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How wide is a stream? Spatial extent of the potential "stream signature" in terrestrial food webs using meta-analysis
Abstract
The magnitude of cross-ecosystem resource subsidies is increasingly well recognized; however, less is known about the distance these subsidies travel into the recipient landscape. In streams and rivers, this distance can delimit the "biological stream width," complementary to hydro-geomorphic measures (e.g., channel banks) that have typically defined stream ecosystem boundaries. In this study we used meta-analysis to define a "stream signature" on land that relates the stream-to-land subsidy to distance. The 50% stream signature, for example, identifies the point on the landscape where subsidy resources are still at half of their maximum (in- or near-stream) level. The decay curve for these data was best fit by a negative power function in which the 50% stream signature was concentrated near stream banks (1.5 m), but a non-trivial (10%) portion of the maximum subsidy level was still found > 0.5 km from the water's edge. The meta-analysis also identified explanatory variables that affect the stream signature. This improves our understanding of ecosystem conditions that permit spatially extensive subsidy transmission, such as in highly productive, middle-order streams and rivers. Resultant multivariate models from this analysis may be useful to managers implementing buffer rules and conservation strategies for stream and riparian function, as they facilitate prediction of the extent of subsidies. Our results stress that much of the subsidy remains near the stream, but also that subsidies (and aquatic organisms) are capable of long-distance dispersal into adjacent environments, and that the effective "biological stream width" of stream and river ecosystems is often much larger than has been defined by hydro-geomorphic metrics alone. Limited data available from marine and lake sources overlap well with the stream signature data, indicating that the "signature" approach may also be applicable to subsidy spatial dynamics across other ecosystems.
... Almost all imagoes are winged and can fly [7], which allow them to disperse in terrestrial environments. Numerous studies on the dispersal of aquatic insects in terrestrial landscapes have shown a decrease in the abundance of aquatic insects with distance from water [8][9][10], thus defining a "biological stream width" [11]. ...
... We compared the goodness of fit of negative binomial regressions between EPTM abundance and preexisting (and static) hydrographic mapping (i.e., the OpenStreetMap source) and four field hydrographic networks, including or not including ponds and ditches and their evolution over time (i.e., session). Insect abundance is highest near their emerging sources [11]. We therefore hypothesized that the hydrographic network that best explains aquatic insect abundance (i.e., with the highest pseudo R 2 ) is the one that best represents the emergence sources among the studied networks. ...
... Distance to water is the main factor which explains the abundances of emerging aquatic insects [11]. We showed that we can greatly improve the explanatory power of models by more precisely defining the sources of emergence. ...
... Similarly, Walters et al. (2018) found that stonefly emergence in the most productive western USA streams resulted in a mean annual export of 10 g of carbon per m of streambank to riparian ecosystems. In a review of the spatial dispersal of winged stream insects, Muehlbauer et al. (2014) showed that among the aquatic insects that emerge (i.e. Ephemeroptera, Plecoptera, Trichoptera and Diptera), some can disperse up to 50 m or more from streams. ...
... Williams and Hynes, 1976;Hershey et al., 1993), inland dispersal and terrestrial deposition of winged aquatic insects emerging from streams are not well known. Because most studies of dispersal of adult aquatic insects (reviewed by Muehlbauer et al., 2014) were conducted in forested areas, information from intensive agricultural contexts is rare. In a companion study (Raitif et al., 2018), we showed that insect biomass emerging from streams in intensive agricultural landscapes was greater than most of those reported for more pristine systems. ...
... This distance was observed to be only 1 m along forest streams . Similarly, half of the Chironomidae dispersed farther than 25 m from our agricultural streams, which is nearly twice the distance observed by Muehlbauer et al. (2014) along forest streams. Thus, according to Muehlbauer et al.'s definition, the "biological width" of a stream, as opposed to its hydro-geomorphic width, is broader in agricultural contexts than in forest contexts. ...
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.
... Many species depend on both upland and wetland sites for some portions of their life cycles [48,49]. A growing body of literature supports the hypothesis that streams subsidize food webs and energy transfer of terrestrial ecosystems, and that the magnitude of these subsidies can have large positive impacts on habitat quality [50][51][52]. ...
... We applied methods developed by [52], who found that a negative power function best matches the observed relationship between terrestrial habitat condition and distance to aquatic habitats across a wide range of ecosystem types. We applied a normalized aquatic subsidy curve [52] by including a weighted addition to the HCI value of any pixel within one kilometer of a river, lake, or wetland. ...
... We applied methods developed by [52], who found that a negative power function best matches the observed relationship between terrestrial habitat condition and distance to aquatic habitats across a wide range of ecosystem types. We applied a normalized aquatic subsidy curve [52] by including a weighted addition to the HCI value of any pixel within one kilometer of a river, lake, or wetland. ...
In 2021, the Biden administration signed an executive order to protect 30% of American lands by 2030. Accomplishing this ambitious goal in the U.S. requires understanding the relative contribution of public and private lands toward supporting biodiversity. New approaches are needed because existing approaches focus on quantity of habitat without incorporating quality. To fill this need, we developed a 30 m resolution national habitat condition index (HCI) that integrates quality and quantity measures of habitat. We hypothesized that including an evaluation of the quality of habitat at landscape scales, both in conservation-focused preserves and working lands would provide a better assessment of the value of geographies for conservation. We divided the conterminous U.S. by major land cover type and into natural and cultivated lands and then spatially mapped multiple anthropogenic stressors, proximity to aquatic habitat, and vegetation departure from expected natural disturbance regimes. Each map layer was then scored for site impact and distance decay and combined into a final national index. Field observations providing scored relative ecological conditions were used for HCI calibration and validation at both CONUS and regional scales. Finally, we evaluate lands by management (conservation versus working lands) and ownership (public versus private) testing the value of these lands for conservation. While we found regional differences across CONUS, functional habitat was largely independent of protection status: working lands provide clear habitat and other values. These results are relevant for guiding strategies to achieve the U.S. 30 by 30 goals. Where similar data exist in other countries, analogous modeling could be used to meet their national conservation commitments.
... Riparian and aquatic habitats have a range of important functional connections including exchanges of organisms, energy and matter (Baxter et al., 2005;Muehlbauer et al., 2014;Schneider et al., 2002 ). Aquaticterrestrial subsidies include both inputs of organic matter which are fed upon by aquatic detritivores, and the emergence of adult aquatic insects which provide food for terrestrial insectivores. ...
... However, individuals were still caught at the maximum lamp/sampling distance of 80 m from the river. The ALAN impact zone for Ephemeroptera around waterbodies could therefore be relatively wide; under natural conditions individuals have been found as far as 160 m from the river edge (Muehlbauer et al., 2014). This reduction in abundance with distance could be explained by their distinct swarming behaviour. ...
... In our study, Trichoptera were still captured at the maximum sampling distance of 80 m, indicating that the ALAN impact zone for this group could still be quite wide. Muehlbauer et al. (2014) found that Trichoptera dispersed laterally up to 650 m away from the river edge under natural conditions. In contrast, Finn and Poff (2008) did not record a rapid decline in Trichoptera abundance with distance. ...
Artificial light at night (ALAN) is increasing globally, and changing in quality due to the installation of white LED street lighting. ALAN is a threat to biodiversity and ecosystem functioning, yet important knowledge gaps exist regarding the magnitude of impacts and how these vary between habitats and levels of exposure. The disturbance of aquatic habitats by ALAN is of particular concern as human settlements and activities are often located near waterbodies, and many aquatic species are sensitive to ALAN. Focusing on adult aquatic insects, an experimental approach was employed in the riparian zone of a structurally simplified river within a dark rural landscape. Two studies were used to (a) estimate the magnitude of the capture effect of white LED lamps and (b) to explore how captures at lamps vary with their distance from the river, and define any distance thresholds. Both studies sampled mayflies (Ephemeroptera), caddisflies (Trichoptera) and true flies (Diptera) repeatedly during mid‐to‐late summer using modified flight intercept traps positioned adjacent to portable LED lamps. In Study A, lit traps were paired with unlit controls. In Study B, lit traps were positioned at six distances up to a maximum of 80 m from the stream edge. For each of the three study orders, captures were significantly higher in the lit treatment compared to the dark control, with medium to large effect sizes. For all study orders, captures at lamps significantly reduced with increasing distance from the river edge. Rapid declines in captures were recorded for Trichoptera (from 10 m) and Ephemeroptera (40 m), with a more gradual decline in Diptera from 60 m that continued up to the maximum sample distance. Previous research indicates that LED lighting can be less attractive to flying insects than broader spectrum alternatives. However, this study demonstrates that the effects of white LED lamps on flying adult aquatic insects should not be dismissed and can extend far from aquatic habitats. As a precautionary approach, and until finer recommendations are available, we recommend that LED lamps should be excluded from a buffer zone of ca. 40–60 m around rivers. Artificial light at night (ALAN) is increasing globally, and changing in quality due to the installation of white LED street lighting. ALAN is a threat to biodiversity and ecosystem functioning, yet important knowledge gaps exist regarding the magnitude of impacts and how these vary between habitats and levels of exposure. The disturbance of aquatic habitats by ALAN is of particular concern as human settlements and activities are often located near waterbodies, and many aquatic species are sensitive to ALAN. Focusing on adult aquatic insects, an experimental approach was employed in the riparian zone of a structurally simplified river within a dark rural landscape. Two studies were used to (a) estimate the magnitude of the capture effect of white LED lamps and (b) to explore how captures at lamps vary with their distance from the river, and define any distance thresholds. Both studies sampled mayflies (Ephemeroptera), caddisflies (Trichoptera) and true flies (Diptera) repeatedly during mid‐to‐late summer using modified flight intercept traps positioned adjacent to portable LED lamps. In Study A, lit traps were paired with unlit controls. In Study B, lit traps were positioned at six distances up to a maximum of 80 m from the stream edge. For each of the three study orders, captures were significantly higher in the lit treatment compared to the dark control, with medium to large effect sizes. For all study orders, captures at lamps significantly reduced with increasing distance from the river edge. Rapid declines in captures were recorded for Trichoptera (from 10 m) and Ephemeroptera (40 m), with a more gradual decline in Diptera from 60 m that continued up to the maximum sample distance. Previous research indicates that LED lighting can be less attractive to flying insects than broader spectrum alternatives. However, this study demonstrates that the effects of white LED lamps on flying adult aquatic insects should not be dismissed and can extend far from aquatic habitats. As a precautionary approach, and until finer recommendations are available, we recommend that LED lamps should be excluded from a buffer zone of ca. 40–60 m around rivers. This study assesses the magnitude of the capture effect of white LED lamps on flying adult aquatic insects, and explores how captures at lamps vary with their distance from the river. Using modified flight intercept traps we estimated the effects of LED lamps, and defined distance thresholds at which captures significantly reduced. Die nächtliche Beleuchtung nimmt weltweit zu und verändert sich qualitativ aufgrund der Installierung von weissen LED‐Strassenlampen. Lichtverschmutzung ist eine bekannte Bedrohung für die biologische Vielfalt und die Ökosystemfunktionen. Es bestehen jedoch wichtige Wissenslücken bezüglich des Ausmasses der Auswirkungen und wie diese zwischen Lebensräumen und Expositionsniveaus variieren. Die Störung der Gewässerlebensräume durch künstliches Licht in der Nacht ist besonders besorgniserregend, da sich menschliche Siedlungen und Aktivitäten oft in der Nähe von Oberflächengewässern befinden und viele gewässerbewohnende Arten reagieren empfindlich auf Kunstlicht. Mit dem Schwerpunkt auf adulte Wasserinsekten wurde ein experimenteller Ansatz im Uferbereich eines strukturarmen Flusses in einer dunklen ländlichen Landschaft angewandt. Zwei Studien wurden durchgeführt, um a) das Ausmass des Fangeffekts von weissen LED‐Lampen zu quantifizieren und b) zu ermitteln, wie die Fänge an den Lampen mit ihrem Abstand zum Fluss variieren und um Abstandsgrenzwerte zu definieren. In beiden Studien wurden im Hoch‐ bis Spätsommer wiederholt Eintagsfliegen (Ephemeroptera), Köcherfliegen (Trichoptera) und Zweiflügler (Diptera) mithilfe von kombinierten Flugfallen, welche unmittelbar vor tragbaren LED‐Lampen aufgestellt wurden, gesammelt. In Studie A wurden beleuchtete Fallen mit unbeleuchteten Kontrollen verglichen. In Studie B wurden beleuchtete Fallen in sechs Abständen bis zum Maximum von 80 m vom Flussufer aufgestellt. Für jede der drei untersuchten Ordnungen waren die Fangzahlen in der beleuchteten Behandlung signifikant höher als in der dunklen Kontrolle mit mittleren bis grossen Effektgrössen. Für alle untersuchten Ordnungen nahmen die Fangzahlen an den Lampen mit zunehmender Entfernung vom Flussufer signifikant ab. Starke Rückgänge der Fangzahlen wurden für Trichoptera (ab 10 m) und Epheme–+roptera (40 m) beobachtet, mit einem sukzessiveren Rückgang von Diptera ab 60 m, welcher bis zur maximalen Stichprobenentfernung anhielt. Synthese und Anwendungen. Bisherige Forschungsarbeiten haben gezeigt, dass LED‐Beleuchtung für fliegende Insekten weniger anziehend sein kann als Alternativen mit breitem Spektrum. Allerdings beweist diese Studie, dass die Auswirkungen von weissen LED‐Lampen auf fliegende adulte Wasserinsekten nicht abzustreiten sind und sich bis weit über Gewässerlebensräume erstrecken können. Bis genauere Erkenntnisse vorliegen, empfehlen wir gemäss dem Vorsorgeprinzip, dass LED‐Lampen aus einem Pufferstreifen von ca. 40 bis 60 m entlang von Fliessgewässern ausgeschieden werden sollten. Die nächtliche Beleuchtung nimmt weltweit zu und verändert sich qualitativ aufgrund der Installierung von weissen LED‐Strassenlampen. Lichtverschmutzung ist eine bekannte Bedrohung für die biologische Vielfalt und die Ökosystemfunktionen. Es bestehen jedoch wichtige Wissenslücken bezüglich des Ausmasses der Auswirkungen und wie diese zwischen Lebensräumen und Expositionsniveaus variieren. Die Störung der Gewässerlebensräume durch künstliches Licht in der Nacht ist besonders besorgniserregend, da sich menschliche Siedlungen und Aktivitäten oft in der Nähe von Oberflächengewässern befinden und viele gewässerbewohnende Arten reagieren empfindlich auf Kunstlicht. Mit dem Schwerpunkt auf adulte Wasserinsekten wurde ein experimenteller Ansatz im Uferbereich eines strukturarmen Flusses in einer dunklen ländlichen Landschaft angewandt. Zwei Studien wurden durchgeführt, um a) das Ausmass des Fangeffekts von weissen LED‐Lampen zu quantifizieren und b) zu ermitteln, wie die Fänge an den Lampen mit ihrem Abstand zum Fluss variieren und um Abstandsgrenzwerte zu definieren. In beiden Studien wurden im Hoch‐ bis Spätsommer wiederholt Eintagsfliegen (Ephemeroptera), Köcherfliegen (Trichoptera) und Zweiflügler (Diptera) mithilfe von kombinierten Flugfallen, welche unmittelbar vor tragbaren LED‐Lampen aufgestellt wurden, gesammelt. In Studie A wurden beleuchtete Fallen mit unbeleuchteten Kontrollen verglichen. In Studie B wurden beleuchtete Fallen in sechs Abständen bis zum Maximum von 80 m vom Flussufer aufgestellt. Für jede der drei untersuchten Ordnungen waren die Fangzahlen in der beleuchteten Behandlung signifikant höher als in der dunklen Kontrolle mit mittleren bis grossen Effektgrössen. Für alle untersuchten Ordnungen nahmen die Fangzahlen an den Lampen mit zunehmender Entfernung vom Flussufer signifikant ab. Starke Rückgänge der Fangzahlen wurden für Trichoptera (ab 10 m) und Epheme–+roptera (40 m) beobachtet, mit einem sukzessiveren Rückgang von Diptera ab 60 m, welcher bis zur maximalen Stichprobenentfernung anhielt. Synthese und Anwendungen. Bisherige Forschungsarbeiten haben gezeigt, dass LED‐Beleuchtung für fliegende Insekten weniger anziehend sein kann als Alternativen mit breitem Spektrum. Allerdings beweist diese Studie, dass die Auswirkungen von weissen LED‐Lampen auf fliegende adulte Wasserinsekten nicht abzustreiten sind und sich bis weit über Gewässerlebensräume erstrecken können. Bis genauere Erkenntnisse vorliegen, empfehlen wir gemäss dem Vorsorgeprinzip, dass LED‐Lampen aus einem Pufferstreifen von ca. 40 bis 60 m entlang von Fliessgewässern ausgeschieden werden sollten.
... For example, birds, bats, spiders, and carabid beetles can depend on aquatic insects at least sometime during the year and this transfer can include transfer of other ecologically important elements (such as mercury) (Walters et al., 2020). Understanding of how and when adult insects disperse, and how much and to where resources are transferred by those vectors can provide critical insights into studies focusing on subsidy and critical life-cycle stage of insects and ecologically sound management of river-riparian ecosystems (Muehlbauer et al., 2014;Smith et al., 2009). ...
... For instance, large-sized species with long-range dispersal ability would provide different characteristics of transferring resources compared to those with relatively small body size and limited dispersal capability. Aquatic insect dispersal patterns have been reported in studies on reproductive behaviors as well as the potential extent of river-derived subsidy in the riparian zones (Carlson et al., 2016;Muehlbauer et al., 2014). Relatively well-studied insect taxa include Ephemeroptera (E), Plecoptera (P), and Trichoptera (T) (see , Braun et al., 2014;Lancaster & Downes, 2018;Petersen et al., 2004). ...
... important for their population dynamics, as well as potential prey for terrestrial consumers in the recipient riparian zone (Muehlbauer et al., 2014;Smith et al., 2009 A relatively high contribution of hyporheic resources to the riparian zone was caused partially by the trait of flight dispersal of the targeted taxa, which was similar to non-hyporheic plecopteran insects. Regardless of their aquatic habitat, that is, benthic or hyporheic, similar behavioral characteristics in flight dispersal may be common among plecopteran species. ...
Hyporheic zone (HZ) locates below the riverbed providing habitat for macroinvertebrates from where the winged adult insects (i.e., hyporheic insects, HIs) emerge and bring out aquatic resources to the riparian zone. This study estimated mean daily flux as dry biomass (BM), carbon (C), and nitrogen (N) deriving from the dominant HI species Alloperla ishikariana (Plecoptera, Chloroperlidae) for a 4th‐order gravel‐bed river during the early‐summer to summer periods. We hypothesized that HIs were an important contributor in total aquatic resources to the riparian zone. In 2017 and 2018, we set parallelly (May to August) and perpendicularly (June to October) oriented Malaise traps to catch the lateral and longitudinal directional dispersing winged adults of A. ishikariana, and other Ephemeroptera, Plecoptera, Trichoptera, and Diptera from the river and estimated the directional fluxes of them. We further split the directional fluxes as moving away or back to the channel (for lateral) and from down‐ to upstream or up‐ to downstream (for longitudinal). Alloperla ishikariana was similar to other Plecoptera species and differed clearly from Ephemeroptera and Trichoptera in directional characteristics of resources flux, suggesting that the extent and directions of HZ‐derived resource transfer depend on taxon‐specific flight behaviors of HIs. Contributions of A. ishikariana to the riparian zone in total aquatic C and N transfer seasonally varied and were lower in May (5%–6%) and August (2%–4%) and the highest in July (52%–70%). These conservative estimates largely increased (9% in May) after the supplementary inclusion of Diptera (Chironomidae and Tipulidae), part of which were considered HIs. We demonstrated that HZ could seasonally contribute a significant portion of aquatic resources to the riparian zone and highlighted the potential importance of HZ in nutrient balance in the river‐riparian ecosystem.
... The effect of resource subsidies is typically studied at local scales, which limits our understanding of large scale geographical variation in ecosystem linkages (Marcarelli et al. 2011, Subalusky and Post 2018, Lafage et al. 2019. When subsidies are transported by animals, their dispersal and development traits should influence the recipient ecosystem, determining the spatial extent of resource subsidies (i.e. the distance they travel form the donor ecosystem) and the period of time they are available to consumers (Yang et al. 2010, Muehlbauer et al. 2014, Gratton et al. 2017, Subalusky and Post 2018. If the presence of traits varies predictably with environmental conditions, trait distributions could underpin macroscale patterns in ecosystem linkages (Montagano et al. 2018, Lafage et al. 2019. ...
... The distance adult aquatic insects travel from the stream determines whether they are accessible to terrestrial consumers living further from the water's edge (Carlson et al. 2016). Dispersal distance should be related to an individual's flight strength, adult life span, and/or body size (Muehlbauer et al. 2014, McKie et al. 2018, Lancaster et al. 2020. ...
... Ephemeroptera, Plecoptera, Trichoptera, Diptera and Odonata; Baxter et al. 2005) relative to other members of the community as an indicator of the supply of aquatic insect subsidies. We estimated the spatial extent of aquatic insect subsidies using the 'stream signature' concept, where the stream signature is the distance a given proportion of insect subsidies measured at the water's edge travels perpendicular from the stream (Muehlbauer et al. 2014). We calculated the distance that 25% of the subsidies emerging from the stream travel away from the water (i.e. ...
Ecological flows across ecosystem boundaries are typically studied at spatial scales that limit our understanding of broad geographical patterns in ecosystem linkages. Aquatic insects that metamorphose into terrestrial adults are important resource subsidies for terrestrial ecosystems. Traits related to their development and dispersal should determine their availability to terrestrial consumers. Here, we synthesize geospatial, aquatic biomonitoring and biological traits data to quantify the relative importance of several environmental gradients on the potential spatial and temporal characteristics of aquatic insect subsidies across the contiguous United States. We found the trait composition of benthic macroinvertebrate communities varies among hydrologic regions and could affect how aquatic insects transport subsidies as adults. Further, several trait–environment relationships were underpinned by hydrology. Large bodied taxa that could disperse further from the stream were associated with hydrologically stable conditions. Alternatively, hydrologically variable conditions were associated with multivoltine taxa that could extend the duration of subsidies with periodic emergence events throughout the year. We also found that anthropogenic impacts decrease the frequency of individuals with adult flight but potentially extend the distance subsidies travel into the terrestrial ecosystem. Collectively, these results suggest that natural and anthropogenic gradients could affect aquatic insect subsidies by changing the trait composition of benthic macroinvertebrate communities. The conceptual framework and trait–environment relationships we present shows promise for understanding broad geographical patterns in linkages between ecosystems.
... Few studies have quantified how far stream subsidies travel into the terrestrial landscape and the conditions that affect this transmission (but see Muehlbauer et al., 2014). Some studies have demonstrated that aquatic insect productivity and emergence rates are important drivers of subsidy dynamics (Polis et al., 2004;Ballinger and Lake, 2006), but the relative importance of aquatic insect productivity in affecting the distance that stream subsidies penetrate into the landscape remains largely unknown. ...
... Increasing distance from the water was assumed to coincide with gradual changes in prey availability. Some studies suggested that many emergent adult insects disperse no more than 50 m from water (Sanzone et al., 2003;Jonsson and Wardle, 2009), but more recent studies reported distances of up to 5 km (Muehlbauer et al., 2014;Kautza and Sullivan, 2016). During pilot sampling for this study, aquatic insects were very rarely seen in webs more than 64 m from the Kowie River, so seven geometricallyspaced [0, 2 (2 1 ), 4 (2 2 ), 8 (2 3 ), 16 (2 4 ), 32 (2 5 ) and 64 (2 6 ) m from the river bank] transects running parallel to the stream were sampled at each site on the most accessible side of the river. ...
... In a global metaanalysis aimed to define a stream signature on land that relates the stream-to-land subsidy to distance, it was estimated that 50% of the stream signature (i.e. aquatic insects) was concentrated between 0.5 and 4 m from the river, while 10% of the signature occurred more than half a kilometre from the river (Muehlbauer et al., 2014). ...
Stream and riparian food webs can be strongly linked by inputs of aquatic emergent insect prey to terrestrial predators. However, quantifying these linkages and understanding how they vary in time and space is challenging. We investigated the dynamic width of a riverine trophic subsidy zone by determining the relationship between perpendicular distance from a river and dietary contributions of aquatic insect prey to web-building spiders' diets. To assess this relationship, riparian web-building spiders at two river sites were sampled during four seasons and analysed for the fatty acids 16:0, 16:1ω7 and 20:5ω3, their total ω3-fatty acid content and their ω3:ω6 ratio to evaluate trophic subsidies reaching them from an adjacent river. River-derived fatty acids generally declined with increased distance from the river, indicating a diffusion of aquatically derived subsidies into the riparian zone. While the river was only 16 m wide at its broadest, river-derived trophic subsidies were detected up to four times that distance from the river edge. Spiders at a downstream section of the river, characterised by generally higher emergence rates of aquatic insects, contained higher proportions of aquatic indicator fatty acids compared with spiders located upstream, where emergence rates were lower. Similarly, proportions of aquatic indicator fatty acids in spiders were lowest during winter when aquatic insect emergence rates were lowest. The fatty acid 20:5ω3 (eicosapentaenoic acid; EPA) held the best promise as a biomarker of aquatic-derived tropic subsidies and could be developed as a useful tool for riparian research and management.
... Buffer size, forestry disturbances, tree species, canopy coverage, and other biotic and abiotic conditions are some of the drivers of riparian buffer diversity (Biswas and Mallik, 2010;Chellaiah and Kuglerová, 2021;Elliott and Vose, 2016). Riparian invertebrate diversity plays a key role in riparian function through aquatic-terrestrial energy exchange as predators, prey items, and decomposers (Muehlbauer et al., 2014;Lafage et al., 2019b). Similarly, species diverse riparian plant communities support riparian function through enhanced aquatic protection and by providing a complex habitat to local fauna (Tabacchi et al., 1998). ...
... Riparian buffer effectiveness is rarely measured through exploration of species functional diversity and organism responses to changes in α-diversity and functional diversity due to forestry management practices is complex (Lelli et al., 2019). In addition, spatial variables such as distances from streams may shape these riparian communities (Muehlbauer et al. 2014;Kuglerová et al, 2014a). To explore that complexity, we performed an intensive survey of ground dwelling and low shrub spider communities and vascular plant communities in and around riparian systems in Swedish production forests. ...
Retention of forested buffers around streams following forest cutting operations is a common management technique used to protect aquatic resources and conserve the surrounding ecosystem services. Species richness, or α-diversity, is commonly used as an indicator of the effects of forestry management although it provides very little information about those effects on ecosystem processes and function. Functional diversity links species traits and ecosystem function incorporating species diversity, community composition, and functional guild and is more suitable to investigate the direct and indirect effects of forestry on ecosystem function. We sampled spiders and vascular plants in buffered and unbuffered stream-forest systems in southern Sweden and used a trait-based approach to assess the effects of buffer size and environmental variables on functional diversity. We used structural equation modeling (SEM) to explore the effects of buffer size and condition on spider and vascular plant diversity. We found no effect of buffer size on the functional richness or functional redundancy for spiders or vascular plants. Buffer size had a slight effect on the α-diversity of spiders within small buffers and fully forested sites but the effect was small. Other buffer variables including canopy closure, buffer density, bare ground coverage, and soil fertility had direct effects on spider and vascular plant functional diversity. The main driver of functional richness was α-diversity, but our SEM analysis illustrated other environmental variables working jointly to drive functional diversity. Using a trait-based approach, we showed that forested buffers have a minimal overall impact on spider and vascular plant functional diversity. However, it is important to maintain high levels of α-diversity to preserve and promote both spider and plant functional richness in production forests and we suggest that forest management conserves and encourages high levels of α-diversity to increase overall functional diversity.
... Aquatic insects are known to be responsive to environmental degradation and climate change, and their populations are threatened from numerous anthropogenic pressures, including: habitat removal, such as when rivers are dammed for power generation or deepened for navigation (e.g., (Wesner et al., 2019); point source or runoff pollution, such as when waterbodies become polluted with toxic agricultural and industrial chemicals (Henrik Barmentlo et al., 2021); and sedimentation, such as when extractive activities lead to higher sediment loads that promote eutrophication and subsequent oxygen depletion (Carpenter et al., 1998). Moreover, aquatic insects are important sources of food for animals and other invertebrates, as well as a conduit for cycling of nutrients within and between aquatic and terrestrial habitats (Baxter, Fausch, & Saunders, 2005;Muehlbauer, Collins, Doyle, & Tockner, 2014;Nakano & Murakami, 2001). Using the data-rich conterminous USA as a case study, we curated a metadataset of 11,298 sequences of the mitochondrial cytochrome c oxidase subunit I gene, comprising 1,974 populations and 717 species of caddisflies, mayflies, and stoneflies to quantify spatial variation in genetic diversity among these important water quality indicator taxa. ...
... Inferring decline and rebound from genetic meta-data Recent reports of dramatic declines among arthropods in Europe have revived concern that insects and the ecosystem services they provide are becoming critically threatened by increasingly intense human land use and climate change (Hallmann et al., 2017;Powney et al., 2019;Raven & Wagner, 2021;Sánchez-Bayo & Wyckhuys, 2019). Losses among aquatic insects, which are an important prey base and conduit for cycling of nutrients within and between aquatic and terrestrial habitats (Baxter et al., 2005;Muehlbauer et al., 2014;Nakano & Murakami, 2001), could severely degrade ecosystem function. Still, many studies reveal heterogeneity among arthropod taxa and sites in abundance, biomass, or occupancy trends over time (Breed, Stichter, & Crone, 2013;Crossley et al., 2020;Duchenne et al., 2020;Forister et al., 2010;Kuussaari, Heliölä, Pöyry, & Saarinen, 2007;Outhwaite et al., 2020;Rosenheim & Ward, 2020;Roubik, 2001;Seibold et al., 2019;Soroye et al., 2020;van Klink et al., 2020;van Strien, van Swaay, van Strien-van Liempt, Poot, & WallisDeVries, 2019;van Swaay, Warren, & Loïs, 2006;Wagner, Fox, et al., 2021;Wepprich et al., 2019), and some of the longest running insect monitoring networks show drops and rebounds over time (Bell et al., 2020;Macgregor et al., 2019). ...
Recent declines in once‐common species are triggering concern that an environmental crisis point has been reached. Yet, the lack of long abundance time series data for most species can make it difficult to attribute these changes to anthropogenic causes, and to separate them from normal cycles. Genetic diversity, on the other hand, is sensitive to past and recent environmental changes, and reflects a measure of a populations’ potential to adapt to future stressors. Here, we consider whether patterns of genetic diversity among aquatic insects can be linked to historical and recent patterns of land use change. We collated mitochondrial cytochrome c oxidase subunit I (COI) variation for >700 aquatic insect species across the United States, where patterns of agricultural expansion and intensification have been documented since the 1800s. We found that genetic diversity was lowest in regions where cropland was historically (pre‐1950) most extensive, suggesting a legacy of past environmental harm. Genetic diversity further declined where cropland has since expanded, even after accounting for climate and sampling effects. Notably though, genetic diversity also appeared to rebound where cropland has diminished. Our study suggests that genetic diversity at the community level can be a powerful tool to infer potential population declines and rebounds over longer time spans than is typically possible with ecological data. For the aquatic insects that we considered, patterns of land use many decades ago appear to have left long‐lasting damage to genetic diversity that could threaten evolutionary responses to rapid global change.
... At the shoreline, araneids compete with tetragnathids for aquatic prey and their diets there are mainly composed of adult aquatic insects as indicated by insect prey availability, carbon and nitrogen isotope analyses (δ 15 N and δ 13 C, respectively), and aquatic-derived contaminant concentrations (Walters et al., 2010;Speir et al., 2014). This spider taxon also occurs away from shorelines in terrestrial habitats (e.g., forests and grasslands) (Hunt et al., 2020), where a larger fraction of their diet consists of terrestrial insects (Raikow et al., 2011;Muehlbauer et al., 2014). Thus, these tetragnathid and araneid spiders are used as biological sentinels of aquatic PCB contamination, as accumulation of these contaminants into spiders is directly proportional to the level of sediment contamination and their reliance on aquatic carbon sources, both of which are driven by proximity to the shoreline (Raikow et al., 2011;Chumchal et al., 2022). ...
... This could indicate that PCBs have been transported extensively into the terrestrial ecosystem at the high site and that the microbial taxa present are more tolerant of PCBs. Most exposure to contaminants via aquatic insects will occur in near-shore environments because most of their biomass remains in those habitats, but some fraction of this prey will disperse much farther into neighboring terrestrial habitats, potentially exposing the upland spiders we sampled in this study (Muehlbauer et al., 2014;Schindler and Smits, 2017). Currently the degree of PCB exposure required to alter a spider's microbiome remains unknown, but controlled experiments could shed light on our hypothesis that a relatively low biomass of PCB-contaminated adult aquatic insects dispersing to upland habitats is sufficient to alter microbiomes of spiders living farther from the water's edge. ...
Microbial communities, including endosymbionts, play diverse and critical roles in host biology and reproduction, but contaminant exposure may cause an imbalance in the microbiome composition with subsequent impacts on host health. Here, we examined whether there was a significant alteration of the microbiome community within two taxa of riparian spiders (Tetragnathidae and Araneidae) from a site with historical polychlorinated biphenyl (PCB) contamination in southern Ontario, Canada. Riparian spiders specialize in the predation of adult aquatic insects and, as such, their contaminant levels closely track those of nearby aquatic ecosystems. DNA from whole spiders from sites with either low or high PCB contamination was extracted, and spider microbiota profiled by partial 16S rRNA gene amplicon sequencing. The most prevalent shift in microbial communities we observed was a large reduction in endosymbionts in spiders at the high PCB site. The abundance of endosymbionts at the high PCB site was 63% and 98% lower for tetragnathids and araneids, respectively, than at the low PCB site. Overall, this has potential implications for spider reproductive success and food webs, as riparian spiders are critical gatekeepers of energy and material fluxes at the land-water interface.
... A species-specific dispersal kernel defines the probability of a dispersing imago to land in a segment. This rule is in line with Muehlbauer et al. (2014) who underpinned that the majority of riverine macroinvertebrates disperses along the river course and only a small percentage in dispersing overland. ...
... These organisms can only fully develop (as a sign for ecosystem health) if the interplay of these habitat types is appropriate in the river catchment. For this, MASTIFF's implicit way of incorporating the river catchment is still adequate as recent studies have shown (e.g., Muehlbauer et al., 2014) that organismal exchange between a river and its catchment is restricted to a rather narrow zone along the river. ...
Riverine ecosystems provide various ecosystem services. One of these services is the biological control of eutrophication by grazing macroinvertebrates. However, riverine ecosystems are subject to numerous stressors that affect community structure, functions, and stability properties. To manage rivers in response to these stressors, a better understanding of the ecological functions underlying services is needed. This requires consideration of local and regional processes, which requires a metacommunity approach that links local food webs through drift and dispersal. This takes into account long-distance interactions that can compensate for local effects of stressors. Our modular model MASTIFF (Multiple Aquatic STressors In Flowing Food webs) is stage-structured, spatially explicit, and includes coupled food webs consisting of benthic resource-consumer interactions between biofilm and three competing macroinvertebrate functional types. River segments are unidirectionally connected through organismal drift and bidirectionally connected through dispersal. Climate and land use stressors along the river can be accounted for. Biocontrol of biofilm eutrophication is used as an exemplary functional indicator. We present the model and the underlying considerations, and show in an exemplary application that explicit consideration of drift and dispersal is essential for understanding the spatiotemporal biocontrol of eutrophication. The combination of drift and dispersal reduced eutrophication events. While dispersal events were linked to specific periods in the species life cycles and therefore had limited potential to control, drift was ubiquitous and thus responded more readily to changing habitat conditions. This indicates that drift is an important factor for coping with stress situations. Finally, we outline and discuss the potential and possibilities of MASTIFF as a tool for mechanistic, cross-scale analyses of multiple stressors to advance knowledge of riverine ecosystem functioning.
... In another study, Howie et al. (2018) observed that spiders collected hundreds of meters from the contamination source had elevated concentrations of Hg. More studies are needed to understand how contaminant concentrations in the tissues of a given spider taxa change with distance from shore and how this is impacted by the dispersal of aquatic insects inland (Muehlbauer et al., 2014;Vander Zanden & Gratton, 2011), spider hunting strategy (Textbox 1 and Supporting Information, Table S1), spider home range (Supporting Information, Table S1), and terrestrial contaminant cycling (Tsui et al., 2019). Regardless, it is clear from meta-analysis of stream ecosystems (Muehlbauer et al., 2014) that insect dispersal exhibits a negative exponential decay with distance from streams; thus, most aquatic insect biomass (50%) is concentrated near stream banks (1.3 m). ...
... More studies are needed to understand how contaminant concentrations in the tissues of a given spider taxa change with distance from shore and how this is impacted by the dispersal of aquatic insects inland (Muehlbauer et al., 2014;Vander Zanden & Gratton, 2011), spider hunting strategy (Textbox 1 and Supporting Information, Table S1), spider home range (Supporting Information, Table S1), and terrestrial contaminant cycling (Tsui et al., 2019). Regardless, it is clear from meta-analysis of stream ecosystems (Muehlbauer et al., 2014) that insect dispersal exhibits a negative exponential decay with distance from streams; thus, most aquatic insect biomass (50%) is concentrated near stream banks (1.3 m). Whereas some nontrivial amount of insect biomass can extend much farther (10s-100s of meters) into terrestrial habitats, these findings indicate that studies using spiders as sentinels of aquatic contamination should focus on nearshore taxa because riparian habitats are where the contaminant signal is most likely to be detected. ...
Aquatic ecosystems around the world are contaminated with a wide range of anthropogenic chemicals, including metals and organic pollutants, that originate from point and nonpoint sources. Many of these chemical contaminants have complex environmental cycles, are persistent and bioavailable, can be incorporated into aquatic food webs, and pose a threat to the health of wildlife and humans. Identifying appropriate sentinels that reflect bioavailability is critical to assessing and managing aquatic ecosystems impacted by contaminants. The objective of the present study is to review research on riparian spiders as sentinels of persistent and bioavailable chemical contaminants in aquatic ecosystems. Our review of the literature on riparian spiders as sentinels suggests that significant progress has been made during the last two decades of research. We identified 55 published studies conducted around the world in which riparian spiders (primarily of the families Tetragnathidae, Araneidae, Lycosidae, and Pisauridae) were used as sentinels of chemical contamination of lotic, lentic, and estuarine systems. For several contaminants, such as polychlorinated biphenyls (PCBs), Hg, and Se, it is now clear that riparian spiders are appropriate sentinels. However, many contaminants and factors that could impact chemical concentrations in riparian spiders have not been well characterized. Further study of riparian spiders and their potential role as sentinels is critical because it would allow for development of national-scale programs that utilize riparian spiders as sentinels to monitor chemical contaminants in aquatic ecosystems. A riparian spider sentinel program in the United States would be complementary to existing national sentinel programs, including those for fish and immature dragonflies. Environ Toxicol Chem 2022;01:1-16. © 2021 SETAC.
... Animal dispersal behavior is essential to the health of natural ecosystems by supporting the potential for gene flow between spaces as organisms move away from their parental location (Nathan, 2001;Ronce, 2007). It affects the size of local populations, and is a necessary process in metapopulation formation and, thus, the distribution of organisms at a landscape scale (Bilton et al., 2001;Baguette, 2003;Muehlbauer et al., 2014;Lancaster and Downes, 2017). Dispersal via flight at the adult stage is critically important for many species of riverine aquatic insects, for two reasons. ...
... Most adult A. ishikariana individuals traveled over a relatively short distance perpendicular to the river channel, showing a pattern similar to other examined taxa (Muehlbauer et al., 2014). Gradual and exponential decreases in catch abundance with increasing distance from the water body, with the majority being caught within several 100 meters, is a common tendency in aquatic adult dispersals (Sode and Wiberg-Larsen, 1993;Kovats et al., 1996;Collier and Smith, 1998;Griffith et al., 1998;Petersen et al., 1999Petersen et al., , 2004. ...
The life-history traits of amphibitic insects are not well understood. These insects inhabit saturated interstitial areas below the riverbed (hyporheic zone) at the larval stage, mate in terrestrial habitats, and return to rivers for oviposition, but there is no knowledge concerning their dispersal characteristics. We sought to address this by examining how far amphibitic insects disperse away from the channel (laterally) and along upstream or downstream (longitudinally) in a gravel-bed river. Alloperla ishikariana was selected as the focal species because it numerically dominates other amphibites in an 18-km study segment of a 4th-order gravel-bed river in Hokkaido, Northern Japan. Malaise traps were set at various distances from the channel towards the riparian forest to estimate lateral dispersal distances. An elevated stable nitrogen isotope ratio in downstream larvae, caused by the influence of effluent from a wastewater treatment plant, was used to assess longitudinal dispersal by identifying and tracking adult movements. Laterally, 50th and 90th percentile dispersal distances were 11.66 and 35.09 m for female A. ishikariana and 20.59 and 59.20 m for male, respectively; this overlapped with distances previously estimated for other aquatic benthic taxa. Longitudinally, 50th and 90th percentile dispersal distances were 0.74 and 1.43 km for female and 3.11 and 7.87 km for male, respectively. Alloperla ishikariana had one of the longest upstream traveling distances compared with other aquatic insects, and the longest among Plecoptera taxa known thus far where male exhibited a greater dispersal distance. A higher number of adults demonstrated upstream movement, suggesting an upstream bias in the longitudinal dispersal of A. ishikariana. Overall, amphibitic stoneflies did not exhibit distinct dispersal characteristics compared with the results of previous reports on presumably benthic taxa. Our findings support an improved visualization of a multi-dimensionally connected river ecosystem in terms of material flow, including vertical connectivity.
... 1-3 m wide, mean 13° slope; Paillex et al. 2020), and have low productivity (Logue et al. 2004). Riparian zones in Val Roseg, and along alpine headwaters more generally, may thus experience relatively low magnitudes of aquatic subsidies in a global context (Ballinger and Lake 2006;Muehlbauer et al. 2014;Schindler and Smits 2017). We found no correlation between estimated spider diets and streamflow intermittency. ...
... As discussed above, when they do predate on emergent macroinvertebrates, hunting spiders likely rely on larger taxa such as stoneflies (Briers et al. 2005;Stenroth et al. 2015). Smaller aquatic invertebrates such as Chironomidae also often disperse farther from streams than larger taxa (Muehlbauer et al. 2014). In alpine areas, Chironomidae may disperse across relatively open riparian zones to forested areas, where small sheetweaving spiders (Linyphiidae) can be abundant (Zingerle 1999;Thaler 2003). ...
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.
... This transport of materials from land to water is long recognized to have important effects on ecosystem function, community dynamics, and species distributions. More recently, the movement of animals from water to land as part of their ontogeny (i.e., complex life cycles), or other means of dispersal (i.e., movement by predators or flooding), has been identified as an important conduit of energy and nutrients back to land (Muehlbauer et al. 2014;Schindler and Smits 2017). Theory describing these linkages, which merge paradigms from both landscape and food web ecology, is now being used to answer questions regarding the ecological effects of contaminants and food web structure on contaminant flux (Kraus 2019a, b;Chumchal and Drenner this volume;Sullivan and Cristol this volume). ...
... Resource exchange between aquatic and terrestrial ecosystems is a well-studied phenomenon in ecology (Minshall 1967;Polis et al. 1997;Baxter et al. 2005;Gratton and Vander Zanden 2009;Muehlbauer et al. 2014, see Collins and Baxter this volume for a brief overview). Theory describing animals with complex life histories and factors affecting timing and success of ontogenetic switch points (i.e., metamorphosis) is also well developed (Wilbur and Collins 1973;Wilbur 1980;Wilbur 1997;Schreiber and Rudolf 2008;de Roos and Persson 2013). ...
In this chapter, we synthesize the state of the science regarding ecological subsidies and contaminants at the land-water interface and suggest research and management approaches for linked freshwater-terrestrial ecosystems. Specifically, we focus on movements of animals with complex life histories and the detrital inputs associated with animal and plant matter delivered to freshwaters. We present a framework based on the physicochemical parameters of contaminants and how they shape the relationship between contaminant persistence within resource subsidies (“dark side” of subsidies) and movement of resource subsidies (“bright side” of subsidies) across ecosystem boundaries. This relationship between the “dark side” and “bright side” of subsidies defines an important parameter space that allows researchers and practitioners to predict the potential impacts of aquatic contaminants on resource subsidies and their interaction with other stressors on consumers. Ecological factors such as ecosystem productivity, community composition, and consumer prey preference shape the ecotoxicological outcomes of aquatic contamination on subsidies. Landscape factors such as lithology, hydrogeomorphology, hydroperiod, and land use underlie chemical, toxicological, and ecological patterns and provide the context within which effects of contaminants play out. Finally, effects of contaminants combine with effects of other global stressors on timing, quality, and quantity of subsidies that drive responses to contaminants at the land-water interface. Understanding the “dark side” of ecological subsidies requires expertise from multiple disciplines. We attempt to synthesize current knowledge from those disciplines and generate conceptual models that ecologists can use to guide future research in understanding cross-ecosystem subsidies and contaminant fate and effects.
... In death, decomposing salmon carcasses leach nutrients to their surroundings and are also directly consumed by many aquatic and terrestrial consumers (Ben-David et al. 1998;Gende et al. 2002;Janetski et al. 2009;Flecker et al. 2010;Hocking and Reynolds 2011;Collins et al. 2015a). Unfortunately, dams, habitat destruction, and overharvest by humans have all contributed to the widespread decline of Pacific salmon in the Pacific Northwest (Lichatowich 2001, Montgomery 2004), eliminating salmon from approximately 40% of their historic range and dramatically reducing their numbers where connectivity persists (NRC 1996;Gresh et al. 2000). Consequently, many streams, rivers, lakes, and riparian forests no longer experience salmon migrations nor receive the inputs of marinederived organic matter and nutrients contained in their tissues (Collins et al. 2015a, b;Schindler and Smits 2017). ...
... Rather, the reverberating effects of bighead carp, mediated through egested waste particles and subsidized midge larvae, increased the emergence of adult midges from aquatic to terrestrial ecosystems by 228% (Collins and Wahl 2017). By amplifying fluxes from planktonic to benthic habitats and across land-water boundaries, bighead carp may indirectly subsidize terrestrial insectivores as effects propagate (e.g., Muehlbauer et al. 2014), potentially altering terrestrial community dynamics along other food web pathways (e.g., Henschel et al. 2001;Murakami and Nakano 2002). ...
Pervasive environmental degradation has altered biodiversity at a global scale. At smaller scales, species extirpations, invasions, and replacements have greatly influenced how ecosystems function and interact by affecting the exchanges of energy, materials, and organisms. In this chapter, we examine how a variety of environmental stressors, and associated species losses and gains, change the exchange of resources (materials or organisms) within and among ecosystems. We specifically consider how changes that occur within an ecosystem may trigger effects that reverberate (e.g., directly, indirectly, and via feedbacks) back and forth across ecological boundaries and propagate to other ecosystems connected via exchanges of materials and organisms. Our synthesis provides cursory overviews of ecosystem “openness” as it has been addressed by community ecologists and the conceptual development of ecological frameworks used to examine resource exchanges between ecosystems. We then describe four case studies and examine how species losses and gains affect food web structure via resource exchanges between ecosystems, with particular emphasis on effects spanning land-water boundaries. Finally, we discuss the need for more complex conceptual treatment of the interconnectedness of food webs among ecosystems.
... Although river regulation can reduce the magnitude of emergent aquatic insect biomass and abundance (Jonsson et al. 2013, Kennedy et al. 2016, few studies have considered whether this reduced prey subsidy leads to a change in food web structure or nutrient cycling through riparian consumer organisms (but see Sullivan et al. 2018). Although the spatial extent of emergent aquatic insect dispersal can extend from meters to kilometers into the upland environment (Muehlbauer et al. 2014), riparian consumers like birds and bats may transport energy and nutrients derived from aquatic primary production much farther upland -and thereby increase the spatial envelope of stream ecosystems. Therefore, understanding nutrient cycling through highly mobile riparian consumers may expand our understanding of the spatial influence of river regulation on linked aquatic-terrestrial ecosystems. ...
... Whereas floods deliver organic material, sediments, and nutrients into floodplains (Schindler and Smits 2017), and emergent aquatic insects transport carbon and nutrients farther into the riparian zone (Jonsson et al. 2012, Muehlbauer et al. 2014, our results show that highly mobile species such as birds and bats could transport the energy generated by aquatic production even farther than these previous studies have considered. Thus, the spatial envelope of aquatic-terrestrial ecosystems (e.g., Gurnell et al. 2016) may be much larger than previously estimated by examining less-mobile riparian consumers, underscoring the potential importance of river food webs to organisms and ecosystems typically characterized as terrestrial. ...
River regulation can modify natural flow regimes with deleterious effects on aquatic communities. While the effects of flow manipulation on the physical environment and populations and assemblages of aquatic organisms have been described thoroughly, how and to what extent river regulation influences ecosystem processes like food web architecture is less studied. Emergent aquatic insect prey can provide an important food resource to riparian consumers like birds and bats with concomitant consequences for nutrient cycling through aquatic–terrestrial food webs, thus potentially increasing the spatial influence of river regulation into the riparian zone and beyond. We used naturally abundant stable isotopes of carbon and nitrogen to compare food web architecture (trophic position and reliance on an aquatic nutritional pathway) leading to birds and bats between a regulated river, the Tuolumne River downstream of Hetch Hetchy Reservoir, and an adjacent unregulated river, the Merced River, located in Yosemite National Park on the west slope of the Sierra Nevada, California, USA. We found that both birds and bats derived >50% of their nutrition from food webs originating in photosynthesis by algae. In addition, birds and bats occupied a similar trophic position to predatory fish in other systems. Both birds and bats seemed to rely more strongly on an aquatic nutritional pathway during the dryer year of our study period, underscoring the potential importance of emergent aquatic prey as a water subsidy in dry systems and in dry years. In the Tuolumne River, reservoir managers strive to simulate characteristics of the natural flow regime, including seasonal scouring flows and prolonged floodplain inundation. Although we found no conclusive evidence of an effect of river regulation on food web responses, our study suggests that nutrient cycling through aquatic–terrestrial food webs expands the potential influence of river regulation to organisms and ecosystems typically characterized as terrestrial.
... Most aquatic subsidies must go against gravity and therefore often rely on animal movement. One of the most observed examples is the emergence of insects from lakes or rivers; these insects eventually feed terrestrial insectivores (Fig. 29.3; Muehlbauer et al., 2014;Paetzold et al., 2005). Emerging aquatic insects are essential prey, as they have a high content of long-chain PUFA. ...
Aquatic and terrestrial habitats are interdependent components of the boreal forest landscape involving multiple dynamic interactions; these are manifested particularly in riparian areas, which are key components in the forest landscape. However, this interdependence between aquatic and terrestrial habitats is not adequately accounted for in the current management of forest ecosystems. Here we review the impacts of land disturbances on the optical and physicochemical properties of water bodies, aquatic food web health, and the ecological functioning of these freshwaters. We also describe how freshwaters influence the adjacent terrestrial ecosystems. A better understanding of these dynamic biotic and abiotic interactions between land and freshwater of the boreal forest is a first step toward including these freshwaters in the sustainable management of the boreal forest.
... Flight also is important for fitnessrelated functions such as escaping predators, locating food and reproduction (Amat et al., 2012;Chen et al., 2011;David et al., 2015;Dudley, 2002;Tokeshi & Reinhardt, 1996). Variable flight distances have been reported depending on species, ranging from 160 m for some Ephemeroptera to 17 km for some Chironomidae (Muehlbauer et al., 2014). However, current knowledge on insect flight comes from studies that use heterogeneous methods and target one or a few species at once, leading to limited understanding of flight ability. ...
Dispersal is a determining step in the life cycle of insects and a key factor of their energy budget. If the body stores of terrestrial insects are relatively well‐documented, those of merolimnic insects (i.e., with aquatic larvae and terrestrial winged adults) remain poorly investigated. We assayed the body stores (protein, carbohydrate and lipid contents) of 18 families of merolimnic insects, encompassing a wide diversity of biological and ecological traits. We highlighted allometry between body stores and mass. The proportion of triglyceride and free carbohydrate relatively decreases with an increase in absolute insect mass, whereas protein and glycogen contents are proportional. We found a significant effect of insect order, Diptera having relatively more proteins than Trichoptera. For swarming taxa, males have relatively more triglycerides than their conspecific female. Passive dispersers are characterised by higher protein and glycogen contents Each body store was associated with specific life‐history traits, emphasising the importance of considering each body store independently to better understand the associated functions. Dispersal strategies seem to be the main structuring factor underlying body store patterns of merolimnic insects, being significantly associated with the variation in adjusted triglycerides (30%), proteins (21%), glycogen (29%) and free carbohydrates (17%). However, our study estimated the role of flight only indirectly through the biological traits of insects. Future works should focus on identifying which substrates are used for flight, by comparing the proportion of body stores before and after flight, as well as protein identification to distinguish proteins associated with flight or reproduction. These further studies will help researchers in understanding the link between body stores of emerging merolimnic insects and the various facets of their ecology.
... It is largely unclear if this pattern is consistent among different consumer species and different trophic levels within one ecosystem. When the relevance of allochthonous input becomes limited to consumers close to the site of resource input (Muehlbauer et al., 2014;Paetzold et al., 2008), then consumer species with a broad distribution range that occur close to as well as at a distance to the site of allochthonous input might differ in their dependency on allochthonous input. ...
Ecosystems are interconnected by energy fluxes that provide resources for the inhabiting organisms along the transition zone. Especially where in situ resources are scarce, ecosystems can become highly dependent on external resources. The dependency on external input becomes less pronounced in systems with elevated in situ production, where only consumer species close to the site of external input remain subsidized, whereas species distant to the input site rely on the in situ production of the ecosystem. It is largely unclear though if this pattern is consistent over different consumer species and trophic levels in one ecosystem, and whether consumer species that occur both proximate to and at a distance from the input site differ in their dependency on external resource inputs between sites. Using stable isotope analysis, we investigated the dependency on external marine input for common ground-associated consumer taxa on small tropical islands with high in situ production. We show that marine input is only relevant for strict beach-dwelling taxa, while the terrestrial vegetation is the main carbon source for inland-dwelling taxa. Consumer species that occurred both close (beach) and distant (inland) to the site of marine input showed similar proportions of marine input in their diets. This supports earlier findings that the relevance of external resources becomes limited to species close to the input site in systems with sufficient in situ production. However, it also indicates that the relevance of external input is also species-dependent, as consumers occurring close and distant to the input site depended equally strong or weak on marine input.
... It is largely unclear if this pattern is consistent among different consumer species and different trophic levels within one ecosystem. When the relevance of allochthonous input becomes limited to consumers close to the site of resource input(Muehlbauer et al., 2014;, then consumer species with a broad distribution range that occur close to as well as at a distance to the site of allochthonous input might differ in their dependency on allochthonous input. ...
Biodiversity on our planet follows defined patterns. On a global scale, these patterns are organized by few parameters, like latitude and elevation. On a local scale, the organization of biodiversity becomes less predictable as various factors simultaneously determine species’ diversity and distribution. Understanding how biodiversity and species distribution are organized on small spatial scales (i.e., within ecosystems or habitats) is fundamental to ecological research. This relevance stems from the ongoing global change. The increasing necessity for understanding how biodiversity and species distribution is organized exists because the most prevalent threat for biodiversity worldwide is land conversion, which acts primarily on a local scale (i.e., within ecosystems or habitats). Various land uses drive the conversion of former pristine ecosystems into modified and degraded land. A plethora of research has described the loss of habitat area or species richness following different human land uses. However, few studies reach beyond merely describing species losses and investigated the underlying mechanisms by which human activities alter the organization biodiversity and distribution. For a thorough understanding of how different human land uses impact the organization of biodiversity within ecosystems, it is first necessary understand how natural abiotic and biotic factors organize and drive biodiversity and distribution in natural systems free of any direct human disturbance before investigating any human-driven changes. Empirical research aiming to analyse the organization of biodiversity and distribution in ecosystems is often hindered by the overall ecosystem complexity and difficulty of demarcating and replicating communities or ecosystems. In this PhD thesis, I introduce a methodical framework that uses small insular ecosystems for investigating how biotic, abiotic, and anthropogenic factors drive biodiversity and distribution. The small sizes and clear boundaries of islands, together with the possibility to use groups of islands as ecosystem replicates, pose an elegant solution to the abovementioned limitations. This PhD thesis uses this insular framework as a model system to study how natural biotic and abiotic factors drive the organization of biodiversity and disentangle the impacts of different human land uses by investigating islands that hold only one specific type of human land use. For this, I consulted a two-step approach. In the first chapter of this PhD thesis, I used a focal taxon approach, while in the second chapter, I used a community-wide sampling approach. Analysing how abiotic, biotic, and anthropogenic factors drive biodiversity and distribution on two ecological levels enabled a more thorough understanding of the relevant factors, as both sampling approaches each bring their methodical advantages. In chapter A, I identified the key natural abiotic drivers for the distribution patterns of the investigated focal taxon, terrestrial hermit crabs (genus Coenobita), and show which physical conditions predominantly influence its distribution. At the same time, interspecific competition does not drive its biodiversity and distribution patterns. Instead, intrinsic mechanisms of resource partitioning stabilize co-occurrence. The investigation of the focal taxon’s biodiversity and distribution patterns on islands used either solely for touristic purposes (‘tourist islands’) or as permanent settlings by the local population (‘local islands’) disentangled the environmental impact of two different human land uses. I demonstrate that two forms of human land use can impact different aspects of the same taxon in the same overall region. Single-taxon-approaches are limited in their generalizability and give less insight than community-wide analyses. Hence, chapter B focused on the ground-associated faunal community of the investigated insular ecosystem (section B.1-B.4 and B.5-B.6) and the avifauna (section B.4). I demonstrate that the biodiversity and distribution patterns of the ground-associated insular community are organized in a compartmentalized way, both concerning habitat niche occupation and trophic niche occupation. Closely related species cluster within the overall niche space and form compartments attributable to distinct insular habitats (e.g., a beach food web compartment and an inland food web compartment). I show that this compartmentalized organization is not occurring due to an overall depauperate insular fauna, but likely stabilized by relative productivity gradients between compartments. More mobile bird taxa likely also form no relevant link between the distinct compartments of the ground-associated infauna. I further show that both investigated forms of human land use have a negative impact on the insular biodiversity patterns. However, the drivers for the observed losses differ between the two land uses. Ultimately, I show that both land uses are related, at least partly, to significant shifts in the trophic niche occupation of the impacted species, which might indicate a species’ susceptibility to land-use-driven abundance declines. By using islands as the methodical framework to study how biodiversity and species distribution is organized on a local scale and how human land uses alter these patterns, this PhD thesis paves the way for future research in community and disturbance ecology. The novelty of this insular approach is further emphasized by the presentation of two species newly recorded in the investigated insular region in the final chapter of my thesis. This underlines the broad spectrum of possible scientific insights that islands as model systems offer for biodiversity research.
... Alterations to stream-riparian food-web structure of urban streams appear to be multivariate and complex, with hydrogeomorphic change working through multiple avenues (e.g., altering energy flux, physical habitat) to affect the distribution, composition, and feeding habits of both instream and riparian biota. These findings have consequential implications for not just ecosystem stability and function of streams, but also contaminant export to terrestrial environments, as emergent aquatic insects are a major vector of contaminants, and can move into larger, more mobile consumers that can potentially transfer contaminants to uplands (Sullivan and Rodewald, 2012;Muehlbauer et al., 2014;Rowse et al., 2014). ...
Urbanization in stream catchments can have strong effects on stream channel hydrogeomorphic features including channel dimensions, channel-floodplain connectivity, and flood regime. However, the consequences of hydrogeomorphic alterations on aquatic-terrestrial subsidy dynamics are largely unexplored. We examined the associations among hydrogeomorphic characteristics, emergent aquatic insect assemblages, and the density and trophic dynamics of riparian spiders of the family Tetragnathidae at 23 small urban stream reaches in the Columbus, OH (United States) Metropolitan Area. Naturally abundant stable isotopes of ¹³ C and ¹⁵ N were used to quantify the relative contribution of aquatically derived energy (i.e., nutritional pathways deriving from algae) to tetragnathid spiders and their trophic position. Bankfull discharge was negatively related to both emergence rate and family richness. On average, tetragnathid spiders relied on aquatically derived energy for 36% of their nutrition, with the greatest reliance found for spiders next to channels with wider flood-prone widths and proportionally fewer emergent insects of the family Chironomidae. Mean emergent aquatic insect reliance on aquatically derived energy was 32% and explained 44% of the variation in tetragnathid aquatically derived energy. A positive relationship between δ ¹³ C of tetragnathid spiders and emergent insects provides additional evidence of tetragnathid reliance on emergent insects. Mean tetragnathid trophic position was 2.85 and was positively associated with channel sinuosity and negatively associated with aquatic insect emergence rate. Sinuosity was also positively related to aquatically derived energy of emergent aquatic insects; as well as emergent insect family richness; % Ephemeroptera, Plecoptera, and Trichoptera (EPT); and aquatic insect emergence rate; implicating instream habitat-mediated shifts in emergent aquatic insect communities as an indirect mechanistic link between hydrogeomorphic processes and spiders. Our findings underscore that the impacts of stream hydrogeomorphic alterations can cascade into terrestrial food webs. These results suggest that monitoring and restoration of fluvial geomorphic form and function (e.g., sinuosity, slope, and hydrology) confer benefits to both aquatic and terrestrial riparian ecosystems in urban catchments.
... Our study showed that three of the 10 species discriminating between FOR and AGR sites were non-insects (Gammarus pulex, Pisidium and Oligochaeta) with higher abundances in the AGR streams. On the one hand, a shift in assemblage composition to non-insects constitutes a reduction in the proportion of in-stream production that can be potentially transferred to terrestrial systems by emerging flying insects (Carlson et al., 2016;McKie et al., 2018;Muehlbauer, Collins, Doyle & Tockner, 2014). On the other hand, the higher abundances of stress-tolerant insect species (e.g. ...
The biodiversity of streams and riparian zones is highly interlinked by multiple cross-habitat flows of organisms, nutrients and materials, and are thus increasingly recognised as forming an extended “meta-ecosystem” network spanning both within and across entire catchments. However, current understanding of how these extended networks respond to environmental change is limited by the lack of studies quantifying the local and larger-scale drivers of biodiversity in both terrestrial and aquatic systems simultaneously. Using high-resolution species and environmental data from eight boreal riparian – stream meta-ecosystems, half of which were situated in forest and half in agricultural catchments, we quantified land-use impacts and the importance of catchment and local (riparian, instream) scale variables on the diversity and community composition of three epigeal arthropod groups (spiders and staphylinid and carabid beetles) and aquatic macroinvertebrates. All four organism groups responded to quantifiable environmental variables. Staphylinid beetle and spider assemblages differed significantly between forested and agricultural sites and were strongly correlated with riparian variables such as vegetation type and soil properties, but also instream variables such as conductivity and floating macrophytes. By contrast, carabid beetle and aquatic macroinvertebrate assemblages did not differ between forested and agricultural sites and showed similar responses to catchment and local scale variables. Our results indicate that measures that only address local scale drivers of terrestrial biodiversity might be ineffective if the catchment-scale variables regulating aquatic biodiversity and aquatic – terrestrial linkages are not adequately addressed in ecosystem management.
... Biologically mediated flows from aquatic to terrestrial ecosystems involve animal movements. Specifically, such flows include the emergence of aquatic insects (Bartels et al., 2012;Muehlbauer et al., 2014;Richardson et al., 2009) and the distribution of fish-derived energy by foraging terrestrial predators (Helfield and Naiman, 2006) in the terrestrial ecosystem. Physically mediated flows from the aquatic ecosystem involve the movements of nutrients, sediments, and detritus through floods and subsurface water flows into terrestrial ecosystems (Schindler and Smits, 2016;Schulz et al., 2015;Wölz et al., 2011). ...
In a meta-ecosystem, spatially separated ecosystems are linked by biotic and abiotic cross-ecosystem flows. Hence, food webs in a meta-ecosystem are functionally linked. They are susceptible to multiple stressors threatening ecosystem functions and associated services. Although empirical studies can help understand stressor effects on meta-ecosystem food webs, they are often limited by their narrow spatial and temporal scales. This limitation may be overcome by process-based food web models, which allow variable spatial and temporal scales. We reviewed process-based food web models and their application to aquatic-terrestrial and theoretical meta-ecosystems. We refer to theoretical models as food web models based on theoretical considerations rather than describing a particular natural system. We found nineteen aquatic-terrestrial models that represented aquatic food webs with flows from terrestrial to aquatic ecosystems. Most of the aquatic-terrestrial models can be applied to study the environmental stressors of eutrophication (15 models) and climate change (10 models). Eight theoretical models were found that study ecosystem stability, trophic cascades, source-sink dynamics, co-nutrient limitation and co-existence of primary consumers. The theoretical models are more similar in terms of types of state variables and model complexity (i.e., number of state variables) than the aquatic-terrestrial models. Generally, the applications of the models have shown that environmental changes cause cross-scale effects on food webs in aquatic-terrestrial and theoretical meta-ecosystems. Finally, we outline major research gaps regarding the directionality of cross-ecosystem flows, anthropogenic stressors, and accessibility of models’ codes.
... It is largely unclear if this pattern is consistent among different consumer species and different trophic levels within one ecosystem. When the relevance of allochthonous input becomes limited to consumers close to the site of resource input (Muehlbauer et al., 2014;Paetzold et al., 2008), then consumer species with a broad distribution range that occur close to as well as at a distance to the site of allochthonous input might differ in their dependency on allochthonous input. ...
Abstract Ecosystems are interconnected by energy fluxes that provide resources for the inhabiting organisms along the transition zone. Especially where in situ resources are scarce, ecosystems can become highly dependent on external resources. The dependency on external input becomes less pronounced in systems with elevated in situ production, where only consumer species close to the site of external input remain subsidized, whereas species distant to the input site rely on the in situ production of the ecosystem. It is largely unclear though if this pattern is consistent over different consumer species and trophic levels in one ecosystem, and whether consumer species that occur both proximate to and at a distance from the input site differ in their dependency on external resource inputs between sites. Using stable isotope analysis, we investigated the dependency on external marine input for common ground‐associated consumer taxa on small tropical islands with high in situ production. We show that marine input is only relevant for strict beach‐dwelling taxa, while the terrestrial vegetation is the main carbon source for inland‐dwelling taxa. Consumer species that occurred both close (beach) and distant (inland) to the site of marine input showed similar proportions of marine input in their diets. This supports earlier findings that the relevance of external resources becomes limited to species close to the input site in systems with sufficient in situ production. However, it also indicates that the relevance of external input is also species‐dependent, as consumers occurring close and distant to the input site depended equally strong or weak on marine input.
... Insects that are herbivores and detritivores, by contrast, tend to have shorter generation times and larger population sizes, which can increase their number of dispersal events when compared to predatory insects (Poff et al. 2006). Moreover, even though the often-smaller body sizes of herbivores and detritivores may limit their locomotory ability, it may also greatly expand their dispersal range by passive wind transport (Muehlbauer et al. 2014). In conclusion, the relative decrease in colonization rates across a gradient of spatial isolation may be stronger for predatory insects than for insects at lower trophic levels (Guzman and Srivastava 2020). ...
Current conceptual metacommunity models predict that the consequences of local selective pressures on community structure increase with spatial isolation when species favored by local conditions also have higher dispersal rates. This appears to be the case of freshwater insects in the presence of fish. The introduction of predatory fish can produce trophic cascades in freshwater habitats because fish tend to prey upon intermediate predatory taxa, such as predatory insects, indirectly benefiting herbivores and detritivores. Similarly, spatial isolation can limit dispersal and colonization rates of predatory insects more strongly than of herbivores and detritivores, thus generating similar cascading effects. Here we tested the hypothesis that the effect of introduced predatory fish on insect community structure increases with spatial isolation by conducting a field experiment in artificial ponds that manipulated the presence/absence of fish (the Redbreast Tilapia) at three different distances from a source wetland. Our results showed that fish have direct negative effects on the abundance of predatory insects but probably have variable net effects on the abundance of herbivores and detritivores because the direct negative effects of predation by fish may offset indirect positive ones. Spatial isolation also resulted in indirect positive effects on the abundance of herbivores and detritivores but this effect was stronger in the absence rather than in the presence of fish so that insect communities diverged more strongly between fish and fishless ponds at higher spatial isolation. We argue that an important additional mechanism, ignored in our initial hypothesis, was that as spatial isolation increases fish predation pressure upon herbivores and detritivores increases due to the relative scarcity of predatory insects, thus dampening the positive effect that spatial isolation confers to lower trophic levels. Our results highlight the importance of considering interspecific variation in dispersal and multiple trophic levels to better understand the processes generating community and metacommunity patterns.
... Chari et al. (2020) also found a strong decrease of PUFAs associated with the aquatic system with distance from shore. This indicates that access to aquatic insects might be stronger driver of PUFA composition than lifestyle, as aquatic insect biomass steeply declines in the first 50 meters from shoreline (Muehlbauer et al., 2014). ...
Polyunsaturated fatty acids (PUFAs) are essential resources unequally distributed throughout landscapes. Certain PUFAs, such as eicosapentaenoic acid (EPA), are common in aquatic but scarce in terrestrial ecosystems. In environments with low PUFA availability, meeting nutritional needs requires either adaptations in metabolism to PUFA-poor resources or selective foraging for PUFA-rich resources. Amphibiotic organisms that emerge from aquatic ecosystems represent important resources that can be exploited by predators in adjacent terrestrial habitats. Here, we traced PUFA transfer from streams to terrestrial ecosystems, considering benthic algae as the initial PUFA source, through emergent aquatic insects to riparian spiders. We combined carbon stable isotope and fatty acid analyses to follow food web linkages across the ecosystem boundary and investigated the influence of spider lifestyle (web building vs. ground dwelling), season, and ecosystem degradation on PUFA relations. Our data revealed that riparian spiders consumed considerable amounts of aquatic-derived resources. EPA represented on average 15 % of the total fatty acids in riparian spiders. Season had a strong influence on spider PUFA profiles, with highest EPA contents in spring. Isotope data revealed that web-building spiders contain more aquatic-derived carbon than ground dwelling spiders in spring, although both spider types had similarly high EPA levels. Comparing a natural with an anthropogenically degraded fluvial system revealed higher stearidonic acid (SDA) contents and Σω3/Σω6 ratios in spiders collected along the more natural river in spring but no difference in spider EPA content between systems. PUFA profiles of riparian spiders where distinct from other terrestrial organism and more closely resembled that of emergent aquatic insects (higher Σω3/Σω6 ratio). We show here that the extent to which riparian spiders draw on aquatic PUFA subsidies can vary seasonally and depends on the spider’s lifestyle, highlighting the complexity of aquatic-terrestrial linkages.
... For the riparian food web, we collected insects, spiders, and lizards as representative consumers and fresh leaves from the dominant species of trees, grasses, and wetland plants (Table 1). We collected all riparian samples, with the exception of spiders, within 10 m of the river because aquatic resource flux declines rapidly with increasing distance from the water (Muehlbauer et al. 2014). Spiders were collected within 1 m of the river to capture high aquatic resource contribution to a riparian predator. ...
Ecosystems are defined, studied, and managed according to boundaries constructed to conceptualize patterns of interest at a certain scale and scope. The distinction between ecosystems becomes obscured when resources from multiple origins cross porous boundaries and are assimilated into food webs through repeated trophic transfers. Ecosystem compartments can define bounded localities in a heterogeneous landscape that simultaneously retain and exchange energy in the form of organic matter. Here we developed and tested a framework to quantify reciprocal reliance on cross‐boundary resource exchange and calculate the contribution of primary production from adjacent ecosystem compartments cycling through food webs to support consumers at different trophic levels. Under this framework, an integrated ecosystem can be measured and designated when the boundary between spatially distinct compartments is permeable and the bidirectional exchange of resources contributes significantly to sustaining both food webs. Using a desert river and riparian zone as a case study, we demonstrate that resources exchanged across the aquatic‐riparian boundary cycle through multiple trophic levels. Further, predators on both sides of the boundary were supported by externally produced resources to a similar extent, indicating this is a tightly integrated river‐riparian ecosystem and that changes to either compartment will substantially impact the other. Using published data on lake ecosystems, we demonstrated that benthic and pelagic ecosystem compartments are likely not fully integrated, but differences between lakes could be used to test ecological hypotheses. Finally, we discuss how the integrated ecosystem framework could be applied in urban‐preserve and field‐forest ecosystems to address a broad range of ecological concepts. Because few systems function in complete isolation, this novel approach has application to research and management strategies globally as ecosystems continue to face novel pressures that precipitate cascading ecological repercussions well beyond a bounded system of focus.
... Terrestrial-aquatic subsidy impacts can have large spatial extents. Signatures of subsidies to terrestrial environments have been shown to extend up to 5,300 m laterally from stream banks and nearly 1,000 m from lakeshores (Muehlbauer et al., 2014). Bats, for example, consume large quantities of emergent aquatic insects and deposit guano several kilometers away, increasing nutrient concentrations near roosts (Power et al., 2004). ...
Global declines in biodiversity have the potential to affect ecosystem function, and vice versa, in both terrestrial and aquatic ecological realms. While many studies have considered biodiversity-ecosystem function (BEF) relationships at local scales within single realms, there is a critical need for more studies examining BEF linkages among ecological realms, across scales, and across trophic levels. We present a framework linking abiotic attributes, productivity, and biodiversity across terrestrial and inland aquatic realms. We review examples of the major ways that BEF linkages form across realms–cross-system subsidies, ecosystem engineering, and hydrology. We then formulate testable hypotheses about the relative strength of these connections across spatial scales, realms, and trophic levels. While some studies have addressed these hypotheses individually, to holistically understand and predict the impact of biodiversity loss on ecosystem function, researchers need to move beyond local and simplified systems and explicitly investigate cross-realm and trophic interactions and large-scale patterns and processes. Recent advances in computational power, data synthesis, and geographic information science can facilitate studies spanning multiple ecological realms that will lead to a more comprehensive understanding of BEF connections.
... 9,10 Once emerged, most insects deposit on land directly adjacent to the aquatic origin (i.e., riparian zone), though some taxa (e.g., dragonflies) can disperse over long distances. 11 The response of riparian consumers, such as spiders, to high aquatic insect emergence can be substantial in terms of their density and relative proportion of aquatic prey in their diet. 12,13 Aquatic prey subsidies are not necessarily only beneficial to terrestrial consumers, as they also can contain contaminants that the aquatic insects are exposed to in water and sediment before emerging as adults. ...
Emergent aquatic insects are important food subsidies to riparian food webs but can also transfer waterborne contaminants to the terrestrial environment. This study aimed to quantitatively assess this biodriven transfer for per- and polyfluoroalkyl substances (PFAS).
Aquatic insect larvae, emergent aquatic insects, terrestrial consumers, sediment, and water were collected from a contaminated lake and stream and an uncontaminated pond, and analyzed for PFAS and stable isotopes of carbon and nitrogen. Top predators in this study were spiders, which showed the highest average Σ24PFAS concentration of 1400±80 ng g-1 dry weight (dw) at the lake and 630 ng g-1 dw at the stream. The transfer of PFAS from the lake to the riparian zone, via deposition of emergent aquatic insects, was 280 ng Σ24PFAS m-2d-1 in 2017 and only 23 ng Σ24PFAS m-2d-1in 2018. Because of higher production of emergent aquatic insects, the lake had higher PFAS transfer and higher concentrations in terrestrial consumers compared to the stream, despite the stream having higher PFAS concentration in water and aquatic insect larvae. Our results indicate that biodriven transfer of PFAS from aquatic systems and subsequent uptake in terrestrial food webs depend more on emergence amounts, i.e. aquatic prey availability, rather than on PFAS concentrations in water and aquatic prey.
... However, many amphibiotic insects, for example, chironomids (family Chironomidae, order Diptera), after emergence do not cover long distances, concentrating in a narrow riparian strip [3]. In view of this, it can be assumed that terrestrial consumers inhabiting the riparian area and areas remote from water bodies have different access to food of aquatic origin. ...
Amphibiotic insects, chironomids of the genera Glyptotendipes and Chironomus, which emerged from saline Lake Shira, differed in composition and content of fatty acids, including the essential eicosapentaenoic acid (20:5n-3, EPA), and upon flying out they were concentrated in different territories, the riparian zone and remote arid steppe zone, respectively. Potential consumers of chironomids adults, the orb-weaver spiders Larinoides suspicax, which inhabited both zones, also differed in fatty acid composition. The main difference in their biochemical composition was a significantly higher level of EPA in spiders from the riparian zone that likely to be explained by consumption of the Glyptotendipes adults enriched in this fatty acid and concentrated only within this zone. The higher level of EPA, which is deficient in terrestrial ecosystems, in orb-weaver spiders from the riparian zone of the saline lake may potentially promote a successful survival of the consumers in the arid landscape.
... Our ability to detect this reliance with publicly available biomonitoring data was potentially hampered in several ways. For example, the relationships between emergent insects and aerial insectivorous birds are diffuse in both time and space, such that the timing of the Breeding Bird Survey (early summer) and the precise locations of monitoring routes may have occurred outside the direct influence of emerging insects from nearby streams or lakes, i.e., the majority (>70%) of emergent insect deposition from streams and lakes generally falls within 100 m of the waterbody (Gratton and Vander Zanden, 2009;Muehlbauer et al., 2014;Schilke et al., 2020). Beyond these mismatches inherent to the monitoring data sets, differential access to water and habitat use among riparian-to-upland obligate species, detrimental effects carried over from nonbreeding habitats, or landscape topography (e.g., ravines vs. lowelevation streams) are all expected to affect the foraging ecology of aerial insectivores. ...
Larval aquatic insects are used to assess water quality, but less attention is paid to their adult, terrestrial life stage, which is an important food resource for declining aerial insectivorous birds. We used open-access water-quality, aquatic-invertebrate, and bird-survey data to study how impaired water quality can emanate from streams and lakes through changes in aquatic insect communities across the contiguous United States. Emergent insect relative abundance was highest across the West, in northern New England, and the Carolinas in streams, and highest near the Great Lakes, parts of the Southwest, and northern New England for lakes. Emergent insects declined with sedimentation, roads, and elevated ammonium concentrations in streams, but not lakes. The odds that a given taxon would be non-emergent increased by up to 2.0× as a function of pollution tolerance, underscoring the sensitivity of emergent aquatic insects to water-quality impairment. However, relationships between bird populations and emergent insects were generally weak for both streams and lakes. For streams, we observed the strongest positive relationships for a mixture of upland and riparian aerial insectivorous birds such as Western Wood-Pewee, Olive-sided Flycatcher, and Acadian Flycatcher and the strongest negative association for Purple Martin. Different avian insectivores responded to emergent insect abundances in lakes (e.g., Barn Swallow, Chimney Swift, Eastern Wood-Pewee, Common Nighthawk). In both streams and lakes, we observed stronger, but opposing, relationships between several aerial insectivores and the relative abundance of sensitive insect orders (E)phemeroptera, (P)lecoptera, and (T)richoptera (positive), and pollution tolerant individuals (negative). Overall, our findings indicate that emergent insects are negatively correlated with pollution tolerance, suggesting a large-scale loss of this nutritional subsidy to terrestrial environments from impaired aquatic ecosystems. While some bird populations tracked scarcities of emergent aquatic insects, especially EPT taxa, responses varied among species, suggesting that unique habitat and foraging behaviors likely complicated these relationships. Strengthening spatial and temporal concordance between emergent-insect and bird-survey data will improve our ability to interpret species-level responses over time. Thus, our analysis highlights the need for developing conservation and biomonitoring strategies that consider the cross-ecosystem effects of water quality declines for threatened insectivorous avifauna and other terrestrial wildlife.
... This study, however, also indicated that more mobile social wasps, another terrestrial predator of emerging aquatic prey, show elevated levels of these contaminants about 30 m inland. Muehlbauer et al. (2014) reviewed the literature to define the spatial extent of a stream signature, i.e., the distance typically travelled by stream insects into the terrestrial surrounding. Although the half-maximum subsidy level was found at about 1.5 m, about 10% of the maximum subsidy level was still found at >500 m away from the stream. ...
The role of freshwater ecosystems as a source of nutrients, energy, and contaminants to terrestrial ecosystems is relatively underappreciated compared with the impact of catchment properties on inputs to receiving surface waters. Aquatic contaminants can reach terrestrial ecosystems through biologically mediated pathways, such as emerging adult aquatic insects or aquatic prey consumed by terrestrial predators. Alternatively, contaminant transfer from aquatic to terrestrial ecosystems occurs through abiotic pathways such as aqueous-phase or particle-bound contaminants transported via flood events to the surrounding terrestrial habitats. Once contaminants reach the terrestrial ecosystem, they may lead to top-down (consumer-driven) or bottom-up (producer-driven) effects on the terrestrial ecosystem/food web, yet our knowledge of these effects is still limited. In this chapter, we review the relative importance of these pathways of contaminant exposure in driving effects of aquatic contaminants on terrestrial ecosystems. We also summarize current knowledge about the effects of these exposures on terrestrial food webs. In this context, we discuss how the use of tools, such as stable isotope analysis, that untangle the complexity of aquatic contaminant effects on terrestrial ecosystems may support informed decision-making as part of chemical risk assessment or ecosystem management.
... Water 2020, 12, 2855 2 of 22 insects provide prey and nutrient subsidies for riparian consumers [6]. Importantly, aquatic insects assimilate not only carbon and nutrients in aquatic habitats but also high quality polyunsaturated fatty acids (PUFAs; e.g., ω3 fatty acids), which they export to terrestrial food webs as adults [7][8][9]. PUFAs are essential for several biological functions across a wide range of organisms [7,[10][11][12]. Higher plants and algae, and to some degree fungi, are the main sources of PUFAs for consumers, with several types of especially long-chain (with ≥20 carbon) PUFAs produced almost exclusively in aquatic environments by algae [7,13,14]. ...
Riparian habitats are important ecotones connecting aquatic and terrestrial ecosystems, but are often highly degraded by human activities. Riparian buffers might help support impacted riparian communities, and improve trophic connectivity. We sampled spider communities from riparian habitats in an agricultural catchment, and analyzed their polyunsaturated fatty acid (PUFA) content to quantify trophic connectivity. Specific PUFAs are exclusively produced by stream algae, and thus are used to track uptake of aquatic resources by terrestrial consumers. Riparian spiders were collected from 10 site pairs situated along agricultural streams, and from five forest sites (25 sites total). Each agricultural site pair comprised an unshaded site with predominantly herbaceous vegetation cover, and a second with a woody riparian buffer. Spider communities differed between site types, with web-building spiders dominating woody buffered sites and free-living spiders associated with more open habitats. PUFA concentrations were greatest overall in free-living spiders, but there was also evidence for increased PUFA uptake by some spider groups when a woody riparian buffer was present. Our results reveal the different roles of open and wooded riparian habitats in supporting terrestrial consumers and aquatic-terrestrial connectivity, and highlight the value of incorporating patches of woody vegetation within riparian networks in highly modified landscapes.
... Floodplains in lowland and floodout zones are usually well developed (Fig. 2.3.8C) and may be so vast that during extreme floods, lateral hydrological connectivity of surface water extends far from the main channel or channels (Fig. 2.3.7), transporting sediments, organic matter, and aquatic biota for long distances (Bunn et al., 2006;Muehlbauer et al., 2014). Further, in contrast to upland and piedmont zones, the directionality of hydrological connectivity in lowland and floodout zones can be complex, resulting in variable rates and locations of the delivery of water and materials to adjacent riparian and other terrestrial ecosystems. ...
... Juvenile caddisflies primarily disperse downstream with river current, but winged adults can fly up, down, and perpendicular to rivers (Svensson 1974). Adults can fly as far as 650 m perpendicular to streams (Muehlbauer et al. 2014), but adult dispersal is probably most concentrated parallel to stream corridors (Peterson et al. 2004, Yaegashi et al. 2014. Markrecapture studies along the Yagi River in Japan found that Stenopsyche fly primarily upstream and individual females can fly up to 12 km in a life cycle (Nishimura 1967(Nishimura , 1981. ...
The watershed-continuum model (WCM) describes fluvial-riparian ecosystems (FREs) as dynamic reach-based ecohydrogeological riverine landscapes linking aquatic, riparian, and upland domains within watersheds. FRE domains include aquatic (channels, hyporheic zones, springs, other groundwater zones and in-channel lakes), riparian, and adjacent upland zones, all of which can interact spatio-temporally. Occupying only a minute proportion of the terrestrial surface, FREs contain and process only a tiny fraction of the Earth’s freshwater, but often are highly productive, flood-disturbed, and ecologically interactive, supporting diverse, densely-packed biotic assemblages and socio-cultural resource uses and functions. FRE biodiversity is influenced by hydrogeomorphology, ecotonal transitions, and shifting habitat mosaics across stage elevation. Thus, the WCM integrates physical, biological, and socio-cultural characteristics, elements, and processes of FREs. Here, we summarize and illustrate the WCM, integrating diverse physical and ecological conceptual models to describe natural (unmanipulated) FRE dynamics. We integrate key processes affecting FRE forms and functions, and illustrate reach-based organization across temporal and spatial scales. Such a holistic approach into natural FRE structure and functions provides a baseline against which to measure and calibrate ecosystem alteration, management, and rehabilitation potential. Integration of groundwater, fluvial, and lacustrine ecological interactions within entire basins supports long-term, seasonally-based sustainable river management, which has never been more urgently needed.
Altering the hydrological connectivity between river channels and their riparian wetlands is a significant threat to river ecosystems. Aquatic insects are sensitive to environmental changes. In this study, community composition and functional traits of aquatic insects were investigated in reaches of the Manjiang and Songjiang Rivers, impacted by a water diversion project in northeast China. The species diversity of the flow-reduced reaches was more than twice as much as that of the reservoir riverine wetlands, and the species diversity of the flow-reduced reaches was less than twice as much as that of the natural riverine wetlands. Compared with the natural riverine wetlands, there was a lower abundance of scrapers in the reservoir wetlands, a higher abundance of filter collectors and shredders in the flow-reduced riverine wetlands, higher abundance of swimmers in the reservoir wetlands, and more sprawlers and swimmers in the flow-reduced wetlands. Co-occurrence network showed interspecific relationships of aquatic insect communities differed between the three types of riverine wetlands. Indicator species differed among wetland types. The environmental factors affecting the aquatic insect were water nitrate-nitrogen content, chlorophyll-a content, and electrical conductivity. In conclusion, proper hydrological connectivity between reaches wetlands should maintain to ensure ecological flow and biological connectivity.
Long-nosed god (LNG) maskettes and iconography have traditionally been seen as a pre-Southern Cult phenomena, placed variously in the tenth to thirteenth centuries. Researchers have suggested they were employed in political and religious interactions or to facilitate trade, but few have looked in detail at their chronology, context, and distribution. Here, an in-depth review of radiocarbon dates and context raises questions about the place of LNGs in midcontinental Native societies. This reassessment illustrates that LNG images do not predate the appearance of Caddo and Cahokian symbolic emergence and can be first securely documented in the late eleventh century. They clearly are objects that signify personal endowments and are inalienable, following that individual to the grave. Their context and distribution indicate that LNG icons are an integral part of the Caddo religious and political networks but are tangential at Cahokia and take on totally different contextual meanings to the north of Cahokia. This study demonstrates that proposing uniform explanations for LNG ideology and implementation does not correlate with the archaeological evidence. Future studies that account for regional variations in LNG chronology, context, and spatial distribution are needed to begin addressing the roles of these unique objects in Native societies.
Mining activities often produce large amounts of pollutants that lead to streams affecting aquatic biota. Aquatic insects have a key role in energy transference from streams to terrestrial systems since emergent insects contribute to the diet of riparian predators. If streams are polluted, emergent insects may act as pollutant conveyors from water to land. Our objective was to investigate if insects inhabiting streams contaminated by heavy metals accumulate, biomagnify, and transfer metals to land. We selected eight streams with different levels of pollution and three metallic pollutants: copper (Cu), manganese (Mn), and zinc (Zn). We sampled (i) water and sediments, (ii) organic matter, macrophytes, and biofilm, (iii) aquatic insects, and (iv) riparian spiders (land predators). We classified the organisms in functional feeding groups (FFG) and used the nitrogen stable isotope (δ15N) to determine the position of organisms in the food web. We found that contaminants in the sediments, but not in the water, were related to contaminant concentrations in biological samples. Biomagnification processes were metal dependent: Cu was biomagnified, Mn underwent biodilution, and no tendency was observed for Zn. The emergence of aquatic insects from metal-polluted rivers is a potential way of Cu, not Mn or Zn, flux to land.
This study tested the hypothesis that color affects the behavior of Ephemeroptera, Plecoptera, and Trichoptera (EPT) adults in the riparian zone of a gravel-bed river in northern Japan. EPT abundance was measured using plot scale surveys and a color-choice experiment that utilized non-shiny sticky traps in two contrasting colors, yellow and blue. Chloroperlidae and Hydrobiosidae were caught more abundantly in yellow and blue traps, respectively, whereas other taxa exhibited little or no color-affected responses. We proposed that Chloroperlidae responses were driven by relatively strong diurnal activity compared with those of other taxa. Hydrobiosidae’s preference of blue remained unknown. Understanding the evolutionary background of color preferences in relation to other possibly interfering factors, such as reflection–polarization characteristics, at the species level will help advance the visual sensory ecology of aquatic insects.
Food webs describe the network of linkages that are responsible for transfers of energy among species, and for the movement of nutrients between organisms and the abiotic environment. Food web networks ideally describe all interactions within a community of species, while recognizing that much of the energy flows through relatively few species. These pathways vary along a river’s length, with season, and among ecosystems. Food webs often receive external inputs, including allochthonous leaf litter, invertebrates from the terrestrial riparian zone, and organic matter transported from upstream, termed spatial subsidies. In turn, riverine ecosystems export material downstream, and the adults of aquatic invertebrates subsidize food webs along the river’s banks. Functional classifications of species’ roles in nutrient and energy flows can be based on a variety of species attributes. Often, organisms will have complementary or overlapping roles for a given functional attribute, but at least in some cases species appear to be functionally irreplaceable. Animals consume and store resources in body tissues, and subsequently mineralize elements through excretion, egestion, and death and decomposition. In this manner they play a critical role in biogeochemical cycling, referred to as consumer-driven nutrient dynamics.
The metacommunity framework has been readily applied to coastal benthic marine environments to examine how larval dispersal affects the dynamics of patchily distributed communities. Transitioning to a meta-ecosystem perspective requires knowledge of interactions between living and non-living compartments occurring across scales in these environments. Here, we synthesize and analyze evidence of non-living resource flows in coastal benthic marine environments. Our objectives are to establish the types of benthic ecosystems that are coupled by resource flows, the spatial scale and directionality of the couplings, and the magnitude of resulting subsidization of recipient organisms. We find that resource flows commonly couple different types of coastal benthic ecosystems and can occur bidirectionally between ecosystems. Our quantitative synthesis yields a frequency distribution of resource flow distance, which suggests they frequently couple ecosystems across smaller distances than larval dispersal and that the probability of resource flows coupling benthic ecosystems decreases exponentially with distance between ecosystems. The magnitude of subsidization of recipient organisms also decreases with distance from the source of the resource flow. Our findings reveal that considering ecosystem heterogeneity and the respective scales of different types of spatial flows will be an important component of extending the marine metacommunity framework to meta-ecosystems. We propose an avenue for integrating ecosystem heterogeneity into meta-ecosystem theory, based upon general differences in functioning across coupled ecosystems revealed by our synthesis, and we argue for the development of a hierarchical meta-ecosystem theory.
Models of interaction networks among species (i.e., network models) can be used to predict behavior of complex adaptive systems, such as ecosystems at the land-water interface. As theoretical and empirical understanding of ecological networks continues to grow, network models are increasingly used to quantify and predict ecosystem stability, metapopulation dynamics, disease spread, and social behavior. Here, we propose that ecological networks can also be used to synthesize many of the dynamics that regulate the movement of contaminants between aquatic and terrestrial systems. We discuss how network models can predict aquatic-terrestrial contaminant transport via food-web interactions, animal movement, and mutualistic interaction webs. In addition, we discuss how knowledge of social-ecological networks can help address contaminant exposure and risk to humans. To illustrate an application of network models in the field of contaminants and subsidies, we provide an empirical example of the responses of ecological networks to dam removal in a temperate river system. In this example, we observed that shifts in species composition (especially losses of larger-bodied, often predatory, sport fish) and subsequent changes to feeding relationships could reduce biomagnification of contaminants in the food web and thus reduce exposure to humans and other terrestrial consumers in the short term. In summary, we propose that ecological networks will be a valuable framework for understanding and forecasting temporally and spatially dynamic aquatic to terrestrial contaminant transport.
Insect-mediated contaminant flux is truly an interdisciplinary concept that merges ideas from many technical areas of science (e.g., environmental chemistry, landscape ecology, and entomology). This chapter introduces risk assessors to this emerging and ecologically relevant concept by distilling the main mechanisms that drive insect-mediated contaminant flux and integrating them together so that more informed decisions can be made on whether the phenomenon presents a potential risk at a site.
At low concentrations, trace metals are critical for sustaining life on Earth. However, at high concentrations, they become a global contaminant with particularly strong effects on freshwater communities. These effects can propagate to terrestrial ecosystems in part by altering production and community structure of adult aquatic insect emergence and aquatic insect-mediated metal fluxes to terrestrial insectivores. Here we highlight mechanisms driving effects of trace metals on aquatic organisms in general, aquatic insects specifically, and insectivorous consumers at the land-water interface. Specifically, we focus on how trace metals impact and bioaccumulate in aquatic organisms and communities and how these changes propagate through aquatic food web interactions and insect metamorphosis to alter fluxes of aquatically derived prey and trace metals to terrestrial consumers. Ultimately, trace metals impact food webs at the land-water interface by altering aquatic insect prey composition and availability for aquatic insectivores and by reducing aquatic insect subsidies to terrestrial consumers, and not by increasing exposure to trace metals in prey. Exposure of terrestrial insectivores to trace metals in prey is decoupled from aqueous concentrations due to high rates of metal excretion during insect metamorphosis from aquatic larvae to terrestrial adult. These effects increase reliance of aquatic insectivores on terrestrial insect prey subsidies and/or lead to declines and behavioral changes in terrestrial insectivore populations.
The occurrence of per- and polyfluoroalkyl substances (PFASs) in aquatic ecosystems is a global concern due to their persistence, potential bioaccumulation and toxicity. In this study, we investigated a PFAS-contaminated pond in Sweden to assess the cross-boundary transfer of PFASs from the aquatic environment to the riparian zone via emergent
aquatic insects. Aquatic and terrestrial invertebrates, surface water, sediment, soil and plants were analysed for PFASs including branched isomers. Stable isotope analysis of carbon and nitrogen was performed to elucidate the importance of diet and trophic position for PFAS uptake. We present the first evidence that PFASs can propagate to the riparian food web via aquatic emergent insects. Elevated Σ24PFAS concentrations were found in aquatic insect larvae, such as dragon- and damselflies, ranging from 1100-4600 ng g-1 dry weight (dw), and remained high in emerged adults (120-3500 ng g-1 dw), indicating exposure risks for top predators that prey in riparian zones. In terrestrial invertebrate consumers, PFAS concentrations increased with the degree of aquatic-based diet and at higher trophic levels. Furthermore, stable isotope data together with calculated bioaccumulations factors indicated that bioconcentration of PFASs was the major pathway of exposure in the aquatic food web, while implying that biomagnification occurred in the riparian food web.
Streams and their adjacent riparian zones are increasingly viewed as interdependent systems linked by reciprocal exchanges of energy, organisms, and materials. We assessed potential associations between the emerging aquatic insect flux and transitions between agricultural land and forest fragments to better understand these stream‐riparian linkages in managed landscapes. We sampled stream environmental conditions and emerging insects at 28 sites distributed along three streams flowing through agriculture‐forest‐agriculture transitions in central Ohio, USA, in the summer of 2012. Ephemeroptera and Trichoptera (ET) taxa had higher flux densities in forests (mean and 95% CI: 0.3 insects m⁻² d⁻¹ [0.1, 0.8]) compared to agriculture (mean and 95% CI: 0 insect m⁻² d⁻¹ [0, 0.1]; p = .004), and ET taxa were found in 67% of forested sites compared to only 15% of agricultural sites. In contrast, Dolichopodidae were more strongly associated with agricultural land (mean and 95% CI: 0.6 insect m⁻² d⁻¹ [0.3 to 1.2]) than forests (mean and 95% CI: 0.1 insects m⁻² d⁻¹ [0.1, 0.2]; p = .002). Although Chironomidae were the most numerically abundant, ET taxa were among the larger bodied insects and comprised >30% of the total biomass flux, illustrating the importance of taxonomic traits in mediating flux dynamics. Mechanisms driving emerging insect flux were related to substrate grain‐size distribution, channel width, and nutrient concentrations. Overall, our results demonstrate that small forest fragments are strongly related to the aquatic‐to‐terrestrial insect flux and thus have important implications for terrestrial biodiversity and food webs in agricultural landscapes.
Anadromous Pacific salmon (Oncorhynchus spp.) transport marine-derived nitrogen (MDN) to the rivers in which they reproduce. Isotopic analyses indicate that trees and shrubs near spawning streams derive ∼22-24% of their foliar nitrogen (N) from spawning salmon. As a consequence of this nutrient subsidy, growth rates are significantly increased in Sitka spruce (Picea sitchensis) near spawning streams. As riparian forests affect the quality of instream habitat through shading, sediment and nutrient filtration, and production of large woody debris (LWD), this fertilization process serves not only to enhance riparian production, but may also act as a positive feedback mechanism by which salmon-borne nutrients improve spawning and rearing habitat for subsequent salmon generations and maintain the long-term productivity Of river corridors along the Pacific coast of North America.
Watersheds are drained by river networks, which route materials and
energy from headwaters to terminal water bodies. River networks likewise
perfuse the terrestrial portion of watershed ecosystems and reroute some
of these materials upslope via material exchange between rivers and
land. Here we develop a model of resource exchange between rivers and
watersheds to predict the spatial extent of material and nutrient fluxes
from aquatic portions of watershed ecosystems. The model is based on a
geomorphic template that includes river network structure, topography,
and channel sinuosity as well as important biological attributes
(productivity and dispersal ability). Analysis of this model suggests
that the geomorphic template strongly influences the spatial extent of
resource flows in watershed ecosystems. The geomorphic template also
predicts the location of areas of concentrated resource exchange,
typically at ridge crests, in meander bends, and tributary junctions. We
contend that these areas represent hotspots of foraging activity for
terrestrial consumers, especially those at the reach scale (meander
bends). More generally, our model suggests that the spatial extent of
aquatic resource flow equal in magnitude to 20% or greater of
terrestrial production may encompass as much as 20%-50% of terrestrial
portions of watersheds. Resource flow from rivers to terrestrial
ecosystems is not merely an edge effect. Instead, the river network may
reroute a substantial flux of materials into watershed ecosystems.
Flight of alpine stream insects has not been well studied but is an important ecological process that ensures successful mating and allows gene flow among relatively isolated populations. In this study, we collected actively flying insects along a perpendicular transect from an alpine headwater stream in the Colorado Rocky Mountains (U.S.A.) during the summer emergence season in two consecutive years with contrasting hydrology: 2002 had minimal snowfall the previous winter, while 2003 snowfall was above average. Flight activity patterns among four common stream taxa were similar to previously reported results from streams below treeline: Ephemeroptera and Plecoptera declined as an inverse power function, Trichoptera declined as a negative exponential function, and Simuliidae did not decrease with lateral distance. Sex ratios typically were strongly biased, possibly a result of the harsh terrestrial environment negatively influencing the naturally more sedentary sex (which varies among taxa). In 2003, the majority of common species emerged approximately one month later than in 2002, and abundance and diversity were greater in 2003 than 2002, patterns potentially attributable to increased snowpack amount and duration. Late-emerging species, by contrast, were less abundant in 2003, likely because that year emergence was delayed to later in the season, when cooler air temperatures reduce flight activity. Our results suggest that alpine streams are sensitive to interannual variation in snowpack, and therefore more research will be needed to address the potential effects of climate change and associated winter snowfall trends on these unexpectedly diverse aquatic systems.
River otters (Lutra canadensis Schreber) inhabiting coastal environments scent, mark specific locations along the coast, known as latrine sites. In this study, we used stable isotope techniques to investigate the effects of this scent-marking behavior on terrestrial vegetation at the terrestrial-marine interface. Our analysis of stable isotope ratios of fur and feces indicated that river otters fed mainly on intertidal and subtidal fish. Eight different species of plants, growing in latrine sites of river otters, had significantly higher values of δ15N compared with the same plant species growing on nonlatrine sites. Elevated N concentrations occurred only in grasses and mosses growing in latrine sites. Our results indicate that, through their scent-marking behavior, coastal river otters transfer marine-derived nitrogen into the beach-fringe forest and thus fertilize the terrestrial vegetation in the terrestrial-marine interface.
Ecologists long have been aware that there is flux of energy and nutrients from riverine systems to the surrounding terrestrial landscape and vice versa. Riparian ecotones are diverse and ecologically important. Consequently, there is substantial literature examining faunal-mediated transfers of energy and nutrients from rivers into terrestrial food webs. A wide variety of taxa has been shown to utilise riparian resources, from species specialised for existence at the aquatic-terrestrial interface to opportunistic predators and scavengers. Outputs from rivers may be influenced by productivity gradients, channel geometry and the condition of the exchange surface. Until recently, consideration of faunal-transferred, allochthonous inputs has been peripheral to other research questions. The development of general models of inter-habitat transfers, together with advances in technology, has placed questions about the ecological importance of riverine outputs squarely on the research agenda. Researchers now are investigating how aquatic subsidies influence food-web dynamics at landscape scales. However, ecologists continue to largely ignore subsidisation of terrestrial food webs by energy and nutrients from floodwaters in lowland river floodplain systems. The dearth of information about the benefits of flooding to terrestrial consumers appears to have resulted in underestimation of the gross ecological impacts of river regulation.
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Summary 1. There have been widespread changes in land use in the uplands of the UK but the implications for dispersal of adult stages of aquatic invertebrates are poorly known. We estimated the lateral dispersal of adult aquatic insects (Plecoptera, Trichoptera, Ephemeroptera) in seven small, upland streams draining catchments under three cat- egories of land use (coniferous plantation forest, cleared forest, moorland). 2. Malaise traps were set out in transects perpendicular to each stream. More than 29 000 adult insects were taken, distributed among 15 species of stoneflies, 40 species of caddisflies and eight taxa of mayflies. Overall species diversity and equitability were highest in the moorland catchments, and few species were numerous in all catchments. 3. Nearly all the mayflies were taken in the moorland catchments, where caddisflies were also most abundant. Fewest stoneflies were taken in the forested catchments. 4. The vast majority of insects were taken either directly over, or very close to, the stream channel. Half the stoneflies were taken within 18 m of the channel, while 90% had travelled less than 60 m. Caddisflies and mayflies travelled even shorter distances. Although there were differences in lateral dispersal between some catchments, there was no overall effect of land use. 5. The overall sex ratio in stoneflies and mayflies in the riparian zone was close to 1 : 1 and lateral dispersal was similar between the sexes. Male mayflies outnumbered females in the riparian zone and males travelled further from the stream, on average, than females. In catches taken directly over the stream, female stoneflies outnumbered males. 6. Regardless of land use, the flight of mayflies and caddisflies was concentrated along the stream, rather than perpendicular to it. This was also true for two numerous stoneflies ( Amphinemura sulcilcollis and Protonemura meyeri ) and for female stoneflies overall. 7. Synthesis and applications . The stream corridor, including the riparian strip extending 10-20 m on either side of the channel, is the main habitat for adult aquatic insects, and its management may affect the biodiversity of aquatic communities. The stream corridor is also revealed as the main 'highway' for adult dispersal. While there is no evidence from this study of an effect on interstream dispersal of land use elsewhere in the catchment, such an effect cannot yet be refuted because rare long-distance dispersal events are difficult to record.
1. Populations in different locations can exchange individuals depending on the distribution and connectivity of suitable habitat, and the dispersal capabilities and behaviour of the organisms. We used an isotopic tracer, 15N, to label stoneflies (Leuctra ferruginea) to determine the extent of adult flight along stream corridors and between streams where their larvae live.
2. In four mass, mark-capture experiments we added 15NH4Cl continuously for several weeks to label specific regions of streams within the Hubbard Brook Experimental Forest, NH, U.S.A. We collected adult stoneflies along the labelled streams (up to 1.5 km of stream length), on transects through the forest away from labelled sections (up to 500 m), and along an 800-m reach of adjacent tributary that flows into a labelled stream.
3. Of 966 individual adult stoneflies collected and analysed for 15N, 20% were labelled. Most labelled stoneflies were captured along stream corridors and had flown upstream a mean distance of 211 m; the net movement of the population (upstream + downstream) estimated from the midpoint of the labelled sections was 126 m upstream. The furthest male and female travelled approximately 730 m and approximately 663 m upstream, respectively. We also captured labelled mature females along an unlabelled tributary and along a forest transect 500 m from the labelled stream, thus demonstrating cross-watershed dispersal.
4. We conclude that the adjacent forest was not a barrier to dispersal between catchments, and adult dispersal linked stonefly populations among streams across a landscape within one generation. Our data on the extent of adult dispersal provide a basis for a conceptual model identifying the boundaries of these populations, whose larvae are restricted to stream channels, and whose females must return to streams to oviposit.
Summary1. Streams and their adjacent riparian zones are closely linked by reciprocal flows of invertebrate prey. We review characteristics of these prey subsidies and their strong direct and indirect effects on consumers and recipient food webs.2. Fluxes of terrestrial invertebrates to streams can provide up to half the annual energy budget for drift-feeding fishes such as salmonids, despite the fact that input occurs principally in summer. Inputs appear highest from closed-canopy riparian zones with deciduous vegetation and vary markedly with invertebrate phenology and weather. Two field experiments that manipulated this prey subsidy showed that it affected both foraging and local abundance of stream fishes.3. Emergence of adult insects from streams can constitute a substantial export of benthic production to riparian consumers such as birds, bats, lizards, and spiders, and contributes 25–100% of the energy or carbon to such species. Emergence typically peaks in early summer in the temperate zone, but also provides a low-level flux from autumn to spring in ice-free streams. This flux varies with in-stream productivity, and declines exponentially with distance from the stream edge. Some predators aggregate near streams and forage on these prey during periods of peak emergence, whereas others rely on the lower subsidy from autumn through spring when terrestrial prey are scarce. Several field experiments that manipulated this subsidy showed that it affected the short-term behaviour, growth, and abundance of terrestrial consumers.4. Reciprocal prey subsidies also have important indirect effects on both stream and riparian food webs. Theory predicts that allochthonous prey should increase density of subsidised predators, thereby increasing predation on in situ prey and causing a negative indirect effect via apparent competition. However, short-term experiments have produced either positive or negative indirect effects. These contrasting results may be due to characteristics of the subsidies and individual consumers, but could also result from differences in experimental designs.5. New study approaches are needed to better determine the direct and indirect effects of reciprocal prey subsidies. Experiments coupled with comparative research will be required to measure their effects on individual consumer fitness and population demographics. Future work should investigate whether reciprocal prey fluxes stabilise linked stream–riparian ecosystems, explore how landscape context affects the magnitude and importance of subsidies, and determine how impacts of human disturbance can propagate between streams and riparian zones via these trophic linkages. Study of these reciprocal connections is helping to define a more holistic perspective of catchments, and has the potential to shape new directions for ecology in general.
Fluxes of resource subsidies, such as terrestrial leaf litter to streams and adult aquatic insects to riparian predators, are examples of important links between adjacent ecosystems. The importance of these cross-ecosystem resource flows from donor systems to recipient consumers is increasingly recognized. Streams, especially small streams with their high edge ratio with the terrestrial system, provide excellent models for the study of subsidies and a large portion of this literature has been produced by aquatic scientists. Field experiments manipulating flows between small streams and their riparian areas (e.g. leaf litter, terrestrial invertebrates, and adult aquatic insects to riparian areas) have indicated that consumers in streams and riparian areas are highly dependent upon such subsidies and the value of the subsidies are further modified by patterns of retention and pathways of use. Experiments typically indicate rapid growth or demographic responses by consumers, indicating these populations are resource limited or at levels of incipient population limitation, and can capitalize on short-term resource pulses. More press manipulations are still necessary to determine the dynamical consequences of subsidies for recipient communities. The nature of the subsidy (e.g. species of litter or invertebrates) and its timing are also important details that need further study. Finally, there are opportunities to consider the evolution of life cycle timing (modelling), interception strategies by recipient populations and short-term and long-term responses of communities. Copyright © 2009 John Wiley & Sons, Ltd.
Riparian habitats are important for the maintenance of regional biodiversity. Many studies have compared bird distributions between riparian and non-riparian habitats but have not established how wide riparian habitats used by birds are, as measured by distance from the nearest stream. We investigated the distribution of understory birds along gradients of distance from streams, soil clay content, and slope in a central Amazonian forest, by mist-netting birds three times in 45 plots. We used nonmetric multidimensional scaling (NMDS) to reduce the dimensionality of species quantitative (abundance) and qualitative (presence-absence) composition to one multivariate axis. Estimates of the width of riparian habitats as indicated by understory birds depended on the community attribute considered, measuring 90 m for species quantitative composition and 140 m for species qualitative composition. Species distributions were correlated with clay content but were independent of slope, while distance from streams was positively correlated with clay content but independent of slope. Clay content affects plant species composition, which in turn, may influence bird species composition. However, distribution patterns of birds in relation to distance from streams are consistent among studies carried out in many different temperate and tropical regions, indicating an effect of distance from streams itself. Protection of riparian habitats is one of the most widely used conservation strategies, and Brazilian environmental legislation mandates the protection of a 30 m wide strip of riparian vegetation on either side of small streams. We show that the protected strip should be much wider and recommend strategies to place other forms of land protection contiguous with riparian areas so that Brazilian environmental legislation better fulfills its role of protecting biodiversity associated with riparian habitats.
Rivers provide important resources for riparian consumers, especially in arid or seasonally arid biomes. Pygmy grasshoppers (Paratettix aztecus & P. mexicanus; Tetrigidae) graze river algae stranded along shorelines of the South Fork Eel River in northern California (39°442N, 123°392W) as the river recedes during the summer drought. Densities of tetrigids during the mid to late summer were highest (1 individual/m2 in July) within 1 m of the river margin, and declined to near zero at 4 m from the margin, especially during peak temperatures in the afternoon. These observations suggested that the distribution of tetrigids was determined by the availability of algae, water, or both. We manipulated the presence/absence of water and beached algae (Cladophora glomerata) in a 2x2 factorial design. All treatments were positioned 2 m upslope from the river's edge (about 30 cm above the water table), where the cobble bar was naturally dry and devoid of algae and densities of tetrigids were lower than at the river margin (0.4 individuals/m2 in July). Tetrigids responded only to the wet Cladophora treatment, which had 30x higher densities than other treatments. Stable isotopic signatures (ä13C) of tetrigids (-19.7‰) collected from the same cobble bars were more similar to those of epilithic algae (-20.4‰) than terrestrial plants (-28.2‰), and higher than those of acridid grasshoppers (-27.9‰) from the same habitats. Mixing models suggest that 88-100% of the C in tetrigid grasshoppers at our study site is derived from riverine algae. A preliminary analysis suggests that tetrigids ingested sufficient quantities of algae to easily meet their energetic demands during the summer. This study supports the idea that algae, produced in stream systems, can determine the distribution and relative abundance of a common terrestrial scavenger and provide an additional pathway for energy exchange between rivers and riparian food webs.
Because nutrient enrichment can increase ecosystem productivity, it may enhance resource flows to adjacent ecosystems as organisms cross ecosystem boundaries and subsidize predators in recipient ecosystems. Here, we quantified the biomass and abundance of aquatic emergence and terrestrial spiders in a reference and treatment stream that had been continuously enriched with nitrogen and phosphorus for 5 years. Because we previously showed that enrichment increased secondary production of stream consumers, we predicted that aquatic emergence flux would be higher in the treatment stream, subsequently increasing the biomass and abundance of terrestrial spiders. Those increases were predicted to be greatest for spiders specializing on aquatic emergence subsidies (e.g., Tetragnathidae). By adding a (15)N stable isotope tracer to both streams, we also quantified nitrogen flow from the stream into the riparian community. Emergence biomass, but not abundance, was higher in the treatment stream. The average body size of emerging adult insects and the relative dominance of Trichoptera adults were also greater in the treatment stream. However, spider biomass did not differ between streams. Spiders also exhibited substantially lower reliance on aquatic emergence nitrogen in the treatment stream. This reduced reliance likely resulted from shifts in the body size distributions and community composition of insect emergence that may have altered predator consumption efficiency in the treatment stream. Despite nutrient enrichment approximately doubling stream productivity and associated cross-ecosystem resource flows, the response of terrestrial predators depended more on the resource subsidy's characteristics that affected the predator's ability to capitalize on such increases.
River and riparian areas provide an important foraging habitat for insectivorous bats owing to high insect availability along waterways. However, structural characteristics of the riverine landscape may also influence the location of foraging bats. We used bat detectors to compare bat activity longitudinally along river reaches with contrasting channel confinement, ratio of valley floor width to active channel width, and riparian vegetation, and laterally with distance from the river along three different reach types. We measured rates of insect emergence from the river and aerial insect availability above the river and laterally up to 50-m into the riparian habitat in order to assess the relationship between food resources and insectivorous bat activity. Longitudinally, bat activity was concentrated along confined reaches in comparison to unconfined reaches but was not related to insect availability. Laterally, bats tracked exponential declines in aquatic insects with distance from the river. These data suggest that along the lateral dimension bats track food resources, but that along the longitudinal dimension channel shape and landscape structure determine bat distributions more than food resources.
Historically, forested riparian buffers have been created to provide protection for aquatic organisms and aquatic ecosystem functions. Increasingly, new and existing riparian buffers are being used also to meet terrestrial conservation requirements. To test the effectiveness of riparian buffers for conserving terrestrial fauna, we conducted a meta-analysis using published data from 397 comparisons of species abundance in riparian buffers and unharvested (reference) riparian sites. The response of terrestrial species to riparian buffers was not consistent between taxonomic groups; bird and arthropod abundances were significantly greater in buffers relative to unharvested areas, whereas amphibian abundance decreased. Edge-preferring species were more abundant in buffer sites than reference sites, whereas species associated with interior habitat were not significantly different in abundance. The degree of buffer effect on animal abundance was unrelated to buffer width; wider buffers did not result in greater similarity between reference and buffer sites. However, responses to buffer treatment were more variable in buffers <50 m wide, a commonly prescribed width in many management plans. Our results indicate that current buffer prescriptions do not maintain most terrestrial organisms in buffer strips at levels comparable to undisturbed sites.
Populations of benthic invertebrates in neighbouring streams are isolated from each other by intervening terrestrial habitat. The adult stages of stream insects that are capable of flight may disperse between streams, although little is known of the extent of inter-stream dispersal, or the degree to which movement is influenced by riparian vegetation. Double-headed Malaise traps were set at differing distances from the channel to measure the numbers of adult stoneflies moving towards or away from three upland streams draining adjacent catchments on the Plynlimon ridge in mid-Wales. Riparian vegetation differed between the streams: open sheep-grazed moorland, buffer strips left free from 15 year old plantation forestry, and mature conifers planted up to the stream bank. Adult abundance was broadly consistent with benthic larval abundance. In three of the five most abundant species (Amphinemura sulcicollis, Leuctra fusca and Siphonoperla torrentium) abundance varied inversely with the amount of forestry in the riparian zone. In the others (L. inermis and L. nigra) numbers caught were unrelated to forestry. Relationships between adult abundance and distance from the channel were best described by inverse power functions. Numbers of adults declined sharply with distance from the stream; 90 % of adults were caught within 11m of the stream channel. The rate of decline of adult numbers with distance did not differ with riparian vegetation. Inter-stream differences in the numbers caught at particular distances were due to differences in overall abundance. There were no differences in the number of males or females caught at different distances from the stream. At all times more stoneflies were caught in the side of the trap facing the stream than that facing away, although the difference was greatest in the middle of the flight period and least at the beginning and end. Overall, the majority of adult stoneflies did not disperse a significant distance from the stream channel, suggesting that only a very limited number of individuals are likely to disperse between streams.
Mutual trophic interactions between contiguous habitats have remained poorly understood despite their potential significance for community maintenance in ecological landscapes. In a deciduous forest and stream ecotone, aquatic insect emergence peaked around spring, when terrestrial invertebrate biomass was low. In contrast, terrestrial invertebrate input to the stream occurred primarily during summer, when aquatic invertebrate biomass was nearly at its lowest. Such reciprocal, across-habitat prey flux alternately subsidized both forest birds and stream fishes, accounting for 25.6% and 44.0% of the annual total energy budget of the bird and fish assemblages, respectively. Seasonal contrasts between allochthonous prey supply and in situ prey biomass determine the importance of reciprocal subsidies.
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In a recent article Caley (1991) outlined a null model for dispersal distributions
against which he suggested empirical data should be compared. He first presented
Waser's geometric model (Waser 1985), which can be derived as follows: Dispersing
individuals move in a straight line from the natal site and settle in the first
unoccupied site they encounter. If unoccupied sites occur independently at random
with probability t as a result of turnover within the habitat, then the distribution
of dispersal distances will follow a geometric distribution in which the probability
of settling at distance i is given by
p(i) = t(l - t)' for i = 0, 1,2,3,. . .
continues..
Adult aquatic insects emerging from streams may be a significant source of energy for terrestrial predators inhabiting riparian zones. In this study, we use natural abundance delta(13)C and delta(15)N values and an isotopic (15)N tracer addition to quantify the flow of carbon and nitrogen from aquatic to terrestrial food webs via emerging aquatic insects. We continuously dripped labeled (15)N-NH(4) for 6 weeks into Sycamore Creek, a Sonoran desert stream in the Tonto National Forest (central Arizona) and traced the flow of tracer (15)N from the stream into spiders living in the riparian zone. After correcting for natural abundance delta(15)N, we used isotopic mixing models to calculate the proportion of (15)N from emerging aquatic insects incorporated into spider biomass. Natural abundance delta(13)C values indicate that orb-web weaving spiders inhabiting riparian vegetation along the stream channel obtain almost 100% of their carbon from instream sources, whereas ground-dwelling hunting spiders obtain on average 68% of their carbon from instream sources. During the 6-week period of the (15)N tracer addition, orb-web weaving spiders obtained on average 39% of their nitrogen from emerging aquatic insects, whereas spider species hunting on the ground obtained on average 25% of their nitrogen from emerging aquatic insects. To determine if stream subsidies might be influencing the spatial distribution of terrestrial predators, we measured the biomass, abundance and diversity of spiders along a gradient from the active stream channel to a distance of 50 m into the upland using pitfall traps and timed sweep net samples. Spider abundance, biomass and richness were highest within the active stream channel but decreased more than three-fold 25 m from the wetted stream margin. Changes in structural complexity of vegetation, ground cover or terrestrial prey abundance could not account for patterns in spider distributions, however nutrient and energy subsidies from the stream could explain elevated spider numbers and richness within the active stream channel and riparian zone of Sycamore Creek.
Studies of the effects of cross-habitat resource subsidies have been a feature of food web ecology over the past decade. To date, most studies have focused on demonstrating the magnitude of a subsidy or documenting its effect in the recipient habitat. Ecologists have yet to develop a satisfactory framework for predicting the magnitude of these effects. We used 115 data sets from 32 studies to compare consumer responses to resource subsidies across recipient habitat type, trophic level, and functional group. Changes in consumer density or biomass in response to subsidies were inconsistent across habitats, trophic, and functional groups. Responses in stream cobble bar and coastline habitats were larger than in other habitats. Contrary to expectation, the magnitude of consumer response was not affected by recipient habitat productivity or the ratio of productivity between donor and recipient habitats. However, consumer response was significantly related to the ratio of subsidy resources to equivalent resources in the recipient habitat. Broad contrasts in productivity are modified by subsidy type, vector, and the physical and biotic characteristics of both donor and recipient habitats. For this reason, the ratio of subsidy to equivalent resources is a more useful tool for predicting the possible effect of a subsidy than coarser contrasts of in situ productivity. The commonness of subsidy effects suggests that many ecosystems need to be studied as open systems.