Article

Can current velocity mediate trophic cascades in a mountain stream?

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Abstract

Trophic cascades can be context-dependent in that they occur under some conditions but not others. An important context for stream benthic communities is near-bed current velocity, which varies at the spatial scale at which benthic species interactions occur.To investigate how centimetre-scale variation in near-bed current velocity influenced the occurrence of benthic cascades, I studied a stonefly/mayfly/algal system in a high-gradient Rocky Mountain stream and examined the effect of 10 and 25 cm s−1 current (i.e. ‘slow’ and ‘fast’, respectively) on stonefly predation and its top-down effects.There were two experiments using stream-side mesocosms and a third using in-stream troughs. The two mesocosm experiments had known numbers of stoneflies and mayflies and examined how near-bed current influenced (i) top-down cascades driven by non-trophic, behaviourally mediated predator effects (stonefly mouthparts were glued shut) and (ii) direct predation. The in-stream experiment allowed mayfly grazers to arrive and depart freely and examined how near-bed current and stoneflies influenced algal accumulation and community structure.Results from these experiments showed that top-down cascades occurred in slow, but not fast current, and that these cascades are driven largely by the non-trophic effects of predation. The mesocosm experiment showed that mayflies removed less algae in slow current when stoneflies with glued mouthparts were present. There was 33% greater algal biomass in slow compared with fast current, and algal biomass in the slow current did not differ from that in the grazer-free controls. The predation experiment showed that stoneflies captured approximately twice as many mayflies in slow as compared to fast current.The in-stream experiment found that algal biomass increased when stoneflies were present, but grazer densities did not differ from stonefly-free controls, supporting the hypothesis that effects mediated by behaviour drove the cascade.This study suggests that near-bed current can turn stonefly/mayfly/algae cascades ‘on’ and ‘off’ at small spatial scales and provides a new perspective for understanding algal heterogeneity on natural streambeds.

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... Both direct and indirect interactions such as those documented in stream food webs can substantially alter community composition of both periphyton and insect grazers (Peckarsky 1980(Peckarsky , 1982McIntosh and Townsend 1994). However, few studies have empirically measured the indirect effects of invertebrate predators on algal production through altered foraging behavior of grazers (McIntosh and Peckarsky 1999;Wellnitz 2014). The influence of direct and indirect effects on basal resources may differ depending on environmental factors, such as water temperature. ...
... We compared changes in algal biomass after a 24 hr grazing period (Peckarsky et al. 1993) along the thermal gradient for the 3 treatments of grazer alone, lethal predators present, and non-lethal predators present. The non-lethal predator treatment was achieved by gluing the mouth parts of individual stoneflies together, as has been done by others (e.g., Peckarsky et al. 1993;Peckarsky and McIntosh 1998;Wellnitz 2014). ...
... We saw no difference in algal biomass between non-lethal predator and grazer-alone treatments (Figure 3). This contrasts with previous research which has shown both strong and weak indirect effects of stonefly predators on grazers (Peckarsky and McIntosh 1998;McIntosh and Peckarsky 1999;Wellnitz 2014). However, these studies occurred over longer timeframes, our relatively short 24 h experimental period may have masked the detection of subtle indirect effects (but see Peckarsky et al. 1993). ...
... Grazer manipulations have included electrified wire exclusions (Pringle and Blake 1994;Moulton et al. 2004), exclusion/inclusion chambers or Handling Editor: Piet Spaak Electronic supplementary material The online version of this article (doi:10.1007/s10452-017-9635-2) contains supplementary material, which is available to authorized users. cages (Malmqvist 1993;Hillebrand and Kahlert 2001;Donato-Rondón et al. 2010;Murdock et al. 2010), block nets (Power et al. 1985;Bertrand and Gido 2007), streamside/in-stream mesocosms or troughs (Perrin et al. 1992;Wellnitz 2014), or a combination of these methods (Hill and Knight 1987;Connolly and Pearson 2013;Wellnitz 2014). Of the many grazer exclusion techniques used in freshwater systems, exclusion cages provide a compromise in ease of use, grazer control, and ability to be placed directly into the study system. ...
... Grazer manipulations have included electrified wire exclusions (Pringle and Blake 1994;Moulton et al. 2004), exclusion/inclusion chambers or Handling Editor: Piet Spaak Electronic supplementary material The online version of this article (doi:10.1007/s10452-017-9635-2) contains supplementary material, which is available to authorized users. cages (Malmqvist 1993;Hillebrand and Kahlert 2001;Donato-Rondón et al. 2010;Murdock et al. 2010), block nets (Power et al. 1985;Bertrand and Gido 2007), streamside/in-stream mesocosms or troughs (Perrin et al. 1992;Wellnitz 2014), or a combination of these methods (Hill and Knight 1987;Connolly and Pearson 2013;Wellnitz 2014). Of the many grazer exclusion techniques used in freshwater systems, exclusion cages provide a compromise in ease of use, grazer control, and ability to be placed directly into the study system. ...
... Organisms living within or near the flow boundary layer must balance the delivery of resources with the stress of drag (subsidy-stress hypothesis, Odum et al. 1979;Biggs et al. 1998). Near-bed velocity can also mediate trophic interactions (Hart and Merz 1998;Hintz and Wellnitz 2013;Wellnitz 2014). Wellnitz (2014) demonstrated that predation events on grazing invertebrates were more frequent in slow (approximately 10 cm s -1 ) velocities than in fast (approximately 25 cm s -1 ) velocities, which in turn led to 33% more algal biomass in slow velocities. ...
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... The baetid, O. hickmani, is likely to use similar mechanisms when drifting. Offadens hickmani may have actively chosen to exit the drift when initially exposed to slow or zero velocity areas by swimming to the substrate (Otto & Sj€ ostr€ om, 1986), possibly to avoid the risk of settlement in slow flow habitats where the risk of predation can be greater (Wellnitz, 2014). Actively settling from the drift may well have increased when encountering progressively slower flows in deeper pools. ...
... A more likely explanation is that an individual will settle from the drift in the lateral margins of the river (Ciborowski, 1983) and crawl or swim upstream, returning to the upstream riffle, as small-scale upstream movements of benthic invertebrates are reasonably common (Elliott, 2003). Alternatively the adverse environmental conditions and/or increased predation may result in many of these individuals dying before moving out of pools (Wellnitz, 2014). ...
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... For example, the construction of hydropower plants has destroyed the natural conditions of streams. In addition, the original ecological balance and seasonality of streams have been disrupted by the decrease in flow, cutoff of water flow, and dry up of rivers (Wellnitz, 2014;Ali et al., 2019;Branco et al., 2019;Ruocco et al., 2019). The construction of small hydropower plants has a small impact on the chemical factors, but it has a large impact on physical factors such as the flow velocity and water depth of the stream . ...
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... However, for mobile grazing taxa like Baetis spp., the mechanisms linking reduced velocity to drift are less clear. Biotic factors associated with reduced velocity, such as elevated benthic predation risk (Wellnitz, 2014) or biofilm depletion (Hintz & Wellnitz, 2013), may have played a role, but we are unable to distinguish among them. ...
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The harsh-benign model of community dynamics predicts that the impact of predation will decline as abiotic conditions become more stressful to biota. Experiments were conducted to determine whether hydrological disturbance altered the impact of an invertebrate predator in stream benthic communities. The impact of a predatory stonefly, Cosmioperla kuna, on its mayfly prey was measured in experimental stream channels receiving constant or variable flow (flooding) regimes over a one-week period. Contrary to predictions of the harsh-benign hypothesis, the impact of Cosmioperla oil its two major prey taxa was either unchanged or increased by artificial floods, despite increased predator emigration from variable-flow channels. Predator impacts in variable-flush treatments were apparently strongly influenced by predator-induced prey emigration during floods. The results of this study show that nonlethal predator effects may be important during abiotic disturbance, and that it may not be reasonable to predict the impact of predation solely on the basis of the relative tolerances of predators and prey to prevailing abiotic conditions.
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We performed laboratory experiments to test the hypothesis that predator species influenced a forager's trade-off between predation risk and food reward. Larvae of the stream mayfly, Baetis tricaudatus Dodds, were provided with simultaneous access to feeding patches in laboratory streams that differed in predation risk (safe, risky) and food reward (low, high). Predation risk was imposed using either: (1) a fish predator, the longnose dace, Rhinichthys cataractae (Valenciennes), (2) a stonefly predator, Claassenia sabulosa (Banks), and (3) both predators in risky patches at the same time. We hypothesized that predator species would influence the behavioural trade-off between predation risk and food reward because mortality risk on Baetis from Rhinichthys exceeds that from Claassenia. Moreover, the presence of Rhinichthys reduces the foraging activities of Claassenia. Although Baetis spent the majority of time in safe patches, the use of risky patches increased three to five-fold when they contained high food, compared with low food reward. Increased use of risky, high food patches by Baetis was associated with a food-dependent, anti-predator flight response. The mean distance at which large Baetis reacted to Claassenia and Rhinichthys by drifting out of patches was almost four times lower in high food, compared to low food, risky patches. Based on the four trade-off models described by Fraser and Huntingford, Baetis adopted a risk-adjusting trade-off irrespective of whether risk was imposed by Rhinichthys, Claassenia or both predator species at the same time. Thus, the behavioural trade-off between predation risk and food reward adopted by Baetis was independent of predator species.
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A recent model, designed with stream systems in mind, suggested that prey exchange (movement of prey among patches) tends to reduce predator impacts on prey density; that is, rapid prey immigration into a patch with predators can swamp local effects of predators on prey density. The previous model, however, included two assumptions that influence the model's qualitative predictions. First, it assumed that the system's focal patches are surrounded by an environment that has no predators and a constant prey density. More importantly, it assumed that prey do not alter their per capita emigration rates in response to the presence of predators. We extended the earlier model by relaxing these assumptions. Specifically, we: (1) addressed predator impacts in patches surrounded by background environments that do not have predators, (2) allowed the background environment to have a constant or decreasing prey density (i.e., we examined situations in which predators deplete prey), and (3) accounted for the fact that prey per capita emigration rates are often altered by the presence of predators. Our most interesting results concerned the profound effects of prey emigration behavior on the relationship between prey exchange and predator impact. If prey do not alter their emigration rates in response to predators, then, as predicted by the earlier model, high prey exchange should result in very low predator impact. If, however, prey disperse out of patches in response to the presence of predators (i.e., if prey per capita emigration rates are higher out of patches with predators than out of predator-free patches), then even very high prey exchange rates cannot swamp predator impact; instead, prey emigration adds to predator impact. Thus, depending on prey emigration behavior, increased overall prey exchange can either decrease or increase predator impact. Predators can also suppress prey emigration (e.g., if prey hide in refuges so effectively that they disperse at low rates from predator patches). In that case, high prey exchange rates tend to result in "negative predator impacts" (i.e., higher prey density in patches with predators). The details of the above relationships are influenced by whether or not the background environment has predators. If the background has predators, but predators do not deplete prey (e.g., if predation is offset by recruitment), then predator impact depends only on the ratio of per capita emigration rates out of predator and predator-free patches; that is, attack rates do not influence impact. In contrast, if predators can deplete prey, then attack rates influence predator impact. In that situation, if attack rates are highly relative to prey emigration rates out of predator-free patches, then predator impact steadily increases over time. A literature review suggested that prey alterations in per capita emigration rates in response to the presence of predators can potentially explain some surprising natural phenonomena, including the existence of "negative predator impacts," and the apparent tendency for invertebrate predators (primarily, stoneflies) to have stronger impacts, and the apparent tendency for vertebrate predators (primarily, fish). Finally, we discussed possible adaptive links between prey escape success, refuge use, dispersal behavior, and predator impacts. This discussion raised a "paradox of danger": that due to their effects on prey exchange, more dangerous predators might often have unexpectedly weak effects on local prey density. In this context, we suggest a framework for studying relationships between prey behavior, prey exchange, and predator impacts.
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Field experiments and surveys were carried out in a Rocky Mountain alpine stream during the summers of 1990 and 1992 to estimate the proportion of natural losses of the mayfly Baetis bicaudatus resulting from the co-occurring, predatory stonefly, Kogotus modestus. Functional response experiments determined the number of prey consumed by male and female Kogotus by manipulating the densities of prey and the presence or absence of predators in stream-side chambers. Estimates of losses of Baetis and density of Kogotus were based on ten benthic samples collected weekly (except for last sampling date) from one study reach. Concurrently, drift density of Baetis was estimated upstream and downstream of the reach to determine gains or losses of Baetis resulting from migration. In the functional response experiments Kogotus consumed the same number of prey regardless of prey densities. Female predators tended to consume more prey (similar to 2 d(-1)) than males (similar to 1 d(-1)), although the result was only statistically significant in one out of three experiments. Per-capita mortality rates of Baetis declined from 0.01 to 0.001 d(-1) (predator m(-2))(-1) with increasing prey density. In the study reach Baetis density declined 70% during the 4.5 wk and per-capita losses averaged 3.8% d(-1). We estimated that predation by Kogotus could cause between 1.6 and 9.5% of the losses of Baetis from the study reach. This occurred because high losses of Baetis were combined with low consumption rates and densities (1.83 individuals m(-2)) of Kogotus. Baetis drift density was higher at night than during the day. Drift densities of Baetis tended to be higher leaving than entering the reach in nighttime estimates, although the results were not statistically significant. Few replicates resulted in low power to detect differences in upstream and downstream drift densities; therefore, it is possible that some losses could be the result of drift out of the study reach. Nonetheless our results suggest that Kogotus populations had little direct, lethal effect on Baetis populations in this study reach.
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Experiments at the East River and Benthette Brook, Gunnison County, Colorado, compared instantaneous mortality rates due to predation by 2 species of stream-dwelling stonefly nymphs on different mayfly species. Megarcys signata consistently selected Baetis bicaudatus over Cinygmula mimus and Ephemerella infrequens in single-prey and mixed-prey trials, when starved for 6 d, 1 d, or fed prior to experiments, but substrate removal increased Cinygmula's absolute mortality rate, resulting in significant selectivity on this species. Kogotus modestus selectively consumed either Baetis or Epeorus deceptivus over Cinygmula. Substrate removal resulted in selection of Baetis over Epeorus, and increased absolute mortality rates of all 3 prey species. Selective predation was more a consequence of active behavioral selection than of prey vulnerability. However, prey defensive behavior and morphology affect not only encounter rates and capture success, but also attacks per encounter. Also, although significant differences were not seen among prey types in encounter rates or captures per attack, multiple regression analysis showed that these factors, in addition to attacks per encounter, contributed significantly to observed variation in capture rates among prey types in behavioral trials. Differential prey vulnerability or fixed behavioral selection is thus a more probable proximate cause of selective predation than is active behavioral selection, since stoneflies showed a fixed attack response to swimming prey. -from Authors
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We summarized studies on the impacts and scale effects of negative (competition, predation, parasitism, herbivory) and positive (mutualism, commensalism, indirect facilitation) species interactions in freshwater benthic habitats since ∼1986 and focused on organisms with mainly or entirely aquatic life cycles. Benthologists publishing in J-NABS have contributed robustly to our overall knowledge of predation and herbivory but less so of other species interactions. Predators can limit the abundance of benthic prey and affect prey size or age structure, behavior, and morphology, and these effects can be transmitted through food webs and ecosystems. Herbivores can limit biomass of benthic algae, alter physiognomy, species composition and diversity, and stoichiometry, and exert strong indirect effects within food webs and nutrient cycles. Parasites can alter host behavior or morphology, but few studies have shown that lethal/sublethal effects of parasites on their hosts have population- or community-scale consequences. Fishes and macroinvertebrates occasionally experience competition, but the effect of competition on demographies and assemblages appears restricted to local scales, perhaps because competition can be modulated by many biotic (bioenergetic efficiency, parasitism, predation) and abiotic (floods, drought, resource distribution) factors. Positive interactions have been the least studied species interaction by benthologists, but interest is growing. Future study of population-scale positive interactions and nontraditional interactions at larger scales (e.g., riparian effects on benthic habitat stabilization, cross-system recruitment of different life stages) will improve our understanding of freshwater benthic ecosystems and their conservation. We urge benthologists to explore how populations evolve in response to biotic interactions embedded in benthic communities and to assess how these responses might redefine trophic and community structure and their emergent properties.
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Proliferations of periphyton associated with the ecological degradation of streams have been linked to changes in flow regime, most frequently as a product of water abstraction and impoundment. We used descriptive and experimental studies to examine the relationship between water velocity and the distribution of two distinct, often high biomass, periphyton patch types and associated macroinvertebrate communities in the Waipara River, a meso‐eutrophic stream in Canterbury, New Zealand.One patch type (‘Phormidium’) consisted mainly of two filamentous cyanobacterial taxa: a Phormidium/Lyngbya complex and Oscillatoria. The other type (‘filamentous greens’) consisted mainly of several filamentous chlorophyte taxa (Mougeotia sp.) along with various diatom epiphytes. Ash‐free dry mass, chlorophyll a and particulate N and P concentrations were significantly greater in the Phormidium than in the filamentous green patches.There was a significant relationship between patch cover and water velocity, with filamentous greens negatively associated with velocity and dominant in areas with a velocity −1, while Phormidium was dominant at velocities >∼0.40 m s−1. A 7‐day experimental reduction in velocity in Phormidium‐dominated habitats resulted in a significant reduction in% cover (84–21%) with a corresponding increase in filamentous greens (16–79%). There was a small but significant reduction in the % cover of filamentous greens (90–82%) with experimentally increased velocity, but no significant change in Phormidium cover (7–2%), perhaps due in part to the slow growth rate of taxa in this patch type.Characterisation of macroinvertebrate assemblages collected from Phormidium and filamentous green patches and the substratum immediately below showed patch‐specific assemblages, although patch type was a less important determinant than vertical location, with 85% of the macroinvertebrates found in the substratum below algal mats.The differential responses of Phormidium and filamentous green patches to velocity are considered in the context of a subsidy‐stress model that examines how trade‐offs between flow‐mediated biomass accrual and loss processes vary for different algal growth forms.Collectively, these results indicated that key ecological characteristics of rivers can be quite sensitive to spatial and temporal variations in hydraulic conditions that are much smaller than those typical of floods and droughts.
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In this paper we review the empirical studies documenting trait-mediated indirect interactions (TMIIs) in food webs. Basic models and empirical approaches that form the foundation of our conceptualization of species interactions generally assume that interactions, are an intrinsic property of the two interacting species and therefore are governed by their respective densities. However, if a species reacts to the presence of a second species by altering its phenotype, then the trait changes in the reacting species can alter the per capita effect of the reacting species on other species and, consequently, population density or fitness of the other species. Such trait-mediated indirect interactions can reinforce or oppose density-mediated effects and have been largely overlooked by community ecologists. We first briefly develop the case for the broad mechanistic basis for TMIIs and then review the direct evidence for TMIIs in various permutations of simple three- to four-species food webs. We find strong evidence for quantitatively significant effects of TMIIs in a variety of aquatic and terrestrial systems. We further highlight those few studies that address the question of the relative magnitudes of density- and trait-mediated effects and the role of species densities in their transmission. These studies indicate that trait effects are often as strong or stronger than density effects. We conclude that ecological communities are replete with TMIIs arising from trait plasticity and that these effects are quantitatively important to community dynamics. Finally, we synthesize our results and indicate profitable directions for future research.
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Predator-prey studies in streams have traditionally focused on mayfly-stonefly interactions in relatively constant flow conditions. In reality, however, lotic prey encounter multiple types of predators, most of which are restricted to low-velocity microhabitats. By contrast, some invertebrate prey may occur in very high current velocities. For example, many blackfly species are able to feed at velocities of 100 cm s-1, whereas even moderate currents reduce the hunting efficiency of their invertebrate predators. The caddisfly larvae of the genus Rhyacophila, however, may be an exception to the pattern of reducing predator efficiency with increasing velocity. Using a combination of laboratory and field experiments and behavioral field observations, we examined the interaction between predatory Rhyacophila caddis larvae and larval blackflies along a velocity gradient of 20–120 cm s-1. In laboratory experiments, Rhyacophila preferred currents slower than 50 cm s-1 while blackflies exhibited a wide tolerance of currents and frequently occurred in currents exceeding 100 cm s-1. In direct field observations, total activity and distance moved by Rhyacophila were similar at all current velocity regimes tested, but frequency of predation attempts on blackflies was lowest at the highest velocities (> 100 cm s-1). In a field colonization study, blackflies avoided substrates with the slowest velocities (< 40 cm s-1), as also did the caddis larvae. Only velocities approaching 100 cm s-1 provide blackflies with refuge from predation by Rhyacophila. Being able to maneuver across a wide range of velocities, Rhyacophila may have more pervasive effects on their prey than other lotic invertebrate predators.
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Experiments were conducted to determine whether overlap between microhabitat preferences and activity periodicities of four mayfly species and their stonefly preda- tors could explain species-specific differences in predator-prey encounter frequencies. Preferences for rock type (slate or granite), flow microhabitat (high or low), rock sur- face (top, bottom, upstream or downstream sides), and periodicity of drift and the use of rock tops were measured in a stream-side system of flow-through circular Plexiglas chambers receiving natural stream water and light levels. These parameters were com- pared among the predatory stoneflies, Megarcys signata or Kogotus modestus, and four species of mayflies that vary in their encounter rates with the stoneflies. Based on predator-prey encounter rates previously observed in similar chambers, we expected greater overlap between Megarcys and Ephemerella infrequens and the overwintering generation of the bivoltine mayfly, Baetis bicaudatus than with Cinygmula sp. Like- wise, we expected Kogotus microhabitat use to overlap more strongly with that of summer generation Baetis than with later instars of Cinygmula and Epeorus decepti- vus. Results ran counter to our predictions, indicating that microhabitats of the prey species with high predator encounter rates did not overlap more strongly with the stoneflies than did mayflies with low predator encounter rates. Most mayflies and stoneflies preferred the bottom surfaces of granite rocks, and showed few flow prefer- ences. Most were nocturnal in their use of top rock surfaces, in drift and feeding activ- ity periodicity. Therefore, nocturnal activity periodicities of both mayflies and stone- flies confirm that mayflies have not evolved feeding periodicity to avoid encounters with foraging stonefly predators. We conclude therefore, that neither temporal nor spatial microhabitat overlap is a reasonable explanation of differential encounter rates between predatory stoneflies and their mayfly prey. Alternative explanations for dif- ferential encounter rates are that more abundant or more mobile mayflies have higher encounter rates with predators, and effective pre-contact predator avoidance responses of other mayflies reduce their encounter rates with stoneflies.
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We used hot-film anemometry to quantify fine-scale spatial and temporal flow variations near the surfaces of stones inhabited by suspension-feeding larval blackflies (Simulium vittatum). We focused especially on within-stone patterns of covariation between patchy microdistributions of larvae and local spatial variations in current speed. Current speeds were sampled at 256 Hz for heights between 1 and 10 mm above the bed. Profiles of current speed exhibited complex shapes, and boundary-layer thicknesses ranged from < 1 to > 5 mm. Average current speeds measured 2 mm above the bed (the approximate height of larval feeding appendages) ranged between 7 and 59 cm s- '. Current speeds measured 10 mm above the bed were very poor predictors of speeds measured at the 2-mm height. Larval abundance exhibited a significant positive relationship to current speed at 2-mm height, and within-stone variations in speed explained -59% of the variation in abundance. Time series of current speed exhibited marked fine-scale temporal heterogeneity, fluctuating by as much as 80 cm s-l in ~0.1 s. Maximum accelerations sometimes exceeded 1 x lo4 cm s-*, which suggests that the forces tending to dislodge benthic organisms from the bed may be greater than previous estimates based on assumptions of steady flow. Observed levels of turbulence were greater than predicted from traditional boundary-layer theory. We suggest that much of the turbulence evident on indi- vidual stones is not produced by local shear but is inherited from upstream roughness elements that cause flow separation.
Article
Behavior of Baetis bicaudatus (Ephemeroptera: Baetidae) was video taped in a W Colorado, stream to identify the specific stimuli eliciting noncontact responses to predatory stoneflies (Kogotus modestus; Plecoptera; Perlodidae). Intact Baetis primarily turned their cerci (tail curl behavior) with similar frequencies toward live or model stoneflies that were 1-2cm downstream, and primarily moved when stoneflies were within 1cm even before contact. Removal of Baetis' cerci reduced the frequency of tail curl responses and the reactive distance, but increased frequency of movement responses to noncontact with Kogotus. Results suggest that stimuli associated with the whole stonefly, such as visual or more probably hydrodynamic cues, cause Baetis tail curl behavior. -from Authors
Article
Feeding (algal grazing) rates, growth rates, and fecundities of Baetis bicaudatus (Baetidae) larvae were measured on fitness correlates of the presence of predatory stoneflies (Perlodidae). Predator avoidance significantly reduced levels of Baetis gut fullness, over the short term, to levels intermediate between the high and low resource treatments. Longer term residence with predatory stoneflies caused larvae of both male and female Baetis to mature at significantly smaller sizes than in the treatment with ambient food but no predators. May flies with predators matured at sizes similar to those held with low food levels over the long term. Neither starved larvae nor those avoiding predators during the experimental period in contrast to about 50% growth rate of larvae experiencing ambient food levels, but no predators. Similarly, egg biomass per mature female larva was significantly reduced in both starved and predator treatments. Females avoiding predators accrued less total mass, but allocated a similar proportion to eggs as did well-fed Baetis. Data demonstrate that predator avoidance can have dramatic consequences on mayfly fitness. The mechanism explaining those fitness consquences is probably a disruptive drift-swim response by Baetis to encounters with stoneflies, which reduced Baetis' feeding rates. -from Authors
Article
Despite long-standing interest in foraging modes as an important element of animal space use, few studies document and compare individual foraging mode differences among species and ecological conditions in the wild. We observed and compared foraging modes of 61 wild Arctic charr, Salvelinus alpinus, 42 brown trout, Salmo trutta, and 50 Atlantic salmon, Salmo salar, in their first growing season over a range of habitats in 10 Icelandic streams. We found that although stream salmonids typically sit-and-wait to ambush prey from short distances, Arctic charr were more mobile during prey search and prior to prey attack than Atlantic salmon, whereas brown trout were intermediate. In all three species, individuals that were mobile during search were more likely to be moving when initiating attacks on prey, although the strength and the slope of this relationship differed among species. Arctic charr also differed from salmon and trout as more mobile individuals travelled longer distances during prey pursuits. Finally, coupled with published data from the literature, salmonid foraging mobility (both during search and prior to attack) clearly decreased from still water habitats (e.g., brook charr), to slow-running waters (e.g., Arctic charr) to fast-running waters (e.g., Atlantic salmon). Hence, our study suggests that foraging mode of young salmonids can vary distinctly among related species and furthers our understanding of the behavioural mechanisms shaping the geographical distribution of wild salmonids.
Article
▪ Abstract Flowing water has profound effects on a diverse array of ecological processes and patterns in streams and rivers. We propose a conceptual framework for investigating the multiple causal pathways by which flow influences benthic biota and focus particular attention on the local scales at which these organisms respond to flow. Flow (especially characteristics linked to the velocity field) can strongly affect habitat characteristics, dispersal, resource acquisition, competition, and predation; creative experiments will be needed to disentangle these complex interactions. Benthic organisms usually reside within the roughness layer, where the unique arrangement of sediment particles produces strongly sheared and highly three-dimensional flow patterns. Thus, accurate characterization of the local flow environments experienced by benthic organisms often requires the use of flow measurement technology with high spatial and temporal resolution. Because flow exhibits variation across a broad range of sca...
Article
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.
Article
Heterogeneous distributions of resources and organisms are characteristic of most ecosystems, but empirical understanding of the causes and consequences of hetero-geneity is limited. We investigated whether predatory fish influenced the heterogeneity (spatial variability) of resources (algae) by modifying the behavior of primary consumers (mayflies). We hypothesized that fish would indirectly increase resource heterogeneity by reducing grazer activity, and that higher densities of grazers would reduce resource het-erogeneity only in the absence of fish. We measured the effects of predator cues (brook trout odor) on grazer behavior (Baetis) and algal heterogeneity in mesocosms (1 m 2) and in simple natural systems (30-m reaches of adjacent fishless streams). In addition, we measured grazer and algal distributions in complex natural streams that varied in grazer density, presence of fish, and physical conditions. Fish odor altered mayfly grazing behavior (measured using a simple behavioral bioassay) and increased algal heterogeneity (measured by Morisita's index at the scale of individual rocks) in mesocosms and manipulated streams. Furthermore, natural streams with higher grazer densities had lower algal heterogeneity, but only if they were fishless. Interestingly, the presence of brook trout decoupled the link between grazer density and algal heterogeneity in natural streams. These observations indicate that release from the threat of predation or increased densities of grazers can homogenize algal resources in fishless streams. We hypothesize that altered foraging in environments with predatory fish, independent of grazer density, led to increased resource patchiness, possibly by allowing the influence of variation in physical characteristics (e.g., flow and substratum) to predominate or by changing grazer microhabitat use. These results support theoretical predictions that factors affecting primary consumer behavior also alter resource heterogeneity.
Article
In this paper we review the empirical studies documenting trait-mediated indirect interactions (TMIIs) in food webs. Basic models and empirical approaches that form the foundation of our conceptualization of species interactions generally assume that interactions are an intrinsic property of the two interacting species and therefore are gov-erned by their respective densities. However, if a species reacts to the presence of a second species by altering its phenotype, then the trait changes in the reacting species can alter the per capita effect of the reacting species on other species and, consequently, population density or fitness of the other species. Such trait-mediated indirect interactions can reinforce or oppose density-mediated effects and have been largely overlooked by community ecol-ogists. We first briefly develop the case for the broad mechanistic basis for TMIIs and then review the direct evidence for TMIIs in various permutations of simple three-to four-species food webs. We find strong evidence for quantitatively significant effects of TMIIs in a variety of aquatic and terrestrial systems. We further highlight those few studies that address the question of the relative magnitudes of density-and trait-mediated effects and the role of species densities in their transmission. These studies indicate that trait effects are often as strong or stronger than density effects. We conclude that ecological communities are replete with TMIIs arising from trait plasticity and that these effects are quantitatively important to community dynamics. Finally, we synthesize our results and indicate profitable directions for future research.
Article
SUMMARY1 Behavioural experiments were performed in artificial stream channels to determine if nymphs of the mayfly Paraleptophlebia adoptiva (Ephemeroptera: Leptophlebiidae) respond to (i) chemical cues from stonefly nymphs, (ii) chemical cues from disturbed conspecific mayflies, and (iii) body fluids of injured conspecifics, and (iv) whether responses to these chemical cues are different when combined with contact by foraging Acroneuria carolinensis stonefly nymphs (Plecoptera: Perlidae).2 Although none of the chemical cues elicited changes in mayfly behaviour in the absence of predator contact, stonefly chemical cues significantly enhanced the avoidance response of mayflies to stonefly contact. Mayfly nymphs swam more frequently and crawled further in response to predator encounters when chemical cues were present than when chemical cues were absent. Thus, direct stonefly precontact chemical cues appear to sensitize P. adoptiva to the potential for an encounter with A. carolinensis, enabling stronger escape or avoidance responses upon contact.3 Mayfly chemical cues that provide indirect information about the proximity of a predator (disturbed or injured conspecifics) did not stimulate increased mayfly response to stonefly contact. Indirect cues may provide less reliable information about the proximity of a stonefly than chemicals emitted directly from an adjacent predator. Mayflies might use chemical cues only in the presence of predator contact because of the high cost of premature escape in a running water system.
Article
Previous studies have shown major differences in the way biomass of stream periphyton is controlled by spatial variations in velocity. We hypothesize that these differences may be the result of different growth forms within the community. Some dense and coherent growth forms (e.g. mucilaginous diatom/cyanobacterial mats) may be resistant to diffusion and also resistant to dislodgment by shear stress. Higher velocities applied to such communities could therefore be expected to enhance biomass accrual by increasing rates of mass transfer, but without greatly increasing losses through sloughing. Conversely, other growth forms (e.g. long filamentous green algae) have an open matrix, and high rates of diffusion into the mats can potentially occur even at low velocity. However, as velocities increase, high skin friction and form drag should lead to higher rates of sloughing. The overall result of these processes should be that maximum biomass occurs at low velocities. This “subsidy-stress” hypothesis was tested twice with each of three different periphytal growth forms: a coherent, mucilaginous, diatom community; a moderately coherent, stalked/ short, filamentous diatom community; and an open-weave, long, filamentous green algal community. A monotonic increase in chl a biomass occurred as a function of near-bed velocities for the first of the two mucilaginous diatom communities investigated. No biomass-velocity relationship was found, however, with the second mucilaginous community, probably because the waters were highly enriched and mass transfer driven by molecular diffusion was probably high throughout the velocity gradient. Biomass was moderate at low velocities, peaked at near-bed velocities from 0.18 to 0.2 m·s−1 (∼0.40–0.45 m·s−1 mean column velocity), and then decreased at higher velocities in both of the stalked/ short filament communities of diatoms analyzed. With the long filamentous green algal communities, a monotonic reduction in biomass occurred as a function of increases in velocity. Proliferations greater than 100 mg·m−2 chl a occurred at low near-bed velocities (i.e. <0.2 m·s−1), after which biomass declined nearly exponentially as a function of increasing velocity to less than 10 mg·m−2 chl a at velocities greater than 0.4 m·s−1. These biomass-velocity trends support our hypothesis that community growth form determines periphytal responses to spatial variations in velocity within stream reaches.
Article
Knowledge of the population biology of invading species will often be necessary to develop effective management procedures and policies. But because invaders can have unexpected indirect effects in food webs, invasion ecologists need to integrate processes at the population level and other ecological levels. I describe a series of coordinated studies in New Zealand streams that address the effect of an exotic fish on individual behavior, population, community, and ecosystem patterns. Such case studies are important as an aid to the formulation of policy about invasions that are especially likely to become problematic. At the individual level, grazing invertebrates showed changes in behavior as a result of the introduction of brown trout ( Salmo trutta), a predator that exerts a very different selection pressure than do native fish. At the population level, trout have replaced nonmigratory galaxiid fish in some streams but not others, and have affected the distributions of crayfish and other large invertebrates. At the community level, trout have suppressed grazing pressure from invertebrates and are thus responsible for enhancing algal biomass and changing algal species composition. Finally, at the ecosystem level, essentially all annual production of invertebrates is consumed by trout ( but not by galaxiids), and algal primary productivity is six times higher in a trout stream. This leads, in turn, to an increased flux of nutrients from the water to the benthic community. The trout invasion has led to strong top-down control of community structure and ecosystem functioning via its effects on individual behavior and population distribution and abundance. Particular physiological, behavioral, and demographic traits of invaders can lead to profound ecosystem consequences that managers need to take into account.
Article
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.
Article
1. An experiment conducted in streamside channels was used to document the regrowth of grazed periphyton. Our objective was to determine the relative importance of current velocity, grazing duration, and grazer type in shaping the trajectory of algal and periphytic regrowth. 2. The grazing mayflies Baetis bicaudatus and Epeorus longimanus were used alone and in combination to create three grazing treatments at slow, medium and fast current (2–5, 15–20 and 30–40 cm s−1, respectively). Duration treatments consisted of 2, 4, 6, 8, 10 days of grazing. Chlorophyll a and ash-free dry mass (AFDM) accumulation on grazed tiles was measured (as periphytic AFDM and chlorophyll a, respectively) at 2, 4, 6, 8 and 10 days following the removal of grazers. 3. Chlorophyll a and AFDM was best predicted by interactions between current velocity, grazing duration and regrowth time. 4. The two grazer species did not differ in their effect on Chlorophyll a and AFDM during the period of periphytic regrowth that followed grazing. 5. Longer grazing duration reduced periphytic biomass, but also accelerated algal regrowth, and this growth enhancement was more pronounced at slower current velocities. 6. Data from this study suggest that herbivory can have important historical effects on periphytic accrual.
Article
The effects of hydraulic disturbances on the impact of two predatory benthic invertebrates on their prey were examined in a stream at two distinct spatial scales. At the scale of small habitat patches (0.0625 m2), hydraulic patch type was an important determinant of the microdistribution of prey and predators. Prey abundances were similar across all patch types at baseflow, but local densities were higher in patches identified as low-flow refugia after periods of high and fluctuating flow. The microdistribution pattern of predatory larvae of a caddisfly, Plectrocnemia conspersa, was similar to that of its prey, whereas predatory larvae of an alderfly, Sialis fuliginosa, did not shift their microdistribution significantly with discharge and were always most abundant in lowflow refugia. There was little evidence of an aggregative response of predators with prey, even though both predators and prey are mobile. Both predator species showed similar patch-specific patterns of per capita consumption rates: uniform consumption rates across hydraulic patch types at low and moderate flows, but highest in flow refugia during high flows. Species-specific patterns, however, were apparent in the magnitude and direction of differences between consumption rates during disturbance events, and in comparable patches at base flow: At high flow, consumption rates for P. conspersa were exaggerated (3.9 times higher) in flow refugia but at par in other patches; for S. fuliginosa they were at par in flow refugia but reduced in other patches (up to 3.3. times lower). These differences may be related to species-specific foraging behaviours (search vs ambush predators) and the influence of prey movements on feeding success. Using the patch-scale results only, it is difficult to predict the effects of physical disturbance on predation intensity at the larger scales of whole habitats, populations or communities. At the large scale (>200 m2), net predator impacts were estimated over the stream reach, using a spatially explicit model that accounts, in an additive way, for habitat heterogeneity and patch-specific responses of predators and prey. The relationship between predator impact over the whole reach and hydraulic disturbance differed for the two predators. The predator impact of S. fuliginosa decreased with increasing hydraulic disturbance, as predicted by the harsh-benign hypothesis. There was no directional trend for P. conspersa, however, and maximum predator impact may occur at intermediate disturbance levels. For the prey community in this stream, predation pressure from S. fuliginosa appears to fluctuate directly with the discharge hydrograph, whereas predation from P. conspersa may be more persistent. Flow refugia may play a dual role in the sructure of stream communities by preventing catastrophic mortality of animals (predators and prey) from physical forces during disturbances, and by maintaining (or perhaps increasing) predation pressure. Summing the effects of species interactions in small habitat patches to the larger scale of a whole stream reach indicates that the scale of approach influences the observed patterns and their implied underlying process.
Article
Playback experiments conducted in a Rocky Mountain, USA, stream determined whether predatory stonefly nymphs (Kogotus modestus; Plecoptera: PerlodiMae) used hydrodynamic cues to discriminate prey species from nonprey species. In the laboratory we recorded pressure wave patterns associated with swimming escape behavior of Baetis bicaudatus (Baetidae), the favored mayfly prey species, and those of a nonprey mayfly, Ephemerella infrequens (Ephemerellidae). We video taped the responses of 24-h starved Kogotus to Baetis playbacks, Ephemerella playbacks or no playbacks made by oscillating (or not) live mayflies (Ephemerella) or clear plastic models placed within in situ flow-through observation boxes. The probability of attacks per encounter with Baetis playbacks was highest and independent of the model type used, but Kogotus also showed an unexpected high probability of attacks per encounter when Ephemerella playbacks were made through live Ephemerella. Thus, Kogotus discriminated between Baetis and Ephemerella swimming patterns but only when playbacks were made through the plastic model. Kogotus never attacked motionless mayflies or motionless plastic models. We allowed some Kogotus to successfully capture one small Baetis immediately before playbacks, which resulted in a much higher probability of attacks per encounter with Baetis playbacks on either model and a heightened discrimination of prey versus nonprey playbacks. The probability of attacks per encounter by Kogotus with live Baetis swimming under similar experimental conditions was strikingly similar to its response to Baetis playbacks made by oscillating the plastic model after a successful capture. Order of playback presentation (Baetis first or Ephemerella first) did not influence predatory responses to mayfly swimming patterns. This study is the first to document the use of hydrodynamic cues by stream-dwelling predators for discrimination of prey from nonprey and provides a mechanism to explain selective predation by stoneflies on Baetis in nature.
Article
The impact that predators have on benthic, macroinvertebrate prey density in streams is unclear. While some studies show a strong effect of predators on prey density, others show little or no effect. Two factors appear to influence the detection of predator impact on prey density in streams. First, many field studies have small sample sizes and thus might be unable to detect treatment effects. Second, streams contain two broad classes of predators, invertebrates and vertebrates, which might have different impacts on prey density for a variety of reasons, including availability of refuge for prey and prey emigration responses to the two types of predators. In addition, predatory vertebrates have more complex prey communities than predatory invertebrates; this complexity might reduce the impact that predatory vertebrates have on prey because of indirect effects. I conducted a meta-analysis on the results of field studies that manipulate predator density in enclosures to determine (1) if predators have a significant impact on benthic prey density in streams, (2) if the impacts that predatory invertebrates and vertebrates have differ, and (3) if predatory vertebrates have different impacts on predatory prey versus herbivorous prey. The results of the meta-analysis suggest that on average predators have a significant negative effect on prey density, predatory invertebrates have a significantly stronger impact than predatory vertebrates, and predatory vertebrates do not differ in their impact on predatory versus herbivorous invertebrate prey. Three methodological variables (mesh size of enclosures, size of enclosures, and experimental duration) were examined to determine if cross correlations exist that may explain the differences in impact between predatory invertebrates and vertebrates. No correlation exists between mesh size and predator impact. Over all predators, no correlation exists between experimental duration and predator impact; however, within predatory invertebrates a correlation does exist between these variables. Also, a correlation was found between enclosure size and predator impact. This correlation potentially explains the difference in impact between predatory invertebrates and predatory vertebrates. Results of the meta-analysis suggest two important areas for future research: (1) manipulate both types of predators within the same system, and (2) examine their impacts on the same spatial scale.