The effects of spate-induced disturbance, predation and environmental complexity on macroinvertebrates in a tropical stream

• 1 The combined effects of fish predation, substrate complexity and flow on benthic macroinvertebrates inhabiting riffles was investigated in a Hong Kong stream. Predation was manipulated using fish inclusion/exclusion cages containing complex (= many refuges) or simple (= few refuges) substrates. Experiments were undertaken during the winter dry season, when disturbance due to flow events was minimal, and repeated during the summer monsoon, when the stream experienced spates of varying intensity and duration.
• 2 Predation by the fish, Oreonectes platycephalus, significantly reduced the abundance of macroinvertebrates, especially chironomids and mayflies. Because chironomids (mostly Chironominae) were the dominant cage colonists, there was also a reduction in total macroinvertebrate density.
• 3 Predator impacts were significantly lessened during the wet season, when macroinvertebrate densities increased considerably, but significant reductions in the densities of vulnerable taxa and total macroinvertebrate abundance were nevertheless apparent.
• 4 Substrate complexity (the presence of prey refuges) had no significant effect on the ability of predators to reduce prey abundance.
• 5 Detritus accumulated in cages during the latter part of the study, and densities of most taxa were correlated with detrital standing stocks.
• 6 The results of this experiment indicate that biotic interactions such as predation may be suppressed during periods of spate-induced disturbance, although they can still influence benthic communities significantly. However, the effects of predation are highly taxon specific and may vary among streams in response to changes in predator and prey species composition, or the severity and duration of spates.

     

Is there safety‐in‐numbers for prey?

The abundance of prey affects the rate of predation, but little consensus exists on whether this enhances or reduces per capita mortality. Studies of aggregating prey in marine habitats generally emphasise that the probability of predation of any individual is the reciprocal of the number of prey within a school. A field experiment tested the alternative hypotheses that predation by predatory fish on schooling prey (1) increased with an increase in the number of prey per school and that this caused (2) survival to be lower in schools with more individuals. The number of prey (juvenile Acanthochromis polyacanthus ) per school was manipulated in replicate treatments with natural densities of large predatory fish (open plots) and treatments without large predatory fish (exclusion cages). Large predatory fish preyed on juveniles in a density-dependent manner and this was the key source of density-dependent mortality in plots open to all predators. There was some suggestion that small predatory fish also prey on juveniles in a density-dependent manner, but this was weak and did not translate into density-dependent mortality of juveniles. It would appear that aggregation of prey may be a successful strategy against predation from some predators, but not always every predator, or all predators in combination.     

Stonefly predation along a hydraulic gradient: a field test of the harsh—benign hypothesis

SUMMARY. 1. Microhabitat preferences of predatory stoneflies and four prey taxa were assessed by taking benthic samples along a hydraulic gradient in a Black Forest stream in West Germany. Densities of predator and prey species were estimated at twenty-one hydraulic regimes.
2. Enclosures containing the stonefly, Dinocras cephalotes , and control cages with no predators were placed in the substrate at hydraulic regimes favourable and unfavourable to predators. Cages received initial prey communities that were obtained from benthic samples taken at hydraulic regimes matching those intended for each cage.
3. Population densities of the two most numerically important prey taxa, the mayfly. Baetis rhodani , and the Chironomidae, were reduced in the presence of Dinocras , but only when enclosures were placed in the hydraulic regimes favourable to the predator. Thus, predation effects increased as the hydraulic regime became more benign to the predators.
4. Densities of two other prey species rare in the diets of Dinocras ( Hydropsyche instabilis and Gammarus fossarum ) were generally unaffected by predators regardless of the hydraulic regime.
5. These data provide support for the hypothesis that perception of the abiotic regime as harsh or benign to predators is a good predictor of predator impact on densities of preferred prey species. In harsher abiotic regimes, impact will be low, while impact will be high in benign abiotic regimes.     

The relative importance of lethal and non-lethal effects of fish on insect colonisation of ponds

1. We hypothesised that adult insects actively monitor potential habitats for the presence of fish by means of chemical cues and avoid sites that pose significant risks. This was examined by quantifying colonisation of insects in outdoor pools with no fish (controls), fish (direct predation effect) or caged fish (chemical predator cues).
2. A significant direct effect of predation was found, but no indirect effect (avoidance of chemical cue pools), on the total biomass of colonising insects. However, predatory insects avoided fish-cue pools, thus releasing non-predatory insects from predation. This resulted in significantly greater biomass of non-predatory insects in fish-cue pools than control pools.
3. Fish reduced the number of species of colonising insects in pools through predation. This negative influence of fish implies that caution is necessary when stocking wetlands and ponds with fish if the goal is to maximise biodiversity.
4. Our data suggest that although predatory aquatic insects may use chemical signals to assess the quality of potential habitats with respect to predation risk, direct predation is the main method by which fish affect insect assemblages in ponds. Because fish and invertebrate predators may both have strong effects on prey mortality, behavioural adjustment by insects to the actual predator regime within a habitat should be more important than avoiding colonisation of habitats with fish.     

Stream flow and predation effects on the spatial dynamics of benthic invertebrates

Field experiments were carried out to determine the influence of predation and prey movements on the accumulation of prey in enclosures. Experimental enclosures permitted exchange of prey with the benthos, but not of the large, predatory larvae of the caddisfly, Plectrocnemia conspersa (Curtis). Unseasonally heavy rainfalls during the experiment resulted in high flows and enabled us to examine the effects of a major, abiotic disturbance on invertebrate spatial dynamics. Prey colonization rates of cages without predators were determined in nine 24 h periods. Colonization rates increased exponentially with flow and were species-specific, depending on dispersal behaviour. Prey accumulation and predator impacts were measured in cages, with and without P. conspersa larvae, placed in the stream for 1, 2 or 3 weeks. Prey densities in cages increased with exposure time, but increases were not gradual and depended on flow regime. Flow was reduced within cages and they accumulated large numbers of invertebrates during high discharge. Analogous, naturally occurring refugia in the stream channel could be important for the recovery of lotic communities after major disturbances. Overall, prey densities were lowest in cages with predators. For fast colonizers, predation effects were detectable early in the experiment, but quickly obscured thereafter by continuous exchange of prey. For slow colonists, predation effects were detectable later, but persisted longer. Consumption rates for P. conspersa varied with prey density and flow regime. We suggest that the spatial dynamics of benthic invertebrates, especially as they are influenced by stochastic events, are important in understanding and detecting predation effects in stream communities.     

Dying from the lesser of three evils: facilitation and non‐consumptive effects emerge in a model with multiple predators

Prey modify their behaviour to avoid predation, but dilemmas arise when predators vary in hunting style. Behaviours that successfully evade one predator sometimes facilitate exposure to another predator, forcing the prey to choose the lesser of two evils. In such cases, we need to quantify behavioural strategies in a mix of predators. We model optimal behaviour of Atlantic cod Gadus morhua larvae in a water column, and find the minimal vulnerability from three common predator groups with different hunting modes; 1) ambush predators that sit‐and‐wait for approaching fish larvae; 2) cruising invertebrates that eat larvae in their path; and 3) fish which are visually hunting predators. We use a state‐dependent model to find optimal behaviours (vertical position and swimming speed over a diel light cycle) under any given exposure to the three distinct modes of predation. We then vary abundance of each predator and quantify direct and indirect effects of predation. The nature and strength of direct and indirect effects varied with predator type and abundance. Larvae escaped about half the mortality from fish by swimming deeper to avoid light, but their activity level and cumulative predation from ambush predators increased. When ambush invertebrates dominated, it was optimal to be less active but in more lit habitats, and predation from fish increased. Against cruising predators, there was no remedy. In all cases, the shift in behaviour allowed growth to remain almost the same, while total predation were cut by one third. In early life stages with high and size‐dependent mortality rates, growth rate can be a poor measure of the importance of behavioural strategies.     

Habitat overlap between predatory benthic fish and their invertebrate prey in streams: the relative influence of spatial and temporal factors on predation risk

1. The spatial heterogeneity of ecosystems as well as temporal activity patterns of organisms can have far‐reaching effects on predator–prey relationships. We hypothesised that spatiotemporal constraints in mesohabitat use by benthic fish predators would reduce habitat overlap with benthic invertebrates and lead to mesohabitat‐specific predation risks. 2. We analysed the spatiotemporal activity patterns of two small‐bodied benthivorous fishes, gudgeon (Gobio gobio) and stone loach (Barbatula barbatula), and of benthic invertebrates in a small temperate stream during three 24‐h field experiments. By applying a novel method of field video observation, we monitored the spatiotemporal foraging behaviour of the fish in their natural environment. A parallel analysis of invertebrate mesohabitat use by means of small area Hess sampling allowed a direct estimation of habitat overlap at a pool–riffle scale. 3. Gudgeon showed a dominant spatial activity pattern preferring pools at all times of day, whereas stone loach used both mesohabitats but with a distinct temporal (nocturnal) activity pattern. The patterns of residence were not identical with those of active foraging. Invertebrate community composition differed significantly between mesohabitats but not between times of day. More than half of the total dissimilarity between pools and riffles was accounted for by six invertebrate taxa. Five of these were subject to higher fish predation in pools than in riffles. The total prey consumption of the two fish species together in pools was about three times as high as in riffles. Trophic niche breadth of stone loach and thus its predation range was broader than that of gudgeon. 4. These results indicate that the potential predation risk for stream invertebrates depends on the combination of spatial and temporal patterns of both predator and prey. Given the distinct differences in predation risk found between pools and riffles, we conclude that spatial heterogeneity at the mesohabitat scale can influence mechanisms and consequences of selective predation. We also suggest that the analysis of spatiotemporal predator–prey relationships should not be based on the premise that the main residence habitat and active foraging habitat of a predator are identical.     

Physiological and visual refuges in a metalimnion: an experimental study of effects of clay turbidity and an oxygen minimum on fish predation

1. The effectivity of elevated clay turbidity and low-oxygen concentration in the metalimnion as refuges for chaoborid larvae against fish predation was studied in experimental water columns.
2. When 70–80 nephelometric turbidity unit clay turbidity was combined with 3–4 mg L⁻¹ oxygen concentration, prey capture rate by fish (golden orfe Leuciscus idus ) was reduced by 74% compared with the control treatment with no refuges. Oxygen and turbidity refuges alone did not significantly reduce the feeding rate.
3. All fish in the control treatment dwelled in the metalimnion, but 36% of the fish in the low-oxygen treatment and 23% of the fish in the turbidity treatment stayed in the epilimnion. In the combined treatment, 54% of the fish were in the epilimnion.
4. The results demonstrated that a combination of moderately elevated turbidity and lowered oxygen concentration in the metalimnion is an effective protection against fish predation, while turbidity or oxygen refuge alone are much less effective.
5. In the treatment with the combined refuge, oxygen concentration limited the time fish could spend in the metalimnion and turbidity affected the detection of prey through changes in reactive distance.
6. Because of the combined effects of turbidity and oxygen refuges, planktivorous fish and phantom midge larvae may co-occur in clay-turbid lakes in high densities. Such situation is problematic for biomanipulation, which aims to enhance the grazing rate of zooplankton through reduction of planktivorous fish.     

Substitution of top predators: effects of pike invasion in a subarctic lake

1. Invasions of top predators may have strong cascading effects in ecosystems affecting both prey species abundance and lower trophic levels. A recently discussed factor that may enhance species invasion is climate change and in this context, we studied the effects of an invasion of northern pike into a subarctic lake ecosystem formerly inhabited by the native top predator Arctic char and its prey fish, ninespined stickleback. 2. Our study demonstrated a strong change in fish community composition from a system with Arctic char as top predator and high densities of sticklebacks to a system with northern pike as top predator and very low densities of sticklebacks. A combination of both predation and competition from pike is the likely cause of the extinction of char. 3. The change in top predator species also cascaded down to primary consumers as both zooplankton and predator‐sensitive macroinvertebrates increased in abundance. 4. Although the pike invasion coincided with increasing summer temperatures in the study area we have no conclusive evidence that the temperature increase is the causal mechanism behind the pike invasion. But still, our study provides possible effects of future pike invasions in mountain lakes related to climate change. We suggest that future pike invasions will have strong effects in lake ecosystems, both by replacing native top consumers and through cascading effects on lower trophic levels.     

The effect of habitat structure on prey mortality depends on predator and prey microhabitat use

Structurally complex habitats provide cover and may hinder the movement of animals. In predator–prey relationships, habitat structure can decrease predation risk when it provides refuges for prey or hinders foraging activity of predators. However, it may also provide shelter, supporting structures and perches for sit-and-wait predators and hence increase their predation rates. We tested the effect of habitat structure on prey mortality in aquatic invertebrates in short-term laboratory predation trials that differed in the presence or absence of artificial vegetation. The effect of habitat structure on prey mortality was context dependent as it changed with predator and prey microhabitat use. Specifically, we observed an ‘anti-refuge’ effect of added vegetation: phytophilous predators that perched on the plants imposed higher predation pressure on planktonic prey, while mortality of benthic prey decreased. Predation by benthic and planktonic predators on either type of prey remained unaffected by the presence of vegetation. Our results show that the effects of habitat structure on predator–prey interactions are more complex than simply providing prey refuges or cover for predators. Such context-specific effects of habitat complexity may alter the coupling of different parts of the ecosystem, such as pelagic and benthic habitats, and ultimately affect food web stability through cascading effects on individual life histories and trophic link strengths.     

Non‐lethal predator effects on the performance of a native and an exotic crayfish species

1. I tested the hypothesis that the potential for non‐lethal effects of predators are more important for overall performance of the fast‐growing exotic signal crayfish (Pacifastacus leniusculus Dana) than for the slower growing native noble crayfish (Astacus astacus L.). I further tested if omnivorous crayfish switched to feed on less risky food sources in the presence of predators, a behaviour that could reduce the feeding costs associated with predator avoidance. 2. In a 2 month long outdoor pool experiment, I measured behaviour, survival, cheliped loss, growth, and food consumption in juvenile noble or signal crayfish in pools with either a caged predatory dragonfly larvae (Aeshna sp.), a planktivorous fish that do not feed on crayfish (sunbleak, Leucaspius delineatus Heckel), or predator‐free controls. Crayfish had access to multiple food sources: live zooplankton, detritus and periphyton. Frozen chironomid larvae were also supplied ad libitum outside crayfish refuges, simulating food in a risky habitat. 3. Crayfish were mainly active during hours of darkness, with signal crayfish spending significantly more time outside refuges than noble crayfish. The proportion of crayfish outside refuges varied between crayfish species, time and predator treatment, with signal crayfish spending more time in refuges at night in the presence of fish. 4. Survival in noble crayfish was higher than in signal crayfish, and signal crayfish had a higher frequency of lost chelipeds, indicating a high level of intraspecific interactions. Crayfish survival was not affected by the presence of predators. 5. Gut‐contents analysis and stable isotope values of carbon (δ¹³C) and nitrogen (δ¹⁵N) indicated that the two crayfish species had similar food preferences, and that crayfish received most of their energy from feeding on invertebrates (e.g. chironomid larvae), although detritus was the most frequent food item in their guts. Signal crayfish guts were more full than those of noble crayfish, but signal crayfish in pools with fish contained significantly less food and fewer had consumed chironomids compared with predator‐free controls. Length increase of signal crayfish (35%) was significantly higher than of noble crayfish (20%), but signal crayfish in pools with fish grew less than in control pools. 6. This short‐term study indicates that fish species that do not pose a lethal threat to an organism may indirectly cause reductions in growth by affecting behaviour and feeding. This may occur even though prey are omnivorous and have access to and consume multiple food sources. These non‐lethal effects of predators are expected to be particularly important in exotic crayfish species that show a general response to fish, have high individual growth rates, and when their feeding on the most profitable food source is reduced.     

Fleeing towards death – leech‐induced behavioural defences increase freshwater snail susceptibility to predatory fish

Prey species are often exposed to multiple predators, which presents several difficulties to prey species. This is especially true when the response to one predator influences the prey’s susceptibility to other predators. Predator‐induced defences have evolved in a wide range of prey species, and experiments involving predators with different hunting strategies allow researchers to evaluate how prey respond to multiple threats. Freshwater snails are known to respond to a variety of predators with both morphological and behavioural defences. Here we studied how freshwater snails Radix balthica responded behaviourally to fish and leech predators, both separately and together. Our aim was to explore whether conflicting predator‐induced responses existed and, if so, what effect they had on snail survival when both predatory fish and leeches were present. We found that although R. balthica increased refuge use when exposed to predatory fish, they decreased refuge use when exposed to predatory leeches. When both predators were present, snails showed a stronger response towards leech than fish and responded by leaving the refuge. This response made the snails more susceptible to fish predation, which increased snail mortality when exposed to both fish and leech compared to fish only. We show that predators that have a relatively low predation rate can substantially increase mortality rates by indirect effects. By forcing snails out of refuges such as rock and macrophyte habitats, leeches can indirectly increase predation from molluscivorous fish and may thus affect snail densities.     

Effects of stream predator richness on the prey community and ecosystem attributes

It is important to understand the role that different predators can have to be able to predict how changes in the predator assemblage may affect the prey community and ecosystem attributes. We tested the effects of different stream predators on macroinvertebrates and ecosystem attributes, in terms of benthic algal biomass and accumulation of detritus, in artificial stream channels. Predator richness was manipulated from zero to three predators, using two fish and one crayfish species, while density was kept equal (n = 6) in all treatments with predators. Predators differed in their foraging strategies (benthic vs. drift feeding fish and omnivorous crayfish) but had overlapping food preferences. We found effects of both predator species richness and identity, but the direction of effects differed depending on the response variable. While there was no effect on macroinvertebrate biomass, diversity of predatory macroinvertebrates decreased with increasing predator species richness, which suggests complementarity between predators for this functional feeding group. Moreover, the accumulation of detritus was affected by both predator species richness and predator identity. Increasing predator species richness decreased detritus accumulation and presence of the benthic fish resulted in the lowest amounts of detritus. Predator identity (the benthic fish), but not predator species richness had a positive effect on benthic algal biomass. Furthermore, the results indicate indirect negative effects between the two ecosystem attributes, with a negative correlation between the amount of detritus and algal biomass. Hence, interactions between different predators directly affected stream community structure, while predator identity had the strongest impact on ecosystem attributes.     

Prey life-history and bioenergetic responses across a predation gradient

To evaluate the importance of non-consumptive effects of predators on prey life histories under natural conditions, an index of predator abundance was developed for naturally occurring populations of a common prey fish, the yellow perch Perca flavescens, and compared to life-history variables and rates of prey energy acquisition and allocation as estimated from mass balance models. The predation index was positively related to maximum size and size at maturity in both male and female P. flavescens, but not with life span or reproductive investment. The predation index was positively related to size-adjusted specific growth rates and growth efficiencies but negatively related to model estimates of size-adjusted specific consumption and activity rates in both vulnerable (small) and invulnerable (large) size classes of P. flavescens. These observations suggest a trade-off between growth and activity rates, mediated by reduced activity in response to increasing predator densities. Lower growth rates and growth efficiencies in populations with fewer predators, despite increased consumption suggests either 1) a reduction in prey resources at lower predator densities or 2) an intrinsic cost of rapid prey growth that makes it unfavourable unless offset by a perceived threat of predation. This study provides evidence of trade-offs between growth and activity rates induced by predation risk in natural prey fish populations and illustrates how behavioural modification induced through predation can shape the life histories of prey fish species.     

Fishing spiders, green sunfish, and a stream-dwelling water strider: male-female conflict and prey responses to single versus multiple predator environments

Many studies have experimentally addressed the effects of a particular predator species on prey behavior. In nature, however, prey frequently face multiple species of predators that often vary in their predatory mode and in their level of predation risk. Relatively few studies have considered prey responses under these complex conditions. In Kentucky, the stream-dwelling water strider (Aquariusremigis) coexists with many potentially dangerous predators, two of which are the green sunfish (Lepomiscyanellus) and the fishing spider (Dolomedesvittatus). Green sunfish occupy stream pools and attack water striders from below. In contrast, fishing spiders hunt along stream shorelines where they perch on overhanging vegetation or rocks and attack water striders near shore. We compared how A. remigis individuals respond to these two very different predators in pools with one or both predators. The presence of sunfish in pools had strong effects on male water strider behavior, including increased use of three types of refuge from sunfish (riffles, climbing out of the water, sitting on the water but at the edges of pools), decreased activity and a decreased number of aggressive males on the water. Spiders also influenced water strider behavior; male water striders avoided spiders by shifting away from the edges of pools. Comparisons of the effects of the two predator species showed that in general, antipredator responses by male water striders were stronger in pools with fish alone than in those with spiders alone. In the presence of both predators, male water strider behavior (microhabitat use and activity) was generally similar to behavior in the presence of fish alone. In contrast, female water striders showed no significant response to the presence of sunfish, and little response to the presence of spiders. This lack of response could be because females spent much of their time in refuges even in the absence of predators (apparently hiding from harassment by males). Both spiders and fish caused decreases in water strider mating activity. The presence of fish reduced both the number of matings per pool (mating frequency), and mean mating durations. Spiders induced a decrease in mean mating duration, but not in mating frequency. The largest reductions in mating activity occurred in pools with both predators present. Pools with either spiders or fish alone suffered 15–20% water strider mortality during our experiment (versus no mortality in predator-free pools). Extant theory suggests that when prey face conflicting microhabitat responses to two predators (as in this study), the predators should have facilitative effects on predation rates (i.e., prey that avoid one predator are often killed by the other and vice versa). Mortality rates in pools with both predators present, however, were not significantly different from that predicted by a null model of multiple predator effects. The lack of predator facilitation can be explained by the compensatory reductions in water strider activity and mating activity in the presence of both predators. Received: 26 August 1996 / Accepted: 12 June 1998     

Selective foraging by the crayfish Orconectes virilis and its impact on macroinvertebrates

SUMMARY. 1. The impact of crayfish predation on the abundance of macroinvertebrates was examined under semi-natural conditions. Female (Experiment 1) or male (Experiment 2) crayfish (Orconectes virilis) were held for 5 weeks in twelve small pools (4.67 m² surface area) at biomasses of 0. 5, 10 or 18 g m^?2 (live weight). The pools were stocked with known densities of macroinvertebrates. 2. Crayfish significantly affected the abundance of macroinvertebrates in the pools. Differences in the effects of crayfish on macroinvertebrates were related to crayfish sex, the presence of age-0 crayfish, and the species of macroinvertebrate. 3. The abundance of snails (Stagnicola elodes and Physa gyrina) was greatly reduced, in comparison with controls, by biomass of female crayfish ≥10 g m^?2 and by biomasses of male crayfish ≥5 g m^?2. The total density of non-molluscan invertebrates was inversely correlated with the biomass of female crayfish but the total biomass of non-molluscan invertebrates did not differ between treatments. This is consistent with our observation that small invertebrates (<2 mg wet weight) were less numerous, and large amphipods (32–64 mg) were more numerous, in pools stocked with female crayfish. In contrast, male crayfish had little apparent effect on the abundance of non-molluscan invertebrates. 4. Age-0 crayfish hatched at the end of Experiment 1 and were present in each pool at the start of Experiment 2. Surprisingly, male crayfish preyed little on age-0 crayfish. At the end of Experiment 2, the densities of age-0 crayfish varied between six and 116 individuals m^?2 and there was a strong inverse correlation between the mean biomass and density of age-0 crayfish recovered from the pools. This suggests age-0 crayfish were food limited in the pools and may explain the dominance of oligochaetes (which largely escape predation by burrowing) in the invertebrate community at the end of Experiment 2. 5. These results indicate that even relatively low densities of crayfish could greatly affect the abundance of macroinvertebrates in lakes. The introduction of crayfish into lakes (most lakes in Alberta currently have no crayfish) could substantially affect abundance and species composition of the macroinvertebrate community and, ultimately, the fish populations.     

Patch based predation in a southern Appalachian stream

Streams are characterized by high degrees of patchiness that could influence the role of predators in these systems. Here we assess the impact of predatory benthic fishes on benthic macroinvertebrate density, biomass, and community structure at the patch scale in a fourth order stream in the southern Appalachians. We tested the role of predation in two different patch types: patches inhabited by adult mottled sculpin ( Cottus bairdi ) and random patches. We placed 30 basket pairs (one open to fish predation, and one from which fish predators were excluded) in the streambed at each patch type. We also tested for potential basket effects by setting up a basket control area. Although there was some evidence of basket artifacts on macroinvertebrate density in sculpin patches, these artifacts were not consistent and we do not feel that they affected our results because predators did not affect macroinvertebrate density. In random patches, predation did not significantly affect macroinvertebrate density or biomass. Predators significantly reduced macroinvertebrate biomass in sculpin patches but did not affect prey density. When the data-set was size-limited to exclude macroinvertebrates too large for consumption by sculpin, macroinvertebrate biomass did not differ significantly between exclusion and open baskets. This suggests that sculpin can reduce macroinvertebrate biomass through a combination of consumption and by predator-induced emigration of large macroinvertebrates into areas that are protected from sculpin. In addition, invertebrate predator biomass was higher in predator exclusion baskets in sculpin patches indicating that predation pressure remained high in the exclusion baskets despite fish exclusion. These results illustrate the heterogeneity of streams and the effect of small-scale differences (e.g. location of predators' territories) on local processes. Experiments that utilize these differences can provide insights into these stream processes.     

The importance of predation,substrate and spatial refugia in determining lotic insect distributions

An experiment was conducted to evaluate the interaction between predation, substrate, and spatial refugia in the organization of a stream insect community (Reeds Creek, Pendleton Co., West Virginia). Patterns of insect colonization were compared between fish exclusion cages and open controls that allowed access to vertebrate predators. Each cage contained 4 different substrates that varied in the relative amount of spatial refugia. Fish had little influence on the diversity or abundance of any insect taxa, even when spatial refugia were limited. The only significant effect due to predation, was an increased diversity of large (>8 mm) invertebrates in the absence of predators. However, because these taxa were relatively rare, the overall role of fish predation on insect community structure was minimal.In contrast, substrate had a marked effect on insect colonization. Insects were always more abundant (number/basket) on loose substrates containing large numbers of interstitial spaces, compared to cement-embedded substrates with few refuges available. In addition, invertebrates were more abundant on loose gravel compared to loose cobbles. Howver, when substrate preferencesrd were examined according to insect density (number/m²), loose cobbles were generally the preferred substrate. The present experiment rejects the hypothesis that patterns of substrate colonization can be explained as differential insect mortality by predators, due to varying amounts of refugia. Alternative mechanisms such as differing amounts of trapped detritus and substrate surface area may account for substrate preference.     

Scale dependent effects of predatory fish on stream benthos

In open predation experiments the effects of predators on prey densities can be influenced by predator consumption and by prey movements in to and out of experimental arenas. A published model predicts that the predator effects observed in such experiments are scale dependent over the scale range where there is a transition from movement control (of prey densities) to consumption control. The scale dependence follows from the assumption that per capita rate of emigration out of an experimental arena decreases with increasing arena size.
To test this model the effects of a small benthic fish ( Cottus gobio ) on densities of stream invertebrates was investigated in instream channels of different length (0.5, 2 and 8 m). The effect of fish predation was scale dependent for four prey taxa. For three of these taxa predator effects increased with experimental scale, which is in agreement with model predictions. However, this proved to be a case of "making the right prediction for the wrong reason" as the basic assumption of scale dependent emigration rate was not upheld. By analyzing the behaviour of the model, parameterized with emigration and consumption rates observed in the experimental channels, it was found that observed scale effects occurred because prey emigration in response to the predator treatment was modified by the experimental scale. Further analysis of the parameterized model suggested that the densities of most prey taxa were controlled by prey movements and not by consumption by the sculpins.     

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.