Shared Prey Research Articles

Trophic ecology of an aquatic mite (Piona carnea) preying on Daphnia pulex: effects of predator density, nutrient supply and a second predator (Chaoborus americanus)

As compared to other aquatic invertebrates, relatively little is known about the ecology of predaceous, pelagic water mites. Studies to date do, however, show that water mites can be important components of aquatic food webs. Here, we used manipulative field experiments to better understand the trophic ecology of a predaceous water mite (Piona carnea). Three experiments were conducted that examined (i) the effect of P. carnea density on the per-capita interaction strength (PCIS) of P. carnea preying on zooplankton; (ii) how the effects of P. carnea predation change with prey productivity; and (iii) how P. carnea interacts with another pelagic predator (Chaoborus americanus) to affect a shared prey species (Daphnia pulex). Results from the first experiment showed that P. carnea can strongly impact D. pulex populations, and that the PCIS of P. carnea decreases with an increase in P. carnea density. The second experiment showed that the effects of P. carnea on D. pulex populations depend on bottom-up factors that influence D. pulex population biomass and the reproductive potential of a D. pulex population relative to its size. The third experiment uncovered a non-additive interaction between P. carnea and C. americanus that resulted in a risk reducing situation for D. pulex in the presence of both predators. Together these experiments show that P. carnea imposes a strong negative impact on D. pulex, that the magnitude of this negative impact is dependent on the P. carnea density and the productivity of the system, and that the trophic ecology of P. carnea is modified by coexisting predator species.

Multiple predators induce risk reduction in coexisting vole species

Large predators may affect the hunting efficiency of smaller ones directly by decreasing their numbers, or indirectly by altering their behaviour. Either way this may have positive effects on the density of shared prey. Using large outdoor enclosures, we experimentally studied whether the presence of the Tengmalm's owl Aegolius funereus affects the hunting efficiency of the smallest member of the vole‐eating predator guild, the least weasel Mustela nivalis, as measured by population responses of coexisting prey species, the field vole Microtus agrestis and the sibling vole M. levis. We compared the density and survival probability of vole populations exposed to no predation, weasel predation or combined predation by a weasel and an owl. The combined predation of both owl and weasel did not result in obvious changes in the density of sibling and field vole populations compared to the control populations without predators, while predation by least weasel alone decreased the densities of sibling voles and induced a similar trend in field vole densities. Survival of field voles was not affected by predator treatment while sibling vole survival was lower in predator treated populations than in control populations. Our results suggest that weasels are intimidated by avian predators, but without changing the effects of predators on competitive situations between the two vole species. Non‐lethal effects of intraguild predation therefore will not necessarily change competitive interactions between shared prey species.

Intraguild predation and mesopredator release effect on long-lived prey

Complete extirpation of a species can generate cascading effects throughout an ecosystem, yet are precisely the goal of island eradications of pest species. “Mesopredator release effect”, an asymmetrical special case of intraguild predation, has been hypothesised as a possible indirect effect from eradications, where superpredator removal can generate a mesopredator increase which may increase the impact on their shared prey. Theoretically this suggests that for intraguild predators, the superpredator may protect the shared prey from mesopredation, and removal of superpredators alone is not recommended. We create a model of long-lived age-structured shared prey and explore the non-equilibrium dynamics of this system. The superpredator can impact all prey life-stages (adult survival and reproductive success) whereas the smaller mesopredator can only impact early life-stages (reproductive success). This model is independently tested with data from a closed oceanic island system where eradication of introduced intraguild predators is possible for conservation of threatened birds. Mesopredator release only occurs in strongly top-down moderated (resource-abundant) systems. Even when mesopredator release can occur, the negative impact of more mesopredators is outweighed by the benefit of superpredator removal, allowing recovery of the prey population. Results are robust to 10% variation in model parameters. The consideration of age-structured prey contradicts previous theoretical results for mesopredator release effect and intraguild predation. Superpredator eradication is vital for population recovery of long-lived insular species. Nonetheless island conservation must retain a whole-ecosystem perspective given the complex trophic relationships among multiple species on islands.

NUMERICAL AND BEHAVIORAL EFFECTS WITHIN A PULSE-DRIVEN SYSTEM: CONSEQUENCES FOR SHARED PREY

Some of the clearest examples of the ramifying effects of resource pulses exist in deciduous forests dominated by mast-producing trees, such as oaks, beech, and hornbeam. Seed production in these forests represents only the first of several pulsed events. Secondary pulses emerge as mast-consuming small rodents numerically respond to seed production and tertiary pulses emerge as generalist predators numerically respond to rodents. Raptors may also respond behaviorally (i.e., diet shifts) to subsequent crashes in small rodents following the crash phase in seed production. In oak-dominated forest in the Hudson Valley, New York, these various pulse and crash phases act synergistically, although not simultaneously, to influence thrush population dynamics through predation on nests, juveniles, and adults. As a consequence, factors limiting population growth rate and their age-specific action vary as a function of past acorn production. We highlight these interactions based on our eight-year study of thrush demography, acorn production, and small mammal abundance coupled with information on regional adult thrush population trends from the Breeding Bird Survey. We use these data sets to demonstrate the sequence of primary to tertiary pulses and how they influence breeding thrush populations. To extend our discussion beyond masting phenomena in the eastern United States, we briefly review the literature of alternative avian prey within pulsed systems to show (1) numerical and behavioral responses by generalist predators are ubiquitous in pulsed systems, and this contributes to (2) variability in reproduction and survivorship of avian prey linked to the underlying dynamics of the pulse. We conclude by exploring the broad consequences of cascading resource pulses for alternative prey based upon the indirect interaction of apparent competition among shared prey and the nature of temporal variability on populations.

PREDATOR DIVERSITY AND TROPHIC INTERACTIONS

The recognition that predators play important roles in ecosystems has prompted research to resolve how combinations of predator species influence ecosystem functions. Interactions among predator species and their prey can lead to a host of linear and nonlinear effects. Understanding the conditions causing these effects is critical for assigning predator species to functional groups in ways that lead to predictive theory of predator diversity effects on trophic interactions. To this end, I provide a synthesis of experiments examining multiple-predator-species effects on mortality of single shared prey. I show how experimental design and experimental venue can determine the conclusion about the importance of predator diversity on trophic interactions. In addition, I link natural history insights on predator species habitat and hunting behavior with linear and nonlinear multiple-predator effects to derive a new concept of predator diversity effects on trophic interactions. This concept holds that the nature of predator diversity effects is contingent upon predator species hunting mode plus predator and prey species habitat domain (defined as the spatial extent to which a microhabitat is used by a species). This concept allows the classification of multiple-predator effects into four broad functional categories: substitutable, nonlinear due to predator species interference, nonlinear due to intraguild predation, and nonlinear due to predator species synergism. Experimental evidence so far provides ample and comparatively equal support for substitutable, interference, and intraguild effects, and equivocal support for nonlinear synergisms. The paper closes by discussing ways to further a research program aimed at using the building blocks presented here to understand predator functional diversity and trophic interactions in complex ecological systems.

The influence of vigilance on intraguild predation

Theoretical work on intraguild predation suggests that if a top predator and an intermediate predator share prey, the system will be stable only if the intermediate predator is better at exploiting the prey, and the top predator gains significantly from consuming the intermediate predator. In mammalian carnivore systems, however, there are examples of top predator species that attack intermediate predator species, but rarely or never consume the intermediate predator. We suggest that top predators attacking intermediate predators without consuming them may not only reduce competition with the intermediate predators, but may also increase the vigilance of the intermediate predators or alter the vigilance of their shared prey, and that this behavioral response may help to maintain the stability of the system. We examine two models of intraguild predation, one that incorporates prey vigilance, and a second that incorporates intermediate predator vigilance. We find that stable coexistence can occur when the top predator has a very low consumption rate on the intermediate predator, as long as the attack rate on the intermediate predator is relatively large. However, the system is stable when the top predator never consumes the intermediate predator only if the two predators share more than one prey species. If the predators do share two prey species, and those prey are vigilant, increasing top predator attack rates on the intermediate predator reduces competition with the intermediate predator and reduces vigilance by the prey, thereby leading to higher top predator densities. These results suggest that predator and prey behavior may play an important dynamical role in systems with intraguild predation.

Priority effects of the early breeding fire salamander on the late breeding banded newt

Early breeding intraguild predators may have advantages over late breeding predators via priority effects; early breeding predators may reduce shared prey resources before late breeders appear and may also prey upon the late breeders. Here we show that predatory larvae of the late-breeding predatory banded newt, Triturus vittatus vittatus, occupy the same temporary pond toward the end of the developmental period of the early-breeding predatory fire salamander, Salamandra salamandra, resulting in a large size disparity between larvae of these two species while they co-occur. We conducted outdoor artificial pool experiments to assess priority effects of large larval Salamandra at the end of their larval development period, on recently hatched larval Triturus. We also assessed how artificial vegetation may influence larval Triturus performance in the presence or absence of Salamandra Salamandra, introduced into the experimental pools two weeks prior to the newt larvae, strongly reduced invertebrate prey abundance shared by these two predatory urodeles and with only a one week period of overlap, strongly reduced abundance of Triturus larvae. The artificial vegetation had only a small ameliorating effect on Triturus survival when Salamandra was present. Triturus size at metamorphosis (snout-tail length) was significantly larger in the Salamandra pools, presumably due to a combination of a strong “thinning effect” and greater vulnerability of smaller Triturus individuals to predation by Salamandra. Time to metamorphosis was not significantly affected by Salamandra. These results have conservation implications as T. v. vittatus is listed as highly endangered and may also explain the largely negative spatial association of the two species.

Indirect effects between shared prey: Predictions for

Suppression of a target prey by a predator can depend on its surrounding community, including the presence of nontarget, alternative prey. Basic theoretical models of two prey species that interact only via a shared predator predict that adding an alternative prey should increase predator numbers and ultimately lower target pest densities as compared to when the target pest is the only prey. While this is an alluring prediction, it does not explain the numerous responses empirically observed. To better understand and predict the indirect interactions produced by shared predation, we explore how additional prey species affect three broad ecological mechanisms, the predator's reproductive, movement, and functional responses. Specifically, we review current theoretical models of shared predation by focusing on these mechanisms, and make testable predictions about the effects of shared predation. We find that target predation is likely to be higher in the two prey system because of predator reproduction, especially when: predators are prey limited, alternative or total prey density is high, or alternative prey are available over time. Target predation may also be greater because of predator movement, but only under certain movement rules and spatial distributions. Predator foraging behavior is most likely to cause lower target predation in the two-prey system, when per capita predation is limited by something other than prey availability. It is clear from this review that no single theoretical generalization will accurately predict community-level effects for every system. However, we can provide testable hypotheses for future empirical and theoretical investigations of indirect interactions and help enhance their potential use in biological control.

Diet of intraguild predators affects antipredator behavior in intraguild prey

In two-predator, one-prey systems with intraguild predation and patchily distributed prey, the intraguild prey may face a choice between prey patches with and without intraguild predators. To minimize falling victim to intraguild predation, intraguild prey are expected to perceive cues specifically associated with the presence of intraguild predators. We investigate whether intraguild prey avoided intraguild predators and which cues triggered this behavior in a system composed of plant-inhabiting arthropods. We found that intraguild prey recognized intraguild predators from a distance, based on their diet: they avoided odors of intraguild predators that had consumed shared prey but did not avoid odors of intraguild predators that had fed on other diets, including a diet of conspecifics. When intraguild prey were foraging on a patch, detection of intraguild predators led to longer periods of immobility and to fewer captures of the shared prey. However, intraguild predators that were either starved or had previously consumed intraguild prey posed a higher risk to intraguild prey than did intraguild predators that had consumed the shared prey. We conclude that the cues used by intraguild prey to avoid intraguild predators are associated with the circumstances under which they encounter intraguild predators in the field and not to different degrees of danger. Key words: food webs, intraguild predation, predator avoidance, predator diet. [Behav Ecol]

PARTITIONING COMPONENTS OF RISK REDUCTION IN A DRAGONFLY–FISH INTRAGUILD PREDATION SYSTEM

Risk to prey imposed by intraguild predation (IGP) can be influenced by a number of factors, yet to date, few studies have measured the contributions of these factors to overall risk. A three-species IGP system with larvae of the dragonfly Anax junius as IG (top) predators, larvae of the dragonfly Plathemis lydia as IG prey (intermediate predators), and fathead minnow hatchlings (Pimephales promelas) as shared prey was used to estimate the contribution of the following three factors to shared-prey mortality rate in combined predator treatments: (1) the trophic effect of the IG predator on IG prey density; (2) the effect of reduced shared prey consumption by the IG prey in the presence of the IG predator; and (3) the effect of alternative prey for the IG predator. These factors were integrated into a model of multiple predator effects. To quantify minnow mortality, P. promelas were exposed to A. junius only, P. lydia only, A. junius and P. lydia, or neither in a two-by-two factorial design. Additional treatments, in which one or both predators were unable to feed, were used to isolate behavioral (activity level) changes in dragonfly larvae. When predators preyed in combination on P. promelas their impact was less than that of the summed effects of the two predators, each in the absence of the other—a result termed risk reduction. A. junius consumed a significant number of P. lydia when they were present (i.e., IGP), and behavioral interactions between A. junius and P. lydia were asymmetric. The presence of A. junius caused P. lydia to become less active, while the presence of P. lydia elicited a diet shift in A. junius to include some P. lydia. Interactions between predator species, specifically IGP, influenced prey survival. Trophic and behavioral effects of IGP were similar in magnitude. These results highlight the importance of trophic and behavioral interactions in predator–prey systems and also suggest that effects of multiple predators may not be predictable based on the sum of individual effects. Determining the effects of multiple predators requires the identification of mechanisms that contribute to nonadditive prey responses. Corresponding Editor: O. J. Schmitz.

Aggregate Effects of Multiple Predator Species on a Shared Prey

Aggregate Effects of Multiple Predator Species on a Shared Prey

Cats protecting birds: modelling the mesopredator release effect

1. Introduced predators account for a large part of the extinction of endemic insular species, which constitutes a major component of the loss of biodiversity among vertebrates. Eradication of alien predators from these ecosystems is often considered the best solution. 2. In some ecosystems, however, it can generate a greater threat for endemic prey through what is called the ‘mesopredator release’. This process predicts that, once superpredators are suppressed, a burst of mesopredators may follow which leads their shared prey to extinction. 3. This process is studied through a mathematical model describing a three species system (prey–mesopredator–superpredator). Analysis of the model, with and without control of meso‐ and superpredators, shows that this process does indeed exist and can drive shared prey to rapid extinction. 4. This work emphasizes that, although counter‐intuitive, eradication of introduced superpredators, such as feral domestic cats, is not always the best solution to protect endemic prey when introduced mesopredators, such as rats, are also present.

Intraguild Predation and Competition Between Larval Dragonflies: Direct and Indirect Effects on Shared Prey

We conducted manipulative field experiments in artificial ponds to quantify the predatory impact of larvae of a migratory dragonfly (Tramea lacerata) on a common resident dragonfly species (Erythemis simplicicollis), and on damselflies as shared prey of the two dragonflies. We found that the combined predatory effects of these two dragonflies on damselflies were not additive. To determine the underlying cause of non—additive predation rates in the field, we conducted a second experiment in laboratory aquaria to isolate the impact of each predator on the consumption rates of the other. Dragonfly consumption rates of damselflies in single—predator treatments were compared to those in the presence of heterospecifics or conspecifics with their menta (mouthparts) surgically modified so that they could not capture prey. In the laboratory experiment, de—mented Tramea reduced the consumption rates of Erythemis to less than half of that observed when Erythemis foraged alone. Erythemis numbers were also reduced by Tramea predation. Erythemis had neither effect on Tramea. Both of the negative effects of Tramea on Erythemis will have indirect positive effects on damselflies. The "behavioral" component (reduced Erythemis foraging rate) should be more important than the "trophic link" (reduced Erythemis numbers) indirect effect. Together these indirect positive effects will allay, but not completely compensate for, the direct negative effects of Tramea predation on damselflies. These results illustrate how an asymmetric potential for intraguild predation can lead to asymmetries in interference competition and to non—additive effects on prey mortality. The addition of removal of predators that interact in this manner to or from communities should have only a small net effect on prey because of compensating direct and indirect effects. This may explain why predator manipulations have often had unpredictable or undetectable effects on freshwater benthic communities.