The influence of size-specific indirect interactions in predator-prey systems

Nonlethal indirect interactions between predators often lead to nonadditive effects of predator number on prey survival and growth. Previous studies have focused on systems with at least two different predator species and one prey species. However, most predators undergo extreme ontological changes in phenotype such that interactions between different-sized cohorts of a predator and its prey could lead to nonadditive effects in systems with only two species. This may be important since different-sized individuals of the same species can differ more in their ecology than similar-sized individuals of different species. This study examined trait-mediated indirect effects in a two-species system including a cannibalistic predator with different-sized cohorts and its prey. I tested for these effects using larvae of two stream salamanders, Gyrinophilus porphyriticus (predator) and Eurycea cirrigera (prey), by altering the densities and combinations of predator size classes in experimental streams. Results showed that the presence of large individuals can significantly reduce the impact of density changes of smaller conspecifics on prey survival through nonlethal means. In the absence of large conspecifics, an increase in the relative frequency of small predators significantly increased predation rates, thereby reducing prey survival. However, with large conspecifics present, increasing the density of small predators did not decrease prey survival, resulting in a 14.3% lower prey mortality than predicted from the independent effects of both predator size classes. Small predators changed their microhabitat use in the presence of larger conspecifics. Prey individuals reduced activity in response to large predators but did not respond to small predators. Both predators reduced prey growth. These results demonstrate that the impact of a predator can be significantly altered by two different types of trait-mediated indirect effects in two-species systems: between different-sized cohorts and between different cohorts and prey. This study demonstrates that predictions based on simple numerical changes that assume independent effects of different size classes or ignore size structure can be strongly misleading. We need to account for the size structure within predator populations in order to predict how changes in predator abundance will affect predator-prey dynamics.     

The growth-predation risk trade-off under a growing gape-limited predation threat

Growth is a critical ecological trait because it can determine population demography, evolution, and community interactions. Predation risk frequently induces decreased foraging and slow growth in prey. However, such strategies may not always be favored when prey can outgrow a predator's hunting ability. At the same time, a growing gape-limited predator broadens its hunting ability through time by expanding its gape and thereby creates a moving size refuge for susceptible prey. Here, I explore the ramifications of growing gape-limited predators for adaptive prey growth. A discrete demographic model for optimal foraging/growth strategies was derived under the realistic scenario of gape-limited and gape-unconstrained predation threats. Analytic and numerical results demonstrate a novel fitness minimum just above the growth rate of the gape-limited predator. This local fitness minimum separates a slow growth strategy that forages infrequently and accumulates low but constant predation risk from a fast growth strategy that forages frequently and experiences a high early predation risk in return for lower future predation risk and enhanced fecundity. Slow strategies generally were advantageous in communities dominated by gape-unconstrained predators whereas fast strategies were advantageous in gape-limited predator communities. Results were sensitive to the assumed relationships between prey size and fecundity and between prey growth and predation risk. Predator growth increased the parameter space favoring fast prey strategies. The model makes the testable predictions that prey should not grow at the same rate as their gape-limited predator and generally should grow faster than the fastest growing gape-limited predator. By focusing on predator constraints on prey capture, these results integrate the ecological and evolutionary implications of prey growth in diverse predator communities and offer an explanation for empirical growth patterns previously viewed to be anomalies.     

Impact of cannibalism on predator-prey dynamics: size-structured interactions and apparent mutualism

Direct and indirect interactions between two prey species can strongly alter the dynamics of predator-prey systems. Most predators are cannibalistic, and as a consequence, even systems with only one predator and one prey include two prey types: conspecifics and heterospecifics. The effects of the complex direct and indirect interactions that emerge in such cannibalistic systems are still poorly understood. This study examined how the indirect interaction between conspecific and heterospecific prey affects cannibalism and predation rates and how the direct interactions between both species indirectly alter the effect of the cannibalistic predator. I tested for these effects using larvae of the stream salamanders Eurycea cirrigera (prey) and Pseudotriton ruber (cannibalistic predator) by manipulating the relative densities of the conspecific and heterospecific prey in the presence and absence of the predator in experimental streams. The rates of cannibalism and heterospecific predation were proportional to the respective densities and negatively correlated, indicating a positive indirect interaction between conspecific and heterospecific prey, similar to "apparent mutualism." Direct interactions between prey species did not alter the effect of the predator. Although both types of prey showed a similar 30% reduction in night activity and switch in microhabitat use in response to the presence of the predator, cannibalism rates were three times higher than heterospecific predation rates irrespective of the relative densities of the two types of prey. Cumulative predation risks differed even more due to the 48% lower growth rate of conspecific prey. Detailed laboratory experiments suggest that the 3:1 difference in cannibalism and predation rate was due to the higher efficiency of heterospecific prey in escaping immediate attacks. However, no difference was observed when the predator was a closely related salamander species, Gyrinophilus porphyriticus, indicating that this difference is species specific. This demonstrates that cannibalism can result in the coupling of predator and prey mortality rates that strongly determines the dynamics of predator-prey systems.     

Predator identity and additive effects in a treehole community

Multiple predator species can interact as well as strongly affect lower trophic levels, resulting in complex, nonadditive effects on prey populations and community structure. Studies of aquatic systems have shown that interactive effects of predators on prey are not necessarily predictable from the direct effects of each species alone. To test for complex interactions, the individual and combined effects of a top and intermediate predator on larvae of native and invasive mosquito prey were examined in artificial analogues of water-filled treeholes. The combined effects of the two predators were accurately predicted from single predator treatments by a multiplicative risk model, indicating additivity. Overall survivorship of both prey species decreased greatly in the presence of the top predator Toxorhynchites rutilus. By itself, the intermediate predator Corethrella appendiculata increased survivorship of the native prey species Ochlerotatus triseriatus and decreased survivorship of the invasive prey species Aedes albopictus relative to treatments without predators. Intraguild predation did not occur until alternative prey numbers had been reduced by approximately one-half. Owing to changes in size structure accompanying its growth, T. rutilus consumed more prey as time progressed, whereas C. appendiculata consumed less. The intermediate predator, C. appendiculata, changed species composition by preferentially consuming A. albopictus, while the top predator, T. rutilus, reduced prey density, regardless of species. Although species interactions were in most cases predicted from pairwise interactions, risk reduction from predator interference occurred when C. appendiculata densities were increased and when the predators were similarly sized.     

Performance level and efficiency of two differing predator-avoidance strategies depend on nutritional state of the prey fish

Animal prey has developed a variety of behavioural strategies to avoid predation. Many fish species form shoals in the open water or seek refuge in structurally complex habitats. Since anti-predator strategies bear costs and are energy-demanding, we hypothesised that the nutritional state of prey should modify the performance level and efficiency of such strategies. In aquaria either containing or lacking a structured refuge habitat, well-fed or food-deprived juvenile roach (Rutilus rutilus) were exposed to an open-water predator (pikeperch, Sander lucioperca). Controls were run without predators. In the presence of the predator, roach enhanced the performance of the anti-predator strategy and increased the use of the refuge habitat whereby food-deprived roach were encountered more often in the structure than well-fed roach. Nonetheless more starved than well-fed roach were fed upon by the predator. In the treatments offering only open-water areas, roach always formed dense shoals in the presence of the predator. The shoal density, however, was lower in starved roach. Starving fish in shoals experienced the highest predation mortality across all experimental treatments. The experiment confirmed the plasticity of the anti-predator behaviour in roach and demonstrated that food deprivation diminished the efficiency of shoaling more strongly than the efficiency of hiding. The findings may be relevant to spatial distribution of prey and predator–prey interactions under natural conditions because when prey are confronted with phases of reduced resource availability, flexible anti-predator strategies may lead to dynamic habitat use patterns.     

Modelling predation by transient leopard seals for an ecosystem-based management of Southern Ocean fisheries

Correctly quantifying the impacts of rare apex marine predators is essential to ecosystem-based approaches to fisheries management, where harvesting must be sustainable for targeted species and their dependent predators. This requires modelling the uncertainty in such processes as predator life history, seasonal abundance and movement, size-based predation, energetic requirements, and prey vulnerability. We combined these uncertainties to evaluate the predatory impact of transient leopard seals on a community of mesopredators (seals and penguins) and their prey at South Georgia, and assess the implications for an ecosystem-based management. The mesopredators are highly dependent on Antarctic krill and icefish, which are targeted by regional fisheries. We used a state-space formulation to combine (1) a mark-recapture open-population model and individual identification data to assess seasonally variable leopard seal arrival and departure dates, numbers, and residency times; (2) a size-based bioenergetic model; and (3) a size-based prey choice model from a diet analysis. Our models indicated that prey choice and consumption reflected seasonal changes in leopard seal population size and structure, size-selective predation and prey vulnerability. A population of 104 (90–125) leopard seals, of which 64% were juveniles, consumed less than 2% of the Antarctic fur seal pup production of the area (50% of total ingested energy, IE), but ca. 12–16% of the local gentoo penguin population (20% IE). Antarctic krill (28% IE) were the only observed food of leopard seal pups and supplemented the diet of older individuals. Direct impacts on krill and fish were negligible, but the “escapement” due to leopard seal predation on fur seal pups and penguins could be significant for the mackerel icefish fishery at South Georgia. These results suggest that: (1) rare apex predators like leopard seals may control, and may depend on, populations of mesopredators dependent on prey species targeted by fisheries; and (2) predatory impacts and community control may vary throughout the predator's geographic range, and differ across ecosystems and management areas, depending on the seasonal abundance of the prey and the predator's dispersal movements. This understanding is important to integrate the predator needs as natural mortality of its prey in models to set prey catch limits for fisheries. Reliable estimates of the variability of these needs are essential for a precautionary interpretation in the context of an ecosystem-based management.     

Food webs and intraguild predation: community interactions of a native mesocarnivore

Trophic level interactions between predators create complex relationships such as intraguild predation. Theoretical research has predicted two possible paths to stability in intraguild systems: intermediate predators either outcompete higher-order predators for shared resources or select habitat based on security. The effects of intraguild predation on intermediate mammalian predators such as swift foxes (Vulpes velox) are not well understood. We examined the relationships between swift foxes and both their predators and prey, as well the effect of vegetation structure on swift fox-coyote (Canis latrans) interactions, between August 2001 and August 2004. In a natural experiment created by the Pinon Canyon Maneuver Site in southeastern Colorado, USA, we documented swift fox survival and density in a variety of landscapes and compared these parameters in relation to prey availability, coyote abundance, and vegetation structure. Swift fox density varied significantly between study sites, while survival did not. Coyote abundance was positively related to the basal prey species and vegetation structure, while swift fox density was negatively related to coyote abundance, basal prey species, and vegetation structure. Our results support the prediction that, under intraguild predation in terrestrial systems, top predator distribution matches resource availability (resource match), while intermediate predator distribution inversely matches predation risk (safety match). While predation by coyotes may be the specific cause of swift fox mortality in this system, the more general mechanism appears to be exposure to predation moderated by shrub density.     

The effects of opportunistic and intentional predators on the herding behavior of prey

In this article, we study how predator behavior influences the aggregation of prey into herds. Game-theoretic models of herd formation are developed based on different survival probabilities of solitary prey and prey that join the herd and on the predator's preference of what type of prey to search for. For an intentional predator that will only pursue its preferred type of prey, a single herd with no solitaries cannot emerge unless the herd acts as a prey refuge. If neither prey choice provides a refuge, it is shown that an equilibrium always exists where there are both types of prey and the predator does not always search for the same type of prey (i.e., a mixed equilibrium exists). On the other hand, if the predator is opportunistic in that it sometimes shifts to pursue the type of prey that is observed first, there may be a single herd equilibrium that does not act as a prey refuge when there is a high level of opportunistic behavior. For low opportunistic levels, a mixed equilibrium is again the only outcome. The evolutionary stability of each equilibrium is tested to see if it predicts the eventual herding behavior of prey in its corresponding model. Our analysis confirms that both predator and prey preferences (for herd or solitary) have strong effects on why prey aggregate. In particular, in our models, only the opportunistic predator can maintain all prey in a single herd that is under predation risk.     

Size- and food-dependent growth drives patterns of competitive dominance along productivity gradients

Patterns of coexistence among competing species exhibiting size- and food-dependent growth remain largely unexplored. Here we studied mechanisms behind coexistence and shifts in competitive dominance in a size-structured fish guild, representing sprat and herring stocks in the Baltic Sea, using a physiologically structured model of competing populations. The influence of degree of resource overlap and the possibility of undergoing ontogenetic diet shifts were studied as functions of zooplankton and zoobenthos productivity. By imposing different size-dependent mortalities, we could study the outcome of competition under contrasting environmental regimes representing poor and favorable growth conditions. We found that the identity of the dominant species shifted between low and high productivity. Adding a herring-exclusive benthos resource only provided a competitive advantage over sprat when size-dependent mortality was high enough to allow for rapid growth in the zooplankton niche. Hence, the importance of a bottom-up effect of varying productivity was dependent on a strong top-down effect. Although herring could depress shared resources to lower levels than could sprat and also could access an exclusive resource, the smaller size at maturation of sprat allowed it to coexist with herring and, in some cases, exclude it. Our model system, characterized by interactions among size cohorts, allowed for consumer coexistence even at full resource overlap at intermediate productivities when size-dependent mortality was low. Observed shifts in community patterns were crucially dependent on the explicit consideration of size- and food-dependent growth. Accordingly, we argue that accounting for food-dependent growth and size-dependent interactions is necessary to better predict changes in community structure and dynamics following changes in major ecosystem drivers such as resource productivity and mortality, which are fundamental for our ability to manage exploitation of living resources in, e.g., fisheries.     

Functional responses: a question of alternative prey and predator density

Throughout the study of ecology, there has been a growing realization that indirect effects among species cause complexity in food webs. Understanding and predicting the behavior of ecosystems consequently depends on our ability to identify indirect effects and their mechanisms. The present study experimentally investigates indirect interactions arising between two prey species that share a common predator. In a natural field experiment, we introduced different densities of mealworms (Tenebrio molitor), an alternative prey, to a previously studied predator-prey system in which paper wasps (Polistes dominulus) preyed on shield beetle larvae (Cassida rubiginosa). We tested if alternative prey affects predation on the first prey (i.e., the predator-dependent functional response of paper wasps) by modifying either interference among predators or the effective number of predators foraging on shield beetles. Presence of mealworms significantly reduced the effective number of predators, whereas predator interference was not affected. In this way, the experimentally introduced alternative prey altered the wasps' functional response and thereby indirectly influenced C. rubiginosa density. In all prey-density combinations offered, paper wasps constantly preferred T. molitor. This led to an asymmetrical, indirect interaction between both prey species: an increase in mealworm density significantly relaxed predation on C. rubiginosa, whereas an increase in C. rubiginosa density intensified predation on mealworms. Such asymmetrical outcomes of a fixed food preference can significantly affect the population dynamics of the species involved. In spite of the repeated finding of a Type III functional response in this system, our experiment did not reveal switching behavior in paper wasps. The variety of mechanisms underlying direct and indirect interactions within our study system exemplifies the importance of incorporating alternative prey when investigating the impact of a generalist predator on a focal prey population under realistic field conditions.     

The effect of predation on artificial reef juvenile demersal fish species

There is a concern that artificial reefs (AR) may act purely as fishing aggregation devices. Predators attracted to ARs can influence the distribution and abundance of prey fish species. Determining the role of predators in AR is important in advancing the understanding of community interactions. This paper documents the effects of predation on fish assemblages of AR located near a coastal lagoon fish nursery. The Dicentrarchus labrax is a very opportunistic species preying on juveniles (0⁺ and 1⁺ age classes) of several demersal fish species on the ARs. Reef prey and sea bass abundance were negatively correlated. The mean numbers of prey per sea bass stomach increased with the increase of reef fish prey abundance, suggesting that predation has a significant influence, resulting in a decrease in prey abundance. Prey mortality (4–48%) of demersal reef fish associated species depends on bass density. Prey selection was related both with prey abundance and vulnerability. Results showed that D. labrax predation on AR-fish associated species can increase prey natural mortality. However, the role of bass predation on the ecological functioning of exploited ARs is not clear. There may be increases in local fishing yields due either to an increase in predator biomass through aggregation of sea bass attracted to ARs or to greater production. In contrast, predation on juveniles of economically important reef fish preys, especially the most frequent and abundant (Boops boops), can contribute to a decrease in recruitment to the fishery. Our results indicate that inter-specific interactions (predator–prey) are important in terms of conservation and management, as well as for the evaluation of the long-term effects of reef deployment. Thus, it is necessary to consider ecological interactions, such as predation, prior to the development and deployment of artificial habitats as a tool for rehabilitation.     

Predicting functional response and size selectivity of juvenile Notonecta maculata foraging on Daphnia magna

A mechanistic model was developed to assess the impact of predation of juvenile Notonecta maculata on size structured Daphnia magna populations and to provide a framework for quantifying the backswimmers uptake of food. Results of experiments and model predictions clearly demonstrate selective predation of backswimmers when fed with a choice of daphnid size classes, with patterns of selectivity differing across N. maculata instars. The model describes the foraging process empirically on the base of a general predation cycle including four conditional events instead of using classic functional response curves. For model parameterisation components of predation, namely probability of encounter, attack and success as well as time spent on handling prey was directly observed by means of video tracking experiments. Since attack rate, capture success and handling time appeared to be a function of prey size differing between Notonecta instars, the model takes into account ontogenic changes in both predator and prey characteristics. Independent data of functional response and size selectivity experiments were used for model validation and proved the model outcome to be consistent with observations.     

Size-based predation by kookaburras (Dacelo novaeguineae) on lizards (Eulamprus tympanum: Scincidae): what determines prey vulnerability?

Lizards and birds are both popular ”model organisms” in behavioural ecology, but the interactions between them have attracted little study. Given the putative importance of birds as predators of diurnal lizards, it is of considerable interest to know which traits (of lizards as well as birds) influence the outcome of a predatory attempt. We studied predation by giant terrestrial kingfishers (kookaburras, Dacelo novaeguineae: Alcedinidae) on heliothermic diurnal lizards (highland water skinks, Eulamprus tympanum: Scincidae), with particular reference to the role of prey (lizard) size. Our approach was twofold: to gather direct evidence (sizes of lizards consumed in the field, compared to those available) and indirect evidence (size-related shifts in lizard behaviour). We quantified the size structure of a natural population of skinks (determined by an extensive mark-recapture program), and compared it to the sizes of wild lizards taken by kookaburras (determined by analysis of prey remains left at the birds’ nests). Kookaburras showed size-based predation: they preyed mainly on small and medium-sized rather than large lizards in the field. However, the mechanism producing this bias remains elusive. It is not due to any distinctive behavioural attributes (locomotor ability, activity level, habitat usage) of the lizards of the size class disproportionately taken by the kookaburras. The greater vulnerability of subadult lizards may reflect subtle ontogenetic shifts in ecological and behavioural traits, but our data suggest that great caution is needed in inferring patterns of vulnerability to predation from indirect measures based on either the prey or the predator alone. Instead, we need direct observations on the interaction between the two. Received: 30 May 2000 / Revised: 29 July 2000 / Accepted: 26 August 2000     

Predator diversity and trophic interactions

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

Gizzard shad put a freeze on winter mortality of age-0 yellow perch but not white perch.

Four decades of observations on the limnology and fishes of Oneida Lake, New York, USA, provided an opportunity to investigate causes of mortality during winter, a period of resource scarcity for most juvenile fishes, in age-0 yellow perch (Perca flavescens) and age-0 white perch (Morone americana). This time series contains several environmental (e.g., winter severity) and biological (e.g., predator abundance) signals that can be used to disentangle multiple effects on overwinter mortality of these fishes. A multiple regression analysis indicated that age-0 yellow perch winter mortality was inversely related to fish length in autumn and to the abundance of alternative prey (gizzard shad [Dorosoma cepedianum] and white perch). However, no length-selective predation of yellow perch by one of the main predators, adult walleye (Sander vitreus), was detected. In contrast, white perch mortality was directly associated with total predator biomass and abundance of white perch in autumn, and inversely related to yellow perch abundance as a potential buffer species, but not to the abundance of gizzard shad. Winter severity was not a significant predictor of mortality for either perch species. Predicted winter starvation mortality, from a model described in the literature, was much lower than observed mortality for yellow perch. Similar models for white perch were correlated with observed mortality. These results collectively suggest that predation is the main mechanism shaping winter mortality of yellow perch, while both predation and starvation may be important for white perch. This analysis also revealed that gizzard shad buffer winter mortality of yellow perch. Although winter duration determines the northern limit of fish distributions, in mid-latitude Oneida Lake and for these species, predator-prey interactions seem to exert a greater influence on winter mortality than starvation.     

Predators of juvenile sea urchins and the effect of habitat refuges

We evaluated the effects of potential predators from intertidal habitats on Strongylocentrotus purpuratus survival using laboratory experiments and assessed abundances of main predatory species along the Pacific coast of North America. The interactive effects of urchins’ and predators’ sizes in mediating predation were quantified. Habitat complexity (substrate pits, adult spine canopy) was manipulated to examine its effects on predation of most susceptible individuals (<14 mm). Pachygrapsus crassipes was identified as a major predator of urchins up to ≈30 mm. A positive effect of predator size on consumption of progressively larger urchins was detected, probably due to a mechanical limitation on crabs’ ability to consume large prey. Larger claws of males with respect to females of comparable sizes facilitated the handling of larger prey. Substrate refuges significantly reduced mortality on juvenile urchins. These results show that crab predation may be important in organizing intertidal communities, despite multiple ecological mechanisms promoting sea urchin survival.     

Consequences of patch reef spacing for density-dependent mortality of coral-reef fishes

The spatial configuration of habitat patches can profoundly affect a number of ecological interactions, including those between predators and prey. I examined the effects of reef spacing on predator-prey interactions within coral-reef fish assemblages in the Bahamas. Using manipulative field experiments, I determined that reef spacing influences whether and how density-dependent predation occurs. Mortality rates of juveniles of two ecologically dissimilar species (beaugregory damselfish and yellowhead wrasse) were similarly affected by reef spacing; for both species, mortality was density dependent on reef patches that were spatially isolated (separated by 50 m), and density independent on reef patches that were aggregated (separated by 5 m). A subsequent experiment with the damselfish demonstrated that a common resident predator (coney) caused a substantial proportion of the observed mortality, independent of reef spacing. Compared to isolated reefs, aggregated reefs were much more likely to be visited by transient predators (mostly yellowtail snappers), regardless of prey density, and on these reefs, mortality rates approached 100% for both prey species. Transient predators exhibited neither an aggregative response nor a type 3 functional response, and consequently were not the source of density dependence observed on the isolated reefs. These patterns suggest that resident predators caused density-dependent mortality in their prey through type 3 functional responses on all reefs, but on aggregated reefs, this density dependence was overwhelmed by high, density-independent mortality caused by transient predators. Thus, the spatial configuration of reef habitat affected both the magnitude of total predation and the existence of density-dependent mortality. The combined effects of the increasing fragmentation of coral reef habitats at numerous scales and global declines in predatory fish may have important consequences for the regulation of resident fish populations.     

Indirect effects of prey swamping: differential seed predation during a bamboo masting event

Resource pulses often involve extraordinary increases in prey availability that "swamp" consumers and reverberate through indirect interactions affecting other community members. We developed a model that predicts predator-mediated indirect effects induced by an epidemic prey on co-occurring prey types differing in relative profitability/preference and validated our model by examining current-season and delayed effects of a bamboo mass seeding event on seed survival of canopy tree species in mixed Patagonian forests. The model shows that predator foraging behavior, prey profitability, and the scale of prey swamping influence the character and strength of short-term indirect effects on various alternative prey. When in large prey-swamped patches, nonselective predators decrease predation on all prey types. Selective predators, instead, only benefit prey of similar quality to the swamping species, while very low or high preference prey remain unaffected. Negative indirect effects (apparent competition) may override such positive effects (apparent mutualism), especially for highly preferred prey, when prey-swamped patches are small enough to allow predator aggregation and/or predators show a reproductive numerical response to elevated food supply. Seed predation patterns during bamboo (Chusquea culeou) masting were consistent with predicted short-term indirect effects mediated by a selective predator foraging in large prey-swamped patches. Bamboo seeds and similarly-sized Austrocedrus chilensis (ciprés) and Nothofagus obliqua (roble) seeds suffered lower predation in bamboo flowered than nonflowered patches. Predation rates on the small-seeded Nothofagus dombeyi (coihue) and the large-seeded Nothofagus alpina (rauli) were independent of bamboo flowering. Indirect positive effects were transient; three months after bamboo seeding, granivores preyed heavily upon all seed types, irrespective of patch flowering condition. Moreover, one year after bamboo seeding, predation rates on the most preferred seed (rauli) was higher in flowered than in nonflowered patches. Despite rapid predator numerical responses, short-term positive effects can still influence community recruitment dynamics because surviving seeds may find refuge beneath the litter produced by bamboo dieback. Together, our theoretical analysis and experiments indicate that indirect effects experienced by alternative prey during and after prey-swamping episodes need not be universal but can change across a prey quality spectrum, and they critically depend on predator-foraging rules and the spatial scale of swamping.     

Diverse trait-mediated interactions in a multi-predator, multi-prey community

Trait-mediated interactions (TMII) can alter the outcome or magnitude of species interactions. We examined how the interaction between a guild of ground and rove beetles and their fly egg prey was altered by a larger predator, the ground beetle Pterostichus melanarius, and an additional prey, aphids. In field and laboratory experiments, we manipulated the presence or absence of P. melanarius and aphids and recorded the impact of these manipulations on beetle activity and fly egg predation. Individually, aphids, by serving as preferred prey, and P. melanarius, by reducing focal beetle activity, weakened egg predation. However, egg predation was restored when both aphids and P. melanarius were present together, because aphids triggered greater foraging activity, and thus increased incidental predation of fly eggs, by P. melanarius. Thus, TMII among subsets of the community that were disruptive to predation on fly eggs could not be summed to predict the dominant, positive TMII within a more diverse community. Future TMII studies should include more realistic representations of species diversity, and should not ignore the influence of prey on predator behavior.     

Patterns of shell repair in articulate brachiopods indicate size constitutes a refuge from predation

The cost of overcoming prey defenses relative to the value of internal tissues is a key criterion in predator/prey interactions. Optimal foraging theory predicts: (1) specific sizes of prey will result in the best returns to predators, and (2) there will often be a size at which the cost/benefit balance is low enough to effectively exclude predation. Data presented here on styles of repaired shell damage and size at which injury had been sustained was collected from samples of terebratulide brachiopods from the Antarctic Peninisula (Liothyrella uva), Falkland Islands (Magellania venosa and Terebratella dorsata) and Chile (M. venosa). The predominant form of damage on shells was indicative of predators attacking the valve margins. The modal size for repaired damage was more than 10 mm smaller than the modal size for the overall size distribution in each species and there were no repaired attacks in the largest size classes of any species. These data suggest that size forms a refuge from predation, as would be predicted by optimal foraging theory. The optimal sizes that predators appeared to attack vary between species, as do the sizes that provided a refuge from predation. High levels of multiple repairs (19% of the M. venosa population from the Falkland Islands sampled had 2 or more repairs) suggest that the mortality following attack is low, suggesting that many predators abandon their attacks.