Aquatic Subsidies Research Articles

Bridging the Gaps: Exploring Aquatic–Terrestrial Connectivity through the Trait-Based Ecology of Riparian Predatory Arthropods

Riparian predatory arthropods represent one of the main trophic links between lotic and terrestrial ecosystems along riverine landscapes. The use of the trait-based approach promises to enhance our understanding of how these predatory communities interact with their environment through their response to various drivers of change and through their trophic interactions. We reviewed the scientific literature focused on the interaction between drivers of community change (natural and anthropogenic) and the functional traits and functional diversity components that characterize riparian ground beetles and spiders and, ultimately, on their role as cross-ecosystem trophic links. We highlight land use changes and river regulations as the strongest drivers that change the communities we study, often through various interacting mechanisms that favor the replacement of riparian specialists with generalist species, thus altering aquatic–terrestrial connectivity and the resilience of riverine arthropod consumers. Tropical regions and traits related to community responses to extreme climatic events (e.g., submersion tolerance and desiccation resistance) are less studied, while inconsistent patterns are noticed for well-studied traits, especially for spiders (e.g., their feeding preference response to aquatic subsidy availability and their body size response to flooding and bank hydrological connectivity). Future research should focus on the aforementioned drivers and knowledge gaps, along with the functional diversity changes in predatory arthropod communities along environmental and anthropogenic impact gradients, in order to improve riparian conservation.

Cascading impacts of changes in subsidy quality on recipient ecosystem functioning.

Resource quantity and quality can differ between adjacent ecosystems, and these differences can impact subsidies exchanged between ecosystems. The quantity and quality of subsidies are rapidly changing in response to stressors associated with global environmental change, but while we have models to predict the effects of changes in subsidy quantity, we currently lack models to predict the effects of changes in subsidy quality on recipient ecosystem functioning. We developed a novel model to predict the effects of subsidy quality on recipient ecosystem biomass distribution, recycling, production, and efficiency. We parameterized the model for a case study of a riparian ecosystem subsidized by pulsed emergent aquatic insects. In this case study we focused on a common measure of subsidy quality that differs between riparian and aquatic ecosystems: the higher content of long-chain polyunsaturated fatty acids (PUFAs) in aquatic ecosystems. We analyzed how changes in the PUFA concentration of aquatic subsidies affect the dynamics in biomass stocks and functions of the riparian ecosystem. We also conducted a global sensitivity analysis to identify key drivers of subsidy impacts. Our analysis showed that subsidy quality increased the functioning of the recipient ecosystem. Recycling increased more strongly than production per unit subsidy quality increase, meaning there was a threshold where an increase in subsidy quality led to stronger effects of subsidies on recycling relative to the production of the recipient ecosystem. Our predictions were most sensitive to basal nutrient input, highlighting the relevance of recipient ecosystem nutrient levels to understanding the effects of ecosystem connections. We argue that recipient ecosystems that rely on high-quality subsidies, such as aquatic-terrestrial ecotones, are highly sensitive to changes in subsidy-recipient ecosystem connections. Our novel model unifies the subsidy hypothesis and food quality hypothesis and provides testable predictions to understand the effects of ecosystem connections on ecosystem functioning under global changes.

Impact across ecosystem boundaries – Does Bti application change quality and composition of the diet of riparian spiders?

Emerging aquatic insects link aquatic and adjacent terrestrial food webs by subsidizing terrestrial predators with high-quality prey. One of the main constituents of aquatic subsidy, the non-biting midges (Chironomidae), showed altered emergence dynamics in response to the mosquito control agent Bacillus thuringiensis var. israelensis (Bti). As riparian spiders depend on aquatic subsidy, they may be affected by such changes in prey availability. Thus, we conducted a field study in twelve floodplain pond mesocosms (FPMs), six were treated with Bti (2.88 × 109 ITU/ha, VectoBac WDG) three times, to investigate if the Bti-induced shift in chironomid emergence dynamics is reflected in their nutritional value and in the diet of riparian spiders. We measured the content of proteins, lipids, glycogen, and carbohydrates in emerged Chironomidae, and determined the stable isotope ratios of female Tetragnatha extensa, a web-building spider living in the riparian vegetation of the FPMs. We analysed the proportion of aquatic prey in spiders' diet, niche size, and trophic position. While the content of nutrients and thus the prey quality was not significantly altered by Bti, effects on the spiders' diet were observed. The trophic position of T. extensa from Bti-treated FPMs was lower compared to the control while the aquatic proportion was only minimally reduced. We assume that spiders fed more on terrestrial prey but also on other aquatic organisms such as Baetidae, whose emergence was unaffected by Bti. In contrast to the partly predaceous Chironomidae, consumption of aquatic and terrestrial primary consumers potentially explains the observed lower trophic position of spiders from Bti-treated FPMs. As prey organisms vary in their quality the suggested dietary shift could transfer previously observed effects of Bti to riparian spiders conceivably affecting their populations. Our results further support that anthropogenic stressors in aquatic ecosystems may translate to terrestrial predators through aquatic subsidy.

Metal Exposure and Sex Shape the Fatty Acid Profile of Midges and Reduce the Aquatic Subsidy to Terrestrial Food Webs.

Aquatic micropollutants can be transported to terrestrial systems and their consumers by emergent aquatic insects. However, micropollutants, such as metals, may also affect the flux of physiologically important polyunsaturated fatty acids (PUFAs). As certain PUFAs have been linked to physiological fitness and breeding success of terrestrial consumers, reduced fluxes from aquatic systems could affect terrestrial populations and food webs. We chronically exposed larvae of the aquatic insect Chironomus riparius to a range of environmentally relevant sediment contents of cadmium (Cd) or copper (Cu) in a 28-day microcosm study. Since elevated water temperatures can enhance metals' toxic effects, we used two temperature regimes, control and periodically elevated temperatures (heat waves) reflecting an aspect of climate change. Cd and Cu significantly reduced adult emergence by up to 95% and 45%, respectively, while elevated temperatures had negligible effects. Both metal contents were strongly reduced (∼90%) during metamorphosis. Furthermore, the chironomid FA profile was significantly altered during metamorphosis with the factors sex and metal exposure being relevant predictors. Consequently, fluxes of physiologically important PUFAs by emergent adults were reduced by up to ∼80%. Our results suggest that considering fluxes of physiologically important compounds, such as PUFAs, by emergent aquatic insects is important to understand the implications of aquatic micropollutants on aquatic-terrestrial meta-ecosystems.

Combining bulk stable H isotope (δ2H) measurements with fatty acid profiles to examine differential use of aquatic vs. terrestrial prey by three sympatric species of aerial insectivorous birds

Aerial insectivorous songbirds such as swallows and martins have declined substantially in North America in recent decades. Aquatic-emergent insects provide more beneficial omega-3 fatty acids than terrestrial insects, and thus, diet quality is expected to vary among aerial insectivores with differential access to aquatic-emergent insects. We compared the stable hydrogen isotope (δ2H) values of feathers and bulk blood plasma fatty acids of nestling purple martins (Progne subis), tree swallows (Tachycineta bicolor), and barn swallows (Hirundo rustica), at lakeshore and inland sites near Lake Erie, Ontario, Canada. We found that diet quality differed between inland and lakeshore nesting habitats, but differences depended on species. Overall, purple martin and tree swallow nestlings had lower feather δ2H values, indicating a more aquatic-emergent diet, and lakeshore populations of both species had higher omega-3 fatty acid levels in their blood plasma compared to inland populations. Conversely, higher plasma levels of omega-6 fatty acids were found in inland birds. Tree swallows have a low omega-3 conversion efficiency from precursor substrates and so depend on aquatic subsidies to fulfill their nutritional needs. We suggest this may also be the case with purple martins. Barn swallows had the most positive feather δ2H values, regardless of proximity to the lakeshore, indicating a more terrestrial diet. However, barn swallow nestlings had consistently higher plasma omega-3 docosahexaenoic acid (DHA) regardless of nesting location, suggesting that barn swallows can efficiently convert omega-3 precursors into their beneficial elongated fatty acid chains. Our study indicates the benefit of combining plasma fatty acid compositional analyses with bulk feather δ2H values to decipher interspecific differences in adaptations to availability of aquatic-emergent insects.

Assessing spatial deposition of aquatic subsidies by insects emerging from agricultural streams

The role of winged aquatic insects that emerge from streams and subsidize terrestrial ecosystems has been demonstrated for natural forest landscapes, but almost no information is available for intensive agricultural landscapes. This study is the first to estimate aquatic subsidies provided by flying insects that emerge from streams and land on cropland. We investigated three major groups of aquatic insects - Trichoptera, Ephemeroptera and Chironomidae (Diptera) - that emerged from 12 third-order temperate, agricultural streams. We simultaneously monitored their emergence using floating traps and their terrestrial dispersal using passive interception traps. We estimated that the annual aquatic emerging dry mass (DM) of these groups varied from 1.4–7.5 g m−2 yr−1, depending on the stream. We used a Bayesian approach to estimate parameters of the terrestrial dispersal function of each group. We combined emerging DM and the dispersal parameters to estimate how terrestrial deposition of aquatic insect DM varied with increasing distance from streams. The results highlighted that emerging DM and dispersal to land could be higher in intensive agricultural landscapes than that previously described in natural settings. We estimated that 12.5 kg ha−1 yr−1 of winged aquatic insect DM fell to the ground 0–10 m from stream edges, composed mainly of Ephemeroptera and Trichoptera. We also estimated that 2.2 kg DM ha−1 yr−1 fell 10–50 m from the stream, especially small-bodied species of Chironomidae, throughout the year, except for the coldest weeks of winter. By influencing aquatic insect communities that emerge from streams, intensive agricultural practices change the magnitude and spatial extent of aquatic subsidy deposition on land. Implications for terrestrial food webs and ecosystem services provided to agriculture are discussed.

Impacts of Warming on Reciprocal Subsidies Between Aquatic and Terrestrial Ecosystems

Cross-ecosystem subsidies are important as their recipients often rely on them to supplementin situresource availability. Global warming has the potential to alter the quality and quantity of these subsidies, but our knowledge of these effects is currently limited. Here, we quantified the biomass and diversity of the invertebrates exchanged between freshwater streams and terrestrial grasslands in a natural warming experiment in Iceland. We sampled invertebrates emerging from the streams, those landing on the water surface, ground-dwelling invertebrates falling into the streams, and those drifting through the streams. Emerging invertebrate biomass or diversity did not change with increasing temperature, suggesting no effect of warming on aquatic subsidies to the terrestrial environment over the 1-month duration of the study. The biomass and diversity of aerial invertebrates of terrestrial origin landing on the streams increased with temperature, underpinned by increasing abundance and species richness, indicating that the greater productivity of the warmer streams may attract more foraging insects. The biomass of ground-dwelling invertebrates falling into the streams also increased with temperature, underpinned by increasing body mass and species evenness, suggesting that soil warming leads to terrestrial communities dominated by larger, more mobile organisms, and thus more in-fall to the streams. The biomass and diversity of terrestrial invertebrates in the drift decreased with temperature, however, underpinned by decreasing abundance and species richness, reflecting upstream consumption due to the higher energetic demands of aquatic consumers in warmer environments. These results highlight the potential for asynchronous responses to warming for reciprocal subsidies between aquatic and terrestrial environments and the importance of further research on warming impacts at the interface of these interdependent ecosystems.

Extensive aquatic subsidies lead to territorial breakdown and high density of an apex predator.

Energetic subsidies between terrestrial and aquatic ecosystems can strongly influence food webs and population dynamics. Our objective was to study how aquatic subsidies affected jaguar (Panthera onca) diet, sociality, and population density in a seasonally flooded protected area in the Brazilian Pantanal. The diet (n = 138 scats) was dominated by fish (46%) and aquatic reptiles (55%), representing the first jaguar population known to feed extensively on fish and to minimally consume mammals (11%). These aquatic subsidies supported the highest jaguar population density estimate to date (12.4 jaguars/100 km²) derived from camera traps (8,065 trap nights) and GPS collars (n = 13). Contrary to their mostly solitary behavior elsewhere, we documented social interactions previously unobserved between same-sex adults including cooperative fishing, co-traveling, and play. Our study demonstrates that aquatic subsidies, frequently described in omnivores, can also transform the ecology and behavior of obligate carnivores.

Riparian hunting spiders do not rely on aquatic subsidies from intermittent alpine streams

Drying in alpine streams might decrease aquatic-terrestrial trophic linkages by reducing terrestrial predation on aquatic prey. We tested this hypothesis by investigating whether a common riparian predator (hunting spiders) in alpine environments assimilated a lower proportion of aquatic prey with increasing stream intermittency. We used high temporal-resolution data from electrical resistance sensors to map patterns of naturally-occurring flow intermittency across 30 headwater streams of Val Roseg, a glacierized catchment in the Swiss Alps. We collected riparian hunting spiders, as well as potential terrestrial and aquatic macroinvertebrate prey, from streams and their associated riparian zones across two seasons (Alpine spring and summer). We estimated aquatic contributions to spider diets (pA) using (i) a gradient approach with aquatic invertebrate and spider carbon stable isotope ratio values (δ13C), and (ii) Bayesian carbon and nitrogen (δ15N) isotope mixing models. Spider pA from the gradient method were not statistically different from zero in spring (0.08 ± 0.10) and low in summer (0.16 ± 0.04). Mixing models also estimated low dependence on aquatic prey in both seasons, although with potentially higher contributions in summer. Spider diet did not vary with increasing flow intermittency in either season. Our results suggested that alpine hunting spiders obtain most of their carbon from terrestrial prey. The slight increase in spider pA during summer may correlate with peak emergence periods for aquatic insects, indicating opportunistic feeding by this riparian predator.

Riparian condition influences spider community structure and the contribution of aquatic carbon subsidies to terrestrial habitats

Degradation of riparian zones can alter aquatic and terrestrial communities of flora and fauna and disrupt their role in assimilating and mobilising carbon between the two ecosystems. Riparian spiders that predate on emergent aquatic invertebrates can contribute to carbon flux and the structure of aquatic and riparian food webs. The impact of riparian degradation on spiders in temperate rivers of Australia and their role in this broader ecosystem function is poorly understood. We surveyed the riparian zone of four rivers of south-western Australia in areas of natural intact vegetation and degraded agricultural land to explore whether riparian spider abundance, and diversity may be affected by changes to riparian condition. We also assessed the impact of the riparian condition on carbon fluxes between aquatic and terrestrial environments, using stable isotope analysis. We found overall abundance of riparian spiders was higher in degraded agricultural sites compared to natural intact sites and the structure of spider assemblages was different. Orb-weaver spiders (Araneidae and Tetragnathidae) were found to be more abundant in agricultural areas where canopy cover and understory are sparse as a result of livestock grazing and trampling. The contribution of carbon from aquatic invertebrates in a natural intact site was 48.5% for Orb-weavers and 41.6% for Cursorial Hunter spiders but reduced to 19.6% and 39.9% respectively in a degraded agricultural site. These results suggest that the position of spiders in riparian food webs and the amount of aquatic subsidy may change according to the condition and complexity of the riparian zone.

Allochthonous aquatic subsidies alleviate predation pressure in terrestrial ecosystems

The input of external energy and matter in recipient ecosystems can act as a bottom-up force that subsidizes consumers, with subsequent cascading effects throughout the food web. Depending on the amount of input, dietary preference, and the strength of trophic links, allochthonous resources generally play a stabilizing role on food webs. In this study, we investigated the stabilizing role of allochthonous aquatic resources on intraguild predation (IGP) and their consequences on shared prey in a terrestrial ecosystem. To this end, we manipulated the input of emergent aquatic insects (the allochthonous resources) from streams to land, and predation pressure by bats and birds (the top predators), in a multitrophic food web using an orthogonal exclusion experiment. Using stable isotope metrics, we found that bats, birds, and spiders (the mesopredators), were highly subsidized by emergent aquatic insects. Moreover, among terrestrial prey, top predators fed more on spiders than insects. As predicted, spiders were strongly affected by the presence of top predators when allochthonous resources were excluded. Consequently, in this scenario terrestrial insects were two times more abundant. Because spiders showed a higher preference for consuming aquatic resources, we suggest that nonconsumptive effects of spiders upon terrestrial insects could be mediating the strong response of those shared prey. We demonstrate that the input of allochthonous aquatic resources can play a fundamental role in stabilizing terrestrial trophic interactions and trophic cascades in riparian zones via decreasing predation pressure.

The nutrient legacy left by salmon tissue on riparian soils in Southeast Alaska

AbstractSalmon‐derived nutrients (SDN) are a distinctive aquatic subsidy to terrestrial ecosystems. Streamwater nutrient increases in response to SDN have been documented but uncertainties about the magnitude and persistence of their effects in riparian areas remain. A key research gap is the response of specific soil types to the nutrient subsidy over time. To address this question, an Entisol and a Spodosol commonly found in riparian zones of Southeast Alaska were delineated adjacent to streams with abundant spawning salmon for experimental nutrient additions. Salmon nutrient additions were applied to track the fate of N and P through time to investigate the patterns of nutrient retention and loss in the Entisol and Spodosol. Salmon additions to Entisols across eight watersheds revealed a large and consistent residual concentration of available NH4+–N but a high variability in NO3−–N after a 30‐d incubation. A comparison of Entisols and Spodosols in two watersheds tracked salmon tissue additions for ∼1 yr, where nutrient concentrations at 240 d decreased relative to 30 d but a flush of available nutrients occurred at 340 d, with higher values in the salmon‐amended soils. The magnitudes of extractable N and P retained in the extractable fraction were similar in the Entisol and Spodosol but dissolved N loss measured via resin beads at the base of the measurement cores indicated higher leaching of NO3−–N from the surface in Entisols. Identification of specific soil types can advance the understanding of nutrient processing, uptake, and loss from SDN additions on riparian landforms.

Stream network geometry and the spatial influence of aquatic insect subsidies across the contiguous United States

AbstractEmergent aquatic insects transport aquatic‐derived resources into terrestrial ecosystems but are rarely studied at landscape or regional scales. Here, we investigate how stream network geometry constrains the spatial influence of aquatic insect subsidies in terrestrial ecosystems. We also explore potential factors (i.e., climate, topography, soils, and vegetation) that could produce variation in stream network geometry and thus change the extent of aquatic insect subsidies from one region to another. The stream signature is the percentage of aquatic insect subsidies traveling a given distance into the terrestrial ecosystem, relative to what comes out of the stream. We use this concept to model the spatial extent (area) and distribution (spatial patterning) of aquatic subsidies in terrestrial ecosystems across the contiguous United States. Our findings suggest that at least 8% of the subsidies measured at the aquatic–terrestrial boundary (i.e., the 8% stream signature) are typically transferred throughout the entire watershed and that variation in this spatial extent is largely influenced by the drainage density of the stream network. Moreover, we found stream signatures from individual stream reaches overlap such that the spatial extent of the 8% stream signature often includes inputs from multiple stream reaches. Landscape‐scale stream network characteristics increased the area of overlapping stream signatures more than reach‐scale channel properties. Finally, we found runoff was an important factor influencing stream network geometry suggesting a potential effect of climate on aquatic‐to‐terrestrial linkages that have been understudied.

Aquatic and terrestrial resources are not nutritionally reciprocal for consumers

Abstract Aquatic and terrestrial ecosystems are connected through reciprocal fluxes of energy and nutrients that can subsidize consumers. Past research on reciprocal aquatic–terrestrial subsidies to consumers has generally focused on subsidy quantity while ignoring major differences in the nutritional composition of aquatic and terrestrial resources. Because aquatic resources contain substantially more highly unsaturated omega‐3 fatty acids (HUFAs) than terrestrial resources, aquatic subsidies may play a unique role by supplying these critical compounds to both aquatic and terrestrial consumers. Here, we first characterized nutritional quality in terms of HUFA content in aquatic and terrestrial insect prey. We then used bulk stable isotope analyses to estimate subsidy use by stream and riparian consumers coupled with compound‐specific stable isotope analyses, which allowed us to document consumer HUFA sources. Finally, in order to understand the nutritional importance of aquatic‐derived HUFAs for riparian consumers, we conducted manipulative diet experiments on Eastern Phoebe (Sayornis phoebe) chicks in the laboratory. Aquatic insects were significantly enriched in HUFAs, mainly in terms of eicosapentaenoic acid (EPA), compared with terrestrial insects. Stream fishes relied mainly upon aquatic resources, while insectivorous birds varied in their use of aquatic subsidies across sites. However, like stream fishes, Eastern Phoebe chicks received HUFAs from aquatic insects, even when they were heavily reliant upon terrestrial insects for their overall diet. In the laboratory, dietary HUFAs, such as those supplied by aquatic insects, increased the growth rate and condition of Eastern Phoebe chicks. This study demonstrates that aquatic and terrestrial subsidies are not nutritionally reciprocal from the perspective of consumers because aquatic resources are the main source of critical fatty acids for both stream and riparian consumers. It also confirms previous findings on the nutritional importance of HUFAs for riparian birds, demonstrating that an insectivorous riparian lifestyle influences avian nutritional needs. Finally, our findings raise the possibility that birds and other riparian insectivores may experience nutritional mismatches with terrestrial prey if they do not have access to high‐quality aquatic subsidies as a consequence of aquatic habitat degradation or shifts in consumer and resource phenology. A free Plain Language Summary can be found within the Supporting Information of this article.

Arthropod Assemblages in a Montane Wetland Complex: Influences of Adjoining Lotic and Lentic Habitat and Temporal Variability

Our objective was to gain an understanding of the influences of habitat context and seasonal and interannual factors on arthropod assemblage structure in a wetland environment. We hypothesized that river and pond riparian habitats in the wetland would have greater diversity and abundance than core wetland habitat, and that these differences would be driven by aquatic subsidy via emerging aquatic insects. We also hypothesized that diversity and abundance of terrestrial fauna would decline through the dry summer. We sampled the study wetland, in Yosemite National Park, California, USA, through the growing seasons of 2013 and 2014; a large wildfire (> 100,000 ha) burned the entire study site during late summer of 2013. Assemblage structure was strongly influenced by habitat context, season, and year. Diversity and abundance were high at the river riparian sites, but these results were driven by a diverse and abundant terrestrial fauna, rather than by large numbers of emerging aquatic insects. Faunal assemblages became increasingly depauperate through the summer, likely due to drying of wetland habitat in this hot Mediterranean-type climate. Fire probably had a strong influence on faunal assemblages and vegetation structure, but we cannot rule out interannual variability independent of the fire.

Seasonal and spatial variations of stream insect emergence in an intensive agricultural landscape

A growing amount of literature exists on reciprocal fluxes of matter and energy between ecosystems. Aquatic subsidies of winged aquatic insects can affect terrestrial ecosystems significantly, but this issue is rarely addressed in agroecosystems. By altering the production of benthic macroinvertebrates, agricultural practices could increase or decrease the strength of aquatic subsidies and subsequently the provision of several ecosystem services to agriculture. Effects of seasons and environmental variables on aquatic insect emergence were investigated in third-order agricultural streams in northwestern France. Most emerging dry mass (DM) of caught insects belonged to Trichoptera (56%), Chironomidae (25%) and Ephemeroptera (19%). We estimated that annual emerging dry mass of aquatic insects ranged between 1445 and 7374 mg/m2/y depending on the stream. Seasonal variations were taxon-specific, with Ephemeroptera emerging only in spring, Trichoptera emerging in spring and early summer, and Chironomidae emerging throughout the year. The percentage of watershed area covered by agriculture, ammonium concentration and hypoxia positively influenced emerging DM of Chironomidae but negatively influenced Ephemeroptera. Emerging DM of Trichoptera and Ephemeroptera increased significantly as water conductivity and temperature increased. Channel openness increased the emerging DM of all taxonomic groups, but Chironomidae were more abundant in narrow, incised streams. Assuming that the biomass of aquatic invertebrates ultimately disperse toward terrestrial habitats, nutrient accumulations on land near streams were estimated to reach 0.5–2.3 kg C ha−1 y−1, 0.1–0.5 kg N ha−1 y−1 and 0.005–0.03 kg P ha−1 y−1, depending on the stream. This suggests a significant flux of aquatic nutrients to agroecosystems and the need for future studies of its potential influence on the ecosystem services provided to agriculture.

DNA metabarcoding of nestling feces reveals provisioning of aquatic prey and resource partitioning among Neotropical migratory songbirds in a riparian habitat.

Riparian habitats are characterized by substantial flows of emergent aquatic insects that cross the stream-forest interface and provide an important source of prey for insectivorous birds. The increased availability of prey arising from aquatic subsidies attracts high densities of Neotropical migratory songbirds that are thought to exploit emergent aquatic insects as a nestling food resource; however, the prey preferences and diets of birds in these communities are only broadly understood. In this study, we utilized DNA metabarcoding to investigate the extent to which three syntopic species of migratory songbirds-Acadian Flycatcher, Louisiana Waterthrush, and Wood Thrush-breeding in Appalachian riparian habitats (Pennsylvania, USA) exploit and partition aquatic prey subsidies as a nestling food resource. Despite substantial differences in adult foraging strategies, nearly every nestling in this study consumed aquatic taxa, suggesting that aquatic subsidies are an important prey resource for Neotropical migrants nesting in riparian habitats. While our results revealed significant interspecific dietary niche divergence, the diets of Acadian Flycatcher and Wood Thrush nestlings were strikingly similar and exhibited significantly more overlap than expected. These results suggest that the dietary niches of Neotropical migrants with divergent foraging strategies may converge due to the opportunistic provisioning of non-limiting prey resources in riparian habitats. In addition to providing the first application of DNA metabarcoding to investigate diet in a community of Neotropical migrants, this study emphasizes the importance of aquatic subsidies in supporting breeding songbirds and improves our understanding of how anthropogenic disturbances to riparian habitats may negatively impact long-term avian conservation.

Contrasting effects of aquatic subsidies on a terrestrial trophic cascade.

Subsidies from adjacent ecosystems can alter recipient food webs and ecosystem functions, such as herbivory. Emerging aquatic insects from streams can be an important prey in the riparian zone. Such aquatic subsidies can enhance predator abundances or cause predators to switch prey, depending on the herbivores. This can lead to an increase or decrease of in situ herbivores and herbivory. We examined the effects of aquatic subsidies on a simplified terrestrial food web consisting of two types of herbivores, plants and predators (spiders). In our six-week experiment, we focused on the prey choice of the spiders by excluding predator immigration and reproduction. In accordance with predator switching, survival of leafhoppers increased in the presence of aquatic subsidies. By contrast, the presence of aquatic subsidies indirectly reduced weevils and herbivory. Our study shows that effects of aquatic subsidies on terrestrial predators can propagate through the food web in contrasting ways. Thereby, the outcome of the trophic cascade is determined by the prey choice of predators.

Subsidies of Aquatic Resources in Terrestrial Ecosystems

Floods, spatially complex water flows, and organism movements all generate important fluxes of aquatic-derived materials into terrestrial habitats, counteracting the gravity-driven downhill transport of matter from terrestrial-to-aquatic ecosystems. The magnitude of these aquatic subsidies is often smaller than terrestrial subsidies to aquatic ecosystems but higher in nutritional quality, energy density, and nutrient concentration. The lateral extent of biological aquatic subsidies is typically small, extending only a few meters into riparian habitat; however, terrestrial consumers often aggregate on shorelines to capitalize on these high-quality resources. Although the ecological effects of aquatic subsidies remain partially understood, it is clear that ongoing human modification to aquatic ecosystems, riparian habitats and river floodplains affect the magnitude, quality, and spatial and temporal patterning of aquatic subsidies in terrestrial landscapes. These changes will alter the character of aquatic–terrestrial coupling and have consequences for terrestrial organisms that rely on these high-quality and temporally dependable resource subsidies. Homogenization of landscapes and flow regimes, eutrophication, exotic species, and contaminants all represent threats to the vital flows of aquatic-derived resources into terrestrial ecosystems. Research emphasizing that landscapes are integrated terrestrial–aquatic systems, characterized by both biological and hydrological flows among habitats, is needed for understanding the consequences of aquatic subsidies and managing ecological risks of ongoing human development.

Prothonotary warbler nestling growth and condition in response to variation in aquatic and terrestrial prey availability.

Aquatic prey subsidies entering terrestrial habitats are well documented, but little is known about the degree to which these resources provide fitness benefits to riparian consumers. Riparian species take advantage of seasonal pulses of both terrestrial and aquatic prey, although aquatic resources are often overlooked in studies of how diet influences the reproductive ecology of these organisms. Ideally, the timing of resource pulses should occur at the time of highest reproductive demand. This study investigates the availability of aquatic (mayfly) and terrestrial (caterpillar) prey resources as well as the nestling diet of the prothonotary warbler (Protonotaria citrea) at two sites along the lower James River in Virginia during the 2014 breeding season. We found large differences in availability of prey items between the two sites, with one having significantly higher mayfly availability. Nestling diet was generally reflective of prey availability, and nestlings had faster mean growth rates at the site with higher aquatic prey availability. Terrestrial prey were fed more readily at the site with lower aquatic prey availability, and at this site, nestlings fed mayflies had higher mean growth rates than nestlings fed only terrestrial prey. Our results suggest that aquatic subsidies are an important resource for nestling birds and are crucial to understanding the breeding ecology of riparian species.