Feeding Behaviour Of Predators Research Articles

Effects of Encarsia formosa (Hymenoptera: Aphelinidae) parasitism on the feeding preference and performance of the whitefly predator Serangium japonicum (Coleoptera: Coccinellidae)

The relationship among the whitefly Bemisia tabaci (Hemiptera: Aleyrodidae), its predator Serangium japonicum (Coleoptera: Coccinellidae), and its parasitoid Encarsia formosa (Hymenoptera: Aphelinidae) can be defined as intraguild predation (IGP). IGP represents an interaction that combines elements of both predation and competition between two species. In our previous study, predators (S. japonicum) often indirectly reduce the number of parasitoids (E. formosa) by consuming shared food resources, such as B. tabaci. To suppress whitefly outbreaks, a combination of parasitoids and predators is often released together using a mixed-species approach. However, the impact of the parasitic behavior of parasitoids on the feeding behavior of specific predators, such as S. japonicum, has not yet been well-studied. We conducted a laboratory study to examine how the parasitoid E. formosa affects the predation rates and performance of S. japonicum. Results indicate that S. japonicum shows a preference for non-parasitized whiteflies over parasitized ones, regardless of the availability of choices. Furthermore, we found a correlation between the preference for non-parasitized whiteflies and the duration of parasitization. As the developmental days of the parasitoid mummies increased, the food intake of the predatory group decreased significantly. Additionally, as the parasitoid mummy develops, S. japonicum spends more time exploring and probing the mummy before deciding whether to consume it. In summary, our study suggests that S. japonicum exhibits a significant preference for consuming non-parasitized whiteflies. Therefore, it is important to coordinate the release timing of S. japonicum and E. formosa in the biological control of B. tabaci.

ROLE OF SPACE IN AN ECO-EPIDEMIC PREDATOR-PREY SYSTEM WITH THE EFFECT OF FEAR AND SELECTIVE PREDATION

In ecological systems, the fear of predation risk asserts a privilege to the prey species by restricting their exposure to the potential predators. It also imposes costs by constraining the exploration of optimal resources. Additional foods for predators play a pivotal role in the biological conservation programs. The predators have ability to distinguish between the susceptible and infected prey items, and they avoid the latter ones to reduce their fitness cost. A predator-prey model with disease in prey is investigated in this study with an aim to explore the effects of fear factor, additional foods and selective predation on the ecological systems. We also investigate the spatio-temporal model to incorporate the facts that the prey and predator populations perform active movements in the spatial directions for their biological relevance. Both the temporal and spatio-temporal models are analyzed through noteworthy mathematical as well as numerical techniques. Our simulation results show that the level of fear responsible for the reduction in the birth rate of susceptible prey, rate of disease transmission and the selective feeding behavior of predators have potentials to create instability in the ecosystem. In contrast, the level of fear responsible for reduction in the disease prevalence can restore stability in the ecosystem by killing the persistent oscillations. Our eco-epidemic system exhibits chaotic nature if the growth of predators due to additional food sources is very low. We find that the spatio-temporal model demonstrates different spatial patterns of the prey and predator populations in the ecosystem.

Predators minimize energy costs, rather than maximize energy gains under warming: Evidence from a microcosm feeding experiment

Abstract Climate warming may alter predator–prey interactions and predator feeding behaviour due to increased metabolic demands. How predators meet these increased demands may depend on trade‐offs in prey energy content and body size, handling time and other functional constraints. We tested hypotheses associated with these trade‐offs with the predatory mite Stratiolaelaps scimitus, and three prey that differed in body size, energy content, and defenses (Folsomia candida, Oppia nitens, and Carpoglyphus lactis). We estimated metabolic rate, predation in choice and no choice feeding trials, movement rate, and lipid and protein content for all four species at 16°C and 24°C. We used these data to estimate the predator's energy demands and compared these to estimated energy intake in the choice feeding trials. Predators had greater metabolic demands at 24°C than at 16°C, but temperature did not affect predator or prey movement rates. Warming decreased lipid content, but not protein content, of all three prey species, leading to lower energy content for C. lactis and O. nitens, but not F. candida. In both feeding trials at 24°C, predators increased their feeding on the smaller, energy‐poor C. lactis, but not the larger, energy‐rich F. candida, resulting in lower estimated energy intake. S. scimitus did not feed on O. nitens at either temperature. Predators increasingly fed on small‐bodied prey under warming and not the large‐bodied prey despite the potential for greater energetic gains from larger prey. We posit that predators minimized energy lost during feeding through lower handling costs associated with C. lactis, rather than maximize energy gain. We conclude that selection of prey based on body size changes with temperature as a trade‐off for predators to balance increased metabolic demands. As predators provide top‐down control and regulate energy flow through the consumption of their prey, changes to predator feeding behaviour with climate warming may affect food web dynamics and ecosystem‐level processes. Read the free Plain Language Summary for this article on the Journal blog.

Assassin snails (Anentome helena) as a biological model for exploring the effects of individual specialisation within generalist predators.

Quantifying feeding behaviour of generalist predators at the population and individual levels is crucial for understanding the structure and functioning of food webs. Individual predator/consumer feeding niches can be significantly narrower than that of the population across animal taxa. In such species, the population of a generalist predator becomes essentially an ensemble of specialist individuals and this often highly affects the dynamics of the prey-predator interactions. Currently, few experimental systems exist that are both easily technically manipulated in a lab and are reliable to accurately assess effects of individual specialisation within generalist predators. Here we argue that a freshwater predaceous snail, Anentome helena (also known as an ‘assassin snail’), is a convenient and reliable experimental system to study feeding of a generalist predator on multiple food types which exhibits well-pronounced specialisation of foraging individuals. Using A. helena we experimentally test: (i) how relative prey abundances in the environment affect the feeding patterns, (ii) whether the feeding patterns are consistent over the duration of the experimental period, and (iii) compare the feeding niche breadth of individuals to that of the laboratory population. By offering four different prey snail species, at a range of relative abundances, we show that there are consistent patterns in feeding. Importantly, the consumption of each prey was independent of the relative abundance at which they were present. Individual predators showed selectivity to a particular prey, i.e. the population of assassin snails seems to be formed of individuals that specialise on different prey. Our findings would contribute to the recent revision and the ongoing debate on the classification of predator species into generalists and specialists.

Penguins and Seals Transport Limiting Nutrients Between Offshore Pelagic and Coastal Regions of Antarctica Under Changing Sea Ice

Large animals such as sea birds and marine mammals can transport limiting nutrients between different regions of the ocean, thereby stimulating and enhancing productivity. In Antarctica this process is influenced by formation and breakup of sea ice and its influence on the feeding behaviour of predators and their prey. We used analyses of bioactive metals (for example, Fe, Co, Mn), macronutrients (for example, N) and stable isotopes (δ13C and δ15N) in the excreta of Adelie (Pygoscelis adeliae) and emperor penguins (Aptenodytes forsteri) as well as Weddell seals (Leptonychotes weddellii) from multiple sites, among multiple years (2012–2014) to resolve how changes in sea ice dynamics, as indicated by MODIS satellite images, were coincident with prey switching and likely changes in nutrient fluxes between the offshore pelagic and coastal zones. We also sampled excreta of the south polar skua (Stercorarius maccormicki), which preys on penguins and scavenges the remains of both penguins and seals. We found strong coincidence of isotopic evidence for prey switching, between euphausiids (Euphausia superba and E. crystallorophias) and pelagic/cryopelagic fishes (for example, Pleuragramma antarcticum) in penguins, and between pelagic/cryopelagic fishes and Antarctic toothfish (Dissostichus mawsoni) in Weddell seals, with changes in sea ice cover among years. Further, prey switching was strongly linked to changes in the concentrations of nutrients (Fe and N) deposited in coastal environments by both penguins and seals. Our findings have important implications for understanding how the roles of large animals in supporting coastal productivity may shift with environmental conditions in polar ecosystems.

A cat in paradise: hunting and feeding behaviour of Eurasian lynx among abundant naive prey

Kill rates of predators typically increase when they come into contact with naive and abundant prey. Such a situation can lead to surplus killing or the occurrence of parallel kills (i.e. additional kills that predator makes while still consuming the carcass from the previous kill). However, there is limited information on the feeding behaviour of predators during such events and how they affect kill rates. Here we report on hunting and feeding behaviour of a male Eurasian lynx (Lynx lynx) that dispersed into a region where this apex predator had been absent for several decades. We also report on the kleptoparasitism by wild boar (Sus scrofa), which effects on lynx prey consumption have not yet been explored. We found 66 ungulates killed by the lynx, among which 39% were part of parallel kills. Compared to the single kills, lynx fed on parallel kills for 2.7-times longer, while the kill rate was 37% higher, resulting in one of the highest kill rates reported so far for male lynx in Europe. We did not detect differences in search times following single or parallel kills and the average distance between consecutive kills was similar in both kill types. We also recorded the highest kleptoparasitism rate by dominant scavengers on Eurasian lynx, as 48% of kills were usurped and consumed by the wild boars. Kleptoparasitism reduced the average time lynx was able to feed on prey for 52% compared to kills not found by wild boars. However, the lynx did not compensate for these losses by increasing the hunting effort, probably due to abundant naive prey available in the area.

Oviposition preference and adult performance of the whitefly predator Serangium japonicum (Coleoptera: Coccinellidae): effect of leaf microstructure associated with ladybeetle attachment ability

The leaf surface microstructure can greatly influence predator feeding behavior. However, its effects on predator oviposition preference, which is crucial for arthropod fitness at the population level, are largely unknown. This study aimed to test leaf discs and plants of five common host plant species of Bemisia tabaci, including Chinese kale, cotton, cucumber, eggplant, and sweetpotato, to determine the oviposition preference and offspring and adult performance of the whitefly predator Serangium japonicum. Cannibalism risk, attachment force, microstructure of the abaxial leaf surface (ALS), and ladybeetle tarsal morphology were examined. Ladybeetle's oviposition preference had no correlation with offspring performance but positively correlated with fecundity. Further, oviposition preference to leaf discs and fecundity positively correlated with attachment force. The cannibalism risk was not significantly different between plant species. The ALS of Chinese kale and eggplant supported the smallest and the largest attachment forces, respectively. The first one had epicuticular wax crystals, whereas the latter had stellate trichomes. The ALS of cotton and sweetpotato did not bear wax crystals or long trichomes. Cucumber leaves were covered with tapered trichomes. Tenant setae on the distal second tarsomere and a pair of curved, tapered claws on the distal fourth tarsomere were the attachment structures of S. japonicum, which interacted with the plant surface structures and generated the attachment force. Plant morphological traits, associated with ladybeetle attachment force and adult performance might be key factors in ladybeetle oviposition preference, and are expected to occur in other host plant herbivore-predator systems.

On the mechanistic understanding of predator feeding behavior using the functional response concept

AbstractFunctional response models describe the relationship between prey density and per capita prey consumption rate by a predator. Type II functional responses, in which density‐dependent predation occurs via a decelerating feeding rate, seem to prevail in nature and are commonly described by Holling’s disk equation. In the derivation of the disk equation, Holling did not include digestion time. Although some authors have later extended the interpretation of handling time by also including digestion time, this violates the key assumption of the disk equation that the processes of searching for and handling prey are mutually exclusive. The steady‐state satiation (SSS) equation is a functional response model that discriminates between handling and digestion time. The application of the SSS equation is underutilized so far in the ecological literature, probably due to its complexity. In this study, we first tested the viability of the SSS equation. Second, we investigated the mechanistic basis of the SSS equation, comparing the model’s predictions with directly observed data. For this purpose, we used predator–prey systems of different taxa, that is, the ladybird beetle Hippodamia variegata preying on the aphid Aphis fabae, the lacewing Chrysoperla agilis preying on the aphid Myzus persicae, and the predatory mite Iphiseius degenerans preying on the thrips Frankliniella occidentalis. Our results show that the SSS equation is viable and can realistically describe type II functional response. In all predator–prey systems we tested, the model fitted the data reasonably well and provided realistic estimation of its parameters.

Timing and duration of phenological resources: Toward a mechanistic understanding of their impacts on community structure and ecosystem processes in stream food chains

AbstractPhenological resources are common across many ecological communities, and can strongly affect community dynamics. Recent field manipulation experiments in stream food chains found that seasonal timing and duration of terrestrial prey inputs affected the feeding behavior, growth, and maturation of fish predators, caused predator‐mediated indirect effects on aquatic prey, and modified trophic‐cascading effects on ecosystem processes (i.e., litter decomposition). These experiments described impacts of resource phenological changes over a few month period, but long‐term impacts of continued changes in resource phenology are unknown. Here, we develop a mathematical model to extrapolate long‐term predictions about the effects of changes in resource phenology from the results of field manipulation experiments. The model predicts that advanced timing generally decreases aquatic prey and litter processing and prolonged duration will either increase or decrease aquatic prey and litter processing depending on the total amount and pre‐disturbed timing and duration of terrestrial prey inputs. Our modeling approach clarifies that stage‐specific responses of life history processes in fish (i.e., growth, maturation, and reproduction) to phenological changes in terrestrial prey inputs mediate indirect effects on aquatic prey and litter processing. We argue that stage‐specific responses of life history processes are an integral part of the mechanisms with which to predict the consequences of phenological species interactions at the community and ecosystem levels.

What five insects told us about how a native plant copes with real-world problems

What five insects told us about how a native plant copes with real-world problems

Woolly Apple Aphid Generalist Predator Feeding Behavior Assessed through Video Observation in an Apple Orchard

Generalist predators are considered effective biological control agents in many agroecosystems. However, characterizing the behavior of different generalist predator species in realistic field settings is difficult due to challenges associated with directly observing predation events in the field. Here we video recorded woolly apple aphid (Erisoma lanigerum) (Hemiptera: Aphididae) colonies to directly observe the feeding behavior of their generalist predators during the day and night. To observe European earwigs (Forficula auricularia) (Dermaptera: Forficulidae), which are thought to be key woolly apple aphid predators but were not present at our study location, we released earwigs into the study area. Across 1413 h of video recorded over 4 weeks, earwigs made the most attacks on woolly apple aphid colonies, but coccinellid larvae spent more total time attacking because their individual attacks were longer. Antagonistic interactions between predators were rare, and earwigs never antagonized other predators. However, ants commonly antagonized earwigs, and incidence of ant-earwig antagonistic interactions was negatively correlated with earwig-aphid attack rates. Overall, these results suggest that coccinellid larvae and earwigs differ in their feeding behavior in the field, and provide new evidence that ants may hinder aphid biological control by antagonizing earwigs.

Hawaiian monk seals exhibit behavioral flexibility when targeting prey of different size and shape.

Animals use diverse feeding strategies to capture and consume prey, with many species switching between strategies to accommodate different prey. Many marine animals exhibit behavioral flexibility when feeding to deal with spatial and temporal heterogeneity in prey resources. However, little is known about flexibility in the feeding behavior of many large marine predators. Here, we documented the feeding behavior and kinematics of the endangered Hawaiian monk seal (Neomonachus schauinslandi, n=7) through controlled feeding trials. Seals were fed multiple prey types (e.g. night smelt, capelin, squid and herring) that varied in size and shape to examine behavioral flexibility in feeding. Hawaiian monk seals primarily used suction feeding (91% of all feeding trials) across all prey types, but biting, specifically pierce feeding, was also observed (9% of all feeding trials). Suction feeding was characterized by shorter temporal events, a smaller maximum gape and gape angle, and a fewer number of jaw motions than pierce feeding; suction feeding kinematic performance was also more variable compared with pierce feeding. Seals showed behavioral flexibility in their use of the two strategies. Suction feeding was used most frequently when targeting small to medium sized prey and biting was used with increasing frequency on larger prey. The feeding kinematics differed between feeding strategies and prey types, showing that Hawaiian monk seals adjusted their behaviors to particular feeding contexts. Hawaiian monk seals are opportunistic marine predators and their ability to adapt their feeding strategy and behavior to specific foraging scenarios allows them to target diverse prey resources.

Predators buffer the effects of variation in prey nutrient content for nutrient deposition

Predator feeding behavior and digestion regulate the flow of nutrients through ecosystems by determining the fate of prey nutrients. Most predators feed on a diversity of prey items, which differ widely in traits including their nutrient content. Yet, relatively little is known of the mechanisms through which variation in prey nutrient content affects the form by which nutrients are deposited into the environment. The overall goal of this study was to test how variation in the nutrient content of prey affected the fate of nutrients following predation by an arthropod carnivore, the Carolina wolf spider Hogna carolinensis. We manipulated the macronutrient content of prey by varying the diet on which crickets were fed to produce prey treatments that differed in lipid and protein content. Nutrients were measured as both macronutrients and elements in prey and elements in excreta. We found that there was no effect of diet treatment on the amounts of elements or macronutrients in prey carcasses and excreta despite significant variation in the nutrient content of those prey. This is in contrast to studies of some aquatic systems where mass balance by consumers results in variation in excreta content depending on the nutrient content of food. Wolf spiders assimilated the majority of prey nutrients and deposited relatively small and similar amounts of nutrients following feeding. Hence, while prey can vary widely in nutrient content, our findings suggest that this variation has little effect on the amounts of nutrients deposited by predators.

Distinct feeding strategies of generalist and specialist spiders

1. Feeding behaviour of generalist and specialist predators is determined by a variety of trophic adaptations. Specialised prey‐capture adaptations allow specialists to catch relatively large prey on a regular basis. As a result, specialists might be adapted to exploit each item of prey more thoroughly than do generalists.2. It was expected that obligatory specialist cursorial spiders would feed less frequently than generalists but for a longer time and, thus, that their foraging pause would be longer. First, the feeding frequencies of three generalist spider species (Cybaeodamus taim, Harpactea hombergi, Hersiliola sternbergsi) were compared with those three phylogenetically related specialist species: myrmecophagous Zodarion rubidum, and araneophagous Nops aff. variabilis and Palpimanus orientalis.3. Generalists captured more prey, exploited each item of prey for a significantly shorter time, and had a shorter foraging pause than was the case for specialists. Generalists also gained significantly less relative amount of prey mass than did specialists.4. Second, the study compared the prey DNA degradation rate in the gut of generalists and specialists by means of PCR. The degradation rate was not significantly different between specialists and generalists: the detectability half‐life was estimated to exist for 14.3 days after feeding.5. This study shows that the feeding strategies of cursorial generalist and obligatory specialist spiders are different. Obligatory specialists have evolved a feeding strategy that is based on thorough exploitation of a few large prey, whereas generalists have evolved a strategy that is based on short exploitation of multiple small items of prey.

Habitat heterogeneity induces rapid changes in the feeding behaviour of generalist arthropod predators.

The “habitat heterogeneity hypothesis” predicts positive effects of structural complexity on species coexistence. Increasing habitat heterogeneity can change the diversity (number of species, abundances) and the functional roles of communities. The latter, however, is not well understood as species and individuals may respond very differently and dynamically to a changing environment.Here, we experimentally test how habitat heterogeneity affects generalist arthropod predators, including epigaeic spiders, carabid and staphylinid beetles, under natural conditions by assessing their diversity and directly measuring their trophic interactions (which provide a proxy for their functional roles). The experiment was conducted in spring barley fields in Southern Sweden where habitat heterogeneity was manipulated by increasing within‐field plant diversity.Increased habitat heterogeneity triggered rapid changes in the feeding behaviour of generalist predators characterized by lower trophic specialization at both network (H2’, degree of interaction specialization in the entire network) and species level (d’, degree of interaction specialization at the species level). We presume that this is because spatial separation resulted in relaxed competition and allowed an increased overlap in resources used among predator species. Predators collected from heterogenous habitats also showed greater individual‐level dietary variability which might be ascribed to relaxed intraspecific competition.Our results provide conclusive evidence that habitat heterogeneity can induce rapid behavioural responses independent of changes in diversity, potentially promoting the stability of ecosystem functions. A plain language summary is available for this article.

Apple Snail Perivitellin Precursor Properties Help Explain Predators' Feeding Behavior.

In contrast with vitellogenin maturation, it is unknown whether gastropod perivitellin precursors are subject to large structural changes. The gastropod reproductive tract includes an accessory organ, the albumen gland (AG), that produces and secretes perivitelline fluid. In the apple snail Pomacea canaliculata, the large, reddish-pink AG provides eggs with perivitellins that are defensive against predators. Although the AG makes a considerable contribution to apple snail biomass, field observations indicate that it is rejected by avian and mammalian predators, although the underlying reason remains unknown. By analyzing the structure-function properties of P. canaliculata perivitellin precursors, we provide insight into perivitellin maturation and its relationship with apple snail predator feeding behavior. Structural analysis using small-angle X-ray scattering, absorption and fluorescence spectroscopy, circular dichroism, electrophoresis, chromatography, and partial proteolysis showed that the size, shape, and structure of perivitellin precursors resemble those of egg mature forms. Functional analysis indicates that the precursors of the defensive perivitellins ovorubin (PcOvo) and perivitellin-2 (PcPV2) are highly stable and antinutritive, withstanding proteinase digestion and displaying structural stability of their quaternary structure under a wide pH range (4.0-10.0). Furthermore, AG extracts limit a predator's ability to digest nutrients and are toxic to mice (median lethal concentration 96 h after administration: 5.9 mg/kg). Treated mice displayed neurologic signs similar to those produced by egg PcPV2. Results indicate that apple snails store active precursors of egg proteins inside the AG, providing evidence that gastropod perivitellin precursors do not experience the large structural processing of invertebrate vitellogenin maturation. These defensive proteins provide the apple snail AG with neurotoxic, antinutritive, and antidigestive activity, a likely explanation for the predators' feeding behavior.

Wrecking the Curve: Altered Functional Response of Tetragoneuria (Odonata: Corduliidae) Naiads Infected with Metacercariae of Haematoloechus floedae.

The ubiquity of host-parasite interactions and their potential for substantial representation, in terms of overall biomass, within ecosystems suggests that parasites have the capacity to influence energy flow within an ecosystem. Although the influence of certain parasites on prey behavior has been well documented, parasites could also exert an influence on ecosystem dynamics by influencing predator feeding behavior. The functional response of Tetragoneuria naiads was characterized by presenting naiads with varying abundances of Daphnia magna , after which a subset of the naiads were exposed to cercariae of Haematoloechus floedae, and the feeding trials repeated for both the control and exposed odonates. A type II functional response was chosen as an appropriate model for comparison. An indicator variable approach to nonlinear regression of the functional response data indicated that infected odonate naiads spent significantly more time foraging than they did before infection, whereas there was no significant change in the functional response of the control naiads. Infected odonates also had a slower rate of growth. These results imply a metabolic cost to infection of Tetragoneuria naiads by H. floedae that might be associated with the encapsulating response to the metacercariae that was observed in infected naiads.

Bayesian inference and model choice for Holling’s disc equation: a case study on an insect predator-prey system

The dynamics of predator-prey systems relate strongly to the density (in)dependent attributes of the predator’s feeding rate, i.e., its functional response. The outcome of functional response models is often used in theoretical or applied ecology in order to extract information about the mechanisms associated with the feeding behavior of predators. The focus of this study centres upon Holling’s type II functional response model, commonly known as the disc equation, which describes an inverse-density dependent mortality caused by a single predator to its prey. A common method to provide inference on functional response data involves nonlinear least squares optimization, assuming independent Gaussian errors, an assumption often violated in practice due to the heteroscedasticity which is typically present in the data. Moreover, as prey depletion is common in functional response experiments, the differential form of disc equation ought to be used in principle. We introduce a related statistical model and adopt a Bayesian approach for estimating parameters in ordinary differential equation models. In addition, we explore model uncertainty via Bayes factors. Our approach is illustrated via the analysis of several data sets concerning the functional response of a widespread ladybird beetle (Propylea quatuordecimpunctata) to its prey (Aphis fabae), predicting the efficiency of this predator on a common and important aphid species. The results showed that the approach developed in this study is towards a direction for accurate estimation of the parameters that determine the shape of the functional response of a predator without having to make unnecessary assumptions. The R (www.r-project.org) code for fitting the proposed model to experimental data is made freely available.

Functional responses to changes in rabbit abundance: is the eagle owl a generalist or a specialist predator?

How foraging behaviour of predators is influenced by prey abundance is often not clear. Predators may behave as generalist or specialist depending on the relationship between the predator feeding behaviour and prey abundance, known as functional response. The European rabbit (Oryctolagus cuniculus) is a key species for most Iberian vertebrate predators, especially for large raptors such as the eagle owl (Bubo bubo). The eagle owl functional response was analysed in central Iberian Peninsula by fitting different models (null, linear, two asymptotic and two sigmoidal models) between rabbit abundance and rabbit consumption by the eagle owl. Rabbit consumption increased as did its abundance until reaching an asymptote. Rabbit-consumed biomass better fitted to a type II functional response (convex, typical of specialist predators), whereas rabbit frequency in eagle owl diet better fitted to a type III functional response (sigmoidal, typical of generalist predators). Trophic diversity increased as rabbit abundance decreased. The consumption of other prey, such as Iberian hare (Lepus granatensis) and medium-size birds (mainly galliformes and columbiformes), increased as rabbit abundance decreased. The eagle owl is highly dependent on rabbits in central Spain, even at low rabbit abundance. In this situation, the eagle owl diversifies its diet, making it difficult to define the eagle owl as a specialist or as a generalist predator. Therefore, the eagle owl in the centre of the Iberian Peninsula should be rather considered as a facultative predator on wild rabbits.

Spatial variation of trace metals within intertidal beds of native mussels (Mytilus edulis) and non-native Pacific oysters (Crassostrea gigas): implications for the food web?

Pollution is of increasing concern within coastal regions and the prevalence of invasive species is also rising. Yet the impact of invasive species on the distribution and potential trophic transfer of metals has rarely been examined. Within European intertidal areas, the non-native Pacific oyster (Crassostrea gigas) is becoming established, forming reefs and displacing beds of the native blue mussel (Mytilus edulis). The main hypothesis tested is that the spatial pattern of metal accumulation within intertidal habitats will change should the abundance and distribution of C. gigas continue to increase. A comparative analysis of trace metal content (cadmium, lead, copper and zinc) in both species was carried out at four shores in south-east England. Metal concentrations in bivalve and sediment samples were determined after acid digestion by inductively coupled plasma-optical emission spectrometry. Although results showed variation in the quantities of zinc, copper and lead (mg m−2) in the two bivalve species, differences in shell thickness are also likely to influence the feeding behaviour of predators and intake of metals. The availability and potential for trophic transfer of metals within the coastal food web, should Pacific oysters transform intertidal habitats, is discussed.