Habitat selection by predators and prey in communities with asymmetrical intraguild predation

Competition and predation have broad ecological consequences as they may influence individual behavior and community structure. In some cases, they are linked and predator and prey are also competitors (intraguild predation). I present a game theoretic model of habitat use by predators and prey under conditions of asymmetrical intraguild predation. This model predicts that when the diet of intraguild predators is restricted to intraguild prey and the resource for which predators and prey compete (the basal resource), co-occurrence is only stable when dietary overlap is low and productivity of the basal resource is not high. The addition of alternative resources for predators results in co-occurrence under all conditions. Variation in alternative resource productivity produces a continuum of intraguild prey distributions from matching relative habitat safety, to one that reflects both food and predation risk. When there is a substantial alternative resource for predators, the distribution of predators matches that of alternative resource availability while the distribution of prey is influenced by both habitat riskiness and food availability. The density and distribution of the predator's alternative resource thus influence habitat selection by the intraguild prey. This stresses the importance of indirect interactions in structuring habitat use in communities and the need to view habitat selection in a community context.     

Threshold levels of generalist predation determine consumer response to resource pulses

Many ecological systems are characterized by brief periods of increased resource availability called resource pulses. Empirical studies suggest that some populations of primary consumers grow rapidly in response to resource pulses, but others instead remain at low abundance despite increases in resource availability. Previous theory suggests that the lack of increase in primary consumers might be due to predators, which can respond to increased prey density both numerically, by increasing their own population, and functionally, by killing prey at a faster rate. The complexity of potential population responses to resource pulses can be assessed with simulations, but analytical conditions determining when one observes qualitatively distinct dynamics have yet to be identified. Here we use a graphical method based on a bifurcation diagram to derive the conditions leading to qualitatively distinct steady state and transient prey population dynamics as levels of predation (abundance and diversity) vary. When predation thresholds are crossed, consumer populations respond numerically to increases in their resources and provide a secondary resource pulse to their predators and parasites. These community dynamics have broad implications for the impact of changing predator communities on insect and rodent population outbreaks, which are economically and epidemiologically important.     

Influence of Predator Presence and Prey Density on Behavior and Growth of Damselfly Larvae (Ischnura elegans) (Odonata: Zygoptera)

Foraging behavior is often determined by the conflicting benefits of energy gain and the risk of mortality from predation or other causes. Theory predicts that animals should have lower activity levels when either the risk of predation or the availability of resources in the environment is high. We investigated the adjustment of the behavior of I. elegans larvae to predator presence (Anax imperator) and prey density (Daphnia sp.) and their interaction in a completely crossed factorial experiment in the lab and the effect of behavior on growth. The foraging activity of the I. elegans larvae was significantly reduced in the presence of a free-swimming predator but not a caged predator. Abdominal movements were significantly reduced at a low prey density. Growth was significantly reduced by the presence of a free swimming predator and low prey densities. These results provide evidence that these damselfly larvae adjust their behavior to the presence of predators to increase their survival at the expense of reduced growth and development.     

The influence of temporally variable predation risk on indirect interactions in an aquatic food chain

We know little about how temporally variable predation risk influences prey behavior. The risk allocation hypothesis predicts that prey facing more frequent risk should show weak anti-predator responses, and should be particularly active foragers during rare periods of safety, compared to prey facing infrequent risk. Several studies offer support for the risk allocation hypothesis, but how these responses might propagate through the larger ecological community remains largely unknown. We experimentally investigated the relative strength of trait- and density-mediated indirect effects of a predator on its prey’s resource across predation treatments that varied the lethality (caged or free-swimming predators) and temporal variability (always, often, or sometimes present) of predation. We performed this experiment in pond mesocosms using a giant water bug predator (Belostoma lutarium), an herbivorous pond snail (Physa gyrina), and algae as the basal resource. Snails greatly reduced the abundance of their algal resource when in the absence of predation. Lethal predation at low and medium intensities had significant positive indirect effects on the abundance of algae, mostly by reducing snail density. Snails responded behaviorally to high levels of deadly predation by foraging more and hiding less than in other situations, as predicted by the risk allocation hypothesis, and thus ameliorated the density-mediated indirect effects of predators on algae. Behavioral responses to caged predators, and the subsequent trait-mediated indirect effects, were negligible regardless of predation intensity. Our previous work has demonstrated that trait-mediated indirect effects are weak when resources are abundant, as they were in this experiment. This work demonstrates that temporal variation in predation intensity plays a key role in determining the relative strength of TMIIs and DMIIs in an aquatic food chain.     

Interactive effects of productivity and predation on zooplankton diversity

Recent studies suggest the necessity of understanding the interactive effects of predation and productivity on species coexistence and prey diversity. Models predict that coexistence of prey species with different competitive abilities can be achieved if inferior resource competitors are less susceptible to predation and if productivity and/or predation pressure are at intermediate levels. Hence, predator effects on prey diversity are predicted to be highly context dependent: enhancing diversity from low to intermediate levels of productivity or predation and reducing diversity of prey at high levels of productivity or predation. While several studies have examined the interactive effects of herbivory and productivity on primary producer diversity, experimental studies of such effects in predator‐prey systems are rare. We tested these predictions using an aquatic field mesocosm experiment in which initial density of the zooplankton predator Notonecta undulata and productivity were manipulated to test their interactive effects on diversity of seven zooplankton, cladoceran species that were common in surrounding ponds. Two productivity levels were imposed via phosphorus enrichment at levels comparable to low and intermediate levels found within neighboring natural ponds. We used open systems to allow for natural dispersal and behaviorally‐mediated numerical responses by the flight‐capable predator. Effects of predators on zooplankton diversity depended on productivity level. At low and high productivity, prey species richness declined while at high productivity it showed a unimodal relationship with increasing the predator density. Effects of treatments were weaker when using Pielou's evenness index or the inverse Simpson index as measures of prey diversity. Our findings are generally consistent with model predictions in which predators can facilitate prey coexistence and diversity at intermediate levels of productivity and predation intensity. Our work also shows that the functional form of the relationship between prey diversity and predation intensity can be complex and highly dependent on environmental context.     

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.     

Effect of scavenging on predation in a food web

Scavenging can have important consequences for food web dynamics, for example, it may support additional consumer species and affect predation on live prey. Still, few food web models include scavenging. We develop a dynamic model that includes two facultative scavenger species, which we refer to as the predator or scavenger species according to their natural scavenging propensity, as well as live prey, and a carrion pool to show ramifications of scavenging for predation in simple food webs. Our modeling suggests that the presence of scavengers can both increase and decrease predator kill rates and overall predation in model food webs and the impact varies (in magnitude and direction) with context. In particular, we explore the impact of the amount of dynamics (exploitative competition) allowed in the predator, scavenger, and prey populations as well as the direction and magnitude of interference competition between predators and scavengers. One fundamental prediction is that scavengers most likely increase predator kill rates, especially if there are exploitative feedback effects on the prey or carrion resources like is normally observed in natural systems. Scavengers only have minimal effects on predator kill rate when predator, scavenger, and prey abundances are kept constant by management. In such controlled systems, interference competition can greatly affect the interactions in contrast to more natural systems, with an increase in interference competition leading to a decrease in predator kill rate. Our study adds to studies that show that the presence of predators affects scavenger behavior, vital rates, and food web structure, by showing that scavengers impact predator kill rates through multiple mechanisms, and therefore indicating that scavenging and predation patterns are tightly intertwined. We provide a road map to the different theoretical outcomes and their support from different empirical studies on vertebrate guilds to provide guidance in wildlife management.     

Non-lethal effects of predators on prey growth rates depend on prey density and nutrient additions

A number of studies show that predators can depress prey growth rates by inducing reductions in foraging activity, but the size of this non-lethal effect is quite variable. Here I investigate how prey density and resource productivity may alter the extent to which predators depress the growth rates of their prey. Theory predicts that when resources are overgrazed, an increase in predation risk will have little net effect on individual food intake because the decline in foraging effort will be offset by an increase in resource level. Thus, the non-lethal effects of predators on prey growth rates should depend upon prey density and resource productivity in a predictable manner, with the growth penalty imposed by predators being strongest when resources are undergrazed and weakest when resources are overgrazed. I tested this hypothesis by manipulating predation risk, prey density, and nutrient additions in a mesocosm experiment with the pulmonate snail Helisoma trivolvis . Refuge use by snails was 45% higher in the presence of caged crayfish than in their absence. Snail growth rates were reduced, on average, by 24% in the presence of caged crayfish. However, the magnitude of the growth penalty exacted by crayfish depended on snail density and nutrient additions. When snails were stocked at high density and nutrient additions were low, growth suppression was just 2.6%. At the other extreme, when snails were at low density and nutrient additions were high, growth suppression was 44.6%. Thus, the non-lethal effects of predators on prey growth depend on environmental context, illustrating an important link between individual traits and system-level properties.     

Consequences of size structure in the prey for predator-prey dynamics: the composite functional response

1. Current formulations of functional responses assume that the prey is homogeneous and independent of intraspecific processes. Most prey populations consist of different coexisting size classes that often engage in asymmetrical intraspecific interactions, including cannibalism, which can lead to nonlinear interaction effects. This may be important as the size structure with the prey could alter the overall density-dependent predation rates. 2. In a field experiment with damselfly and dragonfly larvae, 16 treatments manipulated the density of a small prey stage, the presence of large conspecific prey and the presence of heterospecific predators. 3. Size structure in the prey (i.e. when both prey stages were present) decreased the impact of the predator on overall prey mortality by 25-48% at mid and high prey densities, possibly due to density-dependent size-structured cannibalism in the prey. The predation rates on small prey stages were determined by the interaction of large prey and predators. Predation rates increased with prey density in the absence of large prey, but predation rates were constant across densities when large conspecifics were present. 4. The functional response for unstructured prey followed a Holling type III model, but the predation rate for size-structured prey was completely different and followed a complex pattern that could not be explained with any standard functional response. 5. Using additional laboratory experiments, a mortality model was developed and parameterized. It showed that the overall prey mortality of size-structured prey can be adequately predicted with a composite functional response model that modelled the individual functional responses of each prey stage separately and accounted for their cannibalistic interaction. 6. Thus, treating a prey population as a homogeneous entity will lead to erroneous predictions in most real-world food webs. However, if we account for the effects of size structure and the intraspecific interactions on functional responses by treating size classes as different functional groups, it is possible to reliably predict the dynamics of size-structured predator-prey systems.     

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.     

The effect of predation pressure and predator adaptive foraging on the relative importance of consumptive and non‐consumptive predator net effects in a freshwater model system

An important challenge in community ecology is identifying the functional characteristics capable of predicting the nature and strength of predator effects on food webs. We developed an individual‐based model, based on a shallow lake model system, to evaluate the total, consumptive, and non‐consumptive indirect effect that predators have on basal resources when the predators differ in their foraging types (active adaptive foraging or sedentary foraging). Overall, both predator types caused similar total indirect effects on lower trophic levels. However, the nature net effects of predators diverged between predator foraging types. Active predators caused larger non‐consumptive effects, relative to the total indirect effect, irrespective of predation pressure levels. On the other hand, sedentary predators caused larger non‐consumptive effects for lower predation pressure levels, but consumptive effects became more important as predation pressure increased. Our simulations showed that the reliance on a particular mechanism driving consumer–resource interactions is altered by predator foraging behavior and highlight the importance of both prey and predator foraging behaviors to predict the causes and consequences of cascading effects observed in food webs.     

Effects of Structural Refuge and Density on Foraging Behaviour and Mortality of Hungry Tadpoles Subject to Predation Risk

Theoretical models of prey behaviour predict that food‐limited prey engage in risk‐prone foraging and thereby succumb to increased mortality from predation. However, predation risk also may be influenced by factors including prey density and structural cover, such that the presumed role of prey hunger on predation risk may be obfuscated in many complex predator–prey systems. Using a tadpole (prey) – dragonfly larva (predator) system, we determined relative risk posed to hungry vs. sated prey when both density and structural cover were varied experimentally. Overall, prey response to perceived predation risk was primarily restricted to increased cover use, and hungry prey did not exhibit risk‐prone foraging. Surprisingly, hungry prey showed lower activity than sated prey when exposed to predation risk, perhaps indicating increased effort in search of refuge or spatial avoidance of predator cues among sated animals. An interaction between hunger level and predation risk treatments indicated that prey state affected sensitivity to perceived risk. We also examined the lethal implications of prey hunger by allowing predators to select directly between hungry and sated prey. Although predators qualitatively favoured hungry prey when density was elevated and structural cover was sparse, the overall low observed variation in mortality risk between hunger treatments suggests that preferential selection of hungry prey was weak. This implies that hunger effects on prey mortality risk may not be readily observed in complex landscapes with additional factors influencing risk. Thus, current starvation‐predation trade‐off theory may need to be broadened to account for other mechanisms through which undernourished prey may cope with predation risk.     

You can’t run but you can hide: refuge use in frog tadpoles elicits density-dependent predation by dragonfly larvae

The potential role of prey refuges in stabilizing predator–prey interactions is of longstanding interest to ecologists, but mechanisms underlying a sigmoidal predator functional response remain to be fully elucidated. Authors have disagreed on whether the stabilizing effect of prey refuges is driven by prey- versus predator-centric mechanisms, but to date few studies have married predator and prey behavioural observations to distinguish between these possibilities. We used a dragonfly nymph–tadpole system to study the effect of a structural refuge (leaf litter) on the predator’s functional response, and paired this with behavioural observations of both predator and prey. Our study confirmed that hyperbolic (type II) functional responses were characteristic of foraging predators when structural cover was low or absent, whereas the functional response was sigmoidal (type III) when prey were provided with sufficient refuge. Prey activity and refuge use were density independent across cover treatments, thereby eliminating a prey-centric mechanism as being the genesis for density-dependent predation. In contrast, the predator’s pursuit length, capture success, and handling time were altered by the amount of structure implying that observed shifts in density-dependent predation likely were related to predator hunting efficiency. Our study advances current theory by revealing that despite fixed-proportion refuge use by prey, presence of a prey refuge can induce density-dependent predation through its effect on predator hunting strategy. Ultimately, responses of predator foraging decisions in response to changes in prey availability and search efficiency may be more important in producing density-dependent predation than the form of prey refuge use.     

Stable isotope analyses document intraguild predation in wolf spiders (Araneae: Lycosidae) and underline beneficial effects of alternative prey and microhabitat structure on intraguild prey survival

Intraguild predation (IGP) is common among generalist predators and an important issue in food web theory, because IGP may destabilise communities by increasing extinction of species. Also, IGP may interfere with the effectiveness of generalist predators as biological control agents. In general, occurrence of IGP in laboratory or field studies is inferred from abundance data or direct observations only. We investigated if tracing stable isotopes allows distinction between different types of predation and confirmation of IGP. Wolf spiders were chosen as model organisms for generalist predators; IGP of third instar A. cuneata on second instar P. palustris was investigated in a laboratory experiment. The availability of alternative prey and the complexity of the microhabitat were manipulated, since both factors are thought to facilitate coexistence of predators.
Stable isotope analysis documented predation of A. cuneata on P. palustris and predation on alternative prey by both juveniles. Both the presence of alternative prey and the availability of shelter reduced mortality of juvenile P. palustris during the first week. During the second week mortality increased in complex structure without alternative prey presumably due to enhanced activity and cannibalism among starving P. palustris . Thus, microhabitat complexity and prey abundance may foster coexistence of wolf spiders in the field.
In conclusion, stable isotope analysis was proven a powerful tool to investigate animal behaviour without direct observation. The method allowed disentangling predator feeding behaviour when more than one type of prey was present.     

The Effects of Prey Patchiness, Predator Aggregation, and Mutual Interference on the Functional Response of Phytoseiulus persimilis Feeding on Tetranychus urticae (Acari: Phytoseiidae, Tetranychidae)

The spatial distributions of two-spotted spider mites Tetranychus urticae and their natural enemy, the phytoseiid predator Phytoseiulus persimilis, were studied on six full-grown cucumber plants. Both mite species were very patchily distributed and P. persimilis tended to aggregate on leaves with abundant prey. The effects of non-homogenous distributions and degree of spatial overlap between prey and predators on the per capita predation rate were studied by means of a stage-specific predation model that averages the predation rates over all the local populations inhabiting the individual leaves. The empirical predation rates were compared with predictions assuming random predator search and/or an even distribution of prey. The analysis clearly shows that the ability of the predators to search non-randomly increases their predation rate. On the other hand, the prey may gain if it adopts a more even distribution when its density is low and a more patchy distribution when density increases. Mutual interference between searching predators reduces the predation rate, but the effect is negligible. The stage-specific functional response model was compared with two simpler models without explicit stage structure. Both unstructured models yielded predictions that were quite similar to those of the stage-structured model.     

Prey life-history and bioenergetic responses across a predation gradient

To evaluate the importance of non-consumptive effects of predators on prey life histories under natural conditions, an index of predator abundance was developed for naturally occurring populations of a common prey fish, the yellow perch Perca flavescens, and compared to life-history variables and rates of prey energy acquisition and allocation as estimated from mass balance models. The predation index was positively related to maximum size and size at maturity in both male and female P. flavescens, but not with life span or reproductive investment. The predation index was positively related to size-adjusted specific growth rates and growth efficiencies but negatively related to model estimates of size-adjusted specific consumption and activity rates in both vulnerable (small) and invulnerable (large) size classes of P. flavescens. These observations suggest a trade-off between growth and activity rates, mediated by reduced activity in response to increasing predator densities. Lower growth rates and growth efficiencies in populations with fewer predators, despite increased consumption suggests either 1) a reduction in prey resources at lower predator densities or 2) an intrinsic cost of rapid prey growth that makes it unfavourable unless offset by a perceived threat of predation. This study provides evidence of trade-offs between growth and activity rates induced by predation risk in natural prey fish populations and illustrates how behavioural modification induced through predation can shape the life histories of prey fish species.     

Simple life-history omnivory: responses to enrichment and harvesting in systems with intraguild predation

This article analyzes the nature of top-down and bottom-up effects and alternative states in systems characterized by life-history omnivory. The analysis is based on a three-species food web with intraguild predation (IGP). The top predator population has juvenile and adult stages, which consume the basal resource and the intermediate prey, respectively; the prey consumes only the resource. The per capita reproduction of the adult predators depends on their consumption rate of prey, while the maturation rate of the juvenile predators depends on their resource consumption rate. Enriching the resource can increase or decrease the abundances of one or both of the two consumer species; an increased density is more likely in the intermediate species than in the systems where IGP is not based on stage differences. Alternative states that have or lack the predator occur frequently, particularly when the prey population is capable of reducing the resource to very low densities. These results differ from those of several other recent models of life-history omnivory. They suggest that life-history omnivory may be one of the primary reasons why exploited populations undergo sudden collapses and why collapsed populations fail to recover in spite of large reductions in the exploitation rate.     

Trophic supplements to intraguild predation

Intraguild predation (IGP) is a dominant community module in terrestrial food webs that occurs when multiple consumers feed both on each other and on a shared prey. This specific form of omnivory is common in terrestrial communities and is of particular interest for conservation biology and biological control given its potential to disrupt management of threatened or pest species. Extensive theory exists to describe the dynamics of three-species IGP, but these models have largely overlooked the potential for other, exterior interactions, to alter the dynamics within the IGP module. We investigated how three forms of feeding outside of the IGP module by intraguild predators (i.e. trophic supplementation) affect the dynamics of the predators (both IG predator and IG prey) and their shared resource. Specifically, we examined how the provision of a constant donor-controlled resource, the availability of an alternative prey species, and predator plant-feeding affect the dynamics of IGP models. All three forms of trophic supplements modified the basic expectations of IGP theory in two important ways, and their effects were similar. First, coexistence was possible without the IG prey being a superior competitor for the original shared resource if the IG prey could effectively exploit one of the types of trophic supplements. However, supplements to the IG predator restricted the potential for coexistence. Second, supplements to the IG prey ameliorated the disruptive effects of the IG predator on the suppression of the shared resource, promoting effective control of the resource in the presence of both predators. Consideration of these three forms of trophic supplementation, all well documented in natural communities, adds substantial realism and predictive power to intraguild predation theory.     

The interaction between predation risk and food ration on behavior and morphology of Eurasian perch

The risk of both predation and food level has been shown to affect phenotypic development of organisms. However, these two factors also influence animal behavior that in turn may influence phenotypic development. Hence, it might be difficult to disentangle the behavioral effect from the predator or resource‐level effects. This is because the presence of predators and high resource levels usually results in a lower activity, which in turn affects energy expenditure that is used for development and growth. It is therefore necessary to study how behavior interacts with changes in body shape with regard to resource density and predators. Here, we use the classic predator‐induced morphological defense in fish to study the interaction between predator cues, resource availability, and behavioral activity with the aim to determine their relative contribution to changes in body shape. We show that all three variables, the presence of a predator, food level, and activity, both additively and interactively, affected the body shape of perch. In general, the presence of predators, lower swimming activity, and higher food levels induced a deep body shape, with predation and behavior having similar effect and food treatment the smallest effect. The shape changes seemed to be mediated by changes in growth rate as body condition showed a similar effect as shape with regard to food‐level and predator treatments. Our results suggests that shape changes in animals to one environmental factor, for example, predation risk, can be context dependent, and depend on food levels or behavioral responses. Theoretical and empirical studies should further explore how this context dependence affects fitness components such as resource gain and mortality and their implications for population dynamics.     

Harvesting and predation of a sex- and age-structured population

We study a discrete-time system of equations for a structured ungulate population exploited by human harvesting or a dynamic predator. The population is divided into juveniles, and female and male adults. Harvesting is concentrated on adults (trophy hunting of males or population control measures on females), whereas predation occurs in juveniles. Though the model consists of four nonlinear equations, we find explicit expressions for the steady states. We use these explicit expressions to investigate harvesting rates that allow population persistence, rates that ensure population control, and optimal harvesting efforts. Several reductions of complexity allow for a detailed analysis of the dynamics of the model. Most notably, we find that even compensatory density dependence can lead to a period doubling bifurcation, that the model does not support consumer–resource cycles, and that an Allee effect can emerge from the interplay of stage-specific predation and density-dependent prey reproduction.