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The outcome of predator-prey interactions depends on the characteristics of predators and prey as well as the structure of the environment. In a replicated field enclosure experiment, we tested the effects of quantity and quality of different prey refuges (no structure, structure forming a partial refuge, and structure forming a complete refuge) on the interaction between piscivorous perch (Perca fluviatilis) and juvenile perch and roach (Rutilus rutilus). We quantified the behaviour of the predators and the prey and predator-induced prey mortality. The piscivores stayed in or close to the prey refuge and were more dispersed in the presence than in the absence of prey refuges. Survival of juvenile perch and roach decreased in the presence of predators and was higher for juvenile roach than for juvenile perch. In addition, juvenile perch survival increased with refuge efficiency. Roach formed schools which were denser in the presence of predators, had a higher swimming speed (both in the open water and in the refuge) and used a larger area than juvenile perch. Both prey species decreased their distance to the prey refuge and increased the proportion of their time spent in the refuge in the presence of predators. The number of switches between the open-water habitat and the prey refuge was higher for juvenile roach than for juvenile perch. Juvenile perch used different parts of the prey refuge in a flexible way depending both on presence of predators and refuge type whereas juvenile roach used the different parts of the prey refuge in fixed proportions over all refuge treatments. Our results suggest that juvenile roach had a overall higher capacity to avoid predation than juvenile perch. However, in the presence of qualitatively different prey refuges juvenile perch responded to predators with more flexible refuge use than juvenile roach. The differences in antipredator capacities of juvenile perch and roach when subjected to piscivorous perch predation may depend on differences in life history patterns of the two species.

We investigated experimentally how presence or absence of different piscivores influenced habitat use, diet, and individual growth of two size classes of juvenile perch (Perca fluviatilis) and abundances of juvenile perch resources in different habitats. Pond enclosures encompassed 3 X 6 m of dense vegetation and 9 X 6 m of open habitat, and were stocked with 75 young-of-year and 30 1-yr-old perch and with either three piscivorous perch, three northern pike (Esox lucius), or no piscivores. Northern pike were both larger and possessed a larger gape than piscivorous perch. To isolate behavioral responses of juvenile perch to piscivores, we replaced consumed young-of-year perch. Prey fish mortality was lowest without piscivores and highest with northern pike. Young-of-year perch increased their use of vegetation in the presence of both piscivores, whereas 1-yr-old perch increased their use of vegetation only with northern pike. For both age classes of prey fish, increased use of the vegetation led to reduced individual growth, owing to two complementary mechanisms. First, the physical complexity of submerged macrophytes likely interfered with the benthic feeding of perch, Second, increased use of the (relatively small) vegetated habitat increased the mean density experienced by prey fish. Piscivore-induced changes of prey fish densities in the two habitats had substantial effects on the biomass of prey fish resources in the open habitat, but only minor effects in the vegetation. Sialis lutaria, the major predatory macroinvertebrate (approximate to 50% of total macroinvertebrate biomass in the open habitat), and total predatory macroinvertebrates were positively affected by piscivores in the open habitat, but not in the vegetation. Chironomids (

In size-structured populations, interactions are strongly dependent on size- specific foraging and anti-predator capacities of the organism. Conflicting size-specific selection pressures over the ontogeny often have different effects on different species leading to asymmetries in competitive and predator-prey interactions. Habitat complexity is likely to affect such asymmetric interactions due to species/size-specific competitive abilities in different habitats and due to the fact that habitat structural complexity may act differently as a prey refuge for different species. We experimentally analyzed the impact of a piscivorous predator (adult perch, Perca fluviatilis) on performance of juvenile perch and roach (Rutilus rutilus) at different levels of structural complexity (no structure, structure forming a partial refuge, and structure forming a complete refuge) in enclosures in an experimental pond. We measured predator diet and growth, prey fish habitat use, survival, diet and growth, and prey resource levels in different habitats. Prey fish (perch and roach) were found in the diet of piscivorous perch in no refuge and partial refuge treatments. Growth rate of the piscivorous perch decreased with increased refuge efficiency. Juvenile perch increased their proportional use of the structurally complex refuges in the presence of piscivorous perch and the survival increased with increased refuge efficiency (from partial to complete refuge). The diet of juvenile perch changed from predominantly cyclopoid copepods in the absence of predators to predominantly macroin- vertebrates in the presence of predators. There was no effect of predator-induced habitat restriction on growth of juvenile perch. Roach survival also increased with increased refuge efficiency in the presence of predators, and roach survival in the refuge treatments did not differ from each other or from the treatments with predators absent. Predator-induced habitat restriction in roach was associated with a decreased growth of roach. Our results suggest that, compared to juvenile roach, juvenile perch may compensate more for lost foraging opportunity in the open water via increased exploitation of structure- associated prey in refuges. As a result, predator-induced habitat shifts by juvenile perch and roach may alter competitive interactions between the species. On the other hand, structural complexity may form an almost complete refuge for juvenile roach from predators and thereby affect the predator-prey relationships between piscivorous perch and juvenile perch and roach to the advantage of juvenile roach. The demonstrated effects of structural complexity on competitive and predator-prey interactions between perch and roach can be related to the two species' distributions in lakes with different degrees of structural com- plexity.

While phenotypic responses to direct species interactions are well studied, we know little about the consequences of indirect interactions for phenotypic divergence. In this study we used lakes with and without the zebra mussel to investigate effects of indirect trophic interactions on phenotypic divergence between littoral and pelagic perch. We found a greater phenotypic divergence between littoral and pelagic individuals in lakes with zebra mussels and propose a mussel-mediated increase in pelagic and benthic resource availability as a major factor underlying this divergence. Lakes with zebra mussels contained higher densities of large plankton taxa and large invertebrates. We suggest that this augmented resource availability improved perch foraging opportunities in both the littoral and pelagic zones. Perch in both habitats could hence express a more specialized foraging morphology, leading to an increased divergence of perch forms in lakes with zebra mussels. As perch do not prey on mussels directly, we conclude that the increased divergence results from indirect interactions with the mussels. Our results hence suggest that species at lower food web levels can indirectly affect phenotypic divergence in species at the top of the food chain.

Predation is a major driving force in evolution. Predation has been shown to select for size, morphology, and camouflage. Many animals use camouflage to reduce predation risk. In some cases, individuals can adjust their pigmentation, enabling them a higher survival in a heterogeneous environment. Here, we show that the difference in pigmentation between juvenile perch individuals (Perca fluvuiatilis) occupying different environments (open water and vegetated habitats of lakes) is likely a consequence of predator selection. Lightly pigmented individuals have a higher chance of survival in open water whereas darker pigmented individuals survive better in vegetation. As a response to predators, individuals forced into the vegetation by predators developed darker skin whereas the skin of individuals forced into open water became lighter. In a common garden experiment, in the absence of predation, we found that pigmentation in juvenile perch is only due to plasticity and not to genetic variation. However, contrary to predictions, individuals raised in open water developed darker skin compared to individuals raised in vegetation. This may be a response to UV-stress. Overall, our results suggest that predation can be a strong selective agent on pigmentation differences among conspecifics occupying different habitats.

Cross-ecosystem movements of material and energy are ubiquitous. Aquatic ecosystems typically receive material that also includes organic matter from the surrounding catchment. Terrestrial-derived (allochthonous) organic matter can enter aquatic ecosystems in dissolved or particulate form. Several studies have highlighted the importance of dissolved organic carbon to aquatic consumers, but less is known about allochthonous particulate organic carbon (POC). Similarly, most studies showing the effects of allochthonous organic carbon (OC) on aquatic consumers have investigated pelagic habitats, the effects of allochthonous OC on benthic communities are less well studied. Allochthonous inputs might further decrease primary production through light reduction, thereby potentially affecting autotrophic resource availability to consumers. Here, an enclosure experiment was carried out to test the importance of POC input and light availability on the resource use in a benthic food web of a clearability to consumers. Here, an enclosure experiment was carried out to test the importance of POC input and light availability on the resource use in a benthic food web of a clear-water lake. Corn starch (a C 4 plant) was used as a POC source due to its insoluble nature and its distinct carbon stable isotope value (? 13C). The starch carbon was closely dispersed over the bottom of the enclosures to study the fate of a POC source exclusively available to sediment biota. The addition of starch carbon resulted in a clear shift in the isotopic signature of surface-dwelling herbivorous and predatory invertebrates. Although the starch carbon was added solely to the sediment surface, the carbon originating from the starch reached zooplankton. We suggest that allochthonous POC can subsidize benthic food webs directly and can be further transferred to pelagic systems, thereby highlighting the importance of benthic pathways for pelagic habitats. � 2011 Springer-Verlag.

The zebra mussel (Dreissena polymorpha) provides one example of successful invaders in novel environments. However, little attention has been devoted to exploring the factors regulating zebra mussel density and population size structure at the local scale. We tested effects of physicochemical factors and fish predation on the density of zebra mussels at several sites and between years in a natural lake. Water depth and roach (Rutilus rutilus) density were the most important variables affecting local zebra mussel density. Substrate was also an important factor but affected Dreissena density only at the shallowest depth examined (2 m), which also supported a large population of the mussels. Mean shell length of Dreissena increased with water depth. Our results indicate that predation pressure, intraspecific competition, and food limitation might be responsible for variation in zebra mussel density and population size structure in space and time and that fish predation might have strong top-down effects on zebra mussel populations.

A predator's foraging strategy is generally described as either a sit-and-wait or an actively searching strategy. Two different behaviour patterns, often associated with these strategies, are territorial behaviour for a sit-and-wait predator, and group foraging behaviour for an actively searching predator. In a pond experiment, the effect of predator density on the efficiency of a sit-and-wait predator, pike, and an actively searching predator, perch, was tested. The foraging efficiencies of these two predators were predicted to be density-dependent since perch is a social species with its efficiency enhanced by mutualistic interactions, while pike is a territorial species with its efficiency reduced by competitive interactions. The predators were stocked at two densities (one or five individuals) in enclosures containing the same prey densities (75 individuals of juvenile perch). The behaviour of predators and prey, prey mortality and predator growth were studied. As expected, swimming speed and move frequency were significantly higher for perch than for pike. In the five-pike treatment, predator activity (swimming speed and move frequency) was highly size-dependent with the large pike both swimming faster and moving more frequently than the small pike. The activity levels of the pike were also reflected in the area used, which was larger for the large pike. Perch at both densities and the large pike used similar areas. As hypothesized, the growth rate of the predators corresponded to their foraging strategies. The growth rate of group foraging perch was higher than that of single perch and pike in both density treatments. In the single predator treatments, pike grew better than perch. In contrast to perch, the single pike grew more than the five pike together.

Predators are increasingly recognized as key elements in food webs because of their ability to link the fluxes of nutrients and energy between spatially separated food chains. However, in the context of food web connectivity, predator populations have been mainly treated as homogeneous units, despite compelling evidence of individual specialization in resource use. It is conceivable that individuals of a predatory species use different resources associated with spatially separated food chains, thereby decoupling cross-habitat linkages. We tested whether intrapopulation differences in habitat use in the generalist freshwater predator Eurasian perch (Perca fluviatilis) led to long-term niche partitioning and affected the degree of ecological habitat coupling. We evaluated trophic niche variability at successively larger timescales by analyzing gut contents and stable isotopes (delta13C and delta15N) in liver and muscle, tissues that provide successively longer integration of trophic activity. We found that the use of distinct habitats in perch led to intrapopulation niche partitioning between pelagic and littoral subpopulations, consistent through the various timescales. Pelagic fish showed a narrower niche, lower individual specialization, and more stable trophic behavior than littoral fish, as could be expected from inhabiting a relatively less diverse environment. This result indicated that substantial niche reduction could occur in a generalist predator at the subpopulation level, consistent with the use of a habitat that provides fewer chances of individual specialization. We showed that intrapopulation niche partitioning limits the ability of individual predators to link spatially separated food chains. In addition, we suggest a quantitative, standardized approach based on stable isotopes to measure the degree of habitat coupling mediated by a top predator.

Spatial and temporal heterogeneity in the environment is a common feature affecting many natural populations. For example, both the resource levels and optimal habitat choices of individuals likely change over time. One way for organisms to cope with environmental variation is to display adaptive plasticity in traits such as behavior and morphology. Since trait plasticity is hypothesized to be a prerequisite for character divergence, studies of mechanisms behind such plasticity are warranted. In this study, we looked at the interaction of two potentially important environmental variables on behavioral and morphological plasticity in Eurasian perch (Perca fluviatilis L.). More specifically, the plastic responses in activity and morphology of perch exposed to different resource levels and simulated habitat types were studied in an aquarium experiment. The resource level experienced had a large influence on plasticity in both activity and morphology. Behavioral adaptations have been thought to mediate morphological transitions, and we suggest that the morphological response to the resource level was mediated by differences in activity and growth rates. The habitat type also affected morphological plasticity but to a lesser extent, and there was no effect on activity from habitat type. Based on these results, we suggest that it is essential to include several environmental factors acting in concert when studying mechanisms behind trait plasticity. We also propose that variation in resource levels might play a key role in fostering trait plasticity in at least fish populations, while other environmental variables such as divergent habitat complexities and prey types might be less influential. Dynamics in resource levels and optimal habitat choices might thus be important factors influencing character divergence in natural populations.

Predators can cause a shift in both density and frequency of a prey phenotype that may lead to phenotypic divergence through natural selection. What is less investigated is that predators have a variety of indirect effects on prey that could potentially have large evolutionary responses. We conducted a pond experiment to test whether differences in predation risk in different habitats caused shifts in behavior of prey that, in turn, would affect their morphology. We also tested whether the experimental data could explain the morphological variation of perch in the natural environment. In the experiment, predators caused the prey fish to shift to the habitat with the lower predation risk. The prey specialized on habitat-specific resources, and there was a strong correlation between diet of the prey fish and morphological variation, suggesting that resource specialization ultimately affected the morphology. The lack of differences in competition and mortality suggest that the morphological variation among prey was induced by differences in predation risk among habitats. The field study demonstrated that there are differences in growth related to morphology of perch in two different habitats. Thus, a trade-off between foraging and predator avoidance could be responsible for adaptive morphological variation of young perch.

We evaluated the trade-off between foraging activity and predation risk for larvae of an anuran species presented with different types of food resources. In the laboratory we examined the effects of benthic and phytoplankton resources and the two types combined on the activity and mortality of larvae of the common frog, Rana temporaria, exposed to predatory larva, Dytiscus marginalis. Predation mortality of tadpoles increased with the duration of the experiment and was highest in the presence of the phytoplankton resource alone. This was explained by a decrease in prey activity in the benthic- and combined-resource treatments when the predator was present, whereas in the phytoplankton treatment, the activity of the tadpoles were similar in the presence or absence of the predator. In the presence of the predator, prey mainly used the bottom of the aquarium in the benthic- and combined-resource treatments. In contrast, in the phytoplankton treatment they used the water column more than in the other treatments. In the presence of the predator, the prey had a lower consumption rate in the phytoplankton treatment than in the other treatments. Predator activity did not change among treatments. The results suggest that the trade-off between foraging activity and predation mortality for anuran larvae is strongly affected by the types of food resources present in the environment.

In predator-prey interactions, the efficiency of the predator is dependent on characteristics of both the predator and the prey, as well as the structure of the environment. In a field enclosure experiment, we tested the effects of a prey refuge on predator search mode, predator efficiency and prey behaviour. Replicated enclosures containing young of the year (0+) and 1-year-old (1+) perch were stocked with 3 differentially sized individuals of either of 2 piscivorous species, perch (Perca fluviatilis), pike (Esox lucius) or no piscivorous predators. Each enclosure contained an open predator area with three small vegetation patches, and a vegetated absolute refuge for the prey. We quantified the behaviour of the predators and the prey simultaneously, and at the end of the experiment the growth of the predators and the mortality and habitat use of the prey were estimated. The activity mode of both predator species was stationary. Perch stayed in pairs in the vegetation patches whereas pike remained solitary and occupied the corners of the enclosure. The largest pike individuals stayed closest to the prey refuge whereas the smallest individuals stayed farthest away from the prey refuge, indicating size-dependent interference among pike. Both size classes of prey showed stronger behavioural responses to pike than to perch with respect to refuge use, distance from refuge and distance to the nearest predator. Prey mortality was higher in the presence of pike than in the presence of perch. Predators decreased in body mass in all treatments, and perch showed a relatively stronger decrease in body mass than pike during the experiment. Growth differences of perch and pike, and mortality differences of prey caused by predation, can be explained by predator morphology, predator attack efficiency and social versus interference behaviour of the predators. These considerations suggest that pike are more efficient piscivores around prey refuges such as the littoral zones of lakes, whereas perch have previously been observed to be more efficient in open areas, such as in the pelagic zones of lakes.

PREDATORY EFFICIENCY AND PREY SELECTION - INTERACTIONS BETWEEN PIKE ESOX-LUCIUS, LUCIUS, PERCH PERCA-FLUVIATILIS AND RUDD SCARDINUS-ERYTHROPHTHALMUS