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We test whether the spatial distribution of birds within a habitat is determined by predation risk and also by interspecific competition. The work was carried out in a montane mixed forest of central Spain with four Parus species, the long-tailed tit ( Aegithalos caudatus) and the nuthatch ( Sitta europaea). Experimental feeders, that varied in their risk of predation, were used to control the effect of natural variations in food availability and quality on the habitat use patterns of different species. Tree gleaning passerines avoided feeding on dark inner forest places far from edges, distant from protective cover, outside the inner tree canopy and near the ground; they preferred deciduous, relatively clear forest plots. These effects remained invariable across years and weather conditions. There was a common pattern of selection of foraging locations by the four Parus species: proximity to cover and height above ground and over the lowest branches of the tree canopy positively influenced the use of feeding places. According to these patterns, the vigilance proportion of species was significantly higher when feeding far from cover than when birds were feeding near pine foliage. This pattern was also common for the four studied Parus species. Nevertheless, the interspecific dominance hierarchy of the species was positively correlated with the use of the safest feeders (feeders farther the ground and nearer from protective cover within tree canopy), being the converse with the most exposed ones. Therefore, the results of this paper demonstrate that the selection of feeding locations within habitat follows a pattern minimizing predation risk. Interspecific dominance hierarchies can lead to the exploitation of unfavourable risky patches by subordinate species. [ABSTRACT FROM AUTHOR]

Holling's type II functional response is a cornerstone of community ecology and coevolutionary theory. The so-called disc equation is the most widely used model of the type II response, yet thus far no robust experimental assessment has been achieved in any single system. Fundamental issues that remain to be assessed include whether the assumptions of the disc equation are fulfilled, whether the disc equation yields accurate estimates of predation-related individual traits, and whether differences in disc equation parameters can capture genetic variation in prey behaviour. This paper provides a rigorous approach to all of these questions. The functional response of the predatory mite Pergamasus crassipes on three genetically distinct clones of the springtail Folsomia candida was measured at six levels of prey density in controlled conditions where prey number and arena size were concomitantly manipulated. A crucial assumption of Holling's disc equation was fulfilled by maintaining a constant prey density for the entire experimental period of predation. The timing of each attack and capture, as well as the duration of the handling time, were recorded by constant observation. We contrasted three different methods to calculate functional response curves: (1) indirect estimation of the disc equation's parameters from the number of prey killed by the end of each experimental run; (2) direct estimation of the parameters via a unique protocol of constant observation; and (3) independently deriving a function based on direct measurements of encounter rate and attack success. The basic assumptions of the disk equation were globally fulfilled. Estimations of the functional response's parameters (type II) were remarkably congruent across approach (1) and (2). A single genetic effect was detected – the relationship between the encounter rate and prey density differed significantly between clones – whereas a direct comparison of functional response across clones failed to reveal genetic variation. [ABSTRACT FROM AUTHOR]

This paper reports the effects of food supply, predation and the interaction between them on the population dynamics of root voles,Microtus oeconomus, by adopting factorial experiments in field enclosures. This two-factor experiment proved the general hypothesis that food supply and predation had independent and additive effects on population dynamics of root voles. The experimental results proved the following predictions: (1) predation reduced population density and recruitment significantly; (2) food supply increased population density; (3) predation and food supply influenced spacing behavior of root voles separately and additively: Exposure to predation reduced long movements of root voles between trapping sessions; additional food supply reduced aggression level and home range size of root voles. Less movement of individuals that exposed to predators possibly reduced their opportunity of obtaining food and lessened population survival rate, which led population density to decrease. Smaller home range and lower aggression level could make higher population density tolerable. The interactive effect of predation and food on home range size was highly significant (P=0.0082<0.01). The interactive effect of food and predation on dispersal rate was significant (P<0.01). From the experimental results, we conclude that the external factors (predation, food supply) were more effective than internal factors (spacing behavior) in determining population density of root voles– under the most favorable external conditions (−P, +F treatment), the mean density and mean recruitment of root vole population was the highest; under the most unfavorable external conditions (+P,−F treatment), the mean density and mean recruitment of root vole population was the lowest. [ABSTRACT FROM AUTHOR]

Organisms often perceive predation risk through visual, auditory, or chemical cues that accompany or persist after an attack on other prey individuals. In this paper an argument is developed that suggests that it is adaptive for species that use such indirect cues to include conspecific density in the assessment of predation risk, and to respond to conspecific density by modifying phenotype (e.g. behavior, morphology of life history). A model based on this argument predicts that at equivalent (including negligible) indirect cue levels an organism should adopt less vulnerable phenotypes at lower conspecific density. Further, the phenotypic modifications to differences in conspecific density are predicted to be on the same order of magnitude as phenotypic responses to differences in predator density, to be more pronounced at higher indirect cue levels, and can be extended to responses to differences in the density of heterospecific species that share predators. This “risk assessment” mechanism is qualitatively different from other mechanisms, such as the dilution and “many eyes” effects that predict behavioral responses to conspecific density. If species use conspecific or heterospecific density to assess predation risk as predicted, there may be implications for the role and evolution of traits used to perceive conspecific and heterospecific densities, species aggregation, and population dynamics, and should be considered in the design of experiments of nonlethal effects of predators. [ABSTRACT FROM AUTHOR]

The costs and benefits associated with feeding in the vicinity of a predator should vary within and among species, depending on an individual's vulnerability to a predator. In this paper, we investigate how willingness to risk exposure to a predator in order to gain access to food is expressed within groups of fish. We provided groups of six individually marked brook sticklebacks (Culea inconstans ) and fathead minnows (Pimephales promelas ) with the opportunity to feed in safety, or in the presence of a single predator (walleye, Stizostedion vitreum ). Sticklebacks and fathead minnows are commonly found in the same locations at the same time, but sticklebacks also possess armour and spines that provide defence against some predators. Despite this morphological variation, patterns of individual behaviour were consistent between species. While feeding close to the predator, there was a positive relation between the size of the individual and the proportion of food consumed. No relation existed on the side farthest from the predator. These data suggest that fish may risk exposure to predators to exploit size-related differences in vulnerability to gain a competitive advantage for access to food. [ABSTRACT FROM AUTHOR]

Phenotypic plasticity has primarily been studied in the context of adaptation to environmental fluctuations and its underlying genetic mechanisms. However, its impact on competition and coexistence has rarely been addressed. In this paper we analyzed the ecological and evolutionary implications of phenotypic plasticity in diel vertical migration of competing, physiologically distinguishable, Daphnia populations. We developed a model that evaluates the competitive strength of daphnids which are food limited and which face predation. Competitive strength is assessed by a population's ability to invade and defend a previously established site. Phenotypically plastic individuals were assumed to choose an evolutionarily stable migration strategy at any time during the invasion process. We found plasticity to increase a population's competitive ability and in special cases to support coexistence of similar populations. Including community structure in analyzing the adaptive value of phenotypic plasticity reveals aspects thatare not directly inferable from isolated population models. Hence, selection for plasticity may be more widespread than generally expected. [ABSTRACT FROM AUTHOR]

During the fourth stage of larval development the American lobster (Homarus americanus) leaves the plankton and becomes benthic. Before final settlement lobsters sample different substrates which may be accepted or rejected. Upon rejection the lobster returns to the planktonbefore sampling another substrate. In this paper we present a model of the substrate selection behavior of settling lobsters using dynamic optimization techniques. The model examines the role of substrate quality and availability, postlarval testing of substrates, and mortality associated with testing substrates, upon the decision to accept or reject the substrates sampled and predicts the eventual importance of each substrate in the recruitment of lobsters to reproductive age.Finally we apply the model using recent data on long-term survival of lobsters in the field. The model predicts that the high quality substrate (cobble) accounts for most of the adult lobster population, inspite of the much greater abundance of other, more marginal, substrates. [ABSTRACT FROM AUTHOR]

Data rarely are available to address the level of predation risk faced by diving animals in different parts of the water column. Consequently, most published research on diving behaviour implicitly assumes – like the proverbial ostrich – that ‘unseen’ predators are functionally unimportant. We argue that failure to consider diving in a predation risk framework may have precluded many insights into the ecology of aquatic foragers that breathe air. Using existing literature and a simple model, we suggest that fear from submerged predators in several systems might be influencing patch residence time, and therefore the duration of other dive cycle components. These analyses, along with an earlier model of predation risk faced by diving animals at the surface, suggest that dive cycle organisation can be modified to increase safety from predators, but only at the cost of reduced energy gain. Theoretical arguments presented here can seed hypotheses on factors contributing to population declines of diving species. For instance, adjustments to the dive cycle that reduce predation risk might be unaffordable if resources are scarce. Thus, if animals are to avoid imminent starvation or substantial loss of reproductive potential, resource declines might indirectly increase predation rates by limiting the extent to which dive cycles can deviate from those that would maximize energy gain. We hope that ideas presented in this paper stimulate other researchers to further develop theory and test predictions on how predation risk might influence diving behaviour and its ecological consequences. [ABSTRACT FROM AUTHOR]

The article discusses the use of log-linear models in ecology. Ecologists used logistic regression for modeling categorical and binary data. Poisson log-linear models are seldom used in ecology. However, its specific form is commonly used in ecological studies (Lewis 2004). When it is applied to binary data, it will give equal results to logistic regression. Lewis (2004) recommends authors to perform analysis of data from studies of predation on artificial nests and suggests that logistic regression is better than transformation of data.

Oligophagous and polyphagous predators are confronted with spatially and temporally varying distributions of prey. Their species-specific foraging strategies should be able to cope with this variability. Using an individual based model, we explore how diet breath and the spatial scale at which predators respond to prey affects their capture efficiency in four heterogeneous prey landscapes, and combinations thereof. We interpret the spatial scale of the predator's response as perceptual range, and propose giving-up density as a proxy for diet breadth. Foraging behaviour is evaluated for a total of 121 perceptual range/giving-up density combinations, with four of them reflecting the strategies adopted by real ladybeetle species. Foraging rules of oligophagous ladybeetles were generally not very effective in terms of attained predation rate when foraging in a single prey landscape, but appear to be more effective when foraging in multiple prey landscapes. This finding is compatible with the notion that oligophagous predators do not adopt a foraging strategy that is especially adapted to a specific prey landscape, but to multiple prey landscapes. Simulations further indicated that there was not a ‘best’ foraging rule that resulted in the highest predation rates for a range of spatial prey distributions and prey densities. The findings thus suggest that strategies of four ladybeetle species are effective in generating sufficient prey capture under a broad range of spatial distributions, rather than maximum capture under a narrower set of distributions. [ABSTRACT FROM AUTHOR]

The loss of a predator from an ecological community can cause large changes in community structure and ecosystem processes, or have very little consequence for the remaining species and ecosystem. Understanding when and why the loss of a predator causes large changes in community structure and ecosystem processes is critical for understanding the functional consequences of biodiversity loss. We used experimental microbial communities to investigate how the removal of a large generalist predator affected the extinction frequency, population abundance and total biomass of its prey. We removed this predator in the presence or absence of an alternative, more specialist, predator in order to determine whether the specialist predator affected the outcome of the initial species removal. Removal of the large generalist predator altered some species’ populations but many were unaffected and no secondary extinctions were observed. The specialist predator, though rare, altered the response of the prey community to the removal of the large generalist predator. In the absence of the specialist predator, the effects of the removal were only measurable at the level of individual species. However, when the specialist predator was present, the removal of the large generalist predator affected the total biomass of prey species. The results demonstrate that the effect of species loss from high trophic levels may be very context-dependent, as rare species can have disproportionately large effects in food webs. [ABSTRACT FROM AUTHOR]

Destabilization of one predator–one prey systems with an increase in nutrient input has been viewed as a paradox. We report that enrichment can damp population cycles by a food-web structure that balances inflexible and flexible interaction links (i.e. specialist and generalist predators). We modeled six predator–prey systems involving three or four species in which the predators practice optimal foraging based on prey profitability determined by handling time. In all models, the balance of interaction links simultaneously decreased the amplitude of population oscillations and increased the minimum density with increasing enrichment, leading to a potential theoretical resolution of the paradox of enrichment in non-equilibrium dynamics. The stabilization mechanism was common to all of the models. Important previous studies on the stability of food webs have also demonstrated that a balance of interaction strengths stabilizes systems, suggesting a general rule of ecosystem stability. [ABSTRACT FROM AUTHOR]

It has been suggested that the anthropogenically driven loss of herbivorous mammals might lead to changes in the recruitment patterns of tropical rain forest plants, but few studies have examined the explicit mechanisms that might account for this effect. Here we propose a conceptual model linking differential mammalian defaunation and differential plant recruitment. We posit that in the absence of medium/large herbivores but with small rodent granivores still present (i.e. differential defaunation), predation pressure will be greater on small-seeded species than on large-seeded species. We tested such differential predation hypothesis (DPH) by means of a series of experiments directed to assess: 1) seed consumption by small rodents in laboratory cages; 2) seed-removal rates in small rodent enclosures in the field and 3) removal of seeds placed on the forest floor and exposed to either the full complement of mammals or only small rodents. Seeds used in the experiments were arranged in pairs consisting of species from the same taxonomic family but with a contrasting size (large, small). We found: 1) a significantly greater consumption of smaller seeds (a 2.3- to 20.5-fold difference) in cages and 2) a significantly greater removal of small-seeded species (a 3.7- to 65-fold difference) in field enclosures. Results of seed removal experiments in free-access plots and selective exclosures were more complex, with a general absence of significant differences among treatments but we found that predation was in general concentrated on small-seeded species and small rodents were the predominant visitors to the plots. This, together with the overall short distance of dispersal among large seeds suggest that in differentially defaunated forests large-seeded species are more likely to escape predation. We posit that such size related differential predation may lead to the floristic impoverishment observed in some defaunated forests. [ABSTRACT FROM AUTHOR]

Experimental and field studies suggest that freshwater snail species have negative effects on each other's population growth rates. Because snails share similar diets, these interactions have been interpreted as the result of exploitative competition, but they could also result from intraguild predation. Here we present three experiments aimed at testing the hypothesis that interspecific interactions among three species of freshwater gastropod ( Helisoma trivolvis, Physa acuta and Stagnicola elodes) are mediated by intraguild egg predation. Foraging trials, conducted in a laboratory, showed that some snails readily prey on eggs, but the extent of egg predation depended on both the identity of the snail predator and the identity of the egg mass. Of the three species considered, Stagnicola had the largest effect on egg mortality and Physa had no effect on egg mortality. Foraging trials also showed that the eggs of Physa were the most vulnerable to predators and that the eggs of Stagnicola were largely invulnerable. A study conducted in large outdoor mesocosms assessed the occurrence of egg mortality in an environment of more extensive spatial scale and complexity. The results largely mirrored those of the laboratory study, with Stagnicola being the most voracious predator and the eggs of Physa being most vulnerable to predation. The reproductive success of Physa and Stagnicola raised in sympatry and allopatry was assessed in a mesocosm study conducted over three months. Recruitment of both species was depressed in sympatry, but patterns of growth in the survivors suggest contrasting mechanisms of suppression: Physa suppressed Stagnicola via exploitative competition, but Stagnicola suppressed Physa via egg predation. These experiments support the hypothesis that freshwater snail assemblages are structured by strong interspecific interactions and that a rich interplay of egg predation and interspecific competition underlie interactions among the members of this guild. [ABSTRACT FROM AUTHOR]

The high-arctic tundra ecosystem has the world's simplest vertebrate predator–prey community, with only four predators preying upon one rodent species, the collared lemming ( Dicrostonyx groenlandicus). We document the functional and numerical responses of all the four predators in NE Greenland. Using these data, we assess the impact of predation on the dynamics of the collared lemming with a 4 yr cycle and >100-fold difference between maximum and minimum densities. All predator species feed mostly (>90%) on lemmings when lemming density is >1 ha−1, but the shapes of the predators’ responses vary greatly. The snowy owl ( Nyctea scandiaca) is present and breeds only when lemming densities at snowmelt are >2 ha−1, giving rise to a step-like numerical response. The long-tailed skua ( Stercorarius longicaudus) has a type III functional response and shifts from alternate food (mainly berries and insects) to lemmings with increasing lemming density. The skua surpasses all the other predators in summer by its total response. The type III functional response of the Arctic fox ( Alopex lagopus) starts to increase at much lower lemming densities than the responses of the avian predators, but it has only a weak numerical response. Finally, the stoat ( Mustela erminea) is the most specialized predator and the only one with a clearly delayed numerical response. According to their specific functional and numerical responses, each predator plays a key role at some point of the lemming cycle, but only the stoat has the potential to drive the lemming cycle. Stoat predation is greatly reduced in the winter preceding the lemming peak, and it reaches a maximum in the winter preceding the lowest lemming summer density. Stoat predation appears to maintain low lemming densities for at least two successive years. Our study provides empirical support for the specialist predator hypothesis about small mammal population cycles. [ABSTRACT FROM AUTHOR]

To ecologists, factors such as a forager's encounter rate with prey and its own susceptibility to predation are dominant determinants of foraging. In contrast, digestive physiologists consider foraging to be determined by factors like rates of digestion and absorption of ingested food. We reconcile these views in a model combining encounter rate, external handling, and internal handling of food in the gut. With internal food handling, two food properties become important; energy: external handling time (e/h) and energy: bulk (e/b). We show that internal handling is only one of a suite of rate limiting factors. The gut never reaches full capacity, indicating that spare capacity may be intrinsic to gut structure. Regardless of gut fullness, a food of sufficiently high e/b will always be harvested. Two isolegs in the state space of resource abundances determine diet selectivity. These isolegs, which we call the Mitchell and Pulliam isolegs, divide the state space into regions in which 1) the forager's optimal strategy is opportunism; 2) the forager is always selective on the food with the greater e/h and partially selective on the second food; 3) the forager is always selective on the food with the greater e/h and ignores the second food. The development and analysis of the isolegs thus reconcile the heretofore disparate perspectives of the ecological and the physiological frameworks of foraging. [ABSTRACT FROM AUTHOR]

If environmental conditions vary, plasticity in life-history traits is predicted. A recent model indicates that males and females should differ in life-history traits, because sexes differ in optimal attributes depending on species ecology. In this study we test the impact of two biotic factors in combination (presence/absence of predators and low/high food level) on gender specific life-history traits in the damselfly Coenagrion puella (Odonata). Results show that predator presence and low food density decreased activity in both sexes. Additionally, individuals with less food grew more slowly, emerged later, remained smaller and had a higher mortality. At low food densities, however, and in contrast to former investigations, individuals from treatments with predator presence were the same size or larger than individuals without predators. Gender had a strong impact on larval activity and life-history traits and sexes differed in development. Females were less active and took longer to complete development, but emerged at a larger size, weight and fat content. This study highlights the importance of gender specific approaches in life-history research. [ABSTRACT FROM AUTHOR]

Functional response models differ in which factors limit predation (e.g. searching efficiency, prey handling time, digestion) and whether predation behaviour is governed by an internal physiological state (e.g. satiation). There is now much evidence that satiation is a key factor in understanding changes in foraging behaviour, and that many predators are effectively digestion limited. Here, we ask if predation in a predatory arthropod can be explained from satiation-driven behaviour alone, or if behaviour is also influenced by the density of prey other than via the effect of prey ingestion on satiation. To address this question a satiation-based predation model is formulated, for which parameters are estimated on the basis of observations on digestion rate, satiation-related prey searching rate and prey capture behaviour, basically under high prey density conditions. The model predictions are subsequently tested against longer term predation experiments carried out at high and low prey densities. Since satiation can easily be linked with egg production, these tests are carried out both for predation and oviposition.The predator–prey systems under study consist of females of two predatory mite species (Neoseiulus barkeriandN. cucumeris) and the larvae of two thrips species (Thrips tabaciandFrankliniella occidentalis) as their prey. ForN. barkeriforaging onT. tabaci, the model gives good predictions at both high (4 larvae cm−1) and low (0.1–1 larvae cm−2) prey densities. ForN. cucumerisforaging onF. occidentalis, the predictions hold at the high prey density, but are too low at low prey densities. Thus our analysis indicates that we cannot fully explain density-dependent predation rates from satiation-driven behaviour alone. Different mechanisms are suggested on how prey density may affect foraging efficiency other than via satiation. [ABSTRACT FROM AUTHOR]

This article compares indirect trait-mediated interactions in simple resource-consumer-predator food webs with those that are density-mediated. It focuses on two well documented responses of consumers to predation risk: decrease in consumer activity and habitat switch. These behavioral effects are transmitted to resources and they cause similar indirect effects as those which are mediated by density changes in consumers. Two indirect interactions are studied in this article: trophic cascades, and apparent competition. Results for density only, trait only and combined density and trait mediated interactions are compared and discussed with respect to manipulation with predator density (top-down manipulation) and resource environmental capacity (bottom-up manipulation). The article shows that trait-mediated, effects on species equilibrial densities are similar to those of density-mediated, but they are often highly non-linear. Thus, they may have potential for even stronger impact on food webs than those which are density mediated. [ABSTRACT FROM AUTHOR]

The relationship between the rates of prey capture and predator population growth is a fundamental aspect of predation, yet it is rarely measured for vertebrate predators. For the isolated wolf population on Isle Royale, annual variation in kill rate explains 22% of the variation in wolf population growth rate. From the slope of this relationship, we estimate that the production efficiency (ratio of production to respiration) of wolves is between 0.5% and 1.50/). More generally, we assess the relative extent to which wolf population growth rate is affected by density dependence, prey availability (moose, Alces alces), winter weather, and demographic stochasticity. Prey availability explains the most variation in wolf growth rate (42%), but this is only recognized after accounting for the influence of a disease-induced population crash and age structure of the prey population (i.e. number of vulnerable moose, >9 years of age). Demographic stochasticity accounts for approximately 30% of the variation in wolf growth rate. This recognition is important, but not surprising, given that the average population size of Isle Royale wolves is 22. Previous work indicates that the effect of winter climate, as mediated through prey vulnerability and kill rates, is substantial. This work indicates that the direct effect of winter climate is weak, and explains only about 4% of the variation in wolf growth rate (P =0.10). [ABSTRACT FROM AUTHOR]

The classical theory of the ideal free distribution (IFD) predicts that the spatial distribution of consumers should follow the distribution of the resources they depend on. Here, we study consumer–resource matching in a community context. Our model for the community is a food chain with three levels. We study whether the primary consumers are able to match resources both under predation risk and in its absence. Both prey and predators have varying degrees of knowledge of the global and local resource distribution. We present two versions of the model. In the “resource maximising” model, the consumers consider the availability of their resource only. In the “balancing” model, individual consumers minimise predation risk per unit of resource that they can gain access to. We show that both models can lead to perfect matching of consumers on resources and predators on consumers, assuming that individuals have full knowledge of the whole environment. However, when the consumers’ information and freedom of movement are greater than those of the predators, then the predators generally undermatch the consumers. In the opposite case, we observe overmatching and high consumer movement rates. Furthermore, undermatching of predators on consumers tends to induce overmatching of consumers on resources. [ABSTRACT FROM AUTHOR]

Predation impacts by introduced predators are predicted to be most intense in island ecosystems, and also variable depending on environmental conditions, but large-scale experimental field testing is rare. In this study we examine the factors that determine the distribution and abundance of vole metapopulations preyed upon by feral American mink Mustela vison in the outer Finnish archipelago of the Baltic Sea. Specifically, we follow the dynamics of field voles Microtus agrestis and bank voles Clethrionomys glareolus on 40 small islands under variable rainfall as part of a large-scale mink removal experiment. For both vole species occupancy rates were negatively influenced by island isolation, as were extinction events for field voles. High summer rainfall in 1998 corresponded to large vole populations where mink were absent, populations that then crashed in 1999 and 2000 when below average rains fell during the summer breeding season. Where mink were present however, vole abundance remained more constant between years with no boom-bust apparent. We conclude that weather and predation may drive vole abundance whereas habitat patchiness and metapopulation processes more strongly drive vole distributions. There may also be potential for interaction between these factors: because feral mink prevent rapid vole population growth after good summer rains, and vole dispersal is influenced by population size, feral mink may be changing vole dispersal patterns to disrupt the natural metapopulation dynamic. Hence this indirect impact of mink could lead to gradual erosion of vole populations in the outer archipelago by reducing recolonisation processes. [ABSTRACT FROM AUTHOR]

Birds selecting a nest site have to find the best compromise between the risk of encountering predation, the availability of food near to the nest and microclimatic requirements. As the optimal solution of this problem will vary with changes in predator abundance, we ask whether birds are capable of assessing such changes and of adjusting their nest-site choice accordingly. The reproductive success of dusky warblers Phylloscopus fuscatus, breeding in a mosaic of bushland and tundra habitat in the Russian Far East, varied greatly depending on the abundance of a nest predator, the Siberian chipmunk Tamias sibiricus. Using artificial nests we analysed which strategies dusky warblers should follow to avoid nest predation by chipmunks. We then compared the nest sites which dusky warblers actually had chosen in years with very high chipmunk densities (1998 and 1999) with those chosen in 1997, when chipmunks were almost absent from the study area. We found that safe nest-sites were preferred over those offering other advantages (microclimate, proximity to food) when the risk of predation was high, and we could not detect any confounding factor that might alternatively have caused these striking between-year differences. Our study suggests that even a short-lived passerine may be capable of choosing its nest site according to the actual predation risk. We show that such behavioural plasticity can lead to a paradoxical situation where better-protected nest-sites (selected in years and areas with high risk), on average, suffer greater predation than sites offering low safety. Thus, behavioural plasticity, if undetected, may result in serious misinterpretation of nest-predation patterns. A review of the literature suggests that adaptive plasticity in nest placement may be more widespread than is currently recognized. [ABSTRACT FROM AUTHOR]

Theoretical and empirical ecologists have long acknowledged that information about the intensity or strength of the interaction between species is crucial for an understanding of community dynamics. In communities in which predation is an important structuring process, and some predator species are commercially exploited, quantitative estimates of predation by different predator species within a guild are necessary to make even the simplest recommendations about conservation and resource management. Here, we evaluated per capita and population level components of predation intensity of three intertidal predators that feed on monospecific stands of barnacles and mussels at wave exposed sites in the rocky intertidal zone of central Chile. These prey species represent the two most distinctive stages of the mid-intertidal seascape, with mussels being competitively dominant. Our results showed that the commercially exploited gastropod Concholepas concholepas and the sea star Heliaster helianthus have similarly large per capita and population effects on the competitively dominant mussel Perumytilus purpuratus . Their per capita (by average size individual) and population effects on mussels were more than two orders of magnitude larger than those of Acanthocyclus gayi crabs and likely even larger than the effect of other predator species in this system (other crabs, whelks, birds, fish). The overall pattern of predation on barnacles was similar to that on mussels, but some differences occurred in the way different components of predation intensity were distributed across predator species. Despite the roughly similar pattern of population predation intensity between prey species, the expected consequences for the prey population, and hence the rest of the community, were acutely different for mussels and barnacles. [ABSTRACT FROM AUTHOR]

Many ecological interactions that are called mutualistic are in fact mixtures of antagonistic and mutualistic aspects. For example, plasmids exploit their bacterial hosts but also protect them against external threats. In this study, we analyse the conditions for the evolution of what we call ‘dangerous liaisons’: interactions combining mutualistic and antagonistic aspects. Starting point of our analysis is a model that was proposed as early as 1934. In this model, partners have to form a complex (either temporary or long lasting) in order to interact. Using this model framework we then set out to define and tease apart private interests of the interacting partners from their common good. This dichotomy provides a unifying perspective to classify ecological interactions. We discuss some examples to illustrate how the outcome of the interaction may depend on densities or on other contextual variables. Finally, we note that having a common good is not a necessary condition for partners to have aligned interests. In a dangerous liaison partners may have interest to cooperate even when this does not bolster the common good. [ABSTRACT FROM AUTHOR]

Most mathematical descriptions of predator-prey interactions fail to take into account the spatio-temporal structures of the populations, which can lead to errors or misinterpretations. For example, a compact pulse of prey migrating through a field of quasi-stationary predators may not be well described by standard predator-prey models, because the predators and prey are unlikely to be well mixed; that is, the prey may be exposed to only a fraction of the predator population at a time. This underscores the importance of properly accounting for the ecological neighborhood, or effective feeding range, of predators in models. We illustrate this situation with a series of models of salmon smolts migrating through a reservoir arrayed with predators. The reservoir is divided into a number of longitudinal compartments or spatial cells, the length of each cell representing the upstream-downstream range over which predators can forage. In this series of models a 100-km-long reservoir is divided, successively into 2, 5, 10, 25, 50, 100, 200, and 400 cells, with respective cell lengths of 50, 20, 10, 4, 2, 1, 0.5, and 0.25 km. We used a detailed individual-based simulation model at first, but to ensure robustness of results we supplemented this with a simple analytic model. Both models showed sharp differences in the predicted mortality to a compact pulse of smolt prey moving through the reservoir, depending on the number of spatial cells in the model. In particular, models with fewer than about 10 cells vastly overpredicted the amount of mortality due to predators with activity ranges of not more than a few kilometers. These results corroborate recent theoretical and simulation studies on the importance of spatial scale and behavior in modeling predator-prey dynamics. [ABSTRACT FROM AUTHOR]

Trait-mediated indirect effects (TMIs) are changes in the density of one species that are caused by induced changes in one or more traits of an intervening species. For example, induced defense in a prey species may alter the nature of indirect effects that are mediated through that prey species. In the present study, we investigated the TMIs that stem from an interaction between a carnivorous whelk (Acanthina angelica ) and an intertidal barnacle (Chthamalus anisopoma) . Depending on the timing of the interaction, the predator either kills the barnacle or induces a predation-resistant morph. Based on previous work that addressed the direct interactions between Chthamalus and other species in the community we predicted and subsequently found that community structure varies as a function of these differing effects. Specifically, we found that Acanthina has a positive indirect effect on mussels when it interacts with (kills) undefended adult barnacles. In contrast, the predator has a positive indirect effect on an encrusting algae (Ralfsia ) when it comes into contact with juvenile barnacles, causing the induction of the predator-resistant morph. We suggest that further research should consider the role of environmentally induced polymorphisms in structuring communities. [ABSTRACT FROM AUTHOR]

Spatial autocorrelation is ubiquitous in ecological systems. However, consideration of this phenomenon in seed and seedling predation studies has been limited, perhaps explaining some of the variation observed between studies and the difficulty in generalizing about the impacts of herbivores on successional processes. I examined the prevalence and patterns of spatial autocorrelation in seed and seedling survival between neighboring tree propagules in old-fields. All experiments were conducted within enclosures in which I manipulated the density of meadow voles (Microtus pennsylvanicus ). Voles, and competitively inferior white-footed mice (Peromyscus leucopus ), are the dominant consumers of tree seedlings and seeds in old-fields in the northeastern United States, respectively. An intermediate level of spatial autocorrelation was present in seed and seedling predation trials. Furthermore, the frequency of spatial autocorrelation increased under low-vole density conditions and in study sites with greater heterogeneity in vegetation cover. Survival of neighboring tree propagules was generally positively autocorrelated, suggesting that foraging by mice and voles contributes to a clumped pattern of tree propagule establishment in old-fields. A review of published predation studies found that only 8.3% attempted to test for spatial autocorrelation and that the minimum average spacing used between replicates is well below the average autocorrelation distance of 8.1±6.3 m observed in the current study. Patterns of autocorrelation appear somewhat predictable based on knowledge of the ecology of mice and voles and to provide insights into the factors influencing their foraging behavior and potential impacts on old-field succession. [ABSTRACT FROM AUTHOR]

Predators are usually thought to be rarer (in the sense of having lower population densities) than non-predators. Recent analyses have suggested that this is not the case because a decline in species richness compensates for the well-known decline in number of individuals with increasing trophic rank. I show that a variety of invertebrate communities contain more species of predators than would be expected from the number of predatory individuals. This is not due to differences in dominance or taxonomic resolution between predatory and non-predatory guilds, and implies that predators are indeed relatively rare. I suggest that patterns of energy flow and body size make it likely that there will be a higher proportion of predatory species than individuals in a community, provided that predators have moderately specialized diets. [ABSTRACT FROM AUTHOR]

We studied effects of the non-lethal presence of a predator on behaviour and larval life history of two species of anuran tadpoles, common frog, Rana temporaria, and common toad, Bufo bufo. We manipulated predator diet to study whether differences in chemically perceived predation risk affect the tadpoles. Tadpoles of both species were raisedat two food levels either in the absence of the predator or in the presence of an insect-, frog- or toad-fed larval dragonfly Aeshna juncea. R. temporaria tadpoles lowered their activity in the presence of a predator and also avoided the predator spatially. Furthermore, theyshowed the strongest response to tadpole-fed predators. The number of days elapsed since the last provisioning of food affected the behaviour of both species. In R. temporaria, differences in activity levelbetween predator treatments vanished as the food resources were depleted. In B. bufo, activity was lower in the presence of a toad-fed predator shortly after food had been added to the containers. Spatial avoidance of predators by B. bufo was stronger in the presence of a toad-fed predator than when an insect- or frog-fed predator was present. In both tadpole species growth rate was higher at the high food level but remained unaffected by the predator treatments. In both species individuals at higher food level metamorphosed earlier and at larger size. The metamorphic responses to predator treatments differed between the species. R. temporaria metamorphosed later and at larger size in the presence of tadpole-fed predators than in the control treatment or in the presence of an insect-fed predator. B. bufo metamorphosed earlier in the presence of a toad-fed predator, but this was only slightly correlated to metamorphic size. Manipulations of predator diet affected tadpole life history in both species. However, predator effects on larval life history were not mediated by tadpole behaviour in either species. [ABSTRACT FROM AUTHOR]

I investigated three co-occurring North American alpine plant species for which ants comprise at least 20% of the flower visitors. Ants of the species Formica neorufibarbis gelida pollinate gynodioecious Paronychia pulvinata, are herbivores of the gynodioecious Eritrichum aretioides and appear to have little effect on the hermaphroditic Oreoxis alpina. Given that floral nectar and lipids are important resources for alpine ants, the effect of all three plant species on the ants is positive. In E. aretioides, ants chew on the coronal ring of flowers but are unlikely to contact the stigmas, explaining their negativeeffects on seed production. In ant-pollinated P. pulvinata, ants affect pollination in females more than in hermaphrodites. Single ant visits to unpollinated pistillate flowers show that ants are inefficient pollinators, transferring no pollen (viable or inviable) to stigmas55% of the time. This result is probably explained by low pollen loads and infrequent movement between plants. In contrast, single ant visits to previously unpollinated perfect flowers on hermaphrodites produce similar seed-set similar to that in flowers open to pollination.In O. alpina, the lack of an effect of ants on seed production is probably explained by autogamous seed-set and lack of pollen limitation. In both gynodioecious species, E. aretioides and P. pulvinata, female plants produce more seeds than hermaphrodites in the absence of ants, but this difference is magnified when ants are allowed access to plants, suggesting that ants can influence the evolutionary maintenance of breeding system dimorphism in these species. [ABSTRACT FROM AUTHOR]

Seed dormancy is hypothesized to be a risk-spreading strategy that maximizes plant fitness. By spreading germination time of offspring among years, plants may not only vary the environment to which their offspring are exposed, but may also reduce potential fitness losses dueto sib competition. The sibling competition hypothesis predicts thatlarge families whose offspring are more likely to experience siblingcompetition should stagger germination over several seasons (i.e. have greater dormancy) to a greater extent than small families. This should be observed as a lower initial probability of germination for seeds from large families than those from small families. We examined the relationship between family size and germination behavior in the desert mustard Lesquerella fendleri. In a single population in centralNew Mexico, USA, we estimated family size and germination fraction of 189 randomly selected maternal plants. Although a median test was not supportive, the randomization test showed marginal support for thesibling competition hypothesis. An additional study of twelve large and twelve small families in each of three other populations also showed a trend (significant in one population) for larger families to have lower germination fractions than small families. These results suggest a degree of support for the hypothesis, given that so many selective forces interact to generate patterns of seed dormancy. Thus, we have not ruled out the hypothesis that large families reduce the likelihood of sib competition in good years by producing seed crops with initially low germination. Sibling competition as well as environmental heterogeneity may have influenced the evolution of seed dormancy in this system. [ABSTRACT FROM AUTHOR]

We used trained barn owls to introduce a controlled predation threatto two species of gerbil, Gerbillus allenbyi and G. pyramidum in a system of 2-ha, sandy-substrate field enclosures in the Negev Desert, Israel. Using the principles of optimal density-dependent habitat selection, we estimated several coefficients of population interaction focusing on G. allenbyi. G. allenbyi exhibits strong intraspecific competition. In the absence of owls, G. pyramidum competes with it (Alpha??= -0.35). We estimated the slope of the G. allenbyi victim isocline to be -0.60. The competitive effect of G. pyramidum disappeared in the presence of owl, although the intraspecific competition remained.Our results indicated that in the presence of owls, the threat of predation overwhelms the cost of interspecific competition. [ABSTRACT FROM AUTHOR]

Algae are rare within the bacterial blooms of Leptothrix ochracea that occur within streams possessing zones of iron and manganese oxide deposition. To assess the importance of L. ochracea and metal oxides for constraining algal abundance, and to elucidate the role these bacteria have in maintaining oxide deposition zones, we conducted field studies in a stream containing an extensive bloom of L. ochracea. Transplant experiments revealed a strong, inverse relationship between percent cover of ferro-manganese oxides and the number of algal cells on colonized microscope slides. This negative relationship held whether substrata were colonized first with algae and then placed within the bloom, or were colonized with L. ochracea and then moved outside the bloom; in either case oxide cover was correlated with reduced numbers of algal cells. In another experiment, Leptothrix-bearing slides that had been sterilized with formalin had significantly more algae than did unsterilized controls. Ferro-manganese oxides were more prevalent on unsterilized slides, indicating the bacteria have an active role in maintaining ferro-manganese oxides within the bloom. Active deposition of metal oxides may benefit bacteria by: 1) deterring algal colonization via allelopathic action, and 2) adsorbing phosphorus anddepriving algae of this essential nutrient. Results from this study suggest that an antagonistic relationship exists between stream algaeand L. ochracea and that both bacterial activity and the existence of ferro-manganese oxides mediate the balance between these organisms in nature. [ABSTRACT FROM AUTHOR]

Compensatory mortality or natality can operate as a consequence of seasonally driven mechanisms of density dependence. Our objective is to clarify the relationship between compensation and density dependence in population models for vertebrates when seasonality is present. Field studies of a variety of species have demonstrated that due to compensation, predation or human harvest may not influence spring-breeding or pre-harvest-season densities. Compensation seems to contradictmost harvesting and predation models because these models predict that harvests or predation will always reduce equilibrium population size. In these population models sustainable harvests are attainable because of density dependence. The apparent discrepancy is attributableto the failure of most population models to incorporate the details of environmental seasonality. We review seasonally explicit models ofpopulation dynamics to illustrate how density dependence is the mechanism behind compensatory mortality and natality. Even though spring-breeding or pre-season densities can remain unaffected by harvesting,harvesting or predation reduce the integral of population size throughout the year. Compensatory mortality and natality are often cited as the basis for sustainable harvests of wildlife populations. [ABSTRACT FROM AUTHOR]

Effective animal dispersers of fruit seeds have been primarily considered to be herbivorous birds and bats. To evaluate the seed-dispersal efficacy of a mammalian predator, we studied the American marten (Martes americana) as a disperser of two blueberries (Vaccinium alaskaense, V. ovalifolium) and salmonberry (Rubus spectabilis) on ChichagofIsland, Alaska, by combining data on movement speed and gut-passage speed. Martens traveled a median distance of 133 m in 0 - 1 h and 501m in 4 - 5 h. Median gut-passage time for V. alaskaense was 4.1 h and for R. spectabilis was 4.6 h. Median dispersal distances for both berry species were estimated to be about 500 m. In old-growth stands, V. ovalifolium seeds cleansed of pulp did not germinate differently than seeds cleansed of feces, suggesting that the marten gut does not harm germination. Vaccinium spp. seeds germinated better if cleansed of feces than if inside intact berries, indicating that passage through the gut of a marten improves germination over fruit fall alone. For V. alaskaense in old growth, seeds cleansed of feces germinated less than seeds cleansed of pulp, but still had high (41%) germination rates. We conclude that American martens move ingested seeds long distances, some of the highest reported for a mammal. This efficacy is inpart due to the predatory adaptations of martens. Predators may haveheretofore unrecognized value in shaping landscape patterns of berry-producing plants. [ABSTRACT FROM AUTHOR]

Nest predation, if density dependent, may structure avian communities by selecting for nest-site characteristics that dilute the predators' perception of nest density (e.g., by increasing the number and types of potential nest sites) thereby reducing losses to predators. We tested for density dependent nest predation at a site dominated by predation from raccoons, Procyon lotor, and blue jays, Cyanocitta cristata. We used cohorts of 20 artificial ground nests placed on experimental grids at four densities (1.7, 7, 65, and 167 nests/ha). Predation was density dependent for one of two temporal replicates. In a second experiment, we tested for density dependent predation on both ground and shrub nests using more realistic nest densities (3.5 and 8.9 nests/ha) and excluding avian and small mammal predators by placing nylon screening over the nests. Predation was density dependent on artificial ground nests and density independent on shrub nests. Finally, we analyzed predation on natural shrub nests separating out avian (i.e., jays) and large mammalian predation based on disturbance to the depredated nest. In each of three years there was no evidence for density dependent predation. An outbreak of periodic cicadas in the firstexperiment and the maturation of a large mulberry crop during the second experiment demonstrated the importance of the distribution and abundance of alternative foods on the response of predators toward heterogeneity in nest abundance. [ABSTRACT FROM AUTHOR]

Interactions in size-structured populations are characterized by a mixture of predatory and competitive interactions dependent on the size of the individual organism. We analyzed this mixture of size-structured interactions for a species constellation consisting of perch (Perca fluviatilis) and roach (Rutilus rutilus) in a replicated whole lake experiment over 4 yr. Roach are preyed upon by large perch, but atthe same time compete with small perch for zooplankton. Predictions regarding the effects of roach on perch performance and resource dynamics in the whole-lake experiment were based on results from previouspond and enclosure experiments carried out over short (months) time periods. Variables measured in the whole-lake experiment included both individual level parameters (diet and growth of perch) as well as population level parameters (mortality, population numbers and size structures of perch, abundance and biomass of zooplankton and benthic macroinvertebrates). The population size of perch ? 2 yr old decreasedto less than 10% of the pre-treatment levels in treatment lakes after the introduction of roach and remained low over the whole study period. The mortality in perch one-year-old and older increased with size. Larger perch had a higher growth decrease than smaller perch, but also the individual growth rate of young-of-the-year (YOY) perch was lower in treatment lakes than in control lakes. YOY perch were almostexcluded during the winter and spring following the roach introduction. The few perch of this year class that survived had the following years a higher growth than the corresponding year class in control lakes. This could be related to an increased availability of predator sensitive macroinvertebrates in treatment lakes. Perch in treatment lakes fed to a larger extent on macroinvertebrates and less on pelagic zooplankton than perch in control lakes. As expected, zooplankton abundances decreased in treatment lakes the year following the roach introdu [ABSTRACT FROM AUTHOR]

I examine feeding rate and reproduction of a generalist arthropod predator Pterostichus cupreus (Coleoptera, Carabidae) at two different densities in two agricultural habitats where this beetle is commonly found; perennial ley and annual barley. I also examine effects of predator densities on arthropod community composition in each crop. The aims are to, in each of these habitats, examine if 1) previously observed food limitation in nature is caused by competition for food where prey is depleted by the competitors, or because predators starve because it is inherently difficult for them to find or consume food in a poor habitat where prey abundance is insensitive to the level of predation, and 2) to examine impact, directly or indirectly, of the predator on arthropod community composition. Predator density was manipulated in plots surrounded by barriers (Treatment) in the two habitats(Crop), and the experiment was repeated early and late in the growing season (Time). Large differences in community composition were found between crops and times. Arthropod abundance was always higher in ley, and increased markedly between the first and second experiment inbarley. Predator load was high (44%) in barley in the first experiment compared to the second (26%) and compared to ley (22%). Eggload, amount of stored fat and live body weight of P. cupreus were all markedly higher in ley. The lower fat content in recaptured beetles in barley indicated lower feeding rates and a high degree of food limitation compared to ley. Indications that fat storage was reduced by intraspecific competition were found, but the main difference in fat storage was between crops. No significant treatment effects of initial predator density were detected on arthropod community composition in eachcrop. Food limitation was mainly caused by difficulty to find food in the barley. Adult feeding and reproduction indicates that barley and ley represent widely different habitat qualities to P. cupreus. Thecom [ABSTRACT FROM AUTHOR]

Flowers of the alpine skypilot, Polemonium viscosum, are attacked bynectar-thieving ants of Formica neorufibarbus gelida. Plants in thisspecies display continuous variation in floral form from long, broadly-flared flowers to short, narrow flowers. This study addresses three aspects of the potential for ants to exert selection on floral traits in P. viscosum: the effect of ant visitation on seed production, the relationship between ant visitation patterns and variation in floral form among co-occurring individuals, and the behavioral mechanismsunderlying nonrandom flower choice by ants. Ants visiting flowers ofP. viscosum interfere with seed production by dislodging the style from the ovary. In 1982 and 1997 plants protected from ant visits set significantly more seeds per flower than open controls. Plants with broadly-flared flowers incurred significantly higher rates of ant damage than individuals with narrower flowers in 1995. Similarly, in 1997, damage increased significantly with plant score on a principal component positively correlated to corolla flare and length. Ant abundance declines with elevation above timberline, and, in 1997, damage associated with corolla flare and length was significantly greater for plants in the timberline (krummholz) habitat than for plants higher on the alpine tundra. Laboratory choice trials showed that ants prefer flowers with long broadly-flared corollas over flowers with short narrow corollas. Ant preference may reflect differences in floral reward:broadly-flared flowers contain more nectar sugar than narrow ones inP. viscosum. Additionally energetic costs associated with nectar collection depend in part on flower form. In choice experiments, handling time, or the time required for an ant to move from the corolla opening to the nectary decreased significantly with corolla flare and wasalso less variable for ants choosing broadly-flared flowers than those choosing narrow ones. Ants have the potential to exert selection on flowe [ABSTRACT FROM AUTHOR]