Birds of prey as limiting factors of gamebird populations in Europe: a review

Whether predators can limit their prey has been a topic of scientific debate for decades. Traditionally it was believed that predators take only wounded, sick, old or otherwise low-quality individuals, and thus have little impact on prey populations. However, there is increasing evidence that, at least under certain circumstances, vertebrate predators may indeed limit prey numbers. This potential role of predators as limiting factors of prey populations has created conflicts between predators and human hunters, because the hunters may see predators as competitors for the same resources. A particularly acute conflict has emerged over the past few decades between gamebird hunters and birds of prey in Europe. As a part of a European-wide research project, we reviewed literature on the relationships between birds of prey and gamebirds. We start by analysing available data on the diets of 52 European raptor and owl species. There are some 32 species, mostly specialist predators feeding on small mammals, small passerine birds or insects, which never or very rarely include game animals (e.g. hares, rabbits, gamebirds) in their diet. A second group (20 species) consists of medium-sized and large raptors which prey on game, but for which the proportion in the diet varies temporally and spatially. Only three raptor species can have rather large proportions of gamebirds in their diet, and another seven species may utilise gamebirds locally to a great extent. We point out that the percentage of a given prey species in the diet of an avian predator does not necessarily reflect the impact of that predator on densities of prey populations. Next, we summarise available data on the numerical responses of avian predators to changing gamebird numbers. In half of these studies, no numerical response was found, while in the remainder a response was detected such that either raptor density or breeding success increased with density of gamebirds. Data on the functional responses of raptors were scarce. Most studies of the interaction between raptors and gamebird populations give some estimate of the predation rate (per cent of prey population taken by predator), but less often do they evaluate the subsequent reduction in the pre-harvest population or the potential limiting effect on breeding numbers. The few existing studies indicate that, under certain conditions, raptor predation may limit gamebird populations and reduce gamebird harvests. However, the number and extent of such studies are too modest to draw firm conclusions. Furthermore, their geographical bias to northern Europe, where predator-prey communities are typically simpler than in the south, precludes extrapolation to more diverse southern European ecosystems. There is an urgent need to develop further studies, particularly in southern Europe, to determine the functional and numerical responses of raptors to gamebird populations in species and environments other than those already evaluated in existing studies. Furthermore, additional field experiments are needed in which raptor and possibly also mammalian predator numbers are manipulated on a sufficiently large spatial and temporal scale. Other aspects that have been little studied are the role of predation by the non-breeding part of the raptor population, or floaters, on the breeding success and survival of gamebirds, as well as the effect of intra-guild predation. Finally there is a need for further research on practical methods to reduce raptor predation on gamebirds and thus reduce conflict between raptor conservation and gamebird management.     

A review of predation as a limiting factor for bird populations in mesopredator‐rich landscapes: a case study of the UK

The impact of increasing vertebrate predator numbers on bird populations is widely debated among the general public, game managers and conservationists across Europe. However, there are few systematic reviews of whether predation limits the population sizes of European bird species. Views on the impacts of predation are particularly polarised in the UK, probably because the UK has a globally exceptional culture of intensive, high‐yield gamebird management where predator removal is the norm. In addition, most apex predators have been exterminated or much depleted in numbers, contributing to a widely held perception that the UK has high numbers of mesopredators. This has resulted in many high‐quality studies of mesopredator impacts over several decades. Here we present results from a systematic review of predator trends and abundance, and assess whether predation limits the population sizes of 90 bird species in the UK. Our results confirm that the generalist predators Red Fox (Vulpes vulpes) and Crows (Corvus corone and C. cornix) occur at high densities in the UK compared with other European countries. In addition, some avian and mammalian predators have increased numerically in the UK during recent decades. Despite these high and increasing densities of predators, we found little evidence that predation limits populations of pigeons, woodpeckers and passerines, whereas evidence suggests that ground‐nesting seabirds, waders and gamebirds can be limited by predation. Using life‐history characteristics of prey species, we found that mainly long‐lived species with high adult survival and late onset of breeding were limited by predation. Single‐brooded species were also more likely to be limited by predation than multi‐brooded species. Predators that depredate prey species during all life stages (i.e. from nest to adult stages) limited prey numbers more than predators that depredated only specific life stages (e.g. solely during the nest phase). The Red Fox and non‐native mammals (e.g. the American Mink Neovison vison) were frequently identified as numerically limiting their prey species. Our review has identified predator–prey interactions that are particularly likely to result in population declines of prey species. In the short term, traditional predator‐management techniques (e.g. lethal control or fencing to reduce predation by a small number of predator species) could be used to protect these vulnerable species. However, as these techniques are costly and time‐consuming, we advocate that future research should identify land‐use practices and landscape configurations that would reduce predator numbers and predation rates.     

Impacts of birds of prey on gamebirds in the UK: a review

The influence of predators on the distribution, density and dynamics of their prey species has long been of interest to ecologists and wildlife managers. Where the prey population is also utilized by humans, conflicts may arise through competition for a limited resource. Because gamebird shooting in the UK provides employment, recreation and income, the impact of birds of prey on gamebird populations has been the subject of intense debate for many years. Various approaches have been used to assess the impacts that raptors have on gamebird populations. Here we review the applicability and limitations of the methods used and assess the scientific evidence for population-level and economic impacts of raptors on gamebird populations in the UK. Raptors may, in some situations, take large numbers of gamebirds and may be an important proximate cause of mortality, although few studies have addressed the impacts of raptors on either breeding or pre-shooting densities. Two exceptions are studies of Hen Harrier Circus cyaneus , Peregrine Falcon Falco peregrinus predation on Red Grouse Lagopus lagopus scoticus on moorland in Scotland and Sparrowhawk Accipiter nisus predation on Grey Partridge Perdix perdix on farmland in England. Both these studies suggested that raptors could have population-level impacts when their gamebird prey was already at low density. Studies on predation of captively bred gamebirds suggest that numbers taken by raptors at release pens vary considerably and in a few cases raptors have been documented killing relatively large numbers. On the whole, however, it appears that raptors account for a relatively small proportion of mortality among released birds and the impact on subsequent shooting bags is unknown. We summarize important gaps in current knowledge and recommend specific areas for future research.     

Community context and the influence of non-indigenous species on juvenile salmon survival in a Columbia River reservoir

Non-indigenous species (NIS) have been called biological pollutants, which implies that reducing their numbers should reduce negative impacts. To test this hypothesis, we used food web models, parameterized with data from field studies, to ask how reducing the number of NIS co-occurring with endangered salmon would affect salmon mortality. Our analyses indicate that predation on Upper Columbia River spring chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss juveniles was affected very little by NIS reduction. The effects of removing NIS were partly or totally offset by indirect food web interactions, and were subtle compared to effects of native predator management. We predict that the most effective way of reducing predation on salmon smolts will involve managing native predators and targeted removals of specific NIS. Minimizing impact of established NIS thus entails not only reducing NIS prevalence, but also considering background management practices and community context.     

An assessment of British farmers' and gamekeepers' experiences, attitudes and practices in relation to the European Polecat Mustela putorius

Postal questionnaires were distributed to farmers and gamekeepers within the Polecat’s Mustela putorius main range in Britain. Only 11% of responding farmers had ever experienced damage by Polecats; 28% regarded the species as a threat to livestock. Conversely 53% of farmers believed Polecats control Rabbits Oryctolagus cuniculus, and 39% believed they control rodents on the farm. Two-thirds of responding gamekeepers had experienced Polecat predation of penned game, with the commonest access routes via ‘pop-holes’ and beneath the perimeter wire. Most gamekeepers (68%) regarded the Polecat as a minor pest, but ranked it as a less serious threat to game than predators such as the Fox Vulpes vulpes, Feral Cat Felis catus, Stoat Mustela erminea, corvids and Mink Mustela vison. Majorities of both farmers and gamekeepers would be concerned about an increase in the numbers of Polecats, and most wished to be free to control the species. Trapping was regarded as the main defence against Polecat predation of game; 91% of gamekeepers had trapped Polecats over the preceding 5 years. A minority of farmers carried out Polecat control; this activity was more prevalent on farms near the fringe of the species’ range. Pest-control practices likely to impact accidentally or indirectly upon Polecats, such as rodenticide use, fumigation (‘gassing’) of Rabbit burrows and ferreting, were also more prevalent on farms towards the fringe of the species’ range. These findings are discussed in the light of the Polecat’s status as a Scheduled species recovering its range in Britain. In anticipation of the species’ further spread into areas where game shooting is prevalent, recommendations are made regarding the need to improve game husbandry and to modify trapping practice. In particular, the night-time closure of pop-holes and the effective exclusion of Polecats from tunnel traps are suggested as a means of promoting greater tolerance of Polecats and compliance with the law.     

Predation of released pheasants <Emphasis Type="Italic">Phasianus colchicus</Emphasis> on lowland farmland in the UK and the effect of predator control

We present data accumulated over the last 25 years on predation of radio-tracked released pheasants. In studies of birds during the autumn/winter at six pheasant shoots with high-density releases managed by full-time gamekeepers, predation of released pheasants by foxes before the shooting season began (July–September) averaged 19.2?±?4.0% per site, and during the shooting season (October 1st–February 1st), a further 15.9?±?1.9% were predated. The range in 3-year average predation rates between sites before shooting began was 8.6 to 42.4%. At seven different sites during the spring and summer, between 20 and 71% of released or wild hens that survived the shooting season were predated, mainly by foxes, between mid-March and mid-July. Predation was significantly higher at sites with low-level predator control (59?±?4.7%) compared to those with high-level control (30?±?5.3%). At three of the four sites with low predator control between 5 and 22% of nest failures were caused by incubating hens being predated by foxes. Our data quantify for the first time highly variable predation rates of released pheasants before and during the winter shooting season which we suggest was influenced by a range of site and management factors. During the spring and summer, our data provide evidence that predation of adult hen pheasants as well as nest predation can suppress breeding success and that predator control can reduce these losses.     

Long-term increase in the fecundity of hen harriers in Wales is explained by reduced human interference and warmer weather

The conservation status of the hen harrier Circus cyaneus in Britain, and its pivotal role in the raptor–grouse moor conflict, has rendered research on the factors influencing its population dynamics of crucial importance. Using over 20 years of data, we examine influences on the reproductive output of the small, but recently expanding, population of Welsh hen harriers. Productivity (fledglings per breeding attempt) has steadily increased with time, being the highest in recent years. Available evidence suggested that food availability could not account for these recent increases. Human interference (apparently, largely or entirely in the form of nest destruction by grouse moor gamekeepers) had a large influence on productivity, despite being restricted to a minority of the breeding population. This negative effect, and a positive effect of May temperature, was most influential on variation in breeding productivity. This study quantifies, for the first time, that cessation of persecution can result in a marked improvement in hen harrier reproductive output. Nest predation by red foxes Vulpes vulpes had no effect on variation in breeding productivity and there was no evidence that control of fox numbers by gamekeepers compensated for their depression of productivity through destroying harrier nests. Our analyses also indicated that gamekeepers probably destroyed an unknown number of nests before they were discovered. We argue that a low proportion of territorial females that apparently breed, in cases when clutch size and fledged brood size are high, may be an indication of persecution. The recent increase in the breeding productivity of Welsh harriers has probably been influential in the recent recovery of the Welsh harrier population and has apparently been due to a combination of cessation of human interference and warmer temperatures.     

Hunting‐mediated predator facilitation and superadditive mortality in a European ungulate

Predator‐prey theory predicts that in the presence of multiple types of predators using a common prey, predator facilitation may result as a consequence of contrasting prey defense mechanisms, where reducing the risk from one predator increases the risk from the other. While predator facilitation is well established in natural predator‐prey systems, little attention has been paid to situations where human hunters compete with natural predators for the same prey. Here, we investigate hunting‐mediated predator facilitation in a hunter‐predator‐prey system. We found that hunter avoidance by roe deer (Capreolus capreolus) exposed them to increase predation risk by Eurasian lynx (Lynx lynx). Lynx responded by increasing their activity and predation on deer, providing evidence that superadditive hunting mortality may be occurring through predator facilitation. Our results reveal a new pathway through which human hunters, in their role as top predators, may affect species interactions at lower trophic levels and thus drive ecosystem processes.     

Hen harriers and red grouse: economic aspects of red grouse shooting and the implications for moorland conservation

1.  Thirgood & Redpath (2008) propose ways in which red grouse : hen harrier conflicts could be resolved. It has also been suggested that grouse management could accept lower bag sizes (number of birds shot) thus reducing the need for intensive management of predators and habitats. This would allow hen harriers to co-exist more easily on grouse moors.
2.  We compare the bags, costs and incomes from these less intensive forms of grouse shooting with the more intensive driven shooting.
3.  Allowing high density grouse moors to decline to low density ones will result in greater loss of income than the corresponding saving of costs. This can result in moor owners abandoning grouse management and thus gamekeepers losing their employment.
4.  Losing gamekeepers from the uplands would jeopardize the protection of heather moorland and Special Protection Areas for birds, large areas of which are keepered and which currently support high numbers of breeding waders.
5.   Synthesis and applications . We agree with the study by Thirgood & Redpath that consideration of social and economic factors will be needed to resolve conflict but a reduction in management effort from driven to walked-up shooting is not the answer. A more satisfactory approach to the harrier : grouse conflict could be to try to reduce harrier predation by means of diversionary feeding and to address the problem of the rapid build-up in harrier numbers by exploring the use of a ceiling on harrier densities.     

Killer whale (Orcinus orca) predation in a multi-prey system

Predation can regulate prey numbers but predator behaviour in multiple-prey systems can complicate understanding of control mechanisms. We investigate killer whale (Orcinus orca) predation in an ocean system where multiple marine mammal prey coexist. Using stochastic models with Monte-Carlo simulations, we test the most likely outcome of predator selection and compare scenarios where killer whales: (1) focus predation on larger prey which presumably offer more energy per effort, (2) generalize by feeding on prey as encountered during searches, or (3) follow a mixed foraging strategy based on a combination of encounter rate and prey size selection. We test alternative relationships within the Hudson Bay geographic region, where evidence suggests killer whales seasonally concentrate feeding activities on the large-bodied bowhead whale (Balaena mysticetus). However, model results indicate that killer whales do not show strong prey specialization and instead alternatively feed on narwhal (Monodon monoceros) and beluga (Delphinapterus leucas) whales early and late in the ice-free season. Evidence does support the conjecture that during the peak of the open water season, killer whale predation can differ regionally and feeding techniques can focus on bowhead whale prey. The mixed foraging strategy used by killer whales includes seasonal predator specialization and has management and conservation significance since killer whale predation may not be constrained by a regulatory functional response.     

Two common invertebrate predators show varying predation responses to different types of sentinel prey

Sentinel prey (an artificially manipulated patch of prey) are widely used to assess the level of predation provided by natural enemies in agricultural systems. Whilst a number of different methodologies are currently in use, little is known about how arthropod predators respond to artificially manipulated sentinel prey in comparison with predation on free‐living prey populations. We assessed how attack rates on immobilized (aphids stuck to cards) and artificial (plasticine lepidopteran larvae mimics) sentinel prey differed to predation on free‐moving live prey (aphids). Predation was assessed in response to density of the common invertebrate predators, a foliar‐active ladybird Harmonia axyridis (Coleoptera: Coccinellidae), and a ground‐active beetle Pterostichus madidus (Coleoptera: Carabidae). Significant increases in attack rates were found for the immobilized and artificial prey between the low and high predator density treatments. However, an increased predator density did not significantly reduce numbers of free‐living live aphids included in the mesocosms in addition to the alternate prey. We also found no signs of predation on the artificial prey by the predator H. axyridis. These findings suggest that if our assessment of predation had been based solely on the foliar artificial prey, then no increase in predation would have been found in response to increased predator density. Our results demonstrate that predators differentially respond to sentinel prey items which could affect the level of predation recorded where target pest species are not being used.     

A parasite's modification of host behavior reduces predation on its host

Parasite modification of host behavior is common, and the literature is dominated by demonstrations of enhanced predation on parasitized prey resulting in transmission of parasites to their next host. We present a case in which predation on parasitized prey is reduced. Despite theoretical modeling suggesting that this phenomenon should be common, it has been reported in only a few host–parasite–predator systems. Using a system of gregarine endosymbionts in host mosquitoes, we designed experiments to compare the vulnerability of parasitized and unparasitized mosquito larvae to predation by obligate predatory mosquito larvae and then compared behavioral features known to change in the presence of predatory cues. We exposed Aedes triseriatus larvae to the parasite Ascogregarina barretti and the predator Toxohrynchites rutilus and assessed larval mortality rate under each treatment condition. Further, we assessed behavioral differences in larvae due to infection and predation stimuli by recording larvae and scoring behaviors and positions within microcosms. Infection with gregarines reduced cohort mortality in the presence of the predator, but the parasite did not affect mortality alone. Further, infection by parasites altered behavior such that infected hosts thrashed less frequently than uninfected hosts and were found more frequently on or in a refuge within the microcosm. By reducing predation on their host, gregarines may be acting as mutualists in the presence of predation on their hosts. These results illustrate a higher‐order interaction, in which a relationship between a species pair (host–endosymbiont or predator–prey) is altered by the presence of a third species.     

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.     

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.     

Nest Predation Deviates from Nest Predator Abundance in an Ecologically Trapped Bird

In human-modified environments, ecological traps may result from a preference for low-quality habitat where survival or reproductive success is lower than in high-quality habitat. It has often been shown that low reproductive success for birds in preferred habitat types was due to higher nest predator abundance. However, between-habitat differences in nest predation may only weakly correlate with differences in nest predator abundance. An ecological trap is at work in a farmland bird (Lanius collurio) that recently expanded its breeding habitat into open areas in plantation forests. This passerine bird shows a strong preference for forest habitat, but it has a higher nest success in farmland. We tested whether higher abundance of nest predators in the preferred habitat or, alternatively, a decoupling of nest predator abundance and nest predation explained this observed pattern of maladaptive habitat selection. More than 90% of brood failures were attributed to nest predation. Nest predator abundance was more than 50% higher in farmland, but nest predation was 17% higher in forest. Differences between nest predation on actual shrike nests and on artificial nests suggested that parent shrikes may facilitate nest disclosure for predators in forest more than they do in farmland. The level of caution by parent shrikes when visiting their nest during a simulated nest predator intrusion was the same in the two habitats, but nest concealment was considerably lower in forest, which contributes to explaining the higher nest predation in this habitat. We conclude that a decoupling of nest predator abundance and nest predation may create ecological traps in human-modified environments.     

Effects of perceived predation risk and social environment on the development of three‐spined stickleback (Gasterosteus aculeatus) morphology

Phenotypically plastic changes in response to variation in perceived predation risk are widespread, but little is known about if and how social environment modulates induced responses to predation risk. We investigated the influence of perceived predation risk (i.e. chemical cues from a predator) and social environment (i.e. one, two or 20 individuals reared together) on three‐spined stickleback (Gasterosteus aculeatus) morphology in a factorial common garden experiment. We found that exposure to chemical cues from potential predators did not influence growth or body condition or induce more robust morphological defences (i.e. lateral plate numbers and dorsal spine lengths). However, sticklebacks exposed to predator cues developed longer caudal peduncles and larger eyes as compared with fish from the control treatment. As these responses may improve sticklebacks’ ability to avoid piscine predation, they might be adaptive. Social environment/density also influenced expression of some traits, but these effects were independent of predation‐risk treatment effects. In general, these results suggest that apart from the classic morphological defence structures, which appear mostly constitutive, three‐spined sticklebacks are capable of expressing potentially adaptive morphological responses to chemical cues from potential predators.     

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.     

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.     

Pests controlling pests: does predator control lead to greater European rabbit abundance in Australasia?

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Temporal environmental variation and phenotypic plasticity: a mechanism underlying priority effects

Understanding the role of history in the formation of communities has been a major challenge in community ecology. Here, we explore the role of phenotypic plasticity and its associated trait‐mediated indirect interactions as a mechanism behind priority effects. Using organisms with inducible defenses as a model system, we examine how aquatic communities initially containing different predator environments are affected at the individual and community level by the colonization of a second predator. Snails and tadpoles were established in four different caged‐predator environments (no predator, fish, crayfish or water bugs). These four communities were then crossed with three predator colonization treatments (no colonization, early colonization, or late colonization) using lethal water bugs as the predator. The snails responded to the caged predator environments with predator‐specific behavioral and morphological defenses. In the colonization treatments, snails possessing the wrong phenotype attempted to induce phenotypic changes to defend themselves against the new risk. However, snails initially induced by a different predator environment often suffered high predation rates. Hence, temporal variation in predation risk not only challenged the snail prey to try to track this environmental variation through time by adjusting their defensive phenotypes, but also caused trait‐mediated interactions between snails and the colonizing predator. For tadpoles within these communities, there was little evidence that the morphological responses of snails indirectly effected tadpole predation rates by colonizing water bugs. Unexpectedly, predation rates on tadpoles by colonizing water bugs were generally higher in the three caged‐predator treatments, suggesting that water bugs elevated their foraging activity in response to potentially competing predators. In summary, we demonstrate an important priority effect in which the initial occurrence of one species of predator can facilitate predation by a second predator that colonizes at a later date (i.e. a TMII) suggesting that phenotypic plasticity can be an important driver behind priority effects (i.e. historical exposure to predators).