Seasonal variation in the trophic structure of a spatial prey subsidy linking aquatic and terrestrial food webs: adult aquatic insects

Research over the past decade has established spatial resource subsidies as important determinants of food web dynamics. However, most empirical studies have considered the role of subsidies only in terms of magnitude, ignoring an important property of subsidies that may affect their impact in recipient food webs: the trophic structure of the subsidy relative to in situ resources. This may be especially important when subsidies are composed of organisms, as opposed to nutrient subsidies, because the trophic position of subsidy organisms may differ from in situ prey. I explored the relative magnitude and trophic structure of a cross-habitat prey subsidy, adult aquatic insects, in terrestrial habitats along three streams in the south–central United States. Overall, adult aquatic insects contributed more than one-third of potential insect prey abundance and biomass to the terrestrial habitat. This contribution peaked along a permanent spring stream, reaching as high as 94% of abundance and 86% of biomass in winter. Trophic structure of adult aquatic and terrestrial insects differed. Nearly all adult aquatic insects were non-consumers as adults, whereas all but one taxon of terrestrial insects were consumers. Such a difference created a strong relationship between the relative contribution of the prey subsidy and the trophic structure of the prey assemblage: as the proportion of adult aquatic insects increased, the proportion of consumers in the prey assemblage declined. Specific effects varied seasonally and with distance from the stream as the taxonomic composition of the subsidy changed, but general patterns were consistent. These findings show that adult aquatic insect subsidies to riparian food webs not only elevate prey availability, but also alter the trophic structure of the entire winged insect prey assemblage.     

Variation in active and passive resource inputs to experimental pools: mechanisms and possible consequences for food webs

1. Cross‐ecosystem movements of resources, including detritus, nutrients and living prey, can strongly influence food web dynamics in recipient habitats. Variation in resource inputs is thought to be driven by factors external to the recipient habitat (e.g. donor habitat productivity and boundary conditions). However, inputs of or by ‘active’ living resources may be strongly influenced by recipient habitat quality when organisms exhibit behavioural habitat selection when crossing ecosystem boundaries. 2. To examine whether behavioural responses to recipient habitat quality alter the relative inputs of ‘active’ living and ‘passive’ detrital resources to recipient food webs, we manipulated the presence of caged predatory fish and measured biomass, energy and organic content of inputs to outdoor experimental pools of adult aquatic insects, frog eggs, terrestrial plant matter and terrestrial arthropods. 3. Caged fish reduced the biomass, energy and organic matter donated to pools by tree frog eggs by ～70%, but did not alter insect colonisation or passive allochthonous inputs of terrestrial arthropods and plant material. Terrestrial plant matter and adult aquatic insects provided the most energy and organic matter inputs to the pools (40–50%), while terrestrial arthropods provided the least (7%). Inputs of frog egg were relatively small but varied considerably among pools and over time (3%, range = 0–20%). Absolute and proportional amounts varied by input type. 4. Aquatic predators can strongly affect the magnitude of active, but not passive, inputs and that the effect of recipient habitat quality on active inputs is variable. Furthermore, some active inputs (i.e. aquatic insect colonists) can provide similar amounts of energy and organic matter as passive inputs of terrestrial plant matter, which are well known to be important. Because inputs differ in quality and the trophic level they subsidise, proportional changes in input type could have strong effects on recipient food webs. 5. Cross‐ecosystem resource inputs have previously been characterised as donor‐controlled. However, control by the recipient food web could lead to greater feedback between resource flow and consumer dynamics than has been appreciated so far.     

Invading rainbow trout usurp a terrestrial prey subsidy from native charr and reduce their growth and abundance

Movements of prey organisms across ecosystem boundaries often subsidize consumer populations in adjacent habitats. Human disturbances such as habitat degradation or non-native species invasions may alter the characteristics or fate of these prey subsidies, but few studies have measured the direct effects of this disruption on the growth and local abundance of predators in recipient habitats. Here we present evidence, obtained from a combined experimental and comparative study in northern Japan, that an invading stream fish usurped the flux of allochthonous prey to a native fish, consequently altering the diet and reducing the growth and abundance of the native species. A large-scale field experiment showed that excluding terrestrial invertebrates that fell into the stream with a mesh greenhouse reduced terrestrial prey in diets of native Dolly Varden charr (Salvelinus malma) by 46–70%, and reduced their growth by 25% over six weeks. However, when nonnative rainbow trout (Oncorhynchus mykiss) were introduced, they monopolized these prey and caused an even greater reduction of terrestrial prey in charr diets of 82–93%, and reduced charr growth by 31% over the same period. Adding both greenhouse and rainbow trout treatments together produced similar results to adding either alone. Results from a comparative field study of six other stream sites in the region corroborated the experimental findings, showing that at invaded sites rainbow trout usurped the terrestrial prey subsidy, causing a more than 75% decrease in the biomass of terrestrial invertebrates in Dolly Varden diets and forcing them to shift their foraging to insects on the stream bottom. Moreover, at sites with even low densities of rainbow trout, biomass of Dolly Varden was more than 75% lower than at sites without rainbow trout. Disruption of resource fluxes between habitats may be a common, but unidentified, consequence of invasions, and an additional mechanism contributing to the loss of native species Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Prey Vulnerability Limits Top-Down Control and Alters Reciprocal Feedbacks in a Subsidized Model Food Web

Resource subsidies increase the productivity of recipient food webs and can affect ecosystem dynamics. Subsidies of prey often support elevated predator biomass which may intensify top-down control and reduce the flow of reciprocal subsidies into adjacent ecosystems. However, top-down control in subsidized food webs may be limited if primary consumers posses morphological or behavioral traits that limit vulnerability to predation. In forested streams, terrestrial prey support high predator biomass creating the potential for strong top-down control, however armored primary consumers often dominate the invertebrate assemblage. Using empirically based simulation models, we tested the response of stream food webs to variations in subsidy magnitude, prey vulnerability, and the presence of two top predators. While terrestrial prey inputs increased predator biomass (+12%), the presence of armored primary consumers inhibited top-down control, and diverted most aquatic energy (∼75%) into the riparian forest through aquatic insect emergence. Food webs without armored invertebrates experienced strong trophic cascades, resulting in higher algal (∼50%) and detrital (∼1600%) biomass, and reduced insect emergence (−90%). These results suggest prey vulnerability can mediate food web responses to subsidies, and that top-down control can be arrested even when predator-invulnerable consumers are uncommon (20%) regardless of the level of subsidy.     

What predicts the use by brook trout (Salvelinus fontinalis) of terrestrial invertebrate subsidies in headwater streams?

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Differential support of lake food webs by three types of terrestrial organic carbon

Organic carbon inputs from outside of ecosystem boundaries potentially subsidize recipient food webs. Four whole-lake additions of dissolved inorganic ¹³C were made to reveal the pathways of subsidies to lakes from terrestrial dissolved organic carbon (t-DOC), terrestrial particulate organic carbon (t-POC) and terrestrial prey items. Terrestrial DOC, the largest input, was a major subsidy of pelagic bacterial respiration, but little of this bacterial C was passed up the food web. Zooplankton received <2% of their C from the t-DOC to bacteria pathway. Terrestrial POC significantly subsidized the production of both zooplankton and benthic invertebrates, and was passed up the food web to Chaoborus and fishes. This route supplied 33–73% of carbon flow to zooplankton and 20–50% to fishes in non-fertilized lakes. Terrestrial prey, by far the smallest input, provided some fishes with >20% of their carbon. The results show that impacts of cross-ecosystem subsidies depend on characteristics of the imported material, the route of entry into the food web, the types of consumers present, and the productivity of the recipient system.     

Predator diversity effects cascade across an ecosystem boundary

Food webs in different ecosystems are often connected through spatial resource subsidies. As a result, biodiversity effects in one ecosystem may cascade to adjacent ecosystems. I tested the hypothesis that aquatic predator diversity effects cascade to terrestrial food webs by altering a prey subsidy (biomass and trophic structure of emerging aquatic insects) entering terrestrial food webs, in turn altering the distribution of a terrestrial consumer (spider) that feeds on emerging aquatic insects. Fish presence, but not diversity, altered the trophic structure of emerging aquatic insects by strongly reducing the biomass of emerging predators (dragonflies) relative to non‐feeding taxa (chironomid midges). Fish diversity reduced emerging insect biomass through enhanced effects on the most common prey taxa: predatory dragonflies Pantala flavescens and non‐feeding chironomids. Terrestrial spiders (Tetragnathidae) primarily captured emerging chironomids, which were reduced in the high richness (3 spp.) treatment relative to the 1 and 2 species treatments. As a result, terrestrial spider abundance was lower above pools with high fish richness (3 species) than pools with 1 and 2 species. Synergistic predation effects were mostly limited to the high richness treatment, in which fish occupied each level of vertical microhabitat in the water‐column (benthic, middle, surface). This study demonstrates that predator diversity effects are not limited to the habitat of the predator, but can propagate to adjacent ecosystems, and demonstrates the utility of using simple predator functional traits (foraging domain) to more accurately predict the direction of predator diversity effects.     

Shoreline urbanization reduces terrestrial insect subsidies to fishes in North American lakes

Despite growing recognition of the energetic connections between aquatic and riparian habitats of streams and lakes, there have been few efforts to quantify the importance of terrestrial insect subsidies to fish in lakes. Further, it is unclear whether lakeshore urbanization alters the magnitude of these fluxes. Because lakeshore development has been found to be negatively correlated with riparian vegetation that serves as habitat for terrestrial invertebrates, we expected that shoreline urbanization would reduce the prevalence of terrestrial invertebrates in fish diets. We quantified the effects of lakeshore urbanization on terrestrial insect subsidies to fish at three scales: a focused comparison of annual patterns in four lakes in the Pacific Northwest, a one‐time field survey of 28 Pacific Northwest lakes, and a literature survey of 24 North American lakes. At all geographical scales, terrestrial invertebrate subsidies to fish were negatively correlated with shoreline development. Terrestrial insects comprised up to 100% of fish diet mass in undeveloped lakes, versus an average of 2% of fish diet mass in developed lakes. Trout, Oncorhynchus spp., in undeveloped lakes had an average of 50% greater daily energy intake, up to 50% of which was represented by terrestrial prey. Temporal variability of the terrestrial subsidy suggests that these inputs are distinctly pulsed, and this subsidy is absent or temporally rare in undeveloped lakes.     

Consumer-specific responses to riverine subsidy pulses in a riparian arthropod assemblage

1. Aquatic resource fluxes from streams can provide significant subsidies for riparian consumers. Because aquatic resource fluxes can be highly variable in space and time, the subsidy efficiency (i.e. transfer to the recipient food web) is controlled by the short‐term aggregative response of riparian consumers. 2. Field manipulations of stream‐derived invertebrate prey subsidies were used to examine specific aggregative responses of ground‐dwelling arthropods to riverine subsidy pulses in a braided‐river (Tagliamento River, NE Italy). Subsidy manipulation comprised short‐term reductions of natural stream‐derived subsidies and increased subsidies of stream‐derived invertebrate prey during four seasons. 3. We hypothesised that specific aggregative responses of riparian arthropods depend on their specialisation on aquatic insects which was inferred from stable isotope analysis. Natural riverine subsidy sources including aquatic insect emergence and surface‐drifting organisms were quantified. 4. Arthropods responded significantly with a reduction in abundance by 51%, at reduced subsidies and an increase by 110% at increased subsidies, when averaged over all seasons. Different arthropod taxa responded differently to subsidy manipulations in relation to their specialisation on aquatic subsidies: ground beetles with a diet consisting predominantly of aquatic insects responded only to subsidy reductions, indicating that their local abundance was not limited by natural stream‐derived subsidies; lycosid spiders with a partly aquatic diet showed no significant response; and ants, although relying on a terrestrial diet, responded positively to added stream‐derived invertebrate prey, indicating that stranding of surface‐drifting terrestrial invertebrates represented an important subsidy pathway. 5. Ground beetles and lycosid spiders were seasonally separated in their use of aquatic subsidies. Results indicate that the life‐history characteristics of riparian consumers can control the subsidy efficiency for the recipient community. By the effective uptake of pulsed riverine‐derived subsidies, riparian arthropods can enhance the transfer of riverine food sources to the riparian food web.     

Tangled webs: reciprocal flows of invertebrate prey link streams and riparian zones

1. Streams and their adjacent riparian zones are closely linked by reciprocal flows of invertebrate prey. We review characteristics of these prey subsidies and their strong direct and indirect effects on consumers and recipient food webs. 2. Fluxes of terrestrial invertebrates to streams can provide up to half the annual energy budget for drift‐feeding fishes such as salmonids, despite the fact that input occurs principally in summer. Inputs appear highest from closed‐canopy riparian zones with deciduous vegetation and vary markedly with invertebrate phenology and weather. Two field experiments that manipulated this prey subsidy showed that it affected both foraging and local abundance of stream fishes. 3. Emergence of adult insects from streams can constitute a substantial export of benthic production to riparian consumers such as birds, bats, lizards, and spiders, and contributes 25–100% of the energy or carbon to such species. Emergence typically peaks in early summer in the temperate zone, but also provides a low‐level flux from autumn to spring in ice‐free streams. This flux varies with in‐stream productivity, and declines exponentially with distance from the stream edge. Some predators aggregate near streams and forage on these prey during periods of peak emergence, whereas others rely on the lower subsidy from autumn through spring when terrestrial prey are scarce. Several field experiments that manipulated this subsidy showed that it affected the short‐term behaviour, growth, and abundance of terrestrial consumers. 4. Reciprocal prey subsidies also have important indirect effects on both stream and riparian food webs. Theory predicts that allochthonous prey should increase density of subsidised predators, thereby increasing predation on in situ prey and causing a negative indirect effect via apparent competition. However, short‐term experiments have produced either positive or negative indirect effects. These contrasting results may be due to characteristics of the subsidies and individual consumers, but could also result from differences in experimental designs. 5. New study approaches are needed to better determine the direct and indirect effects of reciprocal prey subsidies. Experiments coupled with comparative research will be required to measure their effects on individual consumer fitness and population demographics. Future work should investigate whether reciprocal prey fluxes stabilise linked stream–riparian ecosystems, explore how landscape context affects the magnitude and importance of subsidies, and determine how impacts of human disturbance can propagate between streams and riparian zones via these trophic linkages. Study of these reciprocal connections is helping to define a more holistic perspective of catchments, and has the potential to shape new directions for ecology in general.     

Species-specific use of allochthonous resources by ground beetles (Carabidae) at a river–land interface

A variety of organisms mediate river–terrestrial linkages through spatial subsidies. However, most empirical studies have classified organisms rather broadly (e.g., by functional group or taxonomic family) and have dismissed species-level linkages at the interface of ecosystems. Here, we show how allochthonous resource use varies among taxonomically similar species of ground beetles (family Carabidae) across seasons (June–September). We investigated seasonal shifts in the distribution of five beetle species and their dietary responses to spatial subsidies (emerging aquatic insects) in a Japanese braided river. Despite their taxonomic closeness, the ground beetles showed species-specific responses to spatial subsidies, and beetle distribution patterns tended to coincide with their diets. Overall, 1–56% of ground beetle diets were derived from aquatic prey. One genus (Bembidion spp.) mainly consumed aquatic prey, while three species fed primarily on terrestrial prey across all seasons. However, one species (Lithochlaenius noguchii) showed shifts in its diet from aquatic to terrestrial prey according to subsidy availability. The observed variation in allochthonous resource use was likely related to species-specific foraging modes, physiological tolerance to dry conditions, and interspecific competition. Our findings suggest that considering species-specific interactions is necessary to fully understand cross-system interactions and recipient food-web dynamics.     

Spiders and subsidies: results from the riparian zone of a coastal temperate rainforest

1. Aquatic insects emerging from streams can provide an important energy subsidy to recipient consumers such as riparian web-building spiders. This subsidy has been hypothesized to be of little importance where the primary productivity of the recipient habitat exceeds that of the donor habitat. 2. To test this hypothesis, we manipulated emerging stream insect abundance in a productive riparian rainforest in a replicated design using greenhouse-type exclosures, contrasted with unmanipulated stream reaches (four exclosures on two streams). 3. Experimental exclosures resulted in a 62.9% decrease in aquatic insect abundance in exclusion reaches compared with control reaches. The overall density of riparian spiders was significantly positively correlated with aquatic insect abundances. Horizontal orb weavers (Tetragnathidae) showed a strong response to aquatic insect reduction - abundance at exclosure sites was 57% lower than at control sites. Several spider families that have not been associated with tracking aquatic insect subsidies also showed significantly decreased abundance when aquatic insects were reduced. 4. This result is contrary to predictions of weak subsidy effects where recipient net primary productivity is high. These results suggest that predicting the importance of resource subsidies for food webs requires a focus on the relative abundance of subsidy materials in recipient and donor habitats and not simply on the total flux of energy between systems.     

Contributions of detrital subsidies to aboveground spiders during secondary succession, revealed by radiocarbon and stable isotope signatures

Prey subsidies originating from detritus add nutrients and energy to arboreal communities. Measurement of this subsidy is required in the understanding of how food web dynamics respond to changes in surrounding environments. Shrub spiders are one of the key predators involved in food web coupling. We evaluate the effects of potential changes in prey availabilities during secondary succession on the contribution of subsidy from detrital food webs to shrub spiders and how different spider feeding guilds used the subsidy of prey from detrital food webs. We measured the relative importance of the subsidy for the spider feeding guilds, using the ratios of stable isotopes of C (δ¹³C), and N (δ¹⁵N) and C isotope discrimination (Δ¹⁴C). Diet age was calculated from Δ¹⁴C values, because old diet ages of spiders indicate that the spiders consume prey from detrital food sources. Dominant aerial prey (Diptera) had a distinctively old diet age compared with arboreal prey, which indicates that aerial prey were subsidized from detrital food webs. Sit-and-wait spiders tended to have an older diet age than active hunting spiders, which indicates that sit-and-wait spiders depended more on subsidies. Diet age varied only slightly for spiders in stands of different ages, indicating that rates at which spiders use grazing and detrital prey are probably determined more by foraging strategies and not by stand age. A dominance of sit-and-wait predators will lead to higher detrital subsidy inputs in shrub habitats. This study highlights the effect of shrub spider community structure (feeding guild composition) on the volume of the subsidy received from the detrital food web.     

Responses to River Inundation Pressures Control Prey Selection of Riparian Beetles

Background

Riparian habitats are subjected to frequent inundation (flooding) and are characterised by food webs that exhibit variability in aquatic/terrestrial subsidies across the ecotone. The strength of this subsidy in active riparian floodplains is thought to underpin local biodiversity. Terrestrial invertebrates dominate the fauna, exhibiting traits that allow exploitation of variable aquatic subsidies while reducing inundation pressures, leading to inter-species micro-spatial positioning. The effect these strategies have on prey selection is not known. This study hypothesised that plasticity in prey choice from either aquatic or terrestrial sources is an important trait linked to inundation tolerance and avoidance.

Method/Principal Findings

We used hydrological, isotopic and habitat analyses to investigate the diet of riparian Coleoptera in relation to inundation risk and relative spatial positioning in the floodplain. The study examined patch scale and longitudinal changes in utilisation of the aquatic subsidy according to species traits. Prey sourced from terrestrial or emerging/stranded aquatic invertebrates varied in relation to traits for inundation avoidance or tolerance strategies. Traits that favoured rapid dispersal corresponded with highest proportions of aquatic prey, with behavioural traits further predicting uptake. Less able dispersers showed minimal use of aquatic subsidy and switched to a terrestrial diet under moderate inundation pressures. All trait groups showed a seasonal shift in diet towards terrestrial prey in the early spring. Prey selection became exaggerated towards aquatic prey in downstream samples.

Conclusions/Significance

Our results suggest that partitioning of resources and habitat creates overlapping niches that increase the processing of external subsidies in riparian habitats. By demonstrating functional complexity, this work advances understanding of floodplain ecosystem processes and highlights the importance of hydrological variability. With an increasing interest in reconnecting rivers to their floodplains, these invertebrates represent a key functional element in ensuring that such reconnections have demonstrable ecological value.     

Forest-stream linkages: effects of terrestrial invertebrate input and light on diet and growth of brown trout (Salmo trutta) in a boreal forest stream

Subsidies of energy and material from the riparian zone have large impacts on recipient stream habitats. Human-induced changes, such as deforestation, may profoundly affect these pathways. However, the strength of individual factors on stream ecosystems is poorly understood since the factors involved often interact in complex ways. We isolated two of these factors, manipulating the flux of terrestrial input and the intensity of light in a 2×2 factorial design, where we followed the growth and diet of two size-classes of brown trout (Salmo trutta) and the development of periphyton, grazer macroinvertebrates, terrestrial invertebrate inputs, and drift in twelve 20 m long enclosed stream reaches in a five-month-long experiment in a boreal coniferous forest stream. We found that light intensity, which was artificially increased 2.5 times above ambient levels, had an effect on grazer density, but no detectable effect on chlorophyll a biomass. We also found a seasonal effect on the amount of drift and that the reduction of terrestrial prey input, accomplished by covering enclosures with transparent plastic, had a negative impact on the amount of terrestrial invertebrates in the drift. Further, trout growth was strongly seasonal and followed the same pattern as drift biomass, and the reduction of terrestrial prey input had a negative effect on trout growth. Diet analysis was consistent with growth differences, showing that trout in open enclosures consumed relatively more terrestrial prey in summer than trout living in covered enclosures. We also predicted ontogenetic differences in the diet and growth of old and young trout, where we expected old fish to be more affected by the terrestrial prey reduction, but we found little evidence of ontogenetic differences. Overall, our results showed that reduced terrestrial prey inputs, as would be expected from forest harvesting, shaped differences in the growth and diet of the top predator, brown trout.     

Influence of fishes on macroinvertebrate communities and insect emergence production in intermittent stream refuges

1. Drying intermittent stream networks often have permanent water refuges that are important for recolonisation. These habitats may be hotspots for interactions between fishes and invertebrates as they become isolated, but densities and diversity of fishes in these refuges can be highly variable across time and space.
2. Insect emergence from streams provides energy and nutrient subsidies to riparian habitats. The magnitude of such subsidies may be influenced by in-stream predators such as fishes.
3. We examined whether benthic macroinvertebrate communities, emerging adult insects, and algal biomass in permanent grassland stream pools differed among sites with naturally varying densities of fishes. We also manipulated fish densities in a mesocosm experiment to address how fishes might affect colonisation during recovery from hydrologic disturbance.
4. Fish biomass had a negative impact on invertebrate abundance, but not biomass or taxa richness, in natural pools. Total fish biomass was not correlated with total insect emergence in natural pools, but orangethroat darter (Etheostoma spectabile) biomass was inversely correlated with emerging Chironomidae biomass and individual midge body size. The interaction in our models between predatory fish biomass and date suggested that fishes may also delay insect emergence from natural pools, altering the timing of aquatic–terrestrial subsidies.
5. There was an increase over time in algal biomass (chlorophyll-a) in mesocosms, but this did not differ among fish density treatments. Regardless, fish presence in mesocosms reduced the abundance of colonising insects and total invertebrate biomass. Mesocosm invertebrate communities in treatments without fishes were characterised by more Chironomidae, Culicidae, and Corduliidae.
6. Results suggest that fishes influence invertebrates in habitats that represent important refuges during hydrologic disturbance, hot spots for subsidy exports to riparian food webs, and source areas for colonists during recovery from hydrologic disturbance. Fish effects in these systems include decreasing invertebrate abundance, shifting community structure, and altering patterns of invertebrate emergence and colonisation.

     

Reduced dispersal propensity in the wingless waterstrider Aquarius najas in a highly fragmented landscape

Dispersal behaviour of animals in fragmented habitats has generated intensive theoretical attention but empirical data on the evolution of dispersal are still relatively scarce. Theory predicts reduced dispersal propensity in small and isolated habitat patches. We tested these predictions in the waterstrider Aquarius najas, a wingless species with special habitat demands. Flightlessness constrains insect dispersal and as a stream specialist A. najas cannot survive on still water. Lakes therefore represent a dispersal barrier for this species. We measured dispersal propensity of the waterstrider A. najas which originated from ten fragmented populations. In the experiment, we transplanted laboratory-grown individuals to the field. We did not find differences between sex in dispersal propensity. However, we found that waterstriders that originated from small and isolated patches moved less than individuals from large and more continuous habitats. This suggests that the cost of dispersal over hostile surrounding habitats may be high. We conclude that a low likelihood of dispersal in A. najas is an adaptation to small and isolated stream habitats.     

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

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

Grazing management influences the subsidy of terrestrial prey to trout in central Rocky Mountain streams (USA)

1. Research in forest and grassland ecosystems indicates that terrestrial invertebrates that fall into streams can be an important prey resource for fish, providing about 50% of their annual energy and having strong effects on growth and abundance. However, the indirect effects of land uses like cattle grazing on this important prey subsidy for stream salmonids are unclear. 2. During summer 2007, we compared the effects of three commonly used grazing systems on terrestrial invertebrate inputs to streams in northern Colorado and their use by trout. Cattle graze individual pastures for about 120 days under traditional season‐long grazing (SLG), about 35–45 days under simple rotational grazing and 10–20 days under intensive rotational grazing in this region. We also compared these effects to a fourth group of sites grazed only by wildlife (i.e. no livestock use). 3. Overall, rotational grazing management (either simple or intensive), resulted in more riparian vegetation, greater inputs of terrestrial invertebrates, greater biomass of terrestrial invertebrate prey in trout diets, a higher input compared to trout metabolic demand and more trout biomass than SLG. However, these differences were frequently not statistically significant owing to high variability, especially for trout diets and biomass. 4. Despite the inherent variability, riparian vegetation and terrestrial invertebrates entering streams and in trout diets at sites managed for rotational grazing were similar to sites managed for wildlife grazing only. 5. These results indicate that rotational grazing systems can be effective for maintaining levels of terrestrial invertebrate subsidies to streams necessary to support robust trout populations. However, factors influencing the effect of riparian grazing on stream subsidies are both spatially variable and complex, owing to differences in microclimate, invertebrate and plant populations and the efforts of ranchers to tailor grazing systems to specific riparian pastures.     

Resource subsidies between stream and terrestrial ecosystems under global change

Streams and adjacent terrestrial ecosystems are characterized by permeable boundaries that are crossed by resource subsidies. Although the importance of these subsidies for riverine ecosystems is increasingly recognized, little is known about how they may be influenced by global environmental change. Drawing from available evidence, in this review we propose a conceptual framework to evaluate the effects of global change on the quality and spatiotemporal dynamics of stream–terrestrial subsidies. We illustrate how changes to hydrological and temperature regimes, atmospheric CO₂ concentration, land use and the distribution of nonindigenous species can influence subsidy fluxes by affecting the biology and ecology of donor and recipient systems and the physical characteristics of stream–riparian boundaries. Climate‐driven changes in the physiology and phenology of organisms with complex life cycles will influence their development time, body size and emergence patterns, with consequences for adjacent terrestrial consumers. Also, novel species interactions can modify subsidy dynamics via complex bottom‐up and top‐down effects. Given the seasonality and pulsed nature of subsidies, alterations of the temporal and spatial synchrony of resource availability to consumers across ecosystems are likely to result in ecological mismatches that can scale up from individual responses, to communities, to ecosystems. Similarly, altered hydrology, temperature, CO₂ concentration and land use will modify the recruitment and quality of riparian vegetation, the timing of leaf abscission and the establishment of invasive riparian species. Along with morphological changes to stream–terrestrial boundaries, these will alter the use and fluxes of allochthonous subsidies associated with stream ecosystems. Future research should aim to understand how subsidy dynamics will be affected by key drivers of global change, including agricultural intensification, increasing water use and biotic homogenization. Our conceptual framework based on the match–mismatch between donor and recipient organisms may facilitate understanding of the multiple effects of global change and aid in the development of future research questions.