Stoneflies as ecological engineers – hungry predators reduce fine sediments in stream beds

1. We conducted experiments in a Colorado Rocky Mountain stream to measure the effect of foraging by predatory stoneflies ( Megarcys signata ) on fine sediment deposition and removal.
2. Cages containing one of four treatments were placed in the stream substratum and removed 3 days after fine sediment load to the stream was experimentally increased. Cages containing a stonefly but no prey accumulated less sediment than cages initially with no benthic invertebrates. Although cages with Megarcys plus prey also tended to have less sediment than controls, sediments were not reduced in cages with prey only.
3. Distance from sediment source, depth and current velocity at cages, final prey density and numbers of prey per predator gut at the end of the experiment had no effects on sediment accumulation in cages.
4. To determine the mechanisms underlying this effect, behavioural tests were conducted in a stream-side artificial stream system. To simulate the different hunger levels of stoneflies in cages, Megarcys were fed or starved for 3 days before behavioural trials that were repeated during high (night) and low (day) stonefly feeding periods. During night trials, foraging movements by starved Megarcys were more prolonged and active than those of fed stoneflies, regardless of the presence of prey. However, neither hunger level nor prey presence affected stonefly foraging behaviour during day trials.
5. Results of field experiments suggest that the presence of stoneflies enhances sediment removal from interstitial spaces. Behavioural observations indicate that nocturnal searching for prey by hungry Megarcys dislodges fine sediments from interstitial spaces.
6. Future studies should explore beyond the direct impacts of predators on stream invertebrate communities, and investigate the potential impact of predators on benthic microhabitat quality.     

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.     

Unidirectional prey-predator facilitation: apparent prey enhance predators' foraging success on cryptic prey

Food availability can strongly affect predator-prey dynamics. When change in habitat condition reduces the availability of one prey type, predators often search for other prey, perhaps in a different habitat. Interactions between behavioural and morphological traits of different prey may influence foraging success of visual predators through trait-mediated indirect interactions (TMIIs), such as prey activity and body coloration. We tested the hypothesis that foraging success of stream-dwelling cutthroat trout (Onchorhyncus clarki) on cryptically coloured, less-active benthic prey (larval mayfly; Paraleptophebia sp.) can be enhanced by the presence of distinctly coloured, active prey (larval stonefly shredder; Despaxia augusta). Cutthroat trout preyed on benthic insects when drifting invertebrates were unavailable. When stonefly larvae were present, the trout ate most of the stoneflies and also consumed a higher proportion of mayflies than under mayfly only treatment. The putative mechanism is that active stonefly larvae supplied visual cues to the predator that alerted trout to the mayfly larvae. Foraging success of visual predators on cryptic prey can be enhanced by distinctly coloured, active benthic taxa through unidirectional facilitation to the predators, which is a functional change of interspecific interaction caused by a third species. This study suggests that prey-predator facilitation through TMIIs can modify species interactions, affecting community dynamics.     

Prey selection by a stonefly: the influence of hunger and prey size

Summary The influences of hunger and prey size on prey selection by the stonefly Hesperoperla pacifica (Perlidae) were explored in the laboratory by observing behavioral responses toward ten prey taxa and three nonprey taxa. Patterns of behavior were consistent with most assumptions and predictions of optimal foraging theory predicting sizebased prey selection by pursuing predators. Handling time appeared to increase as an exponential function of prey mass, and prey profitability (mg/s) was highest for small and intermediate-sized prey. Fasted stoneflies consumed a wide range of prey sizes, whereas well-fed stoneflies concentrated their attacks on intermediate-sized prey. Responses of H. pacifica to nonprey taxa, however, suggest that prey recognition and selection are not based on size alone.     

Fitness and community consequences of avoiding multiple predators

We investigated the fitness and community consequences of behavioural interactions with multiple predators in a four-trophic-level system. We conducted an experiment in oval flow-through artificial-stream tanks to examine the single and interactive sublethal effects of brook trout and stoneflies on the size at emergence of Baetis bicaudatus (Ephemeroptera: Baetidae), and the cascading trophic effects on algal biomass, the food resource of the mayflies. No predation was allowed in the experiment, so that all effects were mediated through predator modifications of prey behaviour. We reared trout stream Baetis larvae from just before egg development until emergence in tanks with four treatments: (1) water from a holding tank with two brook trout (trout odour), (2) no trout odour + eight stoneflies with glued mouthparts, (3) trout odour + stoneflies and (4) no trout odour or stoneflies. We ended the experiment after 3 weeks when ten male and ten female subimagos had emerged from each tank, measured the size of ten male and ten female mature nymphs (with black wing pads), and collected algal samples from rocks at six locations in each tank. To determine the mechanism responsible for sublethal and cascading effects on lower trophic levels we made day and night observations of mayfly behaviour for the first 6 days by counting mayflies drifting in the water column and visible on natural substrata in the artificial streams. Trout odour and stoneflies similarly reduced the size of male and female Baetis emerging from artificial streams, with non-additive effects of both predators. While smaller females are less fecund, a fitness cost of small male size has not been determined. The mechanism causing sublethal effects on Baetis differed between predators. While trout stream Baetis retained their nocturnal periodicity in all treatments, stoneflies increased drift dispersal of mayflies at night, and trout suppressed night-time feeding and drift of mayflies. Stoneflies had less effect on Baetis behaviour when fish odour was present. Thus, we attribute the non-additivity of effects of fish and stoneflies on mayfly growth to an interaction modification whereby trout odour reduced the impact of stoneflies on Baetis behaviour. Since stonefly activity was also reduced in the presence of fish odour, this modification may be attributed to the effect of fish odour on stonefly behaviour. Only stoneflies delayed Baetis emergence, suggesting that stoneflies had a greater sublethal effect on Baetis fitness than did trout. Delayed emergence may reduce Baetis fitness by increasing risks of predation and parasitism on larvae, and increasing competition for mates or oviposition sites among adults. Finally, algal biomass was higher in tanks with both predators than in the other three treatments. These data implicate a behavioural trophic cascade because predators were not allowed to consume prey. Therefore, differences in algal biomass were attributed to predator-induced changes in mayfly behaviour. Our study demonstrates the importance of considering multiple predators when measuring direct sublethal effects of predators on prey fitness and indirect effects on lower trophic levels. Identification of an interaction modification illustrates the value of obtaining detailed information on behavioural mechanisms as an aid to understanding the complex interactions occurring among components of ecological communities. Received: 20 March 1997 / Accepted: 29 September 1997     

The relative importance of refugia in determining the drift and habitat selection of predaceous stoneflies in a sandy-bottomed stream

Patch structure in sandy, compared to rocky streams, is characterized by isolated snags that can only be colonized by drifting. By measuring drift from patches (snags) of various quality we determined the factors that influence habitat selection and drift of the predaceous stoneflies Acroneuria abnormis and Paragnetina fumosa. The presence of refugia (loose bark and leaf packs) was more important than hunger level and modified the effects of increased predator densities and aggressive interactions. Stoneflies concentrated to 8x natural densities with access to refugia remained longer on snags than a single stonefly without access to refugia. During periods of activity, refugia were defended with larger stoneflies always displacing smaller nymphs. During long periods of inactivity, two and sometimes three nymphs would rest side-by-side sharing the same refuge. Hunger level (starved versus satiated stoneflies), an indirect measure of a predator's response to prey availability, had no significant effect on drift or habitat selection regardless of the presence of refugia. Stonefly predators had a uniform distribution while their prey were clumped. Drift was deliberate and almost always delayed until night, usually at dusk or dawn. An examination of previous research plus the results of this study suggest that non-predatory intra- and interspecific interactions can be an important mechanism causing drift in streams.     

A laboratory study of predator-prey interactions of stoneflies and mayflies

SUMMARY. 1. Reactions of individual predatory stoneflies to contact by individual mayfly prey, and vice versa, were studied in the laboratory using two sets of species, one from a stream in southern Ontario. Canada, and the other from a stream in North Wales. 2. Based largely on information received via their antennae, the stoneflies were capable of distinguishing between conspecifics and mayfly prey and between prey species. A clear preference for certain prey species was evident but less preferred species were made more attractive when damaged or when the stoneflies were very hungry. Detection of conspecifics by stoneflies was confused in the presence of prey or the local smell of prey body fluids and this led to interference during hunting and feeding at high predator densities. 3. Some mayfly species were capable of distinguishing between conspecifics. other mayflies and predators. The cerci were important in intra- and interspecific contact encounters between mayflies and were used in a fencing fashion to establish a minimum distance between nymphs. However, the cerci did not appear to play an anti-predation role as stoneflies captured and ate mayflies with and without cerci in equal numbers. 4. Reactions to conspecifics by the Palaearetie mayflies Rhithrogena semicolorata (Curt.) and Baetis rhodani (Pictet) were similar to those of the ecologically-equivalent Nearctic species Stenonema vicarium (Walker) and Baetis vagans McDunnough. 5. In encounters between mayflies, intra- and interspecific responses were similar. However, the responses of some mayfly species to conspeeifics were modified in the physical presence of a predator or the water from its holding tank. Different mayfly species made specific responses to predaeeous stoneflies. Responses to a herbivorous stonefly were either similar to conspeeific encounters or were intermediate between them and encounters with predators. 6. Experiments to reveal the exact nature of the stonefly stimulus detected by the mayfly nymphs strongly suggested that there were two components to the stimulus. One was physical contact by the predator's antennae, the other was chemical in nature and could be detected only over short distances (up to a few mm). Independently each could elicit a response, but combined their effect appeared to be synergistic. 7. Different mayfly species responded differently to the stonefly Phasganophora capitata (Pictet) and this sequence of sensitivity was mirrored by the predator's choice of prey species—the most sensitive to the predator being the favoured prey and the less sensitive the least favoured. The significance of these findings to optimal foraging theory, as applied to tactile predators, is discussed.     

Diet and predation by three leaf-associated stoneflies (Plecoptera) in an Arkansas mountain stream

SUMMARY. 1. Life cycles, food habits, mouthpart morphologies, prey preferences, and predator-prey size relationships were investigated for the three most common stoneflies in leaf packs in the Little Missouri River, Arkansas: Clioperla clio (Newman), Isoperla namata Prison, and Perlesta spp. Each species was univoltine and had a fast seasonal cycle, with extended egg or nymphal diapause, rapid nymphal growth and emergence in spring to early summer.
2. Foregut analysis showed predictable dietary shifts by C. clio and Perlesta spp., from detritus and diatoms in early instars to invertebrate prey in later instars. Isoperla namata fed facultatively on insects, detritus and diatoms, with no conspicuous ontogenetic shifts. The mouthparts of all three species strongly reflected their diet.
3. Electivity analysis indicated opportunistic feeding by the three species, with feeding rarely differing significantly from random.
4. Sizes of stonefly predators and their ingested prey were highly correlated ( P <0.01). However, rather than shifting toward larger prey sizes, growing predators expanded their size thresholds and continued to include numerous small prey items in their diet. Prey choice was governed most directly by prey availability, because stoneflies selected the most abundant groups, irrespective of size or taxon.
5. The likelihood of competition among the three stoneflies for prey was minimized by timing of their life cycles, differential food use and probably the temporary nature of leaf pack habitats.     

Facilitation and interference among three predators affect their consumption of a stream-dwelling mayfly

1. We experimentally tested if a multiplicative risk model accurately predicted the consumption of a common mayfly at risk of predation from three predator species in New Zealand streams. Deviations between model predictions and experimental observations were interpreted as indicators of ecologically important interactions between predators. 2. The predators included a drift‐feeding fish [brown trout (T), Salmo trutta], a benthivorous fish [galaxiid (G), koaro, Galaxias brevipennis] and a benthic predatory stonefly (S; Stenoperla sp.) with Deleatidium sp. mayflies as prey. Eight treatments with all predator species combinations and a predator‐free control were used. Experiments were performed in aquaria with cobbles as predator refuges for mayflies and we measured the proportion of prey consumed after 6 h for both day and night trials. 3. Trout consumed a higher proportion of prey than other predators. For the two predator treatments we found less than expected prey consumption in the galaxiid + trout treatment (G + T) for both day and night trials, whereas a higher than expected proportion of prey was consumed during night time in the stonefly + trout (S + T) treatment. 4. The results indicate interference (G + T) and facilitation (S + T) between predators depending on predator identity and time of day. Thus, to make accurate predictions of interspecific interactions, it is necessary to consider the ecology of individual species and how differences influence the direction and magnitude of interactions.     

Shift between carnivory and omnivory in stream stonefly predators

Omnivory can have profound effects on the trophic dynamics of communities and ecosystems, as they may interact with multiple trophic levels simultaneously. Some species of large-bodied stoneflies may be viewed as omnivores rather than true carnivores even at later nymphal stages. We evaluated the seasonal change in the diet of stonefly predators by analyzing their stable isotope ratio, gut contents, physiological activity, and food availability. A two-source-based mixing model based on stable isotope analysis revealed that stoneflies shifted their diet between carnivory in summer and omnivory in winter—despite the higher availability of animal prey in winter. The gut content analysis showed that swift prey (mayflies) were consumed in the summer, whereas sluggish prey (Chironomidae) were consumed in the winter. The physiological activity of stoneflies also declined markedly in winter. These results suggest that, in winter, stoneflies foraged on a mixture of Chironomidae and algae. It appears that omnivory in some stream consumers is related to the seasonal change in temperature-dependent physiological activity, rather than prey availability.     

Flexible use of patch marks in an insect predator: effect of sex, hunger state, and patch quality

Abstract 1. Patch marks that allow the subsequent avoidance of marked areas may be used by small animals to increase foraging efficiency. This study is the first to demonstrate the presence of a patch-marking system in insect predators. Furthermore, the marking system is found only in females, and factors such as hunger state and patch quality play a key role in determining whether a female will re-investigate a self-marked patch.
2. Females of the insect predator Orius sauteri avoided areas where the female itself had searched previously but did not avoid areas searched by conspecific females when deprived of prey for 24 h . There was no evidence that males use such a patch-marking system, indicating the presence of a sex difference in patch-mark use.
3. Females did not discriminate between patches visited previously and patches not visited when they were either well fed or when patches contained abundant prey.
4. The patch mark used by females was effective for ≤ 1 h and may be a reliable indicator of a recently visited area in which prey have been depleted.
5. These results suggest that O. sauteri females have the flexibility to adjust their behavioural responses to a previously searched area depending on their hunger state and the availability of prey in their foraging environment.     

Mechanisms of intra-and interspecific interference between larval stoneflies

Summary Behavior of focal individuals of two potentially competing sympatric stonefly species, Megarcys signata and Kogotus modestus (Perlodidae), was videotaped in flow-through plexiglass arenas placed in the East River, Gunnison County, Colorado. Focal individuals were observed alone and in pairs with conspecifics and allospecifics at four prey (Baetis bicaudatus, Baetidae, Ephemeroptera) densities to determine whether competitors and prey resource levels affected prey capture rates. Presence of conspecific or allospecific competitors reduced stonefly prey capture rates, especially for Kogotus, the smaller of the two species, due to a significant decline in predator-prey encounter rates with competitors present. This competitive effect was not observed at the lowest and highest prey densities due to very low or very high predator-prey encounter rates, respectively. Thus, interference affected feeding rates only at intermediate prey densities. Competitors had no effect on the probability of attacks per prey encounter or capture success per attack. Within each stonefly species the effects of intra-and interspecific interference on feeding rates were similar, even though behavioral responses by both stoneflies to interspecific encounters were more frequent than to encounters with conspecifics. Kogotus showed the highest levels of response to encounters with other stoneflies, maintaining those high levels of response to Megarcys over all prey densities. Further, male Kogotus, which are the smaller sex, responded more frequently to competitive interactions than did females. These data are consistent with the hypothesis that interspecific interference was asymmetrical with Megarcys, the larger species, being the superior competitor.     

Mechanisms of prey selection by predaceous stoneflies: roles of prey morphology,behavior and predator hunger

Summary Laboratory feeding experiments using Hesperoperla pacifica (Banks), Perlidae, and Megarcys signata (Hagen), Perlodidae, as predators and Baetis tricaudatus Dodds and Ephemerella altana Allen as prey indicate a strong effect of prey morphology and mobility and predator hunger on prey selection by stoneflies. Knowledge of both dietary composition and feeding behavior was necessary to fully understand prey selection by these stoneflies.Fasted stoneflies presented with live prey ate more E. altana while satiated stoneflies ate approximately equal numbers of the two mayfly species. This pattern of dietary composition was the result of a reduction of attack frequency on the slower swimming E. altana with predator satiation and a continued high attack rate on B. tricaudatus regardless of recent feeding history. In contrast, fasted H. pacifica fed fresh frozen mayflies ate more B. tricaudatus indicating the importance of differences in prey mobility in controlling dietary composition.The high degree of similarity in patterns of feeding and mechanisms underlying those patterns for H. pacifica and M. signata suggest that they may be using similar rules for choosing mayfly prey and we suggest that mayfly prey are ranked by stoneflies on the basis of handling times. A general mechanistic model for stoneflies feeding on mayflies is presented and discussed.     

Assessing the functional importance of large‐bodied invertebrates in experimental headwater streams

Recent theoretical advances in food web ecology emphasize the importance of body size disparities among species for the structure, stability and functions of ecosystems. Experimental confirmations of the functional importance of large species, independent of their trophic position, are scarce. We specifically examine the multiple ecological roles of large invertebrates from two distinct trophic levels in headwater streams. We experimentally manipulated the presence of large predatory invertebrates (two Perlid stoneflies) or detritivores (a limnephilid caddisfly and a Pteronarcys stonefly) in a two‐by‐two design in stream channels open to immigration/emigration of smaller biota. We assessed treatment effects on the trophic structure of the benthic invertebrate community, dynamics of basal resources (benthic algae and leaf litter of cedar and alder), and stability of litter decomposition rates against an experimental pulse perturbation (fine sediment input). The presence of the large invertebrates was associated with a ten‐fold decrease in the biomass of invertebrate filterers whereas other trophic groups were unaffected by the large species. The biomass of benthic algae was lower and the rate of mass loss of alder litter was higher in channels lacking the large predators, thus revealing trophic cascades operating along both algal‐based and detritus‐based food chains. The large predators had no detectable effect on the decomposition of cedar whereas both cedar and alder disappeared faster in the presence of the large detritivores. Furthermore, the large predators and large detritivores interactively influenced the decomposition of the cedar–alder mixture through a litter diversity effect and the variability of the rate of alder decomposition after a pulse of fine sediment. Because the large invertebrates affected multiple ecosystem properties, and as their absence was not rapidly compensated for by small immigrant species, our findings support the notion that large species could be critically important in controlling ecosystem structure and functioning.     

Can current velocity mediate trophic cascades in a mountain stream?

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Effect of an abiotic disturbance on a lotic predator-prey interaction

Summary A model derived from marine research, and recently applied to stream communities, suggests that community structure is more likely to be influenced by predators in benign versus harsh abiotic regimes. Experiments were conducted to determine if increasing the harshness of a particular regime would alter the impact of a stream invertebrate predator on prey densities in field enclosures. Density of a stonefly predator, Kogotus nonus, was varied in containers exposed to low (benign) and high (harsh) levels of fine sediment. As predicted by the model, the harsher regime eliminated predator effects in two of three experiments. In the third experiment, however, high levels of sediment actually enhanced the impact of the predator on the prey community. A consideration of the possible mechanisms underlying this model led to the conclusion that increasing the harshness of a regime can be expected to produce outcomes ranging from elimination to enhancement of predator effects, depending on how the regime is perceived by the predator versus the prey.     

Mayfly locomotory responses to endoparasitic infection and predator presence: the effects on predator encounter rate

SUMMARY. 1. The effects of predaceous stoneflies and endoparasilism by mermithid nematodes on the locomotory behaviour of individual mayflies were studied in the laboratory. The effects of predator- and parasite-induced changes in activity on predator encounter, attack and capture rates were also examined.
2. In response to predator presence non-parasitized mayflies decreased activity, while parasitized mayflies increased activity, in comparison to a control group. As a result, the faster-moving parasitized individuals encountered stonefly predators much more frequently. The frequency of attacks per encounter did not change.
3. In view of this and other studies, two predator-avoidance strategies based on mayfly locomotory and defensive capabilities are proposed: (a) Mayflies that are able effectively to flee a predator's attack and those able to employ a behavioural defence may decrease their mean velocity when a predator is detected, thereby reducing the likelihood of an encounter while allowing foraging to continue; (b) Slower-moving or otherwise defenceless mayflies may immediately move away from the area in which a predator is perceived in order to avoid an encounter.
4. In the absence of a predator, parasitized mayflies demonstrated greatly increased activity. This probably reflects increased foraging in response to the nutritional deficit imposed by the endoparasite. A decrease in upstream movement by parasitized mayflies may also indicate a parasite-imposed energy stress.     

The microdistribution of some lotic insect predators in relation to their prey and to abiotic factors

SUMMARY. 1. The microdistribution of three species of insect predators in a Swedish stream was assessed using a multivariate statistical approach. Both abiotic factors and factors pertaining to the prey community were included in the analysis.
2. The factors most strongly correlated to the distribution of large Dinocras cephalotes were the densities of case less caddis larvae ( Wormaldia sp. and Hydropsyche siltalai ) and the median weight of stonefly prey. For small D. cephalotes , the density of H. siltalai and the percentage of moss cover were most important. Isoperla grammatica showed a significant correlation to blackfly density. Rhyacophila nubila occurred predominantly in patches with high prey biomass and with high densities and median weight of case less caddis larvae.
3. It was concluded that the quantity and quality of the prey, such as size, availability and species, influenced the predators'microdistribution. The distributional pattern differed between species of predators and was probably related to their different hunting strategies.     

An experimental study of the effects of predatory fish on macroinvertebrates in a Hong Kong stream

SUMMARY. 1. Predation upon macroinvertebrates by the loach Oreonectes platycephalus Günther (Cobitidae) was studied using predator inclusion/exclusion cages in a series of pools along a Hong Kong stream. Treatments employed were predator exclusion, medium (approximately natural) predator densities (1 fish cage⁻¹) and high predator densities (2 fish cage⁻¹). Macroinvertebrate abundance in cages was monitored after 2 and 4-weeks exposure to predators.
2. The presence of fish was associated with significant declines in the total numbers of macroinvertebrates colonizing cages. However, taxa were influenced differently, with mayflies decreasing by a factor of two while the more mobile shrimps (Atyidae) were unaffected. Chironomid abundance (largely Chironominae) was unaffected by predator density and increased in week 4. Detritus acted as a confounding variable at this time because chironomid abundance was significantly correlated with the weight of accumulated detritus in cages.
3. While invertebrates were more abundant in cages lacking fish, there were no fewer invertebrates in cages with 2 fish than with 1 fish. This may indicate the presence of secure refuges among substrates in the cages, preventing the additional fish from depleting prey further, or a lack of precision of methods due to natural variations in prey densities and spatial patchiness.
4. No significant effects of predators on relative prey abundance or species richness were detected.
5. The impact of predation on prey abundance weakened on week 4, perhaps due to extra refuges among the accumulated detritus. However, drying of the stream increased fish densities in pools so that cages may have become zones of relative safety that were colonized readily by macroinvertebrates. This result highlights the need for year-round investigations to quantify predation effects in Hong Kong's seasonal tropical climate.     

Zebra mussels affect benthic predator foraging success and habitat choice on soft sediments

The introduction of zebra mussels (Dreissena spp.) to North America has resulted in dramatic changes to the complexity of benthic habitats. Changes in habitat complexity may have profound effects on predator-prey interactions in aquatic communities. Increased habitat complexity may affect prey and predator dynamics by reducing encounter rates and foraging success. Zebra mussels form thick contiguous colonies on both hard and soft substrates. While the colonization of substrata by zebra mussels has generally resulted in an increase in both the abundance and diversity of benthic invertebrate communities, it is not well known how these changes affect the foraging efficiencies of predators that prey on benthic invertebrates. We examined the effect of zebra mussels on the foraging success of four benthic predators with diverse prey-detection modalities that commonly forage in soft substrates: slimy sculpin (Cottus cognatus), brown bullhead (Ameirus nebulosus), log perch (Percina caprodes), and crayfish (Orconectes propinquus). We conducted laboratory experiments to assess the impact of zebra mussels on the foraging success of predators using a variety of prey species. We also examined habitat use by each predator over different time periods. Zebra mussel colonization of soft sediments significantly reduced the foraging efficiencies of all predators. However, the effect was dependent upon prey type. All four predators spent more time in zebra mussel habitat than in either gravel or bare sand. The overall effect of zebra mussels on benthic-feeding fishes is likely to involve a trade-off between the advantages of increased density of some prey types balanced against the reduction in foraging success resulting from potential refugia offered in the complex habitat created by zebra mussels.