Indirect food web interactions increase growth of an algivorous stream fish

1. We tested the hypothesis that indirect food web interactions between some common, invertivorous fishes and their prey would positively affect growth of an algivorous fish species. Specifically, we predicted that orangethroat darter (Etheostoma spectabile) would increase periphyton biomass via a top‐down pathway, indirectly enhancing growth of the algivorous central stoneroller minnow (Campostoma anomalum). Moreover, we predicted that sand shiner (Notropis stramineus) would increase periphyton biomass via a bottom‐up pathway and indirectly enhance growth of the stoneroller minnow. 2. In an 83‐day experiment in large, outdoor, stream mesocosms, we stocked two fish species per mesocosm (stoneroller and either darter or shiner), estimated the effects of the invertivorous and grazing fishes on periphyton biomass and estimated growth of the algivorous fish. 3. The darter consumed grazing invertebrates, indirectly increasing periphyton biomass. The shiner consumed terrestrial insects as predicted, but it did not affect periphyton biomass. 4. In support of our hypothesis, the darter indirectly enhanced stoneroller growth. As predicted, stonerollers consumed the increased periphyton in streams with darters, resulting in greater growth, condition and gut fullness compared to streams without darters. No indirect interaction was observed between stonerollers and shiners. 5. Our study suggests that some invertivorous fish species can positively affect growth of algivorous fishes through indirect food web interactions. Thus, in stream communities, it is possible that the loss of a single, invertivorous fish taxon could have negative consequences on algivorous fish populations via the removal of positive indirect food web interactions.     

Testing the differential predation hypothesis for the invasion of rusty crayfish in a stream community: laboratory and field experiments

1. We tested the hypothesis that the non‐native rusty crayfish (Orconectes rusticus) is less vulnerable to predators than two native species (O. propinquus and O. obscurus) it is replacing in streams of the upper Susquehanna River catchment (New York, U.S.A.). 2. We used laboratory experiments to compare species‐specific predation rates by smallmouth bass (Micropterus dolomieu) on crayfish of equal size and field tethering experiments to compare relative predation rates between native O. propinquus and non‐native O. rusticus by the suite of crayfish predators in our system. We predicted that crayfish size would affect predation rate but that predation rates would be equal among species when size was controlled. 3. We also tested for two potential artefacts of tethering. We tethered crayfish in cages to test whether the ability to escape from tethers is size specific, and we tested whether tethering alters differential predation among crayfish species by the smallmouth bass. 4. In the laboratory, smallmouth bass predation on rusty crayfish was lower than on either of the native species. In the field, predation risk for tethered crayfish was inversely related to size but did not differ among species when size was taken into account. Because rusty crayfish in the field experiment were slightly larger than the native species, as in nature, mortality was overall lower for the rusty crayfish. 5. In cages, smaller crayfish were more probably to escape from tethers than larger ones, an artefact that may partially confound results from our tethering experiments. Unexpectedly, tethering nearly eliminated predation by smallmouth bass. This artefact prevented us from testing for an interaction of tethering with differential predation and means that the results of field tethering experiments do not include any contribution from smallmouth bass predation. 6. Our experiments highlight the importance of explicitly considering potential artefacts that could confound results. 7. Our results indicate that differential predation contributes to the rusty crayfish's invasion of a stream community. In our study system, predation rates on rusty crayfish are lower than for native species mostly because of selection by predators for smaller crayfish; species‐specific characteristics such as behaviour that further reduce predation may also contribute, especially where smallmouth bass predation is important.     

Consequences of size structure in the prey for predator-prey dynamics: the composite functional response

1. Current formulations of functional responses assume that the prey is homogeneous and independent of intraspecific processes. Most prey populations consist of different coexisting size classes that often engage in asymmetrical intraspecific interactions, including cannibalism, which can lead to nonlinear interaction effects. This may be important as the size structure with the prey could alter the overall density-dependent predation rates. 2. In a field experiment with damselfly and dragonfly larvae, 16 treatments manipulated the density of a small prey stage, the presence of large conspecific prey and the presence of heterospecific predators. 3. Size structure in the prey (i.e. when both prey stages were present) decreased the impact of the predator on overall prey mortality by 25-48% at mid and high prey densities, possibly due to density-dependent size-structured cannibalism in the prey. The predation rates on small prey stages were determined by the interaction of large prey and predators. Predation rates increased with prey density in the absence of large prey, but predation rates were constant across densities when large conspecifics were present. 4. The functional response for unstructured prey followed a Holling type III model, but the predation rate for size-structured prey was completely different and followed a complex pattern that could not be explained with any standard functional response. 5. Using additional laboratory experiments, a mortality model was developed and parameterized. It showed that the overall prey mortality of size-structured prey can be adequately predicted with a composite functional response model that modelled the individual functional responses of each prey stage separately and accounted for their cannibalistic interaction. 6. Thus, treating a prey population as a homogeneous entity will lead to erroneous predictions in most real-world food webs. However, if we account for the effects of size structure and the intraspecific interactions on functional responses by treating size classes as different functional groups, it is possible to reliably predict the dynamics of size-structured predator-prey systems.     

Population genetic structure reveals terrestrial affinities for a headwater stream insect

1. The spatial distribution of stream‐dwelling organisms is often considered to be limited primarily according to the hierarchical structure of the hydrologic network, and previous conceptual models of population genetic structure have reflected this generality. Headwater specialists, however, are confined to short upstream sections of the network, and therefore are unlikely to respond in the same way as species with a broader range of habitat tolerance. 2. Here, we propose a model to describe spatial patterns of genetic diversity in headwater specialists with a limited ability for among‐stream dispersal. The headwater model predicts a partitioning of genetic variance according to higher‐elevation ‘islands’ of terrestrial habitat that provide required headwater stream conditions. The model therefore expects a geographic pattern of genetic variance similar to that expected for low‐dispersal terrestrial species occupying the adjacent habitat. 3. Using a 1032‐bp mitochondrial DNA fragment encompassing parts of the COI and COII genes, we demonstrate that Madrean Sky Islands populations of the giant water bug Abedus herberti conform to the proposed headwater model. Furthermore, they exhibit phylogeographic patterns broadly concordant with those shown for several terrestrial species in the region, including a major zone of discontinuity in the Chiricahua mountain range. 4. Overall, populations are highly isolated from one another, and a nested clade analysis suggested that A. herberti population structure, similarly to terrestrial Sky Islands species studied previously, has been influenced by Pleistocene climatic cycles causing expansion and contraction of temperate woodland habitat. 5. Because they have no ability to disperse among present‐day mountaintop habitat islands, A. herberti and other headwater species with limited dispersal ability are vulnerable to the projected increasing rate of climatic warming in this region.     

Aquatic insect diversity in a tropical Vietnamese stream in comparison with that in a temperate Korean stream

A comparative investigation on aquatic insect diversity was conducted in a tropical stream in Southeast Asia (the Dak Pri stream in southern Vietnam; stream orders II–V, two sites per stream order) with a reference temperate stream in Northeast Asia (the Gapyeong stream in central Korea) in March 2001 and April 2000, respectively. The numbers of aquatic insect taxa in Dak Pri stream (268 species, mostly undescribed, 230 genera, 91 families, and 9 orders; 110.5 ± 17.1 species per site) were about twice those in Gapyeong stream (133 species, 98 genera, 51 families, and 8 orders; 60.3 ± 8.5 species per site). Coleoptera, Trichoptera, Ephemeroptera, and Diptera were the major aquatic insect orders with high taxonomic richness, and Coleoptera, Odonata, and Hemiptera contributed to the higher degree of aquatic insect diversity in Dak Pri stream. The species diversity indices of Dak Pri stream (4.37 ± 0.19) were higher than those of Gapyeong stream (3.73 ± 0.42), whereas the dominance indices of Dak Pri stream (0.195 ± 0.046) were lower than those of Gapyeong stream (0.346 ± 0.113). Collector-gatherers were predominant in both streams; shredders were more abundant in Dak Pri stream while scrapers were more abundant in Gapyeong stream. Factors affecting the higher degree of aquatic insect diversity in Dak Pri stream are discussed.     

An analysis of changes in the invertebrate community along a southern New Zealand montane stream

Changes in benthic community structure at four sites along a flood-prone montane stream continuum in the South Island of New Zealand were analysed using rank correlation (Kendall's Tau and Spearman's r_s) and a community similarity index (S_c) developed initially for stream pollution assessment. Species richness was highest in a forested headwater stream, which is attributed to its relatively greater physical stability and habitat heterogeneity. Community similarity decreased with distance apart, and pairs of open and forested sites showed the greatest similarity. Species composition and relative abundance along the continuum changed least in the numerically dominant Ephemeroptera and Plecoptera, groups in which several species had broad habitat requirements. In contrast, a number of dipteran and trichopteran species had more restricted distributions. The more specialised niche requirements of some species in these orders are attributed to their generally small size, their ability to spin silk, and their holometabolous development. Physical rather than biological factors appeared to limit the distributions of most invertebrates, and there was little evidence of ecological segregation by either food resource partitioning or differences in life history patterns amongst many closely related species. Theories that stream communities are well ordered entities maximising the efficiency of energy utilisation cannot realistically be applied to such unstable stream systems.     

Experimental studies of exploitative competition in a grazing stream insect

Summary Field and laboratory experiments were conducted to determine whether intraspecific competition for food occurs during the larval stage of the periphyton-grazing caddisfly Glossosoma nigrior (Trichoptera: Glossosomatidae). Larvae were placed in field enclosures at densities less than, equal to, or greater than their natural densities. Most of these individuals began to pupate after 3 weeks, whereupon the mass of each individual was determined. Final mass declined significantly as larval densities increased, whereas neither developmental rate nor mortality/emigration rate was significantly affected by density manipulations. a supplemental experiment comparing the final mass of individuals grown at reduced densities in a laboratory stream with individuals from a natural stream bottom confirmed the results of the more extensive field experiment: reductions in density resulted in significant increases in final mass. Periphyton availability in field enclosures declined according to a negative exponential function as larval densities increased. Over the 25-fold range of larval densities used in these experiments, the final mass of individuals increased linearly with periphyton standing crops. This result suggests that Glossosoma larvae may compete for food even at densities below those employed in this study. Path analysis was used to explore the importance of indirect (i.e., exploitative) and direct (i.e, interference) mechanisms for the observed competitive effects. The analysis indicates that a model based solely on exploitation explains nearly as much of the variance in mass as a model incorporating both interference and exploitation.     

Effects of body size and predators on intracohort competition in wild and domesticated juvenile salmon in a stream

Competition between masu salmon (Oncorhynchus masou) of wild and aquaculture origin was investigated. Fry were individually marked and released in stream enclosures with and without a piscivorous predator. The aim was to assess the effects of predators and salmon body size on survival and growth of the two types of fish under natural conditions. The presence of predaceous Japanese huchen (Hucho perryi) resulted in lower mean growth rates of surviving fry. Relatively large fish survived and grew better than relatively small fish in the absence of predators, but not in their presence. This probably indicates an indirect effect of predation risk on within-cohort competitive behavior among salmon juveniles, with larger fish forced to give up their position as superior competitors. Domesticated fish survived in larger numbers and grew much faster than wild fish, irrespective of predator presence. Comparison with similar field studies revealed a pattern that the pre-experimental environment influenced the outcome of competition between wild and domesticated juvenile salmon. Domesticated fish were superior competitors even in the absence of an initial size advantage, which commonly gives a further advantage to hatchery-raised fish in natural streams. Therefore, caution dictates to avoid the release or escape of such fish into the wild.     

Exotic crayfish in a brown water stream: effects on juvenile trout, invertebrates and algae

SUMMARY 1. The impact of the introduced omnivorous signal crayfish (Pacifastacus leniusculus) on trout fry, macroinvertebrates and algae was evaluated in a brown water stream in southern Sweden using in situ enclosures. We also examined the gut content of all surviving crayfish in the enclosures. Two crayfish densities in addition to a control without crayfish were used in replicate enclosures (1.26 m²) in a 1‐month experiment. Additionally, 20 trout fry (Salmo trutta) were stocked in each enclosure to assess the effects of crayfish on trout survival and growth. 2. Detritus was the most common food item in crayfish guts. Animal fragments were also frequent while algae and macrophytes were scarcer. Crayfish exuviae were found in crayfish guts, but the frequency of cannibalism was low. 3. Trout survival in enclosures was positively related to water velocity but was unaffected by crayfish. 4. Total invertebrate biomass and taxon richness were lower in crayfish treatments. The biomass of all predatory invertebrate taxa was reduced but only three of six non‐predatory taxa were reduced in the crayfish treatments. 5. Epiphytic algal biomass (measured as chlorophyll a, on plastic strips) was not related to crayfish density, whereas the biomass of epilithic algae (measured as chlorophyll a) was enhanced by high water velocity and high crayfish density. The latter was possibly mediated via improved light and nutrient conditions, as active crayfish re‐suspend and/or remove detritus and senescent algal cells during periods of low water velocity. 6. We conclude that the introduced signal crayfish may affect stream communities directly and indirectly. Invaded communities will have reduced macroinvertebrate taxon richness and the signal crayfish will replace vulnerable invertebrate predators such as leeches. In streams that transport large amounts of sediment or organic matter, a high density of crayfish is likely to enhance benthic algal production through physical activity rather than via trophic effects.     

Seasonal diversity in aquatic insect communities in an all-year stream system

Summary The general pattern of the data in this study shows that a riffle zone carries more species and, usually, more organisms/m² than a slower current zone. The slower current zone, however, is still apparently able to support a larger number of organisms and species than a pool of standing water. In the West Fork of the Oak Creek system the dominant insects controlling diversity are the Trichoptera, particularly one species of Helicopsyche. Diversity in the fast current zone is high in the summer, and low during other seasons. Diversity in the slow current zone is highest in spring, but is also fairly high in summer. This reflects to a large extent the seasonal fluctuations in abundance of the dominant Trichopterans in the two habitats.     

Algal-periphyton population and community changes from zinc stress in stream mesocosms

Three treatments of zinc (0.05, 0.5, 1.0 mg Zn l⁻¹) and a control could be identified by different algal communities in outdoor, flow-through, stream mesocosms. Established communities were continuously exposed to Zn, and samples were collected on days 0, 2, 5, 10, 20 and 30 after treatment began. Experiments were conducted in spring, summer, and fall 1984. Control stream mesocosms could be identified by diatoms in all seasons. The 0.05 mg Zn l⁻¹ treatment could be identified by certain diatom taxa being more abundant than in the control in all seasons and by a filamentous green alga in summer and fall. The 0.5 mg Zn l⁻¹ treatment could be identified by a filamentous green alga in fall. The 1.0 mg Zn l⁻¹ treatment was dominated by unicellular green algae in all seasons and by a filamentous blue-green alga in summer. A similarity index (SIMI) indicated that Zn-stressed samples generally became less similar to control samples as Zn concentration increased from 0.05 to 1.0 mg Zn l⁻¹. Total biovolume-density of all taxa responded slower than individual taxa in spring and failed to distinguish between Zn treatments in summer and fall. Zinc bound to periphyton was much better than total Zn in water for identifying Zn treatments. Zinc treatments as low as 0.05 mg Zn l⁻¹ changed algal species composition despite 0.047 mg Zn l⁻¹ being the Criterion of the US Environmental Protection Agency for the 24-h average of total recoverable Zn.     

Indirect effects between shared prey: Predictions for

Suppression of a target prey by a predator can depend on its surrounding community, including the presence of nontarget, alternative prey. Basic theoretical models of two prey species that interact only via a shared predator predict that adding an alternative prey should increase predator numbers and ultimately lower target pest densities as compared to when the target pest is the only prey. While this is an alluring prediction, it does not explain the numerous responses empirically observed. To better understand and predict the indirect interactions produced by shared predation, we explore how additional prey species affect three broad ecological mechanisms, the predator's reproductive, movement, and functional responses. Specifically, we review current theoretical models of shared predation by focusing on these mechanisms, and make testable predictions about the effects of shared predation. We find that target predation is likely to be higher in the two prey system because of predator reproduction, especially when: predators are prey limited, alternative or total prey density is high, or alternative prey are available over time. Target predation may also be greater because of predator movement, but only under certain movement rules and spatial distributions. Predator foraging behavior is most likely to cause lower target predation in the two-prey system, when per capita predation is limited by something other than prey availability. It is clear from this review that no single theoretical generalization will accurately predict community-level effects for every system. However, we can provide testable hypotheses for future empirical and theoretical investigations of indirect interactions and help enhance their potential use in biological control.     

Human-induced changes in the trophic functioning of sandy beaches

The increasing anthropogenic disturbance on coastal ecosystems has threatened ecological interactions and ecosystems functioning. To investigate if human pressure affects the trophic structure of sandy beaches, mass-balanced models were applied on two Brazilian sandy beaches with distinct human impact degree. The food web models included detritus, phytoplankton, macroinvertebrates, fish and seabirds. Macroinvertebrates in non-urbanized sectors represented the highest production fraction consumed by predators. The energy transfer and the cycling indicator were more efficient in the non-urbanized sectors than urbanized ones. The results indicate that macroinvertebrates sensitive to direct human impact such as trampling are important to the trophic functioning of sandy beaches. Establishing a threshold for the number of beachgoers or dispersing recreational activities to avoid crowds may be tangible ways to mitigate the trampling impact on macroinvertebrates.     

Effects of aquatic plant management on stream metabolism and oxygen balance in streams

1. In unshaded, nutrient-rich streams, prolific growth of stream macrophytes often results in flows that over-top the banks and in high primary production and respiration that may result in extreme diel variations in dissolved oxygen. Consequently, water protection authorities commonly remove macrophytes periodically.
2. We investigated the effect of plant removal on stream metabolism and oxygen balance in two Swiss streams with a high macrophyte biomass. We monitored the concentration of dissolved oxygen before and after macrophytes were removed by cutting and dredging, and calculated rates of gross primary production and ecosystem respiration by means of diel oxygen curves.
3. The removal of plants, which had reached a dry biomass of 320–420 g m−2 immediately before plant removal, had a different impact on stream metabolism in the two streams. In the first (plants removed in May), neither primary production nor ecosystem respiration were significantly affected. In the second (plants removed in late July), gross primary production and ecosystem respiration were reduced by about 70%. In this latter stream gross primary production increased in the first 2 weeks after plant removal but never recovered to pre-disturbance levels.
4. The removal of plants coincided with only a moderate increase in nocturnal oxygen concentration (+1 mg L−1). This, and the rapid partial recovery of stream metabolism in the second stream, suggests that an increase in the oxygen concentration after plant cutting is transient in unshaded, nutrient-rich streams.     

Macroinvertebrate community structure in relation to environmental variables in a Swiss glacial stream

1. Benthic macroinvertebrate distribution was examined in relation to channel characteristics (including stability), substratum, hydraulic variables, primary production (chlorophyll a ) and coarse particular organic matter (CPOM) in an alpine glacial stream, the Mutt (Upper Rhône valley, Switzerland). Co-inertia analysis and canonical correspondence analysis were used to identify the major environmental gradients influencing community variations.
2. The Mutt (length: 3.6 km, altitudinal range: 1800–3099 m a.s.l.) exhibited typical characteristics of a kryal stream. Average summer temperature remained below 2 °C immediately downstream from the snout but was on average 5 °C higher 1700 m downstream. Seasonal variations in water sources were evidenced by the high late-summer (September) contribution of groundwater with increased conductivity.
3. Sixty-six taxa were recorded from the five reaches sampled at three periods (snowmelt, ice melt and low water in late summer) in 1996 and 1997, of which 29 were Chironomidae. Three taxa of Diamesinae were the first colonizers of the stream below the glacier, but 16 taxa, including Ephemeroptera, Plecoptera and Trichoptera, were already recorded 200 m downstream. Water depth, channel slope and Pfankuch's Index of channel stability were strongly correlated with the longitudinal faunal gradient. Maximum temperature, current velocity and water conductivity were also correlated, but to a lesser extent.
4. The rapid incorporation of non-chironomid taxa into the stream community represented a departure from Milner & Petts's (1994) conceptual model of invertebrate succession downstream of glacial margins. The results confirmed that glacial stream communities are primarily driven by physical determinants.     

Bacterial community dynamics in the hyporheic zone of an intermittent stream

The dynamics of in situ bacterial communities in the hyporheic zone of an intermittent stream were described in high spatiotemporal detail. We assessed community dynamics in stream sediments and interstitial pore water over a two-year period using terminal-restriction fragment length polymorphism. Here, we show that sediments remained saturated despite months of drought and limited hydrologic connectivity. The intermittency of stream surface water affected interstitial pore water communities more than hyporheic sediment communities. Seasonal changes in bacterial community composition was significantly associated with water intermittency, phosphate concentrations, temperature, nitrate and dissolved organic carbon (DOC) concentrations. During periods of low- to no-surface water, communities changed from being rich in operational taxonomic units (OTUs) in isolated surface pools, to a few OTUs overall, including an overall decline in both common and rare taxa. Individual OTUs were compared between porewater and sediments. A total of 19% of identified OTUs existed in both porewater and sediment samples, suggesting that bacteria use hyporheic sediments as a type of refuge from dessication, transported through hydrologically connected pore spaces. Stream intermittency impacted bacterial diversity on rapid timescales (that is, within days), below-ground and in the hyporheic zone. Owing to the coupling of intermittent streams to the surrounding watershed, we stress the importance of understanding connectivity at the pore scale, consequences for below-ground and above-ground biodiversity and nutrient processing, and across both short- and long-time periods (that is, days to months to years).     

Non-lethal effects of predation in birds

Predators can affect individual fitness and population and community processes through lethal effects (direct consumption or ‘density’ effects), where prey is consumed, or through non‐lethal effects (trait‐mediated effects or interactions), where behavioural compensation to predation risk occurs, such as animals avoiding areas of high predation risk. Studies of invertebrates, fish and amphibians have shown that non‐lethal effects may be larger than lethal effects in determining the behaviour, condition, density and distribution of animals over a range of trophic levels. Although non‐lethal effects have been well described in the behavioural ecology of birds (and also mammals) within the context of anti‐predation behaviour, their role relative to lethal effects is probably underestimated. Birds show many behavioural and physiological changes to reduce direct mortality from predation and these are likely to have negative effects on other aspects of their fitness and population dynamics, as well as affecting the ecology of their own prey and their predators. As a consequence, the effects of predation in birds are best measured by trade‐offs between maximizing instantaneous survival in the presence of predators and acquiring or maintaining resources for long‐term survival or reproduction. Because avoiding predation imposes foraging costs, and foraging behaviour is relatively easy to measure in birds, the foraging–predation risk trade‐off is probably an effective framework for understanding the importance of non‐lethal effects, and so the population and community effects of predation risk in birds and other animals. Using a trade‐off approach allows us to predict better how changes in predator density will impact on population and community dynamics, and how animals perceive and respond to predation risk, when non‐lethal effects decouple the relationship between predator density and direct mortality rate. The trade‐off approach also allows us to identify where predation risk is structuring communities because of avoidance of predators, even when this results in no observable direct mortality rate.