics, and the timescale over which the interactions occur. This context dependence arises because of complex interactions of three mechanisms; (1) the direct negative effect of induced reduction in foraging rates, which is opposed by (2) the potential positive effects of reductions in intra- and interspecific competition, and (3) resource responses to reduced foraging. We present new empirical work, and review previous work, on larval-anuran growth that is in general support of model predictions. The framework presented here can serve to facilitate the design and interpretation of experimental results and predict how the nonlethal predator effect on prey growth in natural systems will vary over time and space. [ABSTRACT FROM AUTHOR]

Many prey modify behaviour in response to predation risk and this modification frequently leads to a foraging rate reduction. Although this reduction can have a clear direct negative effect on prey growth rate, theory predicts that a net positive effect can occur when the combined reduction in foraging by the entire population leads to a large increase in resource level. Here, I present experimental results that corroborate this counterintuitive prediction: the predation threat of ‘nonlethal’ caged larval dragonflies (Anax longipes) caused a net increase in small bullfrog (Rana catesbeiana) growth. A behavioural response (i.e. a reduction in activity level and microhabitat usage) was likely to have negatively affected growth, but was offset by a positive effect on growth from a large increase in resource levels (measured using a bioassay). Further, the positive Anax effect was dependent on nutrient level, illustrating the role of the resource response magnitude. Results of this study are discussed in the context of studies in which Anax had the opposite (i.e. negative) effect on tadpole growth. Predator-induced modifications in prey behaviour can have large negative or positive effects on prey growth, the sign and magnitude of which are dependent on relative species density and resource dynamics. [ABSTRACT FROM AUTHOR]