The impact of predation by burbot (<Emphasis Type="Italic">Lota lota</Emphasis> L.) on the macroinvertebrate community in the littoral zone of a large lake

We studied the effects of predation by juvenile burbot (Lota lota) on the macroinvertebrate community in mesocosm experiments in the stony littoral zone of Lake Constance, a large prealpine lake in Central Europe. Although the growth data of the burbot suggest that the benthivorous fish exerts strong predation pressure on the invertebrate community, the predicted level of consumption is only poorly reflected by changes in biomass and abundance of most invertebrate prey taxa. High exchange rates of the prey between the mesocosms and the ambient littoral environment apparently masked the true predation effects of fish. Also, life-history events such as hatching or synchronised emergence of larvae led to temporal effects that obscured the impact of predation. However, for the dominant prey organism, the freshwater amphipod Gammarus roeseli, direct lethal effects appeared when its migration was limited. When exchange with the ambient littoral zone was possible, abundance and biomass of G. roeseli were unexpectedly high in the mesocosms stocked with burbot, indicating behavioural responses to fish presence. During the experiment, the burbot gained ca. 15% in length and about 60% in body mass. According to stomach content analyses at the end of the experiment, the ingested prey consisted mainly of relatively large and abundant invertebrates. Our study indicates that predation by juvenile burbot should be an important factor in structuring the benthic invertebrate community in terms of qualitative and quantitative effects.     

Temporary pools are not `enemy-free'

Temporary pools are traditionally considered as refuges where the conspicuous anostracans are protected from predation. While this is true for the size-selective predation by fish, there is compelling evidence that invertebrate predation is an important biotic stress regulating temporary pool communities. In rock pools in southeastern Botswana, we studied the impact of some suspected invertebrate predators on populations of the freshwater anostracan Branchipodopsis wolfi by means of observations and manipulative experiments. In a survey of 45 pools, the relationship between B. wolfi natural population sizes and the abundance of suspected predators were never negative for turbellarians and mosquito larvae. When dragonfly larvae, notonectids or tadpoles were present, the anostracan populations were generally non-existent or very small. In enclosure experiments with turbellarians, there was a significant effect of predation within one hour of the start; the average daily predation rate was about 1/4 anostracan per turbellarian. Anostracans from a pool with few turbellarians were slightly less vulnerable than those from a turbellarian-rich pool. Furthermore, there was an indication of males being predated on more than females. With dragonfly larvae and notonectids, the predation effect was marked with all six anostracans in an experiment eaten in less than one day by a single predator (predation rate: about one anostracan every 2 h per predator). In a behavioral study, both sexes of B. wolfi avoided swimming above sediment that held more turbellarians than the open patches; there was no evidence for chemical communication with respect to this behavior.     

Formation of morphological defences in response to YOY perch and invertebrate predation in two <Emphasis Type="Italic">Daphnia</Emphasis> species coexisting in a mesotrophic lake

This study examined the formation of morphological defences by two coexisting Daphnia species, the large-sized D. pulicaria (2 mm) and the small-sized D. mendotae (1.4 mm), in response to the presence of young-of-the-year (YOY) yellow perch (Perca flavescens) and invertebrate predators (Chaoborus, Leptodora) during summer in a mesotrophic lake. We hypothesized that due to differential size-selective predation risk by YOY fish and invertebrates, the large-sized and the small-sized Daphnia species would show different morphological responses to predation threats. We followed changes in two morphological traits (relative length of the tail spine in D. pulicaria and of the helmet in D. mendotae) among different periods during summer according to YOY fish and invertebrate predation. We defined four YOY fish predation periods based on the presence of YOY perch in the pelagic zone of the lake and the relative abundance of Daphnia preys in their gut contents, and two invertebrate predation periods based on exclusive or mutual occurrence of the invertebrate predators. The large-sized (D. pulicaria) and the small-sized (D. mendotae) species showed different morphological responses to YOY fish and invertebrate predators, respectively. The tail spine ratio of the juveniles and adults of D. pulicaria did not change in response to YOY fish predation or to invertebrate predation. A gradual increase in the helmet ratio was observed in the small-sized D. mendotae over the summer period. This change was related to the co-occurrence of the invertebrate predators (Chaoborus and Leptodora) and to YOY fish predation. The warmer temperature cannot be accounted for helmet elongation since it was constant across depths, and not related with the co-occurrence of D. mendotae and YOY perch. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Intra- and interspecific predation on the cocoons of Erpobdella octoculata (L.) (Annelida: Hirudinea)

In laboratory experiments, eleven out of twenty-one invertebrate species and all of the three fish species tested fed on freshly laid flaccid cocoons of Erpobdella octoculata, though predation was only severe from a dytiscid beetle, E. octoculata and the fish species. There was a tendency for more cocoons to be eaten by starved than fed predators. Only snail species damaged one-week-old tougher cocoons.Only one out of 471 E. octoculata collected during their breeding season from the stony shores of two eutrophic lakes had a cocoon in its gut.The role of cannibalism of and interspecific predation on cocoons in the control and regulation of lake-dwelling populations of the erpobdellid is discussed, and assessed to be relatively unimportant. A more likely regulatory mechanism may involve high juvenile mortality from as yet unknown causes.     

Body size and substrate association of littoral insects in relation to vegetation structure

Variation in substrate association types and maximum size of aquatic insects were studied in a vegetated littoral zone of three lake basins. The basins differed from each other in trophic status, biomass of benthivorous fish, and abundance of macrophytes. Four types of substrate association – swimmers, crawlers, semisessiles and burrowers, respectively – were assumed to represent decreasing vulnerability to fish predators. Large-sized species were also hypothesised to be more vulnerable to fish predators. The distributions of species traits were examined in relation to vegetation density. Inferring from ``predation hypothesis' opposite selection pressures on the species traits were expected along the vegetation density. Dense macrophyte beds were thought to be dominated by invertebrate predators and open water by fish predators, since the predation efficiency of fish decreases in complex environments. In the case of invertebrate predator domination, large size and higher activity should be favoured traits among the prey species. Distribution patterns of modes of the two studied traits were explored separately for predatory and non-predatory insects. As expected, swimmers and large-sized crawlers were characteristic of the insect assemblages of dense macrophyte beds. The densities of Odonata, Corixidae, Dytiscidae, Ephemeroptera and Sialidae were higher among macrophytes than in open water, where these insect taxa were possibly depleted by fish. On the other hand, the small-sized and fairly immobile Chironomidae were the most abundant group in open water. These results support the existence of a predator transition zone among littoral vegetation, ranging from domination of invertebrate predation among the dense beds to that of fish predation in open water.     

Impact of Holoshestes heterodon Eigenmann (Pisces: Characidae) on the plankton community of a subtropical reservoir: the importance of predation by Chaoborus larvae

We analyzed the effects of planktivorous Holeshestes heterodon Eigenmann (Characidae) predation on the plankton community of a small subtropical reservoir, using four enclosures (volume about 17.5 m³), open to the sediment, established in the littoral zone. Two enclosures were stocked with fish (mean TL 5.7 cm), at a density of about 4–5 fish m^–3 (approx. 8 g m^–3), whereas two remained fishless. The experiment lasted a little longer than one month. In the fish enclosures, most Crustacea and Chaoborus larvae remained scarce, probably as a result of visually selective fish predation. In both fishless enclosures, Chaoborus larvae became abundant. However, in only one of these did large individuals become relatively numerous; this discrepancy in the demographic structure of the Chaoborus populations between the two fishless enclosures is unexplained. Only in the fishless enclosure without appreciable numbers of large Chaoborus did densities of Crustacea increase greatly. It is suggested that in the enclosure containing large Chaoborus individuals, crustacean populations were prevented from developing due to predation pressure, while the small Chaoborus larvae of the other enclosure could not readily consume these prey. Rotifers were low in abundance in the absence of fish, probably as a consequence of Chaoborus predation. Phytoplankton density increased in all four enclosures, due probably to the lack of water flow. Only in the fishless enclosure with high densities of crustaceans did phytoplankton abundance decrease markedly at the end of the experiment, perhaps because of grazing losses.     

Acidification of a Small Watershed in Southern Norway and Some Characteristics of Acidic Aquatic Environments

Continuous monitoring of a small watershed in southern Norway shows that pH has decreased significantly by almost half a unit (from 5.9 to 5.4), conductivity increased from 23 to 29 μS/cm within the period from 1965–1975. Changes are also detected in sediment analyses where Daphnia and Chaoborus are only found deeper than 3–4 cm. Forest clear-cutting activities have probably accelerated the acidification processes of the already highly vulnerable freshwater localities. The acidic lakes show a solute content below 0.4 meq/1, and the bicarbonate buffer system may no longer work efficiently. Spring thaw and autumn rain episodes are accompanied by strong alternations in the chemical environment of the lakes; by considerable drop in pH and increasing conductivity and sulphate content. With increasing acidification the planktonic community changes from a fish predation governed system (brown trout and Eurasian perch as dominant species) to an invertebrate predation governed one with dominant predators Heterocope and pelagic corixids. Pelagic zooplankton are primarily composed of littoral species: Bosmina longispina, Diaphanosoma brachyurum, Eudiaptomus gracilis and Heterocope saliens. The littoral plant and invertebrate communities are described and discussed.     

Indirect effects of fish on macrophytes in Bays Mountain Lake: evidence for a littoral trophic cascade

Summary We began this experiment to test specific hypotheses regarding direct and indirect effects of fish predation on the littoral macroinvertebrate community of Bays Mountain Lake, Tennessee. We used 24 m² enclosures in which we manipulated the presence and absence of large redear sunfish (Lepomis microlophus>150 mm SL), and small sunfish (L. macrochirus and L. microlophus <50 mm SL) over a 16-mo period. Here we report on effects of fish predation on gastropod grazers that appear to cascade to periphyton and macrophytes.Both large redear sunfish and small sunfish maintained low snail biomass, but snails in fish-free controls increased significantly during the first 2-mo of the experiment. By late summer of the first year of the experiment, the difference in biomass between enclosures with and without fish had increased dramatically (>10×). Midway through the second summer of the experiment, we noted apparent differences in the abundance of periphyton between enclosures containing fish and those that did not. We also noted differences in the macrophyte distribution among enclosures. To document these responses, we estimated periphyton cover, biovolume and cell size frequencies as well as macrophyte distributions among enclosures at the end of the experiment. When fish were absent, periphyton percent cover was significantly reduced compared to when fish were present. Periphyton cell-size distributions in enclosures without fish were skewed toward small cells (only 12% were greater than 200 m³), which is consistent with intense snail grazing. The macrophyte Najas flexilis had more than 60 x higher biomass in the fish-free enclosures than in enclosures containing fish; Potamogeton diversifolius was found only in fish-free enclosures. These results suggest a chain of strong interactions (i.e. from fish to snails to periphyton to macrophytes) that may be important in lake littoral systems. This contrasts sharply with earlier predictions based on cascading trophic interactions that propose that fish predation on snails would enhance macrophyte biomass.     

The Influence of Invertebrate Predators on Daphnia Spatial Distribution and Survival in Laboratory Experiments: Support for Daphnia Horizontal Migration in Shallow Lakes

The behavioural response of Daphnia cucullata to the presence of the pelagic invertebrate predator Leptodora kindtii, and the predation rate of littoral dragonfly nymphs on this species were investigated under laboratory conditions. Results of this study revealed a strong hiding response of Daphnia cucullata in the presence of the predatory cladoceran, L. kindtii, which was similar to the response of Daphnia in the presence of juvenile perch. This suggests that pelagic invertebrate predators may cause Daphnia to hide in the littoral zone which could result in increased exposure to predation by littoral invertebrates. A strong influence of dragonfly nymphs on D. cucullata, both in the presence and absence of macrophytes, was found. The average predation rate of Odonata larvae was about 5 prey ind^–1 h^–1 and did not differ significantly between treatments. Quantification of dragonfly pressure on Daphnia populations will require cross‐verification with field experiments since in the natural conditions Daphnia seeks a shelter in the vegetation stands against predation by Leptodora, despite the occurrence of odonates. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Quantification of invertebrate predation and herbivory in food chains of low complexity

Zooplankton grazing impact on algae, heterotrophic flagellates and bacteria, as well as invertebrate predation on herbivorous zooplankton, were investigated in two sub-Antarctic lakes with extremely simple food chains. The two species of herbivorous zooplankton present in the lakes (the copepods boeckella michaelseni and Pseudoboeckella poppei) exerted substantial grazing pressure on algae. However, the dominant algal species exhibited properties that enabled them to avoid (large size or extruding spines, e.g. Staurastrum sp., Tribonema sp.) or compensate (recruitment from the sediment, Mallomonas sp.) grazing. There are only two potential invertebrate predators on the herbivorous copepods in the two lakes: the copepod Parabroteas sarsi and the diving beetle Lancetes claussi. Vertebrate predators are entirely abscent from sub-Antarctic lakes. Based on our experiments, we estimated that the predators would remove at most about 0.4% of the herbivorous copepods per day, whereas planktivorous fish, if present in the lakes, would have removed 5–17% of the zooplankton each day. Consequently, the invertebrate predators in these high-latitude lakes had only a marginal predation impact compared to the predation pressure on zooplankton in the presence of vertebrate predators in temperate lakes. The study of these simple systems with only two quantitatively functionally important trophic links, suggests that high grazing pressure foreces the algal community towards forms with grazer resistant adaptations such as large size, recruitment from another habitat, and grazer avoidance spines. We propose that due to such adaptations, predictions from food web theory are only partly corroborated, i.e. algal biomass actually increases with increasing productivity, although the grazer community is released from predation. In more species-rich and complex systems, e.g temperate lakes with three functionally important links, such adaptations are likely to be even more important, and, consequently, the observable effects of trophic interactions from top predators on lower trophic levels even more obscured.     

Rotifers,cladocerans and planktivorous fish: What are the major interactions?

Results of ten outdoor tank experiments on the effects of two kinds of planktivorous fish (Dorosoma cepedianum and Menidia beryllina) on phytoplankton and zooplankton were reviewed for evidence of rotifer-cladoceran interactions. Correlation and multiple regression analyses of the responses of populations of six rotifer species in ten experiments were examined for evidence of interference competition with daphnids, resource exploitation, or invertebrate predation by crustaceans associated with daphnid-dominated communities. A separate enclosure experiment was conducted to assess the potential for short-term effects of dense daphnid populations on four rotifer populations. Indirect resource-mediated effects on rotifer populations associated with manipulations of planktivorous fish were more numerous and consistently expressed than direct negative effects of cladocerans or copepods on rotifer populations.     

Fish predation can induce mesohabitat-specific differences in food web structures in small stream ecosystems

Diverse benthic communities in streams include a wide variety of predators with different habitat preferences, e.g. for pools or riffles. We hypothesised that these preferences result in mesohabitat-specific predator community structures with quantitative differences concerning predation intensity by vertebrate and invertebrate predators, importance of intraguild predation, or top–down pressure. This hypothesis was evaluated for a small submontane stream by means of mesohabitat-specific quantification of prey consumption by two benthivorous fish species (Gobio gobio and Barbatula barbatula) and several invertebrate predators. The estimation was based on daily food rations and diet composition of predators and mesohabitat-specific predator biomass. We found clear differences between the two mesohabitat types. Predator food webs were less complex in pools than in riffles. Fish predation was more important than invertebrate predation in pools, and intraguild predation had a higher relative importance in these mesohabitats. These differences were probably caused by the mesohabitat use of G. gobio, the largest top predator, which preferred pools. Consequently, the predator food webs were more similar between the mesohabitats when fish were absent. Top–down pressure on primary consumers by all predators together was lowest in pools without fish, but the effect was not significant. Omnivory (including cannibalism) was intense, but its potentially destabilising effects were probably counterbalanced by mesohabitat connectivity. From the results of our experimental study, we conclude that even in small stream ecosystems, food web structures and predation pathways can differ between mesohabitats and that a mesohabitat-specific consideration will help to explain the variety of top–down effects on benthic communities.     

Stable isotopes indicate that zebra mussels (Dreissena polymorpha) increase dependence of lake food webs on littoral energy sources

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Predator evasion in <Emphasis Type="Italic">Daphnia</Emphasis>: the adaptive value of aggregation associated with attack abatement

To evaluate the adaptive value of daphnian aggregation associated with attack abatement, a combined laboratory and field experiment was performed. As the lake investigated does not contain planktivorous fish, only invertebrate predation is important. In the laboratory, newly collected Chaoborus flavicans, among the most important predators to exploit the Daphnia population in the lake, were individually placed in a spherical flask containing different densities of juvenile Daphnia prey. The number of successful attacks of Chaoborus was independent of prey abundance. Thus, in a given density regime, each individual prey gained protection from a dilution effect. In the field, dense aggregates of Daphnia were created by continuously adding food at a localized point in the littoral. Even though the food input produced aggregates of Daphnia which were maintained for 3 h, the number of invertebrate predators did not increase in the input area. It is concluded that each Daphnia in a group reduces the probability of predation through an effect of attack abatement which is approximately proportional to the number of prey in the group. However, when considering the high median density of Daphnia, together with their strong response to food input, it seems more plausible that natural aggregates are driven by a patchy food distribution, whereas attack abatement is only a secondary effect of aggregation.     

Benthivorous fish reduce stream invertebrate drift in a large-scale field experiment

Drift as a low-energy cost means of migration may enable stream invertebrates to leave risky habitats or to escape after encountering a predator. While the control of the diurnal patterns of invertebrate drift activity by fish predators has received considerable interest, it remains unclear whether benthivorous fish reduce or increase drift activity. We performed a large-scale field experiment in a second-order stream to test if invertebrate drift was controlled by two benthivorous fish species (gudgeon Gobio gobio and stone loach Barbatula barbatula). An almost fishless reference reach was compared with a reach stocked with gudgeon and loach, and density and structure of the invertebrate communities in the benthos and in the drift were quantified in both reaches. The presence of gudgeon and stone loach reduced the nocturnal drift of larvae of the mayfly Baetis rhodani significantly, in contrast to the findings of most previous studies that fish predators induced higher night-time drift. Both drift density and relative drift activity of B. rhodani were lower at the fish reach during the study period that spanned 3 years. Total invertebrate drift was not reduced, by contrast, possibly due to differences in vulnerability to predation or mobility between the common invertebrate taxa. For instance, Chironomidae only showed a slight reduction in drift activity at the fish reach, and Oligochaeta showed no reduction at all. Although benthic community composition was similar at both reaches, drift composition differed significantly between reaches, implying that these differences were caused by behavioural changes of the invertebrates rather than by preferential fish consumption. The direction and intensity of changes in the drift activity of stream invertebrates in response to the presence of benthivorous fish may depend on the extent to which invertebrate taxa can control their drifting behaviour (i.e. active versus passive drift). We conclude that invertebrate drift is not always a mechanism of active escape from fish predators in natural streams, especially when benthos-feeding fish are present.     

Invertebrate zooplankton predator composition and diversity in tropical lentic waters

Invertebrate zooplankton predators are generally less diverse in average species numbers in tropical than in temperate lakes and reservoirs. Predatory Copepoda which comprise the majority of limnetic predators are particularly low in species numbers in the tropics. Predatory Cladocera are confined to the North Temperate zone. Chaoborus appears to be cosmopolitan. Among Rotifera, only the cosmopolitan predator Asplanchna occurs in tropical waters while the other common limnetic carnivorous genus Ploesoma is restricted to higher latitudes. Hydracarina, and insects besides Chaoborus, are generally restricted to the littoral and appear to be more diverse in the tropics. Lakes Awasa and Zwai, Ethiopia, are almost devoid of predators in the limnetic, which is invaded by a littoral chydorid Alona diaphana. Low diversity of lake types and low production of tropical zooplankton could restrict predator diversity too. Very low diversity of invertebrate predators in the limnetic and higher diversity in the littoral may characterize tropical lakes in contrast to temperate lakes, which have more invertebrate predators in the limnetic and perhaps relatively less in the littoral. Tropical zooplankton in freshwaters, appears to be a very immature community. Hence opportunistic species can readily invade the limnetic and even dominate in isolated situations as has been shown for Alona davidi, Hydracarina and some other unconventional forms.     

Predation of the sea urchin Heliocidaris erythrogramma by rock lobsters (Jasus edwardsii) in no-take marine reserves

The formation of sea urchin ‘barrens’ on shallow temperate rocky reefs is well documented. However there has been much conjecture about the underlying mechanisms leading to sea urchin barrens, and relatively little experimentation to test these ideas critically. We conducted a series of manipulative experiments to determine whether predation mortality is an important mechanism structuring populations of the sea urchin Heliocidaris erythrogramma in Tasmania. Tethered juvenile and adult sea urchins experienced much higher rates of mortality inside no-take marine reserves where sea urchin predators were abundant compared to adjacent fished areas where predators were fewer. Mortality of tagged (but not tethered) sea urchins was also notably higher in marine reserves than in adjacent areas open to fishing. When a range of sizes of sea urchins was exposed to three sizes of rock lobsters in a caging experiment, juvenile sea urchins were eaten more frequently than larger sea urchins by all sizes of rock lobster, but only the largest rock lobsters (> 120 mm CL) were able to consume large adult sea urchins. Tagging (but not tethering) juvenile and adult sea urchins in two separate marine reserves indicated that adult sea urchins experience higher predation mortality than juveniles, probably because juveniles can shelter in cryptic microhabitat more effectively. In a field experiment in which exposure of sea urchins to rock lobster (Jasus edwardsii) and demersal reef fish predators was manipulated, rock lobsters were shown to be more important than fish as predators of adult sea urchins in a marine reserve. We conclude that predators, and particularly rock lobsters, exert significant predation mortality on H. erythrogramma in Tasmanian marine reserves, and that adult sea urchins are more vulnerable than smaller cryptic individuals. Fishing of rock lobsters is likely to reduce an important component of mortality in H. erythrogramma populations.     

Estimating predation risk in zooplankton communities: the importance of vertical overlap

In order to estimate predation risk in nature, two basic components of predation need to be quantified: prey vulnerability, and density risk. Prey vulnerability can be estimated from clearance rates obtained from enclosure experiments with and without predators. Density risk is a function of predator density, and the spatial and temporal overlap of the predator and prey populations. In the current study we examine the importance of the vertical component of overlap in making accurate estimates of predation risk from the invertebrate predator Mesocyclops edax on rotifer versus crustacean prey. The results indicate that assumptions of uniform predator and prey densities cause a significant underestimation of predation risk for many crustacean prey due to the coincident vertical migration of these prey with the predator. The assumption of uniformity is more reasonable for estimating predation risk for most rotifer prey.     

Does concentrating chemical defenses within specific regions of marine sponges result in enhanced protection from predators?

Chemical defenses are an effective mode of predator deterrence across benthic marine organisms, but their production may come with associated costs to the organism as limited resources are diverted away from primary processes like growth and reproduction. Organisms concentrating ecologically relevant levels of these defenses in tissues most at risk to predator attack may alleviate this cost while deterring predators. We addressed this hypothesis by investigating the deterrence of chemical extracts from the inner and outer regions of the sponges Aplysina fulva, Ircinia felix, and I. campana from a temperate hard-bottom reef in the South Atlantic Bight. Assays were conducted using natural fish assemblages and sea urchins. Although, A. fulva and I. felix have higher concentrations of defensive metabolites in the outer and inner regions, respectively, extracts from these regions did not display enhanced deterrency against fish or mobile invertebrate predators. Likewise, extracts from both regions of the sponge Ircinia campana, which has a uniform distribution of defensive chemicals throughout, did not differ in their ability to deter either group of predators. Since chemical defenses were effective deterrents at lower concentrations, secondary metabolite allocation patterns observed among these sponges are likely not driven by predation pressure from generalist fish and mobile invertebrate predators on these reefs. Alternatively, these patterns may be driven by other ecological stressors, another suite of predators, or may be more effective at deterring predators when combined with structural defenses.     

The impact of vertebrate and invertebrate predators on a stream benthic community

I assessed the impact of both vertebrate and invertebrate predators on a lotic benthic community in a 1-month-long experiment, using enclosures containing cobble/gravel bottoms, with large-mesh netting that allowed invertebrates to drift freely. Brown trout (Salmo trutta) and leeches (Erpobdella octoculata) were used as predators and four treatments were tested: a predator-free control, leeches only, trout only, and leeches and trout together. A density of 26.7 leeches/m² (20 leeches/enclosure) and 1.3 trout/m² (one trout per enclosure) was stocked into the enclosures. The total biomass of invertebrate prey was significantly lower in the trout and trout plus leech treatments than in the leech and control treatments, which were due to strong negative effects of trout on Gammarus. On the individual prey taxon level, both trout and leeches affected the abundance of Asellus , Baetis and Ephemerella, whereas the abundance of Gammarus was only affected by trout, and the abundance of Orthocladiinae and Limnephilidae was only affected by leeches. In the treatment with trout and leeches together, the abundance of Ephemerella and Baetis was higher than when trout or leeches were alone, which was probably due to predator interactions. Leeches and trout had no effects on prey immigration but did affect per capita emigration rates. Both trout and leeches indirectly increased periphyton biomass in enclosures, probably due to their strong effects on grazers. Both trout and leeches were size-selective predators, with trout selecting large prey, and leeches selecting small prey. Size-selective predation by trout and leeches affected the size structure of five commonly consumed prey taxa. Trout produced prey populations of small sizes owing to consumption of large prey as well as increased emigration out of enclosures by these large prey. Leech predation produced prey assemblages of larger size owing to consumption and increased emigration of small prey. These results suggest that in lotic habits, predatory invertebrates can be as strong interactors as vertebrate predators. Received: 23 June 1997 / Accepted: 4 May 1998