Reciprocal subsidies between freshwater and terrestrial ecosystems structure consumer resource dynamics

Seasonal dynamics of detritus flows and decomposition across ecosystem boundaries

Material fluxes are ubiquitous in nature within and across ecosystems, connecting habitats with vastly different characteristics, like forests to rivers and lakes.1,2,3 Although individual fluxes and their cascading effects are well known,4,5,6 very few studies address the intra-annual phenology of ecosystem processes, despite the pronounced seasonality of fluxes. Here, we empirically quantified and resolved fluxes of recalcitrant and labile types of leaf litter in temperate riparian forests and streams across a year, representing one of the most emblematic examples of seasonal systems. We quantified intra-annual variation in litter inputs from terrestrial plants to forest floors and streams and estimated aquatic and terrestrial decomposition rates across the year at 6-week intervals. Our data show that the autumn pulse of leaf litter is complemented by smaller magnitude but more constant-through-the-year lateral flows to the stream ecosystems. Decomposition of labile litter fluctuated seasonally, on a different phenology, with generally higher rates in summer, but rates of recalcitrant litter decomposition remained largely constant. Microorganisms were the main contributors to the decomposition process in both forests and streams. Overall, our work highlights the asynchronous and seasonally variable changes in decomposition rates between recalcitrant and labile detritus despite their initial synchronized availability and suggests that the dominating presence of recalcitrant litter buffers ecosystem responses to the concentrated temporal distribution of litter resources.7,8 Investigating such ecological processes both across ecosystem borders and at fine intra-annual resolutions is imperative to understand complex system responses in the context of species' shifts in phenologies and resource quality.9,10,11 VIDEO ABSTRACT.

Long-term consequences of Bti applications on aquatic insect emergence: Insights from a 4-year mesocosm study

Aquatic biomass and essential nutrients can subsidize terrestrial food webs. However, stressors in the aquatic ecosystem, such as the non-chemical mosquito control agent Bti (Bacillus thuringiensis israelensis), may affect such subsidies by changing the dynamics and diversity of emergent aquatic insects. Such changes in emergence phenology may influence riparian predators with consequences for the terrestrial food web. Here we asked whether repeated Bti application over four consecutive vegetative periods cause similar impacts over these years (i.e., 2020-2023). We used floodplain pond mesocosms and measured insect emergence from control and Bti-treated ponds between April and August in each year. We found significant effects on insect emergence dynamics and community composition only during the first year. This effect disappeared over the three following years. Our observations suggest a shift in community composition, for example, by replacing more Bti-sensitive (i.e., Chironomidae) with more tolerant insect species - a hypothesis that is partially supported by changes in the wider community of emergent aquatic insects during the study. Alternatively, a reduced Bti activity could explain this pattern: The activity could be affected by changes in environmental factors such as the accumulation of organic carbon and fine sediment. All in all, our data point to a higher annual variability in insect emergence dynamics and the community of emergent aquatic insects relative to the impacts caused by Bti.

Integrating the Bright and Dark Sides of Aquatic Resource Subsidies-A Synthesis

Aquatic and terrestrial ecosystems are linked through the reciprocal exchange of materials and organisms. Aquatic-to-terrestrial subsidies are relatively small in most terrestrial ecosystems, but they can provide high contents of limiting resources that increase consumer fitness and ecosystem production. However, they also may carry significant contaminant loads, particularly in anthropogenically impacted watersheds. Global change processes, including land use change, climate change and biodiversity declines, are altering the quantity and quality of aquatic subsidies, potentially shifting the balance of costs and benefits of aquatic subsidies for terrestrial consumers. Many global change processes interact and impact both the bright and dark sides of aquatic subsidies simultaneously, highlighting the need for future integrative research that bridges ecosystem as well as disciplinary boundaries. We identify key research priorities, including increased quantification of the spatiotemporal variability in aquatic subsidies across a range of ecosystems, greater understanding of the landscape-scale extent of aquatic subsidy impacts and deeper exploration of the relative costs and benefits of aquatic subsidies for consumers.

Quantifying the ecological role of crocodiles: a 50-year review of metabolic requirements and nutrient contributions in northern Australia

The ecological roles of large predators are well recognized, but quantifying their functional impacts remains an active area of research. In this study, we examined the metabolic requirements and nutrient outputs of the estuarine crocodile population (Crocodylus porosus) in northern Australia over a 50-year period, during which the population increased from a few thousand to over 100 000 individuals. Bioenergetic modelling showed that during this period, the crocodile population's annual prey consumption increased from <20 kg km-2 in 1979 to approximately 180 kg km-2 in 2019. Further, the prey consumption increase was accompanied by a significant dietary shift from predominantly aquatic prey (approx. 65% in 1979) to a terrestrial-based diet (approx. 70% in 2019). A substantial portion of these terrestrial-derived nutrients was excreted into the water, significantly increasing the input rates of nitrogen (186-fold) and phosphorus (56-fold). The study shows that, despite being ectothermic, the high biomass of crocodiles within the environment generated nutrient inputs comparable to terrestrial endothermic predator populations. While crocodiles are apex predators, they are not considered to influence ecosystems in the same manner that large-bodied endothermic predators do. However, in the oligotrophic freshwater systems of northern Australia, the large volume of crocodile biomass is likely to impact the ecosystem through top-down and bottom-up processes.

Cause and consequences of Common Snook (Centropomus undecimalis) space use specialization in a subtropical riverscape

Variability in space use among conspecifics can emerge from foraging strategies that track available resources, especially in riverscapes that promote high synchrony between prey pulses and consumers. Projected changes in riverscape hydrological regimes due to water management and climate change accentuate the need to understand the natural variability in animal space use and its implications for population dynamics and ecosystem function. Here, we used long-term tracking of Common Snook (Centropomus undecimalis) movement and trophic dynamics in the Shark River, Everglades National Park from 2012 to 2023 to test how specialization in the space use of individuals (i.e., Eadj) changes seasonally, how it is influenced by yearly hydrological conditions, and its relationship to the between individual trophic niche. Snook exhibited seasonal variability in space use, with maximum individual specialization (high dissimilarity) in the wet season. The degree of individual specialization increased over the years in association with greater marsh flooding duration, which produced important subsidies. Also, there were threshold responses of individual space use specialization as a function of floodplain conditions. Greater specialization in space use results in a decrease in snook trophic niche size. These results show how hydrological regimes in riverscapes influence individual specialization of resource use (both space and prey), providing insight into how forecasted hydroclimatic scenarios may shape habitat selection processes and the trophic dynamics of mobile consumers.

Resource Flow Network Structure Drives Metaecosystem Function

AbstractNonliving resources frequently flow across ecosystem boundaries, which can yield networks of spatially coupled ecosystems. Yet the significance of resource flows for ecosystem function has predominantly been understood by studying two or a few coupled ecosystems, overlooking the broader resource flow network and its spatial structure. Here, we investigate how the spatial structure of larger resource flow networks influences ecosystem function at metaecosystem scales by analyzing metaecosystem models with homogeneously versus heterogeneously distributed resource flow networks but otherwise identical characteristics. We show that metaecosystem function can differ strongly between metaecosystems with contrasting resource flow networks. Differences in function generally arise through the scaling up of nonlinear local processes interacting with spatial variation in local dynamics, the latter of which is influenced by network structure. However, we find that neither network structure guarantees the greatest metaecosystem function. Rather, biotic (organism traits) and abiotic (resource flow rates) properties interact with network structure to determine which yields greater metaecosystem function. Our findings suggest that the spatial structure of resource flow networks coupling ecosystems can be a driver of ecosystem function at landscape scales. Furthermore, our study demonstrates how modifications to the structural, biotic, or abiotic properties of metaecosystem networks can have nontrivial large-scale effects on ecosystem function.

Phenotypic adaptation of Chironomus riparius to chronic Bti exposure: effects on emergence time and nutrient content

Insects with aquatic larval and terrestrial adult life stages are a key component of coupled aquatic-terrestrial ecosystems. Thus, stressors applied to water bodies adversely affecting those larvae have the potential to influence the riparian zone through altered emergence, with differences in prey availability, timing, or nutrition. In this study, the common model organism Chironomus riparius, a species of Chironomidae (Diptera), was used. This selection was further motivated by its wide distribution in European freshwaters and its importance as prey for terrestrial predators. A stressor of high importance in this context is the globally used mosquito control agent Bacillus thuringiensis var. israelensis (Bti) which has been shown to affect Chironomidae. Here, we investigated the ability of chironomid populations to adapt to a regularly applied stressor, leading to a reduced impact of Bti. Therefore, the initial sensitivity of laboratory populations of C. riparius was investigated under the influence of field-relevant Bti treatments (three doses × two application days) and different food sources (high-quality TetraMin vs. low-quality Spirulina). Following a chronic exposure to Bti over six months, the sensitivity of pre-exposed and naïve populations was re-evaluated. Food quality had a strong impact on emergence timing and nutrient content. In addition, alterations in emergence time as well as protein and lipid contents of chronically exposed populations indicated a selection for individuals of advantageous energetics, potentially leading to a more efficient development while combating Bti. Signs of adaptation could be confirmed in five out of 36 tested scenarios suggesting adaptation to Bti at the population level. Adaptive responses of one or several species could theoretically (via eco-evolutionary dynamics) result in a community shift, favouring the prevalence of Bti-tolerant species. (In)direct effects of Bti and the adaptive responses at both population and community levels could affect higher trophic levels and may determine the fate of meta-ecosystems.

Effects of mining activities on fish communities and food web dynamics in a lowland river

Fish communities of streams and rivers might be substantially subsidized by terrestrial insects that fall into the water. Although such animal-mediated fluxes are increasingly recognized, little is known about how anthropogenic perturbations may influence the strength of such exchanges. Intense land use, such as lignite mining, may impact a river ecosystem due to the flocculation of iron (III) oxides, thus altering food web dynamics. We compared sections of the Spree River in North-East Germany that were greatly influenced by iron oxides with sections located downstream of a dam where passive remediation technologies are applied. Compared to locations downstream of the dam, the abundance of benthic macroinvertebrates at locations of high iron concentrations upstream of the dam was significantly reduced. Similarly, catch per unit effort of all fish was significantly higher in locations downstream of the dam compared to locations upstream of the dam, and the condition of juvenile and adult piscivorous pike Esox lucius was significantly lower in sections of high iron concentrations. Using an estimate of short-term (i.e., metabarcoding of the gut content) as well as longer-term (i.e., hydrogen stable isotopes) resource use, we could demonstrate that the three most abundant fish species, perch Perca fluviatilis, roach Rutilus rutilus, and bleak Alburnus alburnus, received higher contributions of terrestrial insects to their diet at locations of high iron concentration. In summary, lotic food webs upstream and downstream of the dam greatly differed in the overall structure with respect to the energy available for the highest tropic levels and the contribution of terrestrial insects to the diet of omnivorous fish. Therefore, human-induced environmental perturbations, such as river damming and mining activities, represent strong pressures that can alter the flow of energy between aquatic and terrestrial systems, indicating a broad impact on the landscape level.

Global patterns of allochthony in stream-riparian meta-ecosystems

Ecosystems that are coupled by reciprocal flows of energy and nutrient subsidies can be viewed as a single "meta-ecosystem." Despite these connections, the reciprocal flow of subsidies is greatly asymmetrical and seasonally pulsed. Here, we synthesize existing literature on stream-riparian meta-ecosystems to quantify global patterns of the amount of subsidy consumption by organisms, known as "allochthony." These resource flows are important since they can comprise a large portion of consumer diets, but can be disrupted by human modification of streams and riparian zones. Despite asymmetrical subsidy flows, we found stream and riparian consumer allochthony to be equivalent. Although both fish and stream invertebrates rely on seasonally pulsed allochthonous resources, we find allochthony varies seasonally only for fish, being nearly three times greater during the summer and fall than during the winter and spring. We also find that consumer allochthony varies with feeding traits for aquatic invertebrates, fish, and terrestrial arthropods, but not for terrestrial vertebrates. Finally, we find that allochthony varies by climate for aquatic invertebrates, being nearly twice as great in arid climates than in tropical climates, but not for fish. These findings are critical to understanding the consequences of global change, as ecosystem connections are being increasingly disrupted.

Sustained use of marine subsidies promotes niche expansion in a wild felid

The use of marine subsidies by terrestrial predators can facilitate substantial transfer of nutrients between marine and terrestrial ecosystems. Marine resource subsidies may have profound effects on predator ecology, influencing population and niche dynamics. Expanding niches of top consumers can impact ecosystem resilience and interspecific interactions, affecting predator-prey dynamics and competition. We investigate the occurrence, importance, and impact of marine resources on trophic ecology and niche dynamics in a highly generalist predator, the caracal (Caracal caracal), on the Cape Peninsula, South Africa. Caracals have flexible diets, feeding across a wide range of terrestrial and aquatic prey. We use carbon and nitrogen stable isotope analysis of fur samples (n = 75) to understand trophic position and niche shifts in coastal and inland foragers, as well as the implications of a diet rich in marine resources. We found significant differences in isotope signatures between these groups, with higher δ13C (P < 0.05) and δ15N values (P < 0.01) in coastal foragers. Isotope mixing models reveal that these elevated signatures were due to non-terrestrial food subsidies, where approximately a third of coastal foraging caracal diet comprised marine prey. The addition of marine prey species to diet increased both the trophic level and isotope niche size of coastal foraging caracals, with potential impacts on prey populations and competition. Our results suggest that marine prey are an important dietary resource for coastal foraging caracals, where seabirds, including two endangered species, are a major component of their diet. However, there are likely risks associated with these resource benefits, as routine consumption of seabirds is linked with higher pollutant burdens, particularly metals. Increased encounters between this terrestrial predator and seabirds may be a result of increased mainland colonies due to changes in habitat availability and the highly opportunistic and generalist foraging behaviour of a native predator.

A synthesis of anthropogenic stress effects on emergence-mediated aquatic-terrestrial linkages and riparian food webs

Anthropogenic stress alters the linkage between aquatic and terrestrial ecosystems in various ways. Here, we review the contemporary literature on how alterations in aquatic systems through environmental pollution, invasive species and hydromorphological changes carry-over to terrestrial ecosystems and the food webs therein. We consider both the aquatic insect emergence and flooding as pathways through which stressors can propagate from the aquatic to the terrestrial system. We specifically synthesize and contextualize results on the roles of pollutants in the emergence pathway and their top-down consequences. Our review revealed that the emergence and flooding pathway are only considered in isolation and that the overall effects of invasive species or pollutants on food webs at the water-land interface require further attention. While very few recent studies looked at invasive species, a larger number of studies focused on metal transfer compared to pesticides, pharmaceuticals or PCBs, and multiple stress studies up to now left aquatic-terrestrial linkages unconsidered. Recent research on pollutants and emergence used aquatic-terrestrial mesocosms to elucidate the effects of aquatic stressors such as the mosquito control agent Bti, metals or pesticides to understand the effects on riparian spiders. Quality parameters, such as the structural and functional composition of emergent insect communities, the fatty acid profiles, yet also the composition of pollutants transferred to land prove to be important for the effects on riparian spiders. Process-based models including quality of emergence are useful to predict the resulting top-down directed food web effects in the terrestrial recipient ecosystem. In conclusion, we present and recommend a combination of empirical and modelling approaches in order to understand the complexity of aquatic-terrestrial stressor propagation and its spatial and temporal variation.

Climate and Environmental Variables Drive Stream Biofilm Bacterial and Fungal Diversity on Tropical Mountainsides

High mountain freshwater systems are particularly sensitive to the impacts of global warming and relevant environmental changes. Microorganisms contribute substantially to biogeochemical processes, yet their distribution patterns and driving mechanism in alpine streams remain understudied. Here, we examined the bacterial and fungal community compositions in stream biofilm along the elevational gradient of 745-1874 m on Mt. Kilimanjaro and explored their alpha and beta diversity patterns and the underlying environmental drivers. We found that the species richness and evenness monotonically increased towards higher elevations for bacteria, while were non-significant for fungi. However, both bacterial and fungal communities showed consistent elevational distance-decay relationships, i.e., the dissimilarity of assemblage composition increased with greater elevational differences. Bacterial alpha diversity patterns were mainly affected by chemical variables such as total nitrogen and phosphorus, while fungi were affected by physical variables such as riparian shading and stream width. Notably, climatic variables such as mean annual temperature strongly affected the elevational succession of bacterial and fungal community compositions. Our study is the first exploration of microbial biodiversity and their underlying driving mechanisms for stream ecosystems in tropical alpine regions. Our findings provide insights on the response patterns of tropical aquatic microbial community composition and diversity under climate change.

Inter- and intrapopulation resource use variation of marine subsidized western fence lizards

Marine resource subsidies alter consumer dynamics of recipient populations in coastal systems. The response to these subsidies by generalist consumers is often not uniform, creating inter- and intrapopulation diet variation and niche diversification that may be intensified across heterogeneous landscapes. We sampled western fence lizards, Sceloporus occidentalis, from Puget Sound beaches and coastal and inland forest habitats, in addition to the lizards' marine and terrestrial prey items to quantify marine and terrestrial resource use with stable isotope analysis and mixing models. Beach lizards had higher average δ13C and δ15N values compared to coastal and inland forest lizards, exhibiting a strong mixing line between marine and terrestrial prey items. Across five beach sites, lizard populations received 20-51% of their diet from marine resources, on average, with individual lizards ranging between 7 and 86% marine diet. The hillslope of the transition zone between marine and terrestrial environments at beach sites was positively associated with marine-based diets, as the steepest sloped beach sites had the highest percent marine diets. Within-beach variation in transition zone slope was positively correlated with the isotopic niche space of beach lizard populations. These results demonstrate that physiography of transitional landscapes can mediate resource flow between environments, and variable habitat topography promotes niche diversification within lizard populations. Marine resource subsidization of Puget Sound beach S. occidentalis populations may facilitate occupation of the northwesternmost edge of the species range. Shoreline restoration and driftwood beach habitat conservation are important to support the unique ecology of Puget Sound S. occidentalis.

The influence of season, hunting mode, and habitat specialization on riparian spiders as key predators in the aquatic-terrestrial linkage

Freshwater ecosystems subsidize riparian zones with high-quality nutrients via the emergence of aquatic insects. Spiders are dominant consumers of these insect subsidies. However, little is known about the variation of aquatic insect consumption across spiders of different hunting modes, habitat specializations, seasons, and systems. To explore this, we assembled a large stable isotope dataset (n > 1000) of aquatic versus terrestrial sources and six spider species over four points in time adjacent to a lotic and a lentic system. The spiders represent three hunting modes each consisting of a wetland specialist and a habitat generalist. We expected that specialists would feed more on aquatic prey than their generalist counterparts. Mixing models showed that spiders' diet consisted of 17-99% of aquatic sources, with no clear effect of habitat specialization. Averaged over the whole study period, web builders (WB) showed the highest proportions (78%) followed by ground hunters (GH, 42%) and vegetation hunters (VH, 31%). Consumption of aquatic prey was highest in June and August, which is most pronounced in GH and WBs, with the latter feeding almost entirely on aquatic sources during this period. Additionally, the elevated importance of high-quality lipids from aquatic origin during fall is indicated by elemental analyses pointing to an accumulation of lipids in October, which represent critical energy reserves during winter. Consequently, this study underlines the importance of aquatic prey irrespective of the habitat specialization of spiders. Furthermore, it suggests that energy flows vary substantially between spider hunting modes and seasons.

Tropical and temperate differences in the trophic structure and aquatic prey use of riparian predators

The influence of aquatic resource-inputs on terrestrial communities is poorly understood, particularly in the tropics. We used stable isotope analysis of carbon and nitrogen to trace aquatic prey use and quantify the impact on trophic structure in 240 riparian arthropod communities in tropical and temperate forests. Riparian predators consumed more aquatic prey and were more trophically diverse in the tropics than temperate regions, indicating tropical riparian communities are both more reliant on and impacted by aquatic resources than temperate communities. This suggests they are more vulnerable to disruption of aquatic-terrestrial linkages. Although aquatic resource use declined strongly with distance from water, we observed no correlated change in trophic structure, suggesting trophic flexibility to changing resource availability within riparian predator communities in both tropical and temperate regions. Our findings highlight the importance of aquatic resources for riparian communities, especially in the tropics, but suggest distance from water is less important than resource diversity in maintaining terrestrial trophic structure.

Quality matters: Stoichiometry of resources modulates spatial feedbacks in aquatic-terrestrial meta-ecosystems

Species dispersal and resource spatial flows greatly affect the dynamics of connected ecosystems. So far, research on meta-ecosystems has mainly focused on the quantitative effect of subsidy flows. Yet, resource exchanges at heterotrophic-autotrophic (e.g. aquatic-terrestrial) ecotones display a stoichiometric asymmetry that likely matters for functioning. Here, we joined ecological stoichiometry and the meta-ecosystem framework to understand how subsidy stoichiometry mediates the response of the meta-ecosystem to subsidy flows. Our model results demonstrate that resource flows between ecosystems can induce a positive spatial feedback loop, leading to higher production at the meta-ecosystem scale by relaxing local ecosystem limitations ('spatial complementarity'). Furthermore, we show that spatial flows can also have an unexpected negative impact on production when accentuating the stoichiometric mismatch between local resources and basal species needs. This study paves the way for studies on the interdependency of ecosystems at the landscape extent.

Connecting distinct realms along multiple dimensions: A meta-ecosystem resilience perspective

Resilience research is central to confront the sustainability challenges to ecosystems and human societies in a rapidly changing world. Given that social-ecological problems span the entire Earth system, there is a critical need for resilience models that account for the connectivity across intricately linked ecosystems (i.e., freshwater, marine, terrestrial, atmosphere). We present a resilience perspective of meta-ecosystems that are connected through the flow of biota, matter and energy within and across aquatic and terrestrial realms, and the atmosphere. We demonstrate ecological resilience sensu Holling using aquatic-terrestrial linkages and riparian ecosystems more generally. A discussion of applications in riparian ecology and meta-ecosystem research (e.g., resilience quantification, panarchy, meta-ecosystem boundary delineations, spatial regime migration, including early warning indications) concludes the paper. Understanding meta-ecosystem resilience may have potential to support decision making for natural resource management (scenario planning, risk and vulnerability assessments).

Spadefoot Pelobates vespertinus (Amphibia, Pelobatidae) as a transmitter of fatty acids from water to land in a forest-steppe floodplain

The transfer of biomass and polyunsaturated fatty acids by the spadefoot P. vespertinus (previously subspecies of P. fuscus) from aquatic to terrestrial ecosystems was studied for five years in small floodplain water bodies of a forest-steppe zone. Average emergence of metamorphs from unit of water area, wet mass was 6.7 g m^-2 year^-1. A ratio of the emergence to biomass was calculated and represented as E/B coefficient (an analog of P/B production/biomass coefficient). The average E/B was found to be 0.038 year^-1. The introduced coefficient can be used for a coarse estimation of the emergence on the basis of tadpole biomass measurements. A considerable partitioning of tadpoles and metamorphs in the composition of fatty acids in their biomass was revealed. Tadpoles had significantly higher mean levels (percent of total fatty acids) of 16:0, 16:1n-9, 18:0, 20:5n-3 and 22:5n-3, while metamorphs had significantly higher levels of 14:0, 15:0, 17:0, 17:1n-8, 18:2n-6, 20:2n-6, 20:4n-6 and 22:5n-6, likely due to the shifting to terrestrial food. Metamorphs had significantly higher content of total fatty acids, mg g^-1 of wet weight, and, in spite of lower level, they had significantly higher content of eicosapentaenoic acid (20:5n-3, EPA) than tadpoles. Metamorphs also had significantly higher content of docosahexaenoic acid (22:6n-3, DHA) and sum of EPA + DHA than tadpoles. Average flux of EPA + DHA from unit of water area with metamorphs was 3.27 mg m^-2 year^-1. The metamorphs appeared to be qualitatively and quantitatively prominent prey for a number of terrestrial consumers.

A temporal perspective on aquatic subsidy: Bti affects emergence of Chironomidae

Emerging aquatic insects serve as one link between aquatic and adjacent riparian food webs via the flux of energy and nutrients. These insects provide high-quality subsidy to terrestrial predators. Thus, any disturbance of emergence processes may cascade to higher trophic levels and lead to effects across ecosystem boundaries. One stressor with potential impact on non-target aquatic insects, especially on non-biting midges (Diptera: Chironomidae), is the widely used mosquito control agent Bacillus thuringiensis var. israelensis (Bti). In a field experiment, we investigated emerging insect communities from Bti-treated (three applications, maximum field rate) and control floodplain pond mesocosms (FPMs) over 3.5 months for changes in their composition, diversity as well as the emergence dynamics and the individual weight of emerged aquatic insects over time. Bti treatments altered community compositions over the entire study duration - an effect mainly attributed to an earlier (∼10 days) and reduced (∼26%) peak in the emergence of Chironomidae, the dominant family (88% of collected individuals). The most reasonable explanation for this significant alteration is less resource competition caused by a decrease in chironomid larval density due to lethal effects of Bti. This is supported by the higher individual weight of Chironomidae emerging from treated FPMs (∼21%) during Bti application (April - May). A temporal shift in the emergence dynamics can cause changes in the availability of prey in linked terrestrial ecosystems. Consequently, terrestrial predators may be affected by a lack of appropriate prey leading to bottom-up and top-down effects in terrestrial food webs. This study indicates the importance of a responsible and elaborated use of Bti and additionally, highlights the need to include a temporal perspective in evaluations of stressors in aquatic-terrestrial meta-ecosystems.

Nonlinear effects of environmental drivers shape macroinvertebrate biodiversity in an agricultural pondscape

Agriculture is a leading cause of biodiversity loss and significantly impacts freshwater biodiversity through many stressors acting locally and on the landscape scale. The individual effects of these numerous stressors are often difficult to disentangle and quantify, as they might have nonlinear impacts on biodiversity. Within agroecosystems, ponds are biodiversity hotspots providing habitat for many freshwater species and resting or feeding places for terrestrial organisms. Ponds are strongly influenced by their terrestrial surroundings, and understanding the determinants of biodiversity in agricultural landscapes remains difficult but crucial for improving conservation policies and actions. We aimed to identify the main effects of environmental and spatial variables on α-, β-, and γ-diversities of macroinvertebrate communities inhabiting ponds (n = 42) in an agricultural landscape in the Northeast Germany, and to quantify the respective roles of taxonomic turnover and nestedness in the pondscape. We disentangled the nonlinear effects of a wide range of environmental and spatial variables on macroinvertebrate α- and β-biodiversity. Our results show that α-diversity is impaired by eutrophication (phosphate and nitrogen) and that overshaded ponds support impoverished macroinvertebrate biota. The share of arable land in the ponds' surroundings decreases β-diversity (i.e., dissimilarity in community), while β-diversity is higher in shallower ponds. Moreover, we found that β-diversity is mainly driven by taxonomic turnover and that ponds embedded in arable fields support local and regional diversity. Our findings highlight the importance of such ponds for supporting biodiversity, identify the main stressors related to human activities (eutrophication), and emphasize the need for a large number of ponds in the landscape to conserve biodiversity. Small freshwater systems in agricultural landscapes challenge us to compromise between human demands and nature conservation worldwide. Identifying and quantifying the effects of environmental variables on biodiversity inhabiting those ecosystems can help address threats impacting freshwater life with more effective management of pondscapes.

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

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

Fatty acid composition differs between emergent aquatic and terrestrial insects—A detailed single system approach

Emergent insects represent a key vector through which aquatic nutrients are transferred to adjacent terrestrial food webs. Aquatic fluxes of polyunsaturated fatty acids (PUFA) from emergent insects are particularly important subsidies for terrestrial ecosystems due to high PUFA contents in several aquatic insect taxa and their physiological importance for riparian predators. While recent meta-analyses have shown the general dichotomy in fatty acid profiles between aquatic and terrestrial ecosystems, differences in fatty acid profiles between aquatic and terrestrial insects have been insufficiently explored. We examined the differences in fatty acid profiles between aquatic and terrestrial insects at a single aquatic-terrestrial interface over an entire growing season to assess the strength and temporal consistency of the dichotomy in fatty acid profiles. Non-metric multidimensional scaling clearly separated aquatic and terrestrial insects based on their fatty acid profiles regardless of season. Aquatic insects were characterized by high proportions of long-chain PUFA, such as eicosapentaenoic acid (20:5n-3), arachidonic acid (20:4n-6), and α-linolenic acid (18:3n-3); whereas terrestrial insects were characterized by high proportions of linoleic acid (18:2n-6). Our results provide detailed information on fatty acid profiles of a diversity of aquatic and terrestrial insect taxa and demonstrate that the fundamental differences in fatty acid content between aquatic and terrestrial insects persist throughout the growing season. However, the higher fatty acid dissimilarity between aquatic and terrestrial insects in spring and early summer emphasizes the importance of aquatic emergence as essential subsidies for riparian predators especially during the breading season.

Seasonal and ontological variation in diet and age-related differences in prey choice, by an insectivorous songbird

The diet of an individual animal is subject to change over time, both in response to short-term food fluctuations and over longer time scales as an individual ages and meets different challenges over its life cycle. A metabarcoding approach was used to elucidate the diet of different life stages of a migratory songbird, the Eurasian reed warbler (Acrocephalus scirpaceus) over the 2017 summer breeding season in Somerset, the United Kingdom. The feces of adult, juvenile, and nestling warblers were screened for invertebrate DNA, enabling the identification of prey species. Dietary analysis was coupled with monitoring of Diptera in the field using yellow sticky traps. Seasonal changes in warbler diet were subtle, whereas age class had a greater influence on overall diet composition. Age classes showed high dietary overlap, but significant dietary differences were mediated through the selection of prey; (i) from different taxonomic groups, (ii) with different habitat origins (aquatic vs. terrestrial), and (iii) of different average approximate sizes. Our results highlight the value of metabarcoding data for enhancing ecological studies of insectivores in dynamic environments.

Impacts of Warming on Reciprocal Subsidies Between Aquatic and Terrestrial Ecosystems

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

Integrated ecosystems: linking food webs through reciprocal resource reliance

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. Furthermore, 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.

Salmon subsidies predict territory size and habitat selection of an avian insectivore

The annual migration and spawning event of Pacific salmon (Oncorhynchus spp.) can lead to cross-boundary delivery of marine-derived nutrients from their carcasses into adjacent terrestrial ecosystems. The densities of some passerine species, including Pacific wrens (Troglodytes pacificus), have been shown to be positively correlated with salmon abundance along streams in Alaska and British Columbia, but mechanisms maintaining these densities remain poorly understood. Riparian areas near salmon streams could provide higher quality habitat for birds through greater food availability and more suitable vegetation structure for foraging and breeding, resulting in wrens maintaining smaller territories. We examined relationships between salmon biomass and Pacific wren territory size, competition, and habitat selection along 11 streams on the coast of British Columbia, Canada. We show that male wren densities increase and territory sizes decrease as salmon-spawning biomass increases. Higher densities result in higher rates of competition as male wrens countersing more frequently to defend their territories along streams with more salmon. Wrens were also more selective of the habitats they defended along streams with higher salmon biomass; they were 68% less likely to select low-quality habitat on streams with salmon compared with 46% less likely at streams without salmon. This suggests a potential trade-off between available high-quality habitat and the cost of competition that structures habitat selection. Thus, the marine-nutrient subsidies provided by salmon carcasses to forests lead to higher densities of wrens while shifting the economics of territorial defence toward smaller territories being defended more vigorously in higher quality habitats.

Cross-Ecosystem Linkages: Transfer of Polyunsaturated Fatty Acids From Streams to Riparian Spiders via Emergent Insects

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.

Short-term apparent mutualism drives responses of aquatic prey to increasing productivity

According to apparent competition theory, sharing a predator should cause indirect interactions among prey that can affect the structure and the dynamics of natural communities. Though shifts in prey dominance and predator resource use along environmental gradients are rather common, empirical evidence on the role of indirect prey-prey interactions through shared predation particularly with increasing productivity, is still scarce. In an 8-week lake mesocosm experiment, we manipulated both the addition of inorganic nutrients and the presence of generalist fish predators (crucian carp, Carassius carassius L.), to test for the effects of indirect interactions through shared predation along a productivity gradient. We found that apparent mutualism (indirect positive interaction) between benthic and pelagic prey strongly affected short-term responses of aquatic food webs to increasing productivity in the presence of a generalist fish. Increasing productivity favoured the relative abundance of benthic prey, following trends in natural productive lake systems. This led to a shift in fish selectivity from pelagic to benthic prey driven by changes in fish behaviour, which resulted in apparent mutualism due to the lower and delayed top-down control of pelagic prey at increasing productivity. Our results show empirical evidence that the coupling of multiple production pathways can lead to strong indirect interactions through shared predation, whereby prey dynamics on short time-scales are highly dependent on the foraging behaviour of generalist predators. This mechanism may play an important role in short-term responses of food webs across environmental gradients.

Cross-ecosystem bottlenecks alter reciprocal subsidies within meta-ecosystems

Reciprocal subsidies link ecosystems into meta-ecosystems, but energy transfer to organisms that do not cross boundaries may create sinks, reducing reciprocal subsidy transfer. We investigated how the type of subsidy and top predator presence influenced reciprocal flows of energy, by manipulating the addition of terrestrial leaf and terrestrial insect subsidies to experimental freshwater pond mesocosms with and without predatory fish. Over 18 months, fortnightly addition of subsidies (terrestrial beetle larvae) to top-predators was crossed with monthly addition of subsidies (willow leaves) to primary consumers in mesocosms with and without top predators (upland bullies) in a 2 × 2 × 2 factorial design in four replicate blocks. Terrestrial insect subsidies increased reciprocal flows, measured as the emergence of aquatic insects out of mesocosms, but leaf subsidies dampened those effects. However, the presence of fish and snails, consumers with no terrestrial life stage, usurped and retained the energy within in the aquatic ecosystem, creating a cross-ecosystem bottleneck to energy flow. Thus, changes in species composition of donor or recipient food webs within a meta-ecosystems can alter reciprocal subsidies through cross-ecosystem bottlenecks.

Stoichiometric control on riparian wetland carbon and nutrient dynamics under different land uses

Riparian wetland provides important ecosystem function, such as water filtration and nutrient retention. When land use change in upland from native forest to sugarcane cultivation have important impacts on carbon (C) and nutrient availability in downstream wetland systems. Here, we examined concentrations and stoichiometry of C and nutrients in total, labile, biomass pools in upland soil, riparian wetland and sediment along two distinct transects (sugarcane versus forest). Sugarcane cultivation significantly reduced total C, nitrogen (N), labile C and N in riparian soils by 69%, 62%, 33% and 45%, respectively, but significantly increased NO₃^--N and δ¹⁵N by 99% and 56% in riparian areas. The presence of native forest resulted in significantly higher NH₄⁺-N concentrations in downstream wetlands. Concentrations of microbial biomass C and N were generally lower, but the abundance of genes associated with nitrifiers (ammonia oxidizing bacteria and archaea) was higher in the sugarcane transect than in the forest transect. These significantly differences between two transects could be attributed to different organic inputs and biogeochemical processes associated with the different vegetation types and management practices in the upland systems. Difference in δ¹³C signature from the two transects further confirmed the significant influence of vegetation type on downstream wetlands. Sugarcane cultivation led to a consistent stoichiometric shift in both resource and microbial biomass towards lower C:P and N:P ratios across upland soils, wetlands and sediment, compared with the forest transect. The average total and microbial biomass C:N:P ratios in soil under sugarcane were 136:9:1 and 180:33:1, respectively. The average total and microbial biomass C:N:P ratios in soil under forest were 410:22:1 and 594:76:1, respectively. It is concluded that since microbial demand of C and nutrients is driven by the stoichiometry of the biomass, which is regulated by the resource stoichiometry, a change of resource induced by upland land use change leads to a shift in the stoichiometry of microbial biomass C, N and P.

Coarse particulate organic matter dynamics in ephemeral tributaries of a Central Appalachian stream network

Headwater ephemeral tributaries are interfaces between uplands and downstream waters. Terrestrial coarse particulate organic matter (CPOM) is important in fueling aquatic ecosystems; however, the extent to which ephemeral tributaries are functionally connected to downstream waters through fluvial transport of CPOM has been little studied. Hydrology and deposition of leaf and wood, and surrogate transport (Ginkgo biloba leaves and wood dowels) were measured over month-long intervals through the winter and spring seasons (6 months) in 10 ephemeral tributaries (1.3–5.4 ha) in eastern Kentucky. Leaf deposition and surrogate transport varied over time, reflecting the seasonality of litterfall and runoff. Leaf deposition was higher in December than February and May but did not differ from January, March, and April. Mean percent of surrogate leaf transport from the ephemeral tributaries was highest in April (3.6% per day) and lowest in February (2.5%) and May (2%). Wood deposition and transport had similar patterns. No CPOM measures were related to flow frequency. Ephemeral tributaries were estimated to annually contribute 110.6 kg AFDM·km⁻¹·yr⁻¹ of leaves to the downstream mainstem. Ephemeral tributaries are functionally connected to downstream waters through CPOM storage and subsequent release that is timed when CPOM is often limited in downstream waters.

Predatory fish invasion induces within and across ecosystem effects in Yellowstone National Park

Predatory fish introduction can cause cascading changes within recipient freshwater ecosystems. Linkages to avian and terrestrial food webs may occur, but effects are thought to attenuate across ecosystem boundaries. Using data spanning more than four decades (1972-2017), we demonstrate that lake trout invasion of Yellowstone Lake added a novel, piscivorous trophic level resulting in a precipitous decline of prey fish, including Yellowstone cutthroat trout. Plankton assemblages within the lake were altered, and nutrient transport to tributary streams was reduced. Effects across the aquatic-terrestrial ecosystem boundary remained strong (log response ratio ≤ 1.07) as grizzly bears and black bears necessarily sought alternative foods. Nest density and success of ospreys greatly declined. Bald eagles shifted their diet to compensate for the cutthroat trout loss. These interactions across multiple trophic levels both within and outside of the invaded lake highlight the potential substantial influence of an introduced predatory fish on otherwise pristine ecosystems.

Metaecosystem Dynamics of Marine Phytoplankton Alters Resource Use Efficiency along Stoichiometric Gradients

Metaecosystem theory addresses the link between local (within habitats) and regional (between habitats) dynamics by simultaneously analyzing spatial community ecology and abiotic matter flow. Here we experimentally address how spatial resource gradients and connectivity affect resource use efficiency (RUE) and stoichiometry in marine phytoplankton as well as the community composition at local and regional scales. We created gradostat metaecosystems consisting of five linearly interconnected patches, which were arranged either in countercurrent gradients of nitrogen (N) and phosphorus (P) supply or with a uniform spatial distribution of nutrients and which had either low or high connectivity. Gradient metaecosystems were characterized by higher remaining N and P concentrations (and N∶P ratios) than uniform ones, a difference reduced by higher connectivity. The position of the patch in the gradient strongly constrained elemental stoichiometry, local biovolume production, and RUE. As expected, algal carbon (C)∶N, biovolume, and N-specific RUE decreased toward the N-rich end of the gradient metaecosystem, whereas the opposite was observed for most of the gradient for C∶P, N∶P, and P-specific RUE. However, at highest N∶P supply, unexpectedly low C∶P, N∶P, and P-specific RUE values were found, indicating that the low availability of P inhibited efficient use of N and biovolume production. Consequently, gradient metaecosystems had lower overall biovolume at the regional scale. Whereas treatment effects on local richness were weak, gradients were characterized by higher dissimilarity in species composition. Thus, the stoichiometry of resource supply and spatial connectivity between patches appeared as decisive elements constraining phytoplankton composition and functioning in metaecosystems.

Dietary nutrient allocation to somatic tissue synthesis in emerging subimago freshwater mayfly Ephemera danica

BACKGROUND

The relative importance of nutrients derived from different sources for tissue synthesis is crucial for predicting a species responds to changes in food availability. The ecological and physiological strategies that govern the incorporation and routing of nutrients for reproduction are often well understood. However, the role and adaptive value of both species and individual variation during early life-stage remain elusive. In freshwater systems, dietary nutrient allocation to somatic tissue should be favoured when dietary source peaks and resource limitation may hinder flexible resource allocation. We used carbon and nitrogen stable isotopes (δ¹³C and δ¹⁵N) to examine metabolic nutrient routing and resource allocation from four dietary sources used to biosynthesize three somatic tissues of emerging subimago Ephemera danica. Aquatic emerging insects, such as the mayfly E. danica, are well suited for such studies. This is because, while burrowing nymph phase is a detritivores feeders with several early life-stages of metamorphosis, adult insects do not feed during this period but do utilize energy.

RESULTS

Constructed models to predict percent proportional contribution of source to tissue showed that terrestrial detritus was the dominant nutrient source for abdomen, head and wing with mean values of 57%, 65% and 73%, respectively. There was evidence for differential resource allocation, as insect partitioned periphyton and sediment (but also seston) elements for tissue synthesis. Utilizing individual-specimen based relationship in isotope value; we derived tissue specific isotopic niche estimates, for the different tissue-source combinations.

CONCLUSIONS

Results indicate that tissue selection is crucial for isotopic ecological measurements in arthropods. Mayfly has long been used as bio-indicator of freshwater ecosystems and their larvae show rapid response to environmental changes. In light of the recent evidence of drastic reduction in flying insect mass in Germany, developing a system using isotopic tools to trace nutrient flow in this important taxon will assist conservation and management efforts.

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

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

Cross-ecosystem carbon flows connecting ecosystems worldwide

Ecosystems are widely interconnected by spatial flows of material, but the overall importance of these flows relative to local ecosystem functioning remains unclear. Here we provide a quantitative synthesis on spatial flows of carbon connecting ecosystems worldwide. Cross-ecosystem flows range over eight orders of magnitude, bringing between 10⁻³ and 10⁵ gC m⁻² year⁻¹ to recipient ecosystems. Magnitudes are similar to local fluxes in freshwater and benthic ecosystems, but two to three orders of magnitude lower in terrestrial systems, demonstrating different dependencies on spatial flows among ecosystem types. The strong spatial couplings also indicate that ecosystems are vulnerable to alterations of cross-ecosystem flows. Thus, a reconsideration of ecosystem functioning, including a spatial perspective, is urgently needed.

Material flows between ecosystems, though the degree to which ecosystems are coupled is under investigation. Here Gounand et al. analyze cross-ecosystem carbon flows and relate them to in situ functions, and report different dependencies on spatial flows across numerous ecosystems.

Fertilizing riparian forests: nutrient repletion across ecotones with trophic rewilding

Trophic rewilding maintains that large mammals are functionally important to resource subsidies and nutrient repletion, yet this prediction is understudied. Here, I report on the potential magnitude and variability of nitrogen that moose populations move from aquatic to terrestrial ecosystems. My aim is to provide justified approximations of the role of moose in the flux of a limiting nutrient across ecotones and to illustrate how this role is linked to wolf predation and climate warming. Using Isle Royale and northeastern Minnesota, USA as contrasting focal systems, I found that the long-term annual N gain for riparian forests likely ranges from 1 to 10 kg N ha^-1 yr^-1, depending on the heterogeneity of moose movements. For these systems, this range is equivalent to approximately 4-30% of net annual N mineralization, approximately 62-625% of annual N runoff, approximately 28-333% of annual atmospheric N deposition and approximately 31-312% of the N sequestered by trees. The N flux approximation is most sensitive to moose population levels and, as such, is influenced by wolves, climate warming and disease. The potential for other terrestrial ungulates that feed on aquatic plants to provide significant nutrient repletion across ecotones is unknown but important to examine in the context of trophic rewilding. The extent to which predators influence ungulate abundance indirectly impacts this nutrient repletion.This article is part of the theme issue 'Trophic rewilding: consequences for ecosystems under global change'.

The community and ecosystem consequences of intraspecific diversity: a meta-analysis

Understanding the relationships between biodiversity and ecosystem functioning has major implications. Biodiversity-ecosystem functioning relationships are generally investigated at the interspecific level, although intraspecific diversity (i.e. within-species diversity) is increasingly perceived as an important ecological facet of biodiversity. Here, we provide a quantitative and integrative synthesis testing, across diverse plant and animal species, whether intraspecific diversity is a major driver of community dynamics and ecosystem functioning. We specifically tested (i) whether the number of genotypes/phenotypes (i.e. intraspecific richness) or the specific identity of genotypes/phenotypes (i.e. intraspecific variation) in populations modulate the structure of communities and the functioning of ecosystems, (ii) whether the ecological effects of intraspecific richness and variation are strong in magnitude, and (iii) whether these effects vary among taxonomic groups and ecological responses. We found a non-linear relationship between intraspecific richness and community and ecosystem dynamics that follows a saturating curve shape, as observed for biodiversity-function relationships measured at the interspecific level. Importantly, intraspecific richness modulated ecological dynamics with a magnitude that was equal to that previously reported for interspecific richness. Our results further confirm, based on a database containing more than 50 species, that intraspecific variation also has substantial effects on ecological dynamics. We demonstrated that the effects of intraspecific variation are twice as high as expected by chance, and that they might have been underestimated previously. Finally, we found that the ecological effects of intraspecific variation are not homogeneous and are actually stronger when intraspecific variation is manipulated in primary producers than in consumer species, and when they are measured at the ecosystem rather than at the community level. Overall, we demonstrated that the two facets of intraspecific diversity (richness and variation) can both strongly affect community and ecosystem dynamics, which reveals the pivotal role of within-species biodiversity for understanding ecological dynamics.