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Emergence of adult aquatic insects from rivers is strongly influenced by water temperature, and emergence timing helps to determine the availability of this ephemeral food resource for birds and other terrestrial insectivores. It is poorly understood how spatial heterogeneity in riverine habitat mediates the timing of emergence. Such spatiotemporal variation may have consequences for terrestrial insectivores that rely on aquatic-derived prey resources. We investigated emergence phenology of the giant salmonfly, Pteronarcys californica, at three spatial scales in two Idaho river networks. We examined the influence of tributary confluences on salmonfly emergence timing and associated insectivorous bird responses. Salmonfly emergence timing was highly variable at the basin-scale during the period we sampled (May–June). Within sub-drainage pathways not punctuated by major tributaries, emergence followed a downstream-to-upstream pattern. At the scale of reaches, abrupt changes in thermal regimes created by 10 major tributary confluences created asynchrony in emergence of 1–6 days among the 20 reaches bracketing the confluences. We observed 10 bird species capturing emerged salmonflies, including 5 species typically associated with upland habitats (e.g., American robin, red-tailed hawk, American kestrel) but that likely aggregated along rivers to take advantage of emerging salmonflies. Some birds (e.g., Lewis’s woodpecker, western tanager, American dipper) captured large numbers of salmonflies, and some of these fed salmonflies to nestlings. Emergence asynchrony created by tributaries was associated with shifts in bird abundance and richness which both nearly doubled, on average, during salmonfly emergence. Thermal heterogeneity in river networks created asynchrony in aquatic insect phenology which prolonged the availability of this pulsed prey resource for insectivorous birds during key breeding times. Such interactions between spatial and temporal heterogeneity and organism phenology may be critical to understanding the consequences of fluxes of resources that link water and land. Shifts in phenology or curtailment of life history diversity in organisms like salmonflies may have implications for these organisms, but could also contribute to mismatches or constrain availability of pulsed resources to dependent consumers. These could be unforeseen consequences, for both aquatic and terrestrial organisms, of human-driven alteration and homogenization of riverscapes.

Stream ecosystem metabolism contributes to global carbon cycling, yet predicting metabolism across ecosystems remains elusive. Even within stream segments, spatial variation in metabolic rates and their controls can be substantial, exhibiting sudden rather than continuous changes. We measured metabolism at 6 sites along a 75-km mainstem segment of Marsh Creek, Idaho. This agricultural stream lacks major geomorphic transitions such as tributaries or changes in valley width, but possesses patchy patterns of turbidity. We asked: (1) How variable is metabolism along this segment, and (2) How do the controls on metabolism vary along this segment? Metabolism varied several-fold along this stream segment. Average rates of gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP) among sites did not correlate with water quality or aquatic macrophyte cover. Rather, reaches along this segment appear to represent different process domains that were characterized by turbidity: More turbid reaches saw negative effects of turbidity on GPP and NEP and positive effects on ER. Turbidity was associated with increased respiration and decoupled ER from GPP at turbid sites. Less turbid sites had stronger coupling between GPP and ER, and GPP was predicted by light and temperature. Heterogeneous sediment supply and transport capacity along Marsh Creek create patchy patterns of turbidity, which affects the controls on metabolism. Geomorphic controls on ecosystem processes are complex but not random. Understanding how metabolism and its controls vary across process domains is needed to scale up process rates and understand how process rates might respond to future changes.

Terrestrial invertebrates are important subsidies to fish diets, though their seasonal dynamics and importance to tropical stream consumers are particularly understudied. In this year-round study of terrestrial invertebrate input to two Trinidadian headwater streams with different forest canopy densities, we sought to (a) measure the mass and composition of terrestrial inputs with fall-in traps to evaluate the influences of seasonality, canopy cover, and rainfall intensity, and; (b) compare terrestrial and benthic prey importance to Anablepsoides hartii (Hart’s Rivulus), the dominant invertivorous fish in these streams, by concurrently measuring benthic and drifting invertebrate standing stocks and the volume and composition of invertebrates in Rivulus guts throughout the year. The biomass of terrestrial invertebrate fall-in was 53% higher in the wet versus dry season; in particular, ant input was 320% higher. Ant biomass fall-in also increased with the density of canopy cover among sampling locations within both streams. Greater precipitation correlated with increased ant inputs to the more open-canopied stream and increased inputs of winged insects in the more closed canopy stream. Concurrently, the biomass of benthic invertebrates was reduced by more than half in the wet season in both streams. We detected no differences in the total volume of terrestrial prey in Rivulus diets between seasons, though ants were a greater proportion of their diet in the wet season. In contrast, benthic prey were nearly absent from Rivulus diets in the wet season in both streams. We conclude that terrestrial invertebrates are a substantial year-round prey subsidy for invertivores in tropical stream ecosystems like those we studied, which may contrast to most temperate streams where such terrestrial inputs are significantly reduced in the cold season. Interestingly, the strongest seasonal pattern in these tropical streams was observed in benthic invertebrate biomass which was greatly reduced and almost absent from Rivulus diets during the wet season. This pattern is essentially the inverse of the pattern observed in many temperate streams and highlights the need for additional studies in tropical ecosystems to better understand how spatial and temporal variation in terrestrial subsidies and benthic prey populations combine to influence consumer diets and the structure of tropical stream food webs.

Many aquatic invertebrates are declining or facing extinction from stressors that compromise physiology, resource consumption, reproduction, and phenology. However, the influence of these common stressors specifically on consumer–resource interactions for aquatic invertebrate consumers is only beginning to be understood. We conducted a field study to investigate Pteronarcys californica (i.e., the “giant salmonfly”), a large-bodied insect that is ecologically and culturally significant to rivers throughout the western United States. We sampled gut contents and polyunsaturated fatty acid composition of salmonflies to compare resource consumption across river (Madison or Gallatin, Montana), sex (male or female), and habitat (rock or woody debris). We found that allochthonous detritus comprised the majority of salmonfly diets in the Gallatin and Madison Rivers, making up 68% of the gut contents on average, followed by amorphous detritus, diatoms, and filamentous algae. Diets showed little variation across river, sex, or length. Minor differences in diets were detected by habitat type, with a higher proportion of diatoms in the diets of salmonflies collected from rocky habitat compared to woody debris. Fatty acid composition generally supported the results of gut content analysis but highlighted the importance of primary producers. The presence of eicosapentaenoic acid (20:5n-3) and alpha linolenic acid (18:3n-3) indicated consumption of diatoms and filamentous green algae, respectively. Our research underscores the importance of a healthy riparian zone that provides allochthonous detritus for invertebrate nutrition as well as the role of algae as an important source of fatty acids. published

Multiple invasive species may interact, influencing one another and generating synergistic effects on food webs and ecosystem processes. We investigated the interaction between two non‐native species widespread in the western USA: common carp (Cyprinus carpio) and Russian olive (Elaeagnus angustifolia), an invasive riparian tree associated with di‐nitrogen fixation. Deep Creek, Idaho, was an International Biological Program site in the early 1970s; at that time, carp were rare and Russian olive was absent. Subsequently, Russian olive was introduced and established a dense stand, increasing allochthonous inputs and nitrogen‐rich benthic organic matter. Since 1971, carp density has increased ~4× (an increase our bioenergetic analysis suggests could not have been sustained in the absence of Russian olive). Carp gut contents in 2013–2014 revealed, on average, ~40% olives, and, similarly, stable isotope analyses revealed ~58% of carp tissues were derived from olives. A small‐scale, short‐term experimental exclusion of these subsidized carp caused ~3× increases in macrophytes and chlorophyll‐a, suggesting they may limit algae and macrophyte biomass. Moreover, carp that consumed olives excreted more nitrogen (~2× more ammonium, ~2× more total dissolved nitrogen, and ~3× more total nitrogen) compared to those that had not, which may amplify recycling and export from streams invaded by both species. This scenario is characteristic of an “invasional meltdown,” with attendant changes in food webs and ecosystem processes.

Networks of direct and indirect biotic interactions underpin the complex dynamics and stability of ecological systems, yet experimental and theoretical studies often yield conflicting evidence regarding the direction (positive or negative) or magnitude of these interactions. We revisited pioneering data sets collected at the deciduous forested Horonai Stream and conducted ecosystem-level syntheses to demonstrate that the direction of direct and indirect interactions can change depending on the timescale of observation. Prior experimental studies showed that terrestrial prey that enter the stream from the adjacent forest caused positive indirect effects on aquatic invertebrates during summer by diverting fish consumption. Seasonal and annual estimates of secondary production and organic matter flows along food web pathways demonstrate that this seasonal input of terrestrial invertebrate prey increases production of certain fish species, reversing the indirect effect on aquatic invertebrates from positive at the seasonal timescale to negative at the annual timescale. Even though terrestrial invertebrate prey contributed 54% of the annual organic matter flux to fishes, primarily during summer, fish still consumed 98% of the aquatic invertebrate annual production, leading to top-down control that is not revealed in short-term experiments and demonstrating that aquatic prey may be a limiting resource for fishes. Changes in the direction or magnitude of interactions may be a key factor creating nonlinear or stabilizing feedbacks in complex systems, and these dynamics can be revealed by merging experimental and comparative approaches at different scales.

The River Continuum Concept (RCC) predicts that food webs (and, in particular, invertebrates) of rivers in temperate, forested drainages should exhibit a longitudinal gradient from reliance on terrestrially derived organic matter (e.g., seasonally shed leaves) in the headwaters to autochthonous sources (e.g., algae) in the mid-orders, to suspended material in larger rivers. This prediction has been evaluated by longitudinal comparisons of macroinvertebrate communities in terms of functional feeding groups (FFGs), but such an approach yields only indirect evidence regarding actual food use. To retest this prediction, we investigated invertebrate diets from the Salmon River, Idaho, by examining the gut contents of archived specimens from the longitudinal set of sites sampled in the original 1976 RCC study and by collecting invertebrates from these same sites in 2009. We detected no apparent shifts in diets over the ∼30-y time span. The importance of allochthonous materials in invertebrate diets diff...

Foodweb analyses can provide insight as to how species persist in naturally dynamic environments and how these communities may respond to anthropogenic alteration. In the remaining undammed rivers of the arid southwestern United States, it is thought that plasticity in feeding behavior allows native fishes to thrive in the context of natural fluctuations in temperature, sediment, and resource availability associated with extreme swings in season and hydrology. There have been few evaluations of this hypothesis, in part owing to the rarity of rivers that retain their natural environmental dynamics and complement of native species. We investigated the feeding ecology of the fish assemblage of one such river, the Little Colorado River, Arizona, focusing on four native species: bluehead sucker (Catostomus discobolus), flannelmouth sucker (Catostomus latipinnis), speckled dace (Rhinichthys osculus), and endangered humpback chub (Gila cypha). Over two years, we sampled fish diets in July, September, January, March, and June, encompassing turbid and clear‐water conditions across seasons. We quantified food resource contributions to fish diets in fish‐centered food webs, comparing diversity of trophic linkages and linkage‐strength distribution across time. We used ordination analyses to investigate differences in diets among species and changes in diet composition across seasons and conducted seasonal comparisons of gut fullness. We observed a high diversity of dietary items (as absolute numbers and consumer–resource ratios) within a fish assemblage that was apparently resistant to extreme and recurrent environmental variability. Fish diet diversity was ~2× that of the nearby dammed Colorado River's tailwater, and over 50% of fish‐resource linkages were weak (

Ecosystems are connected through fluxes of nutrients, organic matter, and organisms. Disturbances that alter structure and function of one ecosystem may have consequences for other linked ecosystems. We investigated how wildfire and subsequent debris flows altered fluxes of invertebrates from tributaries in the Salmon River Basin, Idaho, USA. We compared fluxes of invertebrates downstream through drift and laterally via insect emergence from streams with varying disturbance histories (unburned, burned, and burned + debris flow) during two summers (3–4 years post fire and 2–3 years post debris flow). We observed that the combination of wildfire and debris flow increased the biomass export of invertebrates from tributaries to main-stem ecosystems 2–3 × compared to other streams. In contrast, aquatic insect emergence did not differ in magnitude among streams of different disturbance histories, but instead diverged in timing. Underwater surveys indicated trout in the main-stem river selected confluence habitats, with a tendency for stronger selection of confluences with burned streams. In a behavioral comparison between confluence and non-confluence habitats, rates of agonistic behavior were 4–6 × higher in confluence areas, indicating that confluences may be worth defending. Abundances of web-spinning spiders that depend on emerging insects did not vary with disturbance history in early-mid summer, but tended to be highest in riparian areas along burned streams by August. Because wildfire and debris flows are predicted to increase, our results elucidate potential pathways by which altered disturbance regimes may affect coupled aquatic-terrestrial ecosystems.

Postfire disturbances, such as debris flows, may alter the recovery of stream ecosystems from wildfire. Furthermore, changes to productivity and the dynamics of nutrients and organic matter (OM) in tributaries may affect fluxes of materials and organisms to mainstem habitats. We investigated the consequences of watershed wildfire and debris flows for exports from tributaries to mainstem habitats in the South Fork Salmon River Basin of central Idaho, USA. We compared fluxes from 15 streams (n = 5 unaffected [UA], n = 5 fire-affected [FA], and n = 5 fire + debris flow-affected [FDA]) in summer 2011 (4 y postfire, 3 y postdebris flow). In early June, FDA streams exported 650% higher concentrations of sediment to downstream habitats than did UA streams (mean, FDA: 18.7 g/m3, UA: 1.9 g/m3). Overall, concentrations of dissolved organic C were 75% greater in FDA than in UA streams (FDA: 2.8, UA: 1.6 mg/L), and the quality of OM was higher (i.e., more chlorophyll a [chl a] and higher chl a:OM) in FDA than...

Droughts, fires, and floods are natural disturbances influencing aquatic ecosystems. If drought is accompanied by fires, and if fires are closely followed by floods, teasing apart their distinctive and potentially interactive responses can be difficult. We compared the responses of macroinvertebrate communities to fire via comparisons of streams in burned and unburned catchments in 3 fire-prone biomes that differ biogeographically and climatically (northwestern Mediterranean, southeastern Australia, and northwestern intermountain USA). The responses of macroinvertebrate communities in streams in burned catchments were similar in all biogeographic regions, but the magnitude of these responses varied. Fire combined with high seasonal stream flows, flooding, or drought was associated with reduced measures of taxonomic richness and increased abundance, especially of r-strategist taxa. Differences between sites in burned and unburned catchments were consistently stronger in southeastern Australia than ...

Theory states that both the spatial complexity of landscapes and the strength of interactions between consumers and their resources are important for maintaining biodiversity and the balance of nature. Spatial complexity is hypothesized to promote biodiversity by reducing the potential for competitive exclusion; whereas, models show that weak trophic interactions can enhance stability and maintain biodiversity by dampening destabilizing oscillations associated with strong interactions. Here, we show that spatial complexity can reduce the strength of consumer-resource interactions in natural food webs. By sequentially aggregating food webs of individual aquatic habitat patches across a floodplain mosaic, we found that increasing spatial complexity resulted in decreases in the strength of interactions between predators and prey, owing to a greater proportion of weak interactions and a reduced proportion of strong interactions in the meta-food web. The main mechanism behind this pattern was that some patches provided predation refugia for species that were often strongly preyed upon in other patches. If weak trophic interactions do indeed promote stability, then our findings may signal an additional mechanism by which complexity and stability are linked in nature. In turn, this may have implications for how the values of landscape complexity, and the costs of biophysical homogenization, are assessed.

Nearly all ecosystems have been altered by human activities, and most communities are now composed of interacting species that have not co-evolved. These changes may modify species interactions, energy and material flows, and food-web stability. Although structural changes to ecosystems have been widely reported, few studies have linked such changes to dynamic food-web attributes and patterns of energy flow. Moreover, there have been few tests of food-web stability theory in highly disturbed and intensely managed freshwater ecosystems. Such synthetic approaches are needed for predicting the future trajectory of ecosystems, including how they may respond to natural or anthropogenic perturbations. We constructed flow food webs at six locations along a 386-km segment of the Colorado River in Grand Canyon (Arizona, USA) for three years. We characterized food-web structure and production, trophic basis of production, energy efficiencies, and interaction-strength distributions across a spatial gradient of pertu...

Climate change (CC) is projected to increase the frequency and severity of natural disturbances (wildfires, insect outbreaks, and debris flows) and shift distributions of terrestrial ecosystems on a global basis. Although such terrestrial changes may affect stream ecosystems, they have not been incorporated into predictions of stream responses to CC. Here, we introduce a conceptual framework to evaluate to what extent responses of streams to CC will be driven by not only changes in thermal and hydrologic regimes, but also alterations of terrestrial processes. We focused on forested watersheds of western North America because this region is projected to experience CC-induced alteration of terrestrial processes. This provided a backdrop for investigating interactive effects of climate and terrestrial responses on streams. Because stream responses to terrestrial processes have been well-studied in contexts largely independent of CC research, we synthesized this knowledge to demonstrate how CC-induced alterations of terrestrial ecosystems may affect streams. Our synthesis indicated that altered terrestrial processes will change terrestrial–aquatic linkages and autotrophic production, potentially yielding greater sensitivity of streams to CC than would be expected based on shifts in temperature and precipitation regime alone. Despite uncertainties that currently constrain predictions regarding stream responses to these additional pathways of change, this synthesis highlighted broader effects of CC that require additional research. Based on widespread evidence that CC is linked to changing terrestrial processes, we conclude that accurate predictions of CC effects on streams may be coupled to the accuracy of predictions for long-term changes in terrestrial ecosystems.

River regulation may mediate the interactions among native and nonnative species, potentially favoring nonnative species and contributing to the decline of native populations. We examined food resource use and diet overlap among small-bodied fishes in the Grand Canyon section of the Colorado River as a first step in evaluating potential resource competition. We compared the diets of the predominant small-bodied fishes (native Speckled Dace Rhinichthys osculus, juvenile Flannelmouth Sucker Catostomus latipinnis, and juvenile Bluehead Sucker C. discobolus, and nonnative Fathead Minnow Pimephales promelas) across seasons at four sites downstream of Glen Canyon Dam using nonmetric multidimensional scaling and Schoener's similarity index. The diets of these fishes included diatoms, amorphous detritus, aquatic invertebrates (especially simuliid and chironomid larvae), terrestrial invertebrates, and terrestrial vegetation. Diets varied with season and were affected by high turbidity. Fish consumed more amorphous detritus and terrestrial vegetation during the summer monsoon season (July–September), when turbidity was higher. The diets of all species overlapped, but there was large variation in the degree of overlap. The diets of juvenile suckers and Fathead Minnows were most similar, while Speckled Dace had relatively distinct diets. The differences took the form of higher proportions of diatoms and amorphous detritus in the diets of Bluehead Suckers and Fathead Minnows and higher proportions of simuliids and chironomids in those of Speckled Dace. If food resources are or become limiting, diet overlap suggests that competition may occur among native and nonnative species, which could have implications for the population dynamics of these fishes and for the management of the Colorado River ecosystem in Grand Canyon. Received September 4, 2013; accepted February 11, 2014

Artificial additions of nutrients of differing forms such as salmon carcasses and analog pellets (i.e. pasteurized fishmeal) have been proposed as a means of stimulating aquatic productivity and enhancing populations of anadromous and resident fishes. Nutrient mitigation to enhance fish production in stream ecosystems assumes that the central pathway by which effects occur is bottom‐up, through aquatic primary and secondary production, with little consideration of reciprocal aquatic‐terrestrial pathways. The net outcome (i.e. bottom‐up vs. top‐down) of adding salmon‐derived materials to streams depend on whether or not these subsidies indirectly intensify predation on in situ prey via increases in a shared predator or alleviate such predation pressure. We conducted a 3‐year experiment across nine tributaries of the N. Fork Boise River, Idaho, USA, consisting of 500‐m stream reaches treated with salmon carcasses (n = 3), salmon carcass analog (n = 3), and untreated control reaches (n = 3). We observed 2–8 fold increases in streambed biofilms in the 2–6 weeks following additions of both salmon subsidy treatments in years 1 and 2 and a 1.5‐fold increase in standing crop biomass of aquatic invertebrates to carcass additions in the second year of our experiment. The consumption of benthic invertebrates by stream fishes increased 110–140% and 44–66% in carcass and analog streams in the same time frame, which may have masked invertebrate standing crop responses in years 3 and 4. Resident trout directly consumed 10.0–24.0 g·m−2·yr−1 of salmon carcass and

A disturbance, such as species invasion, can alter the exchange of materials and organisms between ecosystems, with potential consequences for the function of both ecosystems. Russian olive (Elaeagnus angustifolia) is an exotic tree invading riparian corridors in the western United States, and may alter stream organic matter budgets by increasing allochthonous litter and by reducing light via shading, in turn decreasing in-stream primary production. We used a before-after invasion comparison spanning 35 years to show that Russian olive invasion increased allochthonous litter nearly 25-fold to an invaded vs. a control reach of a stream, and we found that this litter decayed more slowly than native willow. Despite a mean 50% increase in canopy cover by Russian olive and associated shading, there were no significant changes in gross primary production. Benthic organic matter storage increased fourfold after Russian olive invasion compared to pre-invasion conditions, but there were no associated changes in stream ecosystem respiration or organic matter export. Thus, estimated stream ecosystem efficiency (ratio of ecosystem respiration to organic matter input) decreased 14%. These findings show that invasions of nonnative plant species in terrestrial habitats can alter resource fluxes to streams with consequences for whole-ecosystem functions.

Frequently nonnative species invade habitats occupied by a similar, even closely related, species, and it is often assumed that the two have functionally redundant ecological roles. However, the consequences of these types of invasions are rarely studied. Eastern brook trout are the most widely distributed and abundant nonnative fish throughout western North America and are assumed to be analogous in terms of ecological function to the native cutthroat trout they replace. However, based on results of previous studies conducted on a few streams, we hypothesized that the two species differ ecologically, and we predicted that brook trout would exhibit greater density, biomass, and annual production than cutthroat trout at the onset of invasion (i.e., in sympatry) and once replacement has occurred (i.e., in allopatry). We compiled data sets (sympatry: n = 169; allopatry: n = 687) from across the historical range of inland cutthroat trout and made comparisons for all sites combined (western scale) and among regions. In sympatry, at the western scale we observed that brook trout exhibited 3.1 times greater density and 2.5 times greater biomass than cutthroat trout, but no difference in production. In allopatry, density and biomass were 1.5 times greater and annual production 1.9 times higher for brook trout than cutthroat trout. Results of comparisons by region were similar, though more variable. In addition, we found that trout density and biomass were correlated with landscape features such as elevation and gradient, but such watershed variables did not account for the differences we observed between the trout species. These results suggest a general pattern of greater density, biomass, and production when brook trout invade and replace cutthroat trout, and highlight the need for more studies that elucidate the mechanisms responsible for this pattern and that investigate the broader consequences of a nonnative species replacing a closely related (and often presumed to be similar) native species.

Mountain whitefish (Prosopium williamsoni) are among the most abundant native fishes in western North America, yet their role in ecosystems is largely unknown. We investigated their ecology in Big Creek, a 7th-order wilderness watershed in central Idaho. Based on underwater visual counts and hook and line surveys conducted during summer months, as well as tissue analysis, we estimated mountain whitefish distribution and abundance, total biomass, production, and energy and nutrient contributions along a 60 km segment of Big Creek. We observed that abundance decreased with distance upstream, and 93% of mountain whitefish surveyed in Big Creek were greater than 200 mm (≥ 3 years); few juveniles were observed, suggesting that rearing occurs in habitats downstream. Mountain whitefish were the dominant fish in Big Creek, in terms of both abundance (33% of fish observed) and biomass (57% of salmonid biomass). We estimated their production averaged across the study segment as 0.51 g m-2 yr-1, though it r...

Replacement of a native species by a nonnative can have strong effects on ecosystem function, such as altering nutrient cycling or disturbance frequency. Replacements may cause shifts in ecosystem function because nonnatives establish at different biomass, or because they differ from native species in traits like foraging behavior. However, no studies have compared effects of wholesale replacement of a native by a nonnative species on subsidies that support consumers in adjacent habitats, nor quantified the magnitude of these effects. We examined whether streams invaded by nonnative brook trout (Salvelinus fontinalis) in two regions of the Rocky Mountains, USA, produced fewer emerging adult aquatic insects compared to paired streams with native cutthroat trout (Oncorhynchus clarkii), and whether riparian spiders that depend on these prey were less abundant along streams with lower total insect emergence. As predicted, emergence density was 36% lower from streams with the nonnative fish. Biomass of brook trout was higher than the cutthroat trout they replaced, but even after accounting for this difference, emergence was 24% lower from brook trout streams. More riparian spiders were counted along streams with greater total emergence across the water surface. Based on these results, we predicted that brook trout replacement would result in 6-20% fewer spiders in the two regions. When brook trout replace cutthroat trout, they reduce cross-habitat resource subsidies and alter ecosystem function in stream-riparian food webs, not only owing to increased biomass but also because traits apparently differ from native cutthroat trout.

Although the study of resource subsidies has emerged as a key topic in both ecosystem and food web ecology, the dialogue over their role has been limited by separate approaches that emphasize either subsidy quantity or quality. Considering quantity and quality together may provide a simple, but previously unexplored, framework for identifying the mechanisms that govern the importance of subsidies for recipient food webs and ecosystems. Using a literature review of >90 studies of open-water metabolism in lakes and streams, we show that high-flux, low-quality subsidies can drive freshwater ecosystem dynamics. Because most of these ecosystems are net heterotrophic, allochthonous inputs must subsidize respiration. Second, using a literature review of subsidy quality and use, we demonstrate that animals select for high-quality food resources in proportions greater than would be predicted based on food quantity, and regardless of allochthonous or autochthonous origin. This finding suggests that low-flux, high-quality subsidies may be selected for by animals, and in turn may disproportionately affect food web and ecosystem processes (e.g., animal production, trophic energy or organic matter flow, trophic cascades). We then synthesize and review approaches that evaluate the role of subsidies and explicitly merge ecosystem and food web perspectives by placing food web measurements in the context of ecosystem budgets, by comparing trophic and ecosystem production and fluxes, and by constructing flow food webs. These tools can and should be used to address future questions about subsidies, such as the relative importance of subsidies to different trophic levels and how subsidies may maintain or disrupt ecosystem stability and food web interactions.

Introductions of nonnative salmonids, such as rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta, have affected native fishes worldwide in unforeseen and undesirable ways. Predation and other interactions with nonnative rainbow trout and brown trout have been hypothesized as contributing to the decline of native fishes (including the endangered humpback chub Gila cypha) in the Colorado River, Grand Canyon. A multiyear study was conducted to remove nonnative fish from a 15-km segment of the Colorado River near the Little Colorado River confluence. We evaluated how sediment, temperature, fish prey availability, and predator abundance influenced the incidence of piscivory (IP) by nonnative salmonids. Study objectives were addressed through spatial (upstream and downstream of the Little Colorado River confluence) and temporal (seasonal and annual) comparisons of prey availability and predator abundance. Data were then evaluated by modeling the quantity of fish prey ingested by trout durin...

Russian olive (Elaeagnus angustifolia) is a non-native riparian tree that has become common and continues to rapidly spread throughout the western United States. Due to its dinitrogen (N2)-fixing ability and proximity to streams, Russian olive has the potential to subsidize stream ecosystems with nitrogen (N), which may in turn alter nutrient processing in these systems. We tested these potential effects by comparing background N concentrations; nutrient limitation of biofilms; and uptake of ammonium (NH4-N), nitrate (NO3-N), and phosphate (PO4-P) in paired upstream-reference and downstream-invaded reaches in streams in southeastern Idaho and central Wyoming. We found that stream reaches invaded by Russian olive had higher organic N concentrations and exhibited reduced N limitation of biofilms compared to reference reaches. However, at low inorganic N background concentrations, reaches invaded by Russian olive exhibited higher demand for both NH4-N and NO3-N compared to their paired reference reaches, suggesting these streams have the potential to retain the N subsidy from Russian olive N2 fixation and diminish its downstream export and effects. Our findings demonstrate the potential for a non-native riparian plant to significantly alter biogeochemical cycling in streams. Finally, we used our results to develop a conceptual model that describes predicted effects of Russian olive and other non-native riparian N2 fixers on in-stream N dynamics.

The effects of wildfire can alter the structure of stream insect assemblages. Post-fire shifts to dominance by r-strategist taxa could drive increases in productivity of primary consumer and predatory insects, but this possibility has not been explicitly investigated. Likewise, the extent and duration of such effects might be mediated by fire severity, but this hypothesis also has not been evaluated. We report results from a comparative study that examined the mid-term (5–10 y post-fire) effects of wildfire of varying severity on stream insect assemblage composition and productivity measured in terms of benthic larval biomass and flux of emerging adults. We compared benthos and emergence in a suite of 2nd- to 3rd-order, unburned streams to those that had experienced low-severity and high-severity wildfire in wilderness watersheds of central Idaho. Reaches that experienced high-severity burn had the greatest biomass of r-strategist, generalist primary consumers that included Chironomidae, Baetis, a...

Increased need to quantify adult insects emerging from streams as a part of foodweb and ecosystem studies has placed new demands on techniques used to sample adults. The population sampled must be better understood to establish the scope of inferences that may be drawn from emergence data. We used data from 2 different studies to compare the structure of insect assemblages represented by benthic samples and emergence-trap samples and to compare adult insect assemblages collected in emergence traps placed at mid-channel vs streambank locations. Nonmetric multidimensional scaling (NMDS) ordination showed that some components of the benthic assemblage were underrepresented in the emergence-trap samples and others were underrepresented in benthic samples. These differences were mainly caused by taxa that emerged by crawling out on the stream bank (e.g., Plecoptera) or whose larvae reside in habitats, such as stream margins, that are underrepresented with traditional benthic sampling (e.g., Dixidae). The flux of insects into traps placed mid-channel did not differ significantly from the flux into traps placed along the stream bank. Taxa collected by mid-channel and streambank traps overlapped considerably, but midchannel traps tended to collect proportionally more Trichoptera, Ephemeroptera, and Diptera, whereas streambank traps collected proportionally more Plecoptera. Our results can be used to improve trapping designs for future assessments of aquatic insect emergence in studies of insect behavior and life histories and as part of foodweb and ecosystem research.

The replacement of native species by nonnative species has been widely documented. In aquatic ecosystems, nonnative trout often replace native trout species to which they are closely related. For example, nonnative brook trout Salvelinus fontinalis have replaced native cutthroat trout Oncorhynchus clarkii throughout western North America. We hypothesized that when brook trout replace cutthroat trout, they exhibit greater density, biomass, and annual production than were achieved by the cutthroat trout before replacement. We tested this hypothesis in five paired streams in the Teton River drainage, Idaho, over a 4-year period. Streams were paired based on similar habitat characteristics and the presence of brook trout or cutthroat trout as the dominant species. Fish were surveyed annually by means of multipass depletion along representative stream reaches. Consistent with our hypothesis, brook trout exhibited 2.4 times greater density, 1.7 times greater biomass, and 2.5 times higher annual product...

We investigated the midterm effects of wildfire (in this case, five years after the fire) of varying severity on periphyton, benthic invertebrates, emerging adult aquatic insects, spiders, and bats by comparing unburned sites with those exposed to low severity (riparian vegetation burned but canopy intact) and high severity (canopy completely removed) wildfire. We observed no difference in periphyton chlorophyll a or ash-free dry mass among different burn categories but did observe significantly greater biomass of benthic invertebrates in both high severity burned and unburned reaches versus low severity burned reaches. Moreover, a significantly greater flux of adult aquatic insect emergence occurred at sites that experienced high severity fire versus low severity burned and unburned sites. The greatest number of spiders and bat echolocation calls were also observed at sites of high severity fire. Our results suggest that fires of different severity may have very different affects on stream-riparian food webs and that high severity wildfire may lead to an extended “fire pulse” that stimulates aquatic productivity and flux of prey to terrestrial habitats, driving local increases in riparian consumers.

Species invasions are often associated with large-scale human alteration of ecosystems. One classic example is the increasing dominance of non-native taxa below and above dams on large rivers. These dams substantially alter the physical template of river ecosystems, and exotic taxa often proliferate with potentially large impacts on coexisting taxa and ecosystem processes. Here we document the invasion of New Zealand mud snails (Potamopyrgus antipodarum) in the Colorado River directly below Lake Powell in Glen Canyon, Arizona, USA. We also quantified the magnitude and variability in growth and secondary production of P. antipodarum during 2006–2007 to gain a functional measure of their role in the ecosystem. Snails were first detected in Glen Canyon in 1995, and have since become a dominant component of the invertebrate fauna. Throughout the invasion of P. antipodarum, biomass of other dominant taxa was variable and did not appear to be positively or negatively influenced by the presence of P. antipodarum. Specific growth rates of P. antipodarum were moderate (0.001–0.030 day−1) and strongly related to body size. Mean annual habitat-weighted biomass and production were relatively high (biomass: 4.4 g/m2; secondary production: 13.3 g m−2 year−1) and similar among habitats. Mean monthly biomass and daily secondary production were much more variable, with highest values occurring in autumn. We show that invasion of a productive aquatic consumer to a highly disturbed river ecosystem had little detectable influence on the biomass of other invertebrate taxa. However, additional research will be necessary to fully understand and predict effects of P. antipodarum on coexisting taxa.

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.

Large-scale patterns of hyporheic exchange are predictable within some river systems, but our understanding of the factors driving hyporheic processes and the magnitude of hyporheic exchange needed to influence biophysical patterns at larger scales remains limited. We investigated the patterns, magnitude, and potential effects on biota of reach-scale hyporheic exchange in an alluvial river of the Pacific Northwest. The river was topographically similar to and in the same geographic region as other systems where large-scale hyporheic exchange and associated biological responses have been observed. We hypothesized that predictable reach-scale patterns of hyporheic exchange would occur in alluvial valley segments of the river and that hyporheic upwelling would be associated with reach-scale patterns of physical and biological characteristics. We used in-channel piezometers and synoptic stream flow measurements to quantify hyporheic exchange. We measured temperature, dissolved O2, pH, specific conduct...

Removal of riparian vegetation and straightening of stream channels (channelization) are the most prevalent forms of habitat degradation in streams and their riparian zones. Both have direct effects on organisms in the habitats where they occur, but also have potential to cause indirect effects by interrupting the flux of invertebrate prey between the two adjacent ecosystems. We measured abundance of web-building riparian spiders along four types of streams in Hokkaido, Japan: relatively undisturbed streams, streams where riparian vegetation had been removed, previously channelized streams where the banks had revegetated, and streams that had been both channelized and had the vegetation removed. Spider abundance was reduced by 70% or more by either habitat disturbance alone, or both combined, and the number of spider families was also reduced. Spiders of the family Tetragnathidae, which specialize in capturing adult insects emerging from streams, were strongly reduced by either form of habitat degradation alone, or in combination. In contrast, abundance of spiders in other families that capture prey from both terrestrial and aquatic sources was reduced more strongly by vegetation loss than channelization. These results indicate that riparian vegetation loss has strong direct effects on spiders by reducing habitat for web sites. They also suggest that channelization can have strong indirect effects on riparian-specialist tetragnathid spiders, probably by reducing the flux of adult aquatic insects from the stream to the riparian zone.

Habitat alteration and biotic invasions are the two leading causes of global environmental change and biodiversity loss. Recent innovative experiments have shown that habitat disturbance can have drastic effects that cascade to adjacent ecosystems by altering the flow of resource subsidies from donor systems. Likewise, exotic species in- vasions could alter subsidies and affect distant food webs, but very few studies have tested this experimentally. Here we report evidence from a large-scale field experiment in northern Japan that invasion of nonnative rainbow trout (Oncorhynchus mykiss) interrupted reciprocal flows of invertebrate prey that drove stream and adjacent riparian forest food webs. Rainbow trout usurped terrestrial prey that fell into the stream, causing native Dolly Varden charr (Salvelinus malma) to shift their foraging to insects that graze algae from the stream bottom. This indirectly increased algal biomass, but also decreased biomass of adult aquatic insects emerging from the stream to the forest. In turn, this led to a 65% reduction in the density of riparian-specialist spiders in the forest. Thus, species invasions can interrupt flows of resources between interconnected ecosystems and have effects that propagate across their boundaries, effects that may be difficult to anticipate without in-depth understanding of food web relationships.

Measurements of groundwater–stream water interactions are increasingly recognized as important to understanding the ecology of fishes and other organisms in stream and riparian ecosystems. However, standard measurement techniques are often feasible only at small spatial scales, in areas with easy access, or in systems with relatively fine substrata. We developed simple new techniques for installing minipiezometers and obtaining estimates of vertical hydraulic gradient, hydraulic conductivity, and specific discharge in gravel and cobble streambeds that allowed for large numbers of measurements to be obtained in remote locations. Our approach yielded values comparable to those obtained through more traditional methods. Consequently, these techniques may provide a labor cost-efficient way for detecting groundwater−stream water interaction patterns that are critical labor-attributes of stream and riparian systems at multiple scales.

The distribution and abundance of bull trout (Salvelinus confluentus) spawning were affected by geomorphology and hyporheic groundwater - stream water exchange across multiple spatial scales in streams of the Swan River basin, northwestern Montana. Among spawning tributary streams, the abundance of bull trout redds increased with increased area of alluvial valley segments that were longitudinally confined by geomorphic knickpoints. Among all valley segment types, bull trout redds were primarily found in these bounded alluvial valley segments, which possessed complex patterns of hyporheic exchange and extensive upwelling zones. Bull trout used stream reaches for spawning that were strongly influenced by upwelling. However, within these selected reaches, bull trout redds were primarily located in transitional bedforms that possessed strong localized downwelling and high intragravel flow rates. The changing relationship of spawning habitat selection, in which bull trout selected upwelling zones at one spatial scale and downwelling zones at another spatial scale, emphasizes the importance of considering multiple spatial scales within a hierarchical geomorphic context when considering the ecology of this species or plans for bull trout conservation and restoration.

The Swan Basin in Montana is considered a stronghold of regional significance for the bull trout Salvelinus confluentus, a native char whose populations are fragmented and declining throughout its range. We used correlation analysis to examine spatial and temporal variation of bull trout redd count data (1982–1995) relative to geomorphic and land-use factors among nine principal spawning tributaries of the Swan River. Bull trout redd numbers were positively correlated with the extent of alluvial valley segments bounded by knickpoints and negatively correlated with the density of logging roads in spawning tributary catchments. The density of logging roads in spawning tributary catchments was not significantly correlated with geomorphic factors. Temporal trends among the principal spawning streams were variable. In four of the nine principal spawning streams, redd numbers increased significantly during the survey period, and in the remaining streams, redd numbers showed no significant change. Chang...

We measured large woody debris (LWD) in 20 known bull trout (Salvelinus confluentus) spawning stream reaches from logged and wilderness watersheds in northwestern Montana. Mean bankfull width of stream reaches was 14.1 m ranging from 3.9 to 36.7 m. Streams were large enough to move LWD and form aggregates. We determined the characteristics of individual pieces of LWD that were interactive with the stream channel. Large, short pieces of LWD attached to the stream bank were the most likely to be positioned perpendicular to stream flow, while large, long pieces either tended to be parallel to the flow or, when attached, were most apt to extend across the channel thalweg. Observations indicated that the majority of pools were formed as scour pools by either very large LWD pieces that were perpendicular to the stream or multipiece LWD aggregates. Among reaches in wilderness watersheds, ratios of large to small LWD, attached to unattached LWD, and with and without rootwads were relatively consistent. However, among reaches with logging in the watershed, these ratios varied substantially. These results suggest that logging can alter the complex balance of delivery, storage, and transport of LWD in northern Rocky Mountain streams, and therefore, the likely substantive change in stream habitats. Resume : Nous avons mesure les gros debris ligneux (GDL) dans 20 troncons de cours d'eau servant de frayeres a l'omble a tete plate dans des bassins exploites par l'industrie forestiere et des bassins sauvages du nord-ouest du Montana. La largeur moyenne des troncons a pleins bords etait de 14,1 m, avec une fourchette de 3,9 m a 36,7 m. Les cours d'eau etaient assez larges pour que les GDL se deplacent et forment des agregats. Nous avons determine les caracteristiques des morceaux de GDL qui interagissaient avec le chenal. Les morceaux gros et courts attaches a la berge etaient les plus susceptibles de se positionner perpendiculairement au courant, tandis que les morceaux gros et longs se placaient parallelement au courant ou, s'ils etaient attaches, etaient les plus susceptibles de se placer en travers du thalweg du chenal. Les observations ont montre que la majorite des fosses sont le resultat de l'affouillement cause soit par de tres gros morceaux de GDL perpendiculaires au courant, soit par des agregats composes de plusieurs morceaux de GDL. Parmi les troncons des bassins sauvages, les rapports des gros aux petits GDL, des GDL attaches aux GDL non attaches, des GDL avec et sans attaches racinaires, etaient relativement constants, alors qu'ils variaient considerablement parmi les troncons des bassins soumis a l'exploitation forestiere. Ces resultats permettent de penser que l'exploitation forestiere peut alterer l'equilibre complexe de l'apport, de l'installation et du transport des GDL dans les cours d'eau du nord des Montagnes Rocheuses, et donc occasionner des modifications potentiellement importantes des habitats lotiques.

The transfer of nutrients and energy across habitat or ecosystem boundaries, or resource subsidies, is important in structuring ecological communities. Variation in biotic and abiotic characteristics of patches receiving resource subsidies may mediate their effects, including responses by consumers, however few studies have investigated this explicitly or the potential effects at broader spatial scales. To test the role recipient patches may play in mediating effects of a subsidy and consequences at larger reach scales, we conducted an experiment to evaluate changes in arthropod community composition and abundance in response to a subsidy of salmon carcasses in vegetated and un-vegetated riparian patches, where carcasses are frequently deposited by animals, and how these patterns in patches mediate responses at reach scales. Arthropod community composition differed between habitats, and salmon additions yielded strong positive changes in arthropod abundance among select families. Four dipteran families responded positively to additions of carcasses to riparian habitats, and effects were generally stronger in vegetated habitats. Salmon carcasses in un-vegetated habitats desiccated whereas in vegetated habitats they remained moist which likely facilitated rapid consumption. Of the five predatory arthropod families observed, only the coleopteran, Staphylinidae, increased in response to the salmon carcass subsidy, and only in vegetated habitats. Differences in effect size between habitats suggest selection of salmon carcasses by riparian arthropods changes with habitat context. Additionally, we detected significant increases in adult Calliphoridae biomass at reach scales, with the largest increases occurring at streams with more carcasses in vegetated patches. Our results show that the responses to the subsidy of salmon carcasses are not ubiquitous among riparian arthropods, nor are they spatially homogenous. Rather, our findings demonstrate that spatial variation in recipient habitat can mediate the responses of primary and secondary consumers to a subsidy, with effects manifesting at broader spatial extents, highlighting an additional means by which landscape heterogeneity influences the dynamics of food webs.

Mitigation activities designed to supplement nutrient and organic matter inputs to streams experiencing decline or loss of Pacific salmon typically presuppose that an important pathway by which salmon nutrients are moved to fish (anadromous and/or resident) is via nutrient incorporation by biofilms and subsequent bottom-up stimulation of biofilm production, which is nutrient-limited in many ecosystems where salmon returns have declined. Our objective was to quantify the magnitude of nutrient incorporation and biofilm dynamics that underpin this indirect pathway in response to experimental additions of salmon carcasses and pelletized fish meal (a.k.a., salmon carcass analogs) to 500-m reaches of central Idaho streams over three years. Biofilm standing crops increased 2–8-fold and incorporated marine-derived nutrients (measured using 15N and 13C) in the month following treatment, but these responses did not persist year-to-year. Biofilms were nitrogen (N) limited before treatments, and remained N limited in analog, but not carcass-treated reaches. Despite these biofilm responses, in the month following treatment total N load was equal to 33–47% of the N added to the treated reaches, and N spiraling measurements suggested that as much as 20%, but more likely 2–3% of added N was taken up by microbes. Design of biologically and cost-effective strategies for nutrient addition will require understanding the rates at which stream microbes take up nutrients and the downstream distance traveled by exported nutrients.