Omega-3 long-chain polyunsaturated fatty acids support aerial insectivore performance more than food quantity

Compound-specific stable isotope analyses of fatty acids indicate feeding zones of zooplankton across the water column of a subalpine lake

Spatial and temporal zooplankton feeding dynamics across the water column of lakes are key for understanding site-specific acquisition of diet sources. During this 6-week lake study, we examined stable carbon (δ13C) and nitrogen (δ15N) isotopes and conducted compound-specific fatty acid (FA) stable isotope analysis (CSIA) of edible seston in the epi-, meta-, and hypolimnion, and zooplankton of Lake Lunz, Austria. We predicted that CSIA of essential FA can discern the foraging grounds of zooplankton more accurately than the commonly used bulk stable isotopes. The δ13C and δ15N values of seston from different lake strata were similar, whereas a dual CSIA approach using stable carbon and hydrogen isotopes of FA (δ13CFA and δ2HFA) provided sufficient isotopic difference in essential FA to discern different lake strata-specific diet sources throughout the study period. We present a CSIA model that suggests strata-specific foraging grounds for different zooplankton groups, indicating higher preference of cladocerans for feeding on epilimnetic diet sources, while calanoid copepods retained more hypolimnetic resources. The CSIA approach thus yields strata-specific information on foraging strategies of different zooplankton taxa and provides more details on the spatial and temporal trophodynamics of planktonic food webs than commonly used bulk stable isotopes.

Dietary fatty acids modulate oxidative stress response to air pollution but not to infection

Anthropogenic changes to the environment expose wildlife to many pollutants. Among these, tropospheric ozone is of global concern and a highly potent pro-oxidant. In addition, human activities include several other implications for wildlife, e.g., changed food availability and changed distribution of pathogens in cities. These co-occurring habitat changes may interact, thereby modulating the physiological responses and costs related to anthropogenic change. For instance, many food items associated with humans (e.g., food waste and feeders for wild birds) contain relatively more ω6-than ω3-polyunsaturated fatty acids (PUFAs). Metabolites derived from ω6-PUFAs can enhance inflammation and oxidative stress towards a stimulus, whereas the opposite response is linked to ω3-derived metabolites. Hence, we hypothesized that differential intake of ω6-and ω3-PUFAs modulates the oxidative stress state of birds and thereby affects the responses towards pro-oxidants. To test this, we manipulated dietary ω6:ω3 ratios and ozone levels in a full-factorial experiment using captive zebra finches (Taeniopygia guttata). Additionally, we simulated an infection, thereby also triggering the immune system's adaptive pro-oxidant release (i.e., oxidative burst), by injecting lipopolysaccharide. Under normal air conditions, the ω3-diet birds had a lower antioxidant ratio (GSH/GSSG ratio) compared to the ω6-diet birds. When exposed to ozone, however, the diet effect disappeared. Instead, ozone exposure overall reduced the total concentration of the key antioxidant glutathione (tGSH). Moreover, the birds on the ω6-rich diet had an overall higher antioxidant capacity (OXY) compared to birds fed a ω3-rich diet. Interestingly, only the immune challenge increased oxidative damage, suggesting the oxidative burst of the immune system overrides the other pro-oxidative processes, including diet. Taken together, our results show that ozone, dietary PUFAs, and infection all affect the redox-system, but in different ways, suggesting that the underlying responses are decoupled despite that they all increase pro-oxidant exposure or generation. Despite lack of apparent cumulative effect in the independent biomarkers, the combined single effects could together reduce overall cellular functioning and efficiency over time in wild birds exposed to pathogens, ozone, and anthropogenic food sources.

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.

Dietary fatty acid transfer in pelagic food webs across trophic and climatic differences of Chinese lakes

In eutrophic lake ecosystems, cyanobacteria typically lead to unbalanced phytoplankton community structure and low dietary quality for consumers at higher trophic levels. However, it still remains poorly understood how zooplankton manage to respond to seasonal and spatial differences in lake trophic gradients and temperature factors to retain highly required dietary nutrients from phytoplankton. In this field study, we investigated seston and different size classes of zooplankton of temperate and subtropical large lakes of different trophic conditions in China. We used fatty acids (FA) as dietary nutrients from seston to zooplankton to investigate how eutrophication affects the FA composition of various zooplankton size classes. This study revealed a curvilinear relationship between total phosphorus (TP) and polyunsaturated fatty acids (PUFA) contents of edible phytoplankton ("seston") across 3 seasons and 2 climatic areas. The PUFA content of seston increased until mesotrophic lake conditions (TP: 11-20 μg L-1), after which the dietary provision of PUFA for respective consumers declined. Seston FA, rather than trophic condition or water temperature, primarily predicted changes in zooplankton FA, while this predictive power decreased with zooplankton size. Despite increasing eutrophic lake conditions, LC-PUFA content of the zooplankton consistently increased per unit biomass. The results indicate that the nutritional value of phytoplankton was highest in mesotrophic lakes, and lake zooplankton selectively increased their LC-PUFA retention with body size and/or were able to convert dietary FA endogenously to meet their size-specific FA demands, independent of lake location or time (season) or the measured trophic condition of the lake (from oligo- to eutrophic).

Experimental evidence that EPA and DHA are dietary requirements in a migratory shorebird, but they do not affect muscle oxidative capacity

Dietary n-3 long chain polyunsaturated fatty acids (LCPUFAs) are hypothesized to be natural doping agents in migratory shorebirds, enabling prolonged flight by increasing membrane fluidity and oxidative capacity of the flight muscles. Animals can obtain n-3 LCPUFAs from the diet or by conversion of dietary α-linolenic acid, 18:3 n-3. However, the capacity to meet n-3 LCPUFA requirements from 18:3 n-3 varies among species. Direct tests of muscle oxidative enhancement and fatty acid conversion capacity are lacking in marine shorebirds that evolved eating diets rich in n-3 LCPUFAs. We tested whether the presence and type of dietary fatty acids influence the fatty acid composition and flight muscle oxidative capacity in western sandpipers (Calidris mauri). Sandpipers were fed diets low in n-3 PUFAs, high in 18:3 n-3, or high in n-3 LCPUFAs. Dietary fatty acid composition was reflected in multiple tissues, and low intake of n-3 LCPUFAs decreased the abundance of these fatty acids in all tissues, even with a high intake of 18:3 n-3. This suggests that 18:3 n-3 cannot replace n-3 LCPUFAs, and dietary n-3 LCPUFAs are required for sandpipers. Flight muscle indicators of enzymatic oxidative capacity and regulators of lipid metabolism did not change. However, the n-3 LCPUFA diet was associated with increased FAT/CD36 mRNA expression, potentially benefitting fatty acid transport during flight. Our study suggests that flight muscle lipid oxidation is not strongly influenced by n-3 PUFA intake. The type of dietary n-3 PUFA strongly influences the abundance of n-3 LCPUFAs in the body and could still impact whole-animal performance.

Dietary fatty acids and flight-training influence the expression of the eicosanoid hormone prostacyclin in songbirds

Diet shifts can alter tissue fatty acid composition in birds, which is subsequently related to metabolic patterns. Eicosanoids, short-lived fatty acid-derived hormones, have been proposed to mediate these relationships but neither baseline concentrations nor the responses to diet and exercise have been measured in songbirds. We quantified a stable derivative of the vasodilatory eicosanoid prostacyclin in the plasma of male European Starlings (Sturnus vulgaris, N = 25) fed semisynthetic diets with either high (PUFA) or low (MUFA) amounts of n6 fatty acid precursors to prostacyclin. Plasma samples were taken from each bird before, immediately after, and two days following a 15-day flight-training regimen that a subset of birds (N = 17) underwent. We found elevated prostacyclin levels in flight-trained birds fed the PUFA diet compared to those fed the MUFA diet and a positive relationship between prostacyclin and body condition, indexed by fat score. Prostacyclin concentrations also significantly decreased at the final time point. These results are consistent with the proposed influences of precursor availability (i.e., dietary fatty acids) and regulatory feedback associated with exercise (i.e., fuel supply and inflammation), and suggest that prostacyclin may be an important mediator of dietary influence on songbird physiology.

The importance of omega-3 polyunsaturated fatty acids as high-quality food in freshwater ecosystems with implications of global change

Traditionally, trophic ecology research on aquatic ecosystems has focused more on the quantity of dietary energy flow within food webs rather than food quality and its effects on organisms at various trophic levels. Recent studies emphasize that food quality is central to consumer growth and reproduction, and the importance of food quality for aquatic ecosystems has become increasingly well recognized. It is timely to synthesise these findings and identify potential future research directions. We conducted a systematic review of omega-3 polyunsaturated fatty acids (ω3-PUFAs) as a crucial component of high-quality food sources in freshwater ecosystems to evaluate their impact on a variety of consumers, and explore the effects of global change on these high-quality food sources and their transfer to higher trophic consumers within and across ecosystems. In freshwater ecosystems, algae rich in ω3 long-chain PUFAs, such as diatoms, dinoflagellates and cryptophytes, represent important high-quality food sources for consumers, whereas cyanobacteria, green algae, terrestrial vascular plants and macrophytes low in ω3 long-chain PUFAs are low-quality food sources. High-quality ω3-PUFA-containing food sources usually lead to increased growth and reproduction of aquatic consumers, e.g. benthic invertebrates, zooplankton and fish, and also provide ω3 long-chain PUFAs to riparian terrestrial consumers via emergent aquatic insects. Consumers feeding on high-quality ω3-PUFA-containing foods in turn represent high-quality food for their own predators. However, the ω3-PUFA content of food sources is sensitive to global environmental changes. Warming, eutrophication, increased light intensity (e.g. from loss of riparian shading), and pollutants potentially inhibit the synthesis of algal ω3-PUFAs while at the same time promoting the growth of lower-quality foods, such as cyanobacteria and green algae. These factors combined could lead to a significant reduction in the availability of ω3-PUFAs for consumers and constrain their overall fitness. Although the effect of individual environmental factors on high-quality ω3-PUFA-containing food sources has been investigated, multiple environmental factors (e.g. climate change, human activities, pollution) will act in combination and any synergistic effects on aquatic food webs remain unclear. Identifying the sources and fate of ω3-PUFAs within and across ecosystems could represent an important approach to understand the impact of multiple environmental factors on trophic relationships and the implications for populations of freshwater and riparian consumers. Maintaining the availability of high-quality ω3-PUFA-containing food sources may also be key to mitigating freshwater biodiversity loss due to global change.

Assessment of the impact of dams on aquatic food webs using stable isotopes: Current progress and future challenges

Dams have disrupted natural river systems worldwide and although population and community level effects on aquatic biota have been well documented, food web responses remain poorly understood and difficult to characterize. The application of stable isotope analysis (SIA) provides a means to assess the effect of dams on food webs. Here we review the effect of dams on aquatic food webs using SIA, aiming to detect knowledge gaps in the field of dam impacts on aquatic food webs and propose a conceptual framework to help formulate hypotheses about dam impacts on food webs guided by food web theory. Dams can affect aquatic food webs via two pathways: a bottom-up pathway with altered basal food sources and their transfer to consumers through changes in flow, nutrients, temperature and sediment, and a top-down pathway with consumer species composition altered mainly through habitat fragmentation and related physiochemical changes. Taking these mechanisms into consideration, the impact of dams on food web attributes derived from SIA was evaluated. These studies generally apply mixing models to determine how dams alter the dominant carbon sources supporting food webs, use δ15N to examine how dams alter food-chain length, or use Layman metrics of isotope variability to assess niche changes for invertebrate and fish assemblages. Most studies compare the patterns of SIA metrics spatially (e.g. upstream vs reservoir vs downstream of dams; regulated vs unregulated rivers) and temporally (before vs after dam construction), without explicit hypotheses and/or links to theoretical concepts of food webs. We propose several steps to make SIA studies of dam impacts more rigorous and enhance their potential for producing novel insights. Future studies should quantify the shape and strength of the effect of dams on SIA-measured food web response, be conducted at larger temporal and spatial scales (particularly along the river longitudinal continuum and the lateral connected ecosystems (e.g., floodplains)), and consider effects of dams on food web resilience and tipping points.

Seasonal changes in diet, immune function, and oxidative stress in three passerines inhabiting a Mediterranean climate

Oxidative status and immune function are energy-demanding traits closely linked to diet composition, particularly resource availability and nutritional value. In seasonal environments, nutrient availability and diet quality fluctuate, potentially influencing these traits. However, limited evidence exists regarding these dietary effects on immune function in seasonal environments. In this study, we employed stable isotope analysis to assess the impact of seasonal changes in niche width and trophic level (i.e., δ15N) on two immune variables (hemolysis and hemagglutination scores) and two oxidative status parameters (lipid peroxidation and total antioxidant capacity) in three passerine species: Zonotrichia capensis (omnivorous), Troglodytes aedon (insectivorous), and Spinus barbatus (granivorous). We found that hemolysis scores varied seasonally in Z. capensis, with higher values in winter compared to summer. Total antioxidant capacity (TAC) also increased during the winter in Z. capensis and S. barbatus. The isotopic niche width for Z. capensis and S. barbatus was smaller in winter than in summer, with the omnivorous species exhibiting a decrease in δ15N. Despite the seasonal shifts in ecological and physiological traits in Z. capensis, we identified no correlation between immune response and TAC with trophic level. In contrast, in the granivorous S. barbatus, the lower trophic level resulted in an increase in TAC without affecting immunity. Our findings revealed that dietary shifts do not uniformly impact oxidative status and immune function across bird species, highlighting species-specific responses to seasonal changes. This underscores the importance of integrating ecological and evolutionary perspectives when examining how diet shapes avian immunity and oxidative balance.

Lack of vitamin B12 impairs innate and adaptive immunity of Cuban tree frog (Osteopilus septentrionalis) tadpoles

Vitamin B12 is a micronutrient required by a variety of organisms for healthy cellular functioning. Despite the systemic effects observed in cases of B12 deficiency, relatively little is known about how vitamin B12 affects immune health, especially in amphibians, which are declining at unprecedented rates. In this study, we tested how supplementing an algae diet with B12 affects the innate and adaptive immunity of Cuban tree frog (Osteopilus septentrionalis) tadpoles. We found that innate immunity, as measured by a bacterial killing assay, was significantly more robust in B12-supplemented tadpoles than control tadpoles, but no significant differences were found in natural antibody production or hematocrit between groups. Adaptive immunity, as measured by Aeromonas hydrophila-specific IgY antibodies, was significantly greater in tadpoles challenged with A. hydrophila and supplemented with B12 than in control tadpoles, those only challenged with A. hydrophila, and those only given B12. Our results suggest that vitamin B12 is an important factor in maintaining a functional immune system in tadpoles, which may also be true for all vertebrates.

The fa(c)ts that matter: Bumble bees differentially allocate and oxidate three common fatty acids in pollen

Pollen serves as a crucial source of protein and lipids for numerous insects. Despite the importance of pollen lipids for nutrient regulation in bees, the digestibility and absorption of different fatty acids (FAs) by bees remain poorly understood. We used 13C labeled fatty acids (FAs) to investigate the absorption and allocation of three common dietary FAs in pollen by bumble bees. Palmitic acid, the most common saturated FA in pollen, was poorly absorbed, even when supplied as tripalmitate, emulsified, or mixed in vegetable oil. In contrast, the essential linoleic acid was absorbed and allocated at the highest rate among the three FAs tested. Oleic acid, a non-essential monounsaturated FA, was absorbed and oxidized at lower rates than linoleic acid. Notably, a feeding rate experiment revealed that different fatty acids did not affect the consumption rate of pollen. This results suggests that the specific FA's absorption efficiency and allocation differ in bumble bees, impacting their utilization. These findings demonstrate the importance of considering the digestibility and absorption of different FAs. Furthermore, the study highlights the influence of pollen lipid composition on the nutritional content for pollinators and raises questions about the utilization of polyunsaturated FAs in insect metabolism.

Omega-3 enriched chick diet reduces the foraging areas of breeders in two closely related shearwaters from contrasting marine environments

Seabirds have evolved several life-history characteristics to help buffer environmental stochasticity. However, particularly during the breeding season, seabirds may be affected by reductions in prey availability and localised oceanographic conditions caused by variations in the environment. The increase in sea surface temperature, triggered by accelerated global warming, is impairing phytoplankton production of omega-3 fatty acids (FAs). Here, we assessed the ecological role of omega-3 FAs on chick development and subsequently on breeder foraging behaviour in two closely related shearwater species foraging in contrasting marine environments. We supplemented chicks with omega-3 FA pills or with control placebo pills and monitored chick growth, chick health status and breeder at-sea foraging behaviour using global positioning system devices. We found that omega-3 chick supplementation reduced the 95% kernel utilization distribution of short trips of Cape Verde shearwaters, but overall, breeders kept a similar foraging pattern between treatments, potentially influenced by predictable prey patches off the West African coast. In contrast, for Cory's shearwaters, the parents of the omega-3 group greatly reduced the foraging effort. This suggests that the proximity to productive prey patches around the colony may help birds to adjust their effort and, therefore, energy expenditure, to changes in the development of their offspring, as driven by their nutritional status. Overall, our results suggest a link between a chick diet enriched in omega-3 FAs and parental foraging effort, providing insight into their ability to cope with a changing and increasingly stochastic marine environment.

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.

Aquatic-terrestrial transfer of neonicotinoid insecticides in riparian food webs

Current-use pesticides are ubiquitous in freshwaters globally, often at very low concentrations. Emerging aquatic insects can accumulate pesticides during their aquatic development, which can be retained through their metamorphosis into terrestrial adults. Emerging insects thus provide a potential, yet largely understudied linkage for exposure of terrestrial insectivores to waterborne pesticides. We measured 82 low to moderately lipophilic organic pesticides (logK_ow: -2.87 to 6.9) in the aquatic environment, emerging insects and web-building riparian spiders from stream sites impacted by agricultural land use. Insecticides, mainly neuro-active neonicotinoids were ubiquitous and had the highest concentrations in emerging insects and spiders (∑ insecticides: 0.1-33 and 1-240 ng/g, respectively), although their concentrations in water were low, even when compared to global levels. Furthermore, neonicotinoids, although not considered to be bioaccumulative, were biomagnified in riparian spiders. In contrast, concentrations of fungicides and most herbicides decreased from the aquatic environment to the spiders. Our results provide evidence for the transfer and accumulation of neonicotinoids across the aquatic-terrestrial ecosystem boundary. This could threaten food webs in ecologically sensitive riparian areas worldwide.

Omega-3 versus Omega-6: Are We Underestimating the Ecological Significance of Arachidonic Acid in Aquatic Systems?

The long-chain polyunsaturated fatty acids (PUFA) eicosapentaenoic acid (EPA, ω-3, or n-3) and arachidonic acid (ARA, ω-6 or n-6) are known to have distinct physiological functions, yet can both support growth and reproduction of consumers, raising the question of whether EPA and ARA are ecologically substitutable dietary resources. We explored the relative importance of EPA and ARA for the growth and reproduction of the freshwater keystone herbivore Daphnia in a life-history experiment. Both PUFA were supplemented in a concentration-dependent manner to a PUFA-free diet, separately and in combination (50% EPA: 50% ARA mixture). The growth-response curves obtained with EPA, ARA, and the mixture were virtually congruent and the thresholds for PUFA limitation did not differ, indicating that EPA (n-3) and ARA (n-6) were substitutable dietary resources under the applied experimental conditions. The actual requirements for EPA and ARA might change with growth conditions, e.g., under the influence of parasites or pathogens. The higher retention of ARA in Daphnia suggests that EPA and ARA are subject to different turnover rates, which also implies different physiological functions. Studies on the ARA requirements of Daphnia could provide valuable information on the presumably underestimated ecological importance of ARA in freshwater food webs.

Trophic transfer of polyunsaturated fatty acids across the aquatic–terrestrial interface: An experimental tritrophic food chain approach

Aquatic and their adjacent terrestrial ecosystems are linked via the flux of organic and inorganic matter. Emergent aquatic insects are recognized as high‐quality food for terrestrial predators, because they provide more physiologically relevant long‐chain polyunsaturated fatty acids (PUFA) than terrestrial insects. The effects of dietary PUFA on terrestrial predators have been explored mainly in feeding trials conducted under controlled laboratory conditions, hampering the assessment of the ecological relevance of dietary PUFA deficiencies under field conditions. We assessed the PUFA transfer across the aquatic–terrestrial interface and the consequences for terrestrial riparian predators in two outdoor microcosm experiments. We established simplified tritrophic food chains, consisting of one of four basic food sources, an intermediary collector gatherer (Chironomus riparius, Chironomidae), and a riparian web‐building spider (Tetragnatha sp.). The four basic food sources (algae, conditioned leaves, oatmeal, and fish food) differed in PUFA profiles and were used to track the trophic transfer of single PUFA along the food chain and to assess their potential effects on spiders, that is, on fresh weight, body condition (size‐controlled measurement of nutritional status), and immune response. The PUFA profiles of the basic food sources, C. riparius and spiders differed between treatments, except for spiders in the second experiment. The PUFA α‐linolenic acid (ALA, 18:3n‐3) and ɣ‐linolenic acid (GLA, 18:3n‐6) were major contributors to the differences between treatments. PUFA profiles of the basic food sources influenced the fresh weight and body condition of spiders in the first experiment, but not in the second experiment, and did not affect the immune response, growth rate, and dry weight in both experiments. Furthermore, our results indicate that the examined responses are dependent on temperature. Future studies including anthropogenic stressors would deepen our understanding of the transfer and role of PUFA in ecosystems.

Quantifying the influence of urban biotic and abiotic environmental factors on great tit nestling physiology

There is a long history of avian studies investigating the impacts of urbanization. While differences in several life-history traits have been documented, either between urban and rural populations or across generalized urbanization gradients, a detailed understanding of which specific environmental variables cause these phenotypic differences is still lacking. Here, we quantified several local environmental variables coupled to urbanization (air pollution, tree composition, ambient temperature, and artificial light at night [ALAN]) within territories of breeding great tits (Parus major). We linked the environmental variables to physiological measures of the nestlings (circulating fatty acid composition [FA], antioxidant capacity and an oxidative damage marker [malondialdehyde; MDA]), to garner a mechanistic understanding of the impact of urbanization. We found that the antioxidant capacity of nestlings decreased with higher numbers of oak trees and levels of PM_2.5 (airborne particulate matter with a diameter < 2.5 μm). Furthermore, the ratio of ω6:ω3 polyunsaturated FAs, important for immune function, was positively correlated with PM_2.5 concentration, while being negatively associated with ambient temperature and number of non-native trees in the territory. Body mass and wing length both increased with the number of local oak trees. We also show, through a principal component analysis, that while the environmental variables fall into an urbanization gradient, this gradient is insufficient to explain the observed physiological responses. Therefore, accounting for individual environmental variables in parallel, and thus allowing for interactions between these, is crucial to fully understand the urban ecosystem.

Stream degradation affects aquatic resource subsidies to riparian ground-dwelling spiders

Freshwater systems have undergone drastic alterations during the last century, potentially affecting cross-boundary resource transfers between aquatic and terrestrial ecosystems. One important connection is the export of biomass by emergent aquatic insects containing omega-3 polyunsaturated fatty acids (PUFAs), especially eicosapentaenoic acid (EPA), that is scarce in terrestrial systems. Because of taxon-specific differences in PUFA content and functional traits, the contribution of different insect groups should be considered, in addition to total biomass export. In this context, one important trait is the emergence mode. Stoneflies, in contrast to other aquatic insects, crawl to land to emerge instead of flying directly from the water surface, making them accessible to ground-dwelling predators. Because stoneflies are especially susceptible to environmental change, stream degradation might cause a mismatch of available and required nutrients, particularly for ground-dwelling predators. In this study, we estimated emergent biomass and EPA export along two streams with different levels of habitat degradation. The EPA content in aquatic insects did not differ with different degrees of habitat degradation and total biomass export in spring was with 7.9 ± 9.6 mg m^-2 day^-1 in the degraded and 7.3 ± 8.5 mg m^-2 day^-1 in the natural system, also unaffected. However, habitat degradation substantially altered the contribution of crawling emergence to the total export in spring, with no biomass export by stoneflies at the most degraded sites. The EPA content in ground-dwelling spiders was correlated with emergent stonefly biomass, making up only 16.0 ± 6.2 % of total fatty acids at sites with no stonefly emergence, but 27.3 ± 3.0 % at sites with highest stonefly emergence. Because immune function in ground-dwelling spiders has been connected to EPA levels, reduced crawling emergence might impact spider fitness. Functional traits, like emergence mode as well as nutritional quality, should be considered when assessing the effects of stream degradation on adjacent terrestrial ecosystems.

Aquatic-Terrestrial Insecticide Fluxes: Midges as Neonicotinoid Vectors

Exposure of freshwater ecosystems to insecticides can negatively impact the development of emerging aquatic insects. These insects serve as an important nutritional subsidy for terrestrial insectivores. Changes in insect emergence phenology (i.e., emergence success and temporal pattern) or fluxes of insecticides retained by the emerging adults have the potential to negatively impact terrestrial food webs. These processes are influenced by contaminant toxicity, lipohilicity, or metabolic processes. The interplay between emergence phenology, contaminant retention through metamorphosis, and associated contaminant flux is not yet understood for current-use insecticides. In a microcosm study, we evaluated the impacts of a 24-h pulse exposure of one of three current-use insecticides, namely pirimicarb, indoxacarb, and thiacloprid, at two environmentally realistic concentration levels on the larval development and emergence of the nonbiting midge Chironomus riparius. In addition, we measured insecticide concentrations in the larvae and adults using ultrahigh performance liquid chromatography coupled to tandem mass spectrometry by electrospray ionization. Exposure to pirimicarb delayed larval development and emergence, and exposure to indoxacarb reduced emergence success. The neonicotinoid thiacloprid had the greatest impact by reducing larval survival and emergence success. At the same time, thiacloprid was the only insecticide measured in the adults with average concentrations of 10.3 and 37.3 ng/g after exposure at 0.1 and 4 µg/L, respectively. In addition, an approximate 30% higher survival to emergence after exposure to 0.1 µg/L relative to a 4-µg/L exposure resulted in a relatively higher flux of thiacloprid, from the aquatic to the terrestrial environment, at the lower exposure. Our experimental results help to explain the impacts of current-use insecticides on aquatic-terrestrial subsidy coupling and indicate the potential for widespread dietary exposure of terrestrial insectivores preying on emerging aquatic insects to the neonicotinoid thiacloprid. Environ Toxicol Chem 2023;42:60-70. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

The role of vital dietary biomolecules in eco-evo-devo dynamics

The physiological dependence of animals on dietary intake of vitamins, amino acids, and fatty acids is ubiquitous. Sharp differences in the availability of these vital dietary biomolecules among different resources mean that consumers must adopt a range of strategies to meet their physiological needs. We review the emerging work on omega-3 long-chain polyunsaturated fatty acids, focusing predominantly on predator-prey interactions, to illustrate that trade-off between capacities to consume resources rich in vital biomolecules and internal synthesis capacity drives differences in phenotype and fitness of consumers. This can then feedback to impact ecosystem functioning. We outline how focus on vital dietary biomolecules in eco-eco-devo dynamics can improve our understanding of anthropogenic changes across multiple levels of biological organization.

Land use changes biomass and temporal patterns of insect cross-ecosystem flows

Emergent aquatic insects constitute an important food source for higher trophic levels, linking aquatic to terrestrial ecosystems. Little is known about how land use affects the biomass or composition of insect emergence. Previous studies are limited to individual time points or seasons, hampering understanding of annual biomass export patterns and detection of phenological changes. Over 1 year's primary emergence period, we continuously determined the biomass, abundance, and identity of >45,000 aquatic insects and recorded land-use-related environmental variables in 20 stream sites using a paired design with upstream forested sites and downstream agricultural sites. Total insect biomass and abundance were 2-7 mg day-1  m-2 and 7-36 ind day-1  m-2 higher in agricultural than forested sites. However, we found turnover of families between forested and agricultural sites, with more insects with shorter generation time in agriculture, indicating lower sensitivity to land-use-related stress because of higher recovery potential. Except for stoneflies, biomass and abundance of major orders were higher in agriculture, but their phenology differed. For different orders, emergence peaked 30 days earlier to 51 days later in agriculture than forest, whereas total abundance and biomass both peaked earlier in agriculture: 3-5 and 3-19 days, respectively. The most important land-use-related drivers were pesticide toxicity and electrical conductivity, which were differentially associated with different aquatic insect order abundances and biomass. Overall, we found that land use was related to changes in composition and phenology of aquatic insect emergence, which is likely to affect food-web dynamics in a cross-ecosystem context.

Content of Docosahexaenoic Acid in Pectoral Muscles of Birds Correlates with Wing Beat Frequency

Docosahexaenoic acid (22:6n-3, DHA) is a structural component of cell membranes and due to a peculiar form of its molecule exerts a high lateral pressure in the membranes enhancing activity of membrane-associated enzymes. A high content of DHA probably provides a high frequency of contraction and a continuous working of skeletal muscles. To estimate the probable physiological and biochemical role of DHA in muscle tissue, a relation of its contents in pectoral muscles of birds with wing beat frequency was evaluated. A high statistically significant correlation between the content of DHA in pectoral muscles of birds and species-specific wing beat frequency was found.

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

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

Two is better than one: Coupling DNA metabarcoding and stable isotope analysis improves dietary characterizations for a riparian-obligate, migratory songbird

While an increasing number of studies are adopting molecular and chemical methods for dietary characterization, these studies often employ only one of these laboratory-based techniques; this approach may yield an incomplete, or even biased, understanding of diet due to each method's inherent limitations. To explore the utility of coupling molecular and chemical techniques for dietary characterizations, we applied DNA metabarcoding alongside stable isotope analysis to characterize the dietary niche of breeding Louisiana waterthrush (Parkesia motacilla), a migratory songbird hypothesized to preferentially provision its offspring with pollution-intolerant, aquatic arthropod prey. While DNA metabarcoding was unable to determine if waterthrush provision aquatic and terrestrial prey in different abundances, we found that specific aquatic taxa were more likely to be detected in successive seasons than their terrestrial counterparts, thus supporting the aquatic specialization hypothesis. Our isotopic analysis added greater context to this hypothesis by concluding that breeding waterthrush provisioned Ephemeroptera and Plecoptera, two pollution-intolerant, aquatic orders, in higher quantities than other prey groups, and expanded their functional trophic niche when such prey were not abundantly provisioned. Finally, we found that the dietary characterizations from each approach were often uncorrelated, indicating that the results gleaned from a diet study can be particularly sensitive to the applied methodologies. Our findings contribute to a growing body of work indicating the importance of high-quality, aquatic habitats for both consumers and their pollution-intolerant prey, while also demonstrating how the application of multiple, laboratory-based techniques can provide insights not offered by either technique alone.

Export of dietary lipids via emergent insects from eutrophic fishponds

Fishponds, despite being globally abundant, have mainly been considered as food production sites and have received little scientific attention in terms of their ecological contributions to the surrounding terrestrial environment. Emergent insects from fishponds may be important contributors of lipids and essential fatty acids to terrestrial ecosystems. In this field study, we investigated nine eutrophic fishponds in Austria from June to September 2020 to examine how Chlorophyll-a concentrations affect the biomass of emergent insect taxa (i.e., quantity of dietary subsidies; n = 108) and their total lipid and long-chain polyunsaturated fatty acid content (LC-PUFA, i.e., quality of dietary subsidies; n = 94). Chironomidae and Chaoboridae were the most abundant emergent insect taxa, followed by Trichoptera, Ephemeroptera, and Odonata. A total of 1068 kg of emergent insect dry mass were exported from these ponds (65.3 hectares). Chironomidae alone exported 103 kg of total lipids and 9.4 kg of omega-3 PUFA. Increasing Chl-a concentrations were associated with decreasing biomass export and a decrease in total lipid and LC-PUFA export via emergent Chironomidae. The PUFA composition of emergent insect taxa differed significantly from dietary algae, suggesting selective PUFA retention by insects. The export of insect biomass from these eutrophic carp ponds was higher than that previously reported from oligotrophic lakes. However, lower biomass and diversity are exported from the fishponds compared to managed ponds. Nonetheless, our data suggest that fishponds provide crucial ecosystem services to terrestrial consumers by contributing essential dietary nutrients to consumer diets via emergent insects.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10750-022-05040-2.

Visitation rate, but not foraging range, responds to brood size manipulation in an aerial insectivore

Abstract

Life history theory predicts that increased investment in current offspring decreases future fecundity or survival. Avian parental investment decisions have been studied either via brood size manipulation or direct manipulation of parental energetic costs (also known as handicapping). However, we have limited experimental data on the potential interactive effects of these manipulations on parent behavior. Additionally, we know little about how these manipulations affect spatial foraging behavior away from the nest. We simultaneously manipulated brood size and parental costs (via added weight in the form of a GPS tag) in wild female barn swallows (Hirundo rustica). We measured multiple aspects of parent behavior at and away from the nest while controlling for measures of weather conditions. We found no significant interactive effects of manipulated brood size and parental costs. Both sexes increased their visitation rate with brood size, but nestlings in enlarged broods grew significantly less post-brood size manipulation than those in reduced broods. Foraging range area was highly variable among GPS-tagged females but was unaffected by brood size. As such, increased visitation rate in response to brood size may be more energetically costly for far-ranging females. GPS-tagged females did not alter their visitation rate relative to un-tagged birds, but their mates had higher visitation rates. This suggests that GPS tagging may affect some unmeasured aspect of female behavior, such as prey delivery. Our findings indicate that investigation of foraging tactics alongside visitation rate is critical to understanding parental investment and the benefits and costs of reproduction.

Significance statement

Avian parental investment decisions have been studied by either brood size manipulation or direct manipulation of parental costs, but rarely both simultaneously. We simultaneously manipulated brood size and parental costs (via addition of a GPS tag) in a wild avian system, allowing us to examine interactive effects of these manipulations. Additionally, studies of parental investment often examine behaviors at the nest, but measurements of parental care behavior away from the nest are rare. Our study is unique in that we measured multiple aspects of parental care, including spatial foraging behavior tracked with GPS tags. We found no interactive effects of manipulated brood size and parental costs on visitation rate or nestling growth, and spatial foraging behavior of females was individually variable. Documenting foraging tactics alongside visitation rate is critical to understanding parental investment because the same visitation rate might be more costly for far-ranging females.

Ecological lipidology

Dietary lipids (DLs), particularly sterols and fatty acids, are precursors for endogenous lipids that, unusually for macronutrients, shape cellular and organismal function long after ingestion. These functions – cell membrane structure, intracellular signalling, and hormonal activity – vary with the identity of DLs, and scale up to influence health, survival, and reproductive fitness, thereby affecting evolutionary change. Our Ecological Lipidology approach integrates biochemical mechanisms and molecular cell biology into evolution and nutritional ecology. It exposes our need to understand environmental impacts on lipidomes, the lipid specificity of cell functions, and predicts the evolution of lipid-based diet choices. Broad interdisciplinary implications of Ecological Lipidology include food web alterations, species responses to environmental change, as well as sex differences and lifestyle impacts on human nutrition, and opportunities for DL-based therapies.

Dark matters: Contrasting responses of stream biofilm to browning and loss of riparian shading

Concentrations of terrestrial-derived dissolved organic carbon (DOC) in freshwater ecosystems have increased consistently, causing freshwater browning. The mechanisms behind browning are complex, but in forestry-intensive regions browning is accelerated by land drainage. Forestry actions in streamside riparian forests alter canopy shading, which together with browning is expected to exert a complex and largely unpredictable control over key ecosystem functions. We conducted a stream mesocosm experiment with three levels of browning (ambient vs. moderate vs. high, with 2.7 and 5.5-fold increase, respectively, in absorbance) crossed with two levels of riparian shading (70% light reduction vs. open canopy) to explore the individual and combined effects of browning and loss of shading on the quantity (algal biomass) and nutritional quality (polyunsaturated fatty acid and sterol content) of the periphytic biofilm. We also conducted a field survey of differently colored (4.7 to 26.2 mg DOC L^-1 ) streams to provide a 'reality check' for our experimental findings. Browning reduced greatly the algal biomass, suppressed the availability of essential polyunsaturated fatty acids, especially eicosapentaenoic acid (EPA), and sterols, but increased the availability of terrestrial-derived long-chain saturated fatty acids (LSAFA). In contrast, loss of shading increased primary productivity, which resulted in elevated sterol and EPA contents of the biofilm. The field survey largely repeated the same pattern: biofilm nutritional quality decreased significantly with increasing DOC, as indicated particularly by a decrease of the ω-3:ω-6 ratio and increase in LSAFA content. Algal biomass, in contrast, was mainly controlled by dissolved inorganic nitrogen (DIN) concentration, while DOC concentration was of minor importance. The ongoing browning process is inducing a dramatic reduction in the nutritional quality of the stream biofilm. Such degradation of the major high-quality food source available for stream consumers may reduce the trophic transfer efficiency in stream ecosystems, potentially extending across the stream-forest ecotone.

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.

Linking omega-3 polyunsaturated fatty acids in natural diet with brain size of wild consumers

Omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA) are key structural lipids and their dietary intake is essential for brain development of virtually all vertebrates. The importance of n-3 LC-PUFA has been demonstrated in clinical and laboratory studies, but little is known about how differences in the availability of n-3 LC-PUFA in natural prey influence brain development of wild consumers. Consumers foraging at the interface of aquatic and terrestrial food webs can differ substantially in their intake of n-3 LC-PUFA, which may lead to differences in brain development, yet this hypothesis remains to be tested. Here we use the previously demonstrated shift towards higher reliance on n-3 LC-PUFA deprived terrestrial prey of native brown trout Salmo trutta living in sympatry with invasive brook trout Salvelinus fontinalis to explore this hypothesis. We found that the content of n-3 LC-PUFA in muscle tissues of brown trout decreased with increasing consumption of n-3 LC-PUFA deprived terrestrial prey. Brain volume was positively related to the content of the n-3 LC-PUFA, docosahexaenoic acid, in muscle tissues of brown trout. Our study thus suggests that increased reliance on diets low in n-3 LC-PUFA, such as terrestrial subsidies, can have a significant negative impact on brain development of wild trout. Our findings provide the first evidence of how brains of wild vertebrate consumers response to scarcity of n-3 LC-PUFA content in natural prey.

Combined effects of cold snaps and agriculture on the growth rates of Tree Swallows (Tachycineta bicolor)

The decline of avian aerial insectivores has been greater than any other foraging guild and both climate change and agricultural intensification are leading hypotheses explaining this decline. Spring cold snaps are predicted to increase in frequency due to climate change, and factors associated with agricultural intensification (e.g., toxicological agents, simplification of agricultural landscapes, and reductions of insect prey) potentially exacerbates the negative effects of cold snaps on aerial insectivore nestling growth and body condition. We evaluated this hypothesis using repeated measures of Tree Swallow (Tachycineta bicolor (Vieillot, 1808)) nestling body mass and 9th primary length across an expansive gradient of agricultural intensification. Growth rate, asymptotic body mass, and near fledging 9th primary length were lower for nestlings in landscapes consisting of more agro-intensive monocultures. This 14-year data set of body measures occurring at 2, 6, 12 and 16 days of age showed that the negative impact of cold snaps on the growth of these two traits was stronger for nestlings reared in more agro-intensive landscapes. Our findings provide further evidence that two of the primary hypothesized drivers for the decline of many aerial insectivores may interact and aggravate their decline by reducing fledging survival.

Nesting success of red-winged blackbirds (Agelaius phoeniceus) in marshes in an anthropogenic landscape

Recent analyses show significant population declines in many abundant avian species, especially marsh-nesting species including the red-winged blackbird (RWBL). Hypothesized causes include reduced nesting success resulting from changing land-use patterns and exposure to contaminants. Our goal was to test the hypothesis that landscape and nest characteristics as well as exposure to polychlorinated biphenyls (PCBs) correlate with nesting success. From 2008 to 2014, we measured clutch size, egg and nestling mass, hatching and fledging success and daily survival of 1293 RWBL nests from 32 marshes in the Hudson River valley of New York. Using generalized linear effect and survival models, we found that: (i) Julian date was negatively related to hatching success and clutch size but positively related to egg mass; (ii) nest height was negatively related to hatching success; (iii) nestling mass decreased with increased nest density and distance to edges; (iv) fledging success was significantly lower in nests closer to the ground that were far from water; and (v) clutch size and daily survival were higher in nests farther from water. Results showed that nesting success was correlated with variables associated with flooding, population density and predation and provided no support for the predicted negative effects of PCB exposure.

Long-chain polyunsaturated fatty acid biosynthesis in a land-crab with advanced terrestrial adaptations: Molecular cloning and functional characterization of two fatty acyl elongases

Depending on the presence and activities of the front-end fatty acyl desaturases and elongation of very long-chain fatty acid (Elovl) enzymes, animals have different capacities for long-chain (≥C₂₀) polyunsaturated fatty acids (LC-PUFA) biosynthesis. Successful land colonisation in brachyuran crabs requires a shift towards terrestrial food chain with limited LC-PUFA availability. We cloned and functionally characterised two elovl genes from the purple land crab Gecarcoidea lalandii. The two Elovl contained all the necessary motifs of a typical polyunsaturated fatty acids (PUFA) Elovl and phylogenetically clustered in the Elovl1 and Elovl6 clades, respectively. The G. lalandii Elovl1 elongated saturated fatty acids, with low activities towards C20 and C22 PUFA substrates. Moreover, the G. lalandii Elovl6 was particularly active in the elongation of C18 PUFA, although it also recognised monounsaturated fatty acids as substrates for elongation. Collectively, the herein characterised G. lalandii elovl paralogues fulfil all the elongation steps involved in the LC-PUFA biosynthetic pathways. Tissue distribution of the G. lalandii elovl genes, along with the FA composition analyses, suggest the hepatopancreas and gill as key metabolic sites for fatty acid elongation. However, current data suggest that G. lalandii is unable to rely solely on biosynthesis to fulfil LC-PUFA requirements, since front-end desaturase appears to be absent in this species and other decapods.

Integrating DNA-based Prey Occurrence Probability Into Stable Isotope Mixing Models

The introduction of laboratory methods to animal dietary studies has allowed researchers to obtain results with accuracy and precision not possible with observational techniques. For example, DNA barcoding, or the identification of prey with taxon-specific DNA sequences, allows researchers to classify digested prey tissues to the species-level, while stable isotope analysis paired with Bayesian mixing models can quantify dietary contributions by comparing a consumer's isotopic values to those derived from their prey. However, DNA-based methods are currently only able to classify, but not quantify, the taxa present in a diet sample, while stable isotope analysis can only quantify dietary taxa that are identified a priori as prey isotopic values are a result of life history traits, not phylogenetic relatedness. Recently, researchers have begun to couple these techniques in dietary studies to capitalize on the reciprocal benefits and drawbacks offered by each approach, with some even integrating DNA-based results directly into Bayesian mixing models as informative priors. As the informative priors used in these models must represent known dietary compositions (e.g., percentages of prey biomasses), researchers have scaled the DNA-based frequency of occurrence of major prey groups so that their normalized frequency of occurrence sums to 100%. Unfortunately, such an approach is problematic as priors stemming from binomial, DNA-based data do not truly reflect quantitative information about the consumer's diet and may skew the posterior distribution of prey quantities as a result. Therefore, we present a novel approach to incorporate DNA-based dietary information into Bayesian stable isotope mixing models that preserves the binomial nature of DNA-based results. This approach uses community-wide frequency of occurrence or logistic regression-based estimates of prey occurrence to dictate the probability that each prey group is included in each mixing model iteration, and, in turn, the probability that each iteration's results are included in the posterior distribution of prey composition possibilities. Here, we demonstrate the utility of this method by using it to quantify the prey composition of nestling Louisiana waterthrush (Parkesia motacilla).

Lowered nutritional quality of prey decrease the growth and biomolecule content of rainbow trout fry

Diet quality is crucial for the development of offspring. Here, we examined how the nutritional quality of prey affects somatic growth and the lipid, carbohydrate, protein, amino acid, and polyunsaturated fatty acid content of rainbow trout (Oncorhynchus mykiss) fry using a three-trophic-level experimental setup. Diets differed especially in their content of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are physiologically essential polyunsaturated fatty acids for a fish fry. Trout were fed with an artificial diet (fish feed, DHA-rich), marine zooplankton diet (krill/Mysis, DHA-rich), or freshwater zooplankton diet (Daphnia, Cladocera, DHA-deficient). The Daphnia were grown either on a poor, intermediate, or high-quality algal/microbial diet simulating potential changes in the nutritional prey quality (EPA-content). Trout fed with the fish feed or marine zooplankton entirely replaced their muscle tissue composition with compounds of dietary origin. In contrast, fish tissue renewal was only partial in fish fed any Daphnia diet. Furthermore, fish grew five times faster on marine zooplankton than on any of the Daphnia diets. This was mainly explained by the higher dietary contents of arachidonic acid (ARA), EPA, and DHA, but also by the higher content of some amino acids in the marine zooplankton than in the Daphnia diets. Moreover, fatty acid-specific carbon isotopes revealed that trout fry could not biosynthesize ARA, EPA, or DHA efficiently from their precursors. Our results suggest that changes in the zooplankton and macroinvertebrate communities' structure in freshwater habitats from DHA-rich to DHA-poor species may reduce the somatic growth of fish fry.

CONSERVING WETLANDS IN AGROECOSYSTEMS CAN SUSTAIN AERIAL INSECTIVORE PRODUCTIVITY AND SURVIVAL

Agricultural intensification simplifies natural landscapes and frequently results in the loss of biodiversity. Wetlands are highly productive and may offset these losses, but the amount of wetland area needed to support declining avian species on farmland is unknown. Using an avian aerial insectivore, the Tree Swallow (Tachycineta bicolor (Vieillot, 1808), we tested whether a gradient of pond area (visible surface water in wetland basins within 500 m of nest boxes; range: 0.2 - 30% pond area) at cropland and grassland sites was related to aquatic insect biomass, reproductive success, and adult female or nestling body condition. Aquatic insect biomass was ~2-8 times higher at the cropland site with intermediate (5.2%) pond area than at sites with the highest (15.6%) and lowest (0.2%) pond area. Swallow clutch initiation date was ~3-4 days earlier and nestling body condition and model-predicted first-year survival were ~10% higher among cropland sites with more pond area and were comparable to birds hatched at grassland sites. Loss of ponds due to agricultural drainage can reduce aquatic insect prey during the breeding season with apparent individual and demographic consequences for insectivorous birds. Overall, results suggest that where wetlands are conserved, intensive croplands may sustain Tree Swallow populations.

Differences in fatty acids composition between Plasmodium infected and uninfected house sparrows along an urbanization gradient

Anthropogenic activities such as intensification of agriculture, animal husbandry and expansion of cities can negatively impact wildlife through its influence on the availability of high-quality food resources and pathogen transmission. The house sparrow (Passer domesticus), an urban exploiter, is undergoing a population decline. Nutritional constrains and infectious diseases has been highlighted as potential causes. Fatty acids (FAs) play an important role in modulating certain immune responses needed to combat parasite infections. FAs are highly influenced by dietary availability and have been shown to vary between urban and rural birds. Habitat anthropization also affects avian malaria epidemiology but little attention has been given to the relationship between blood parasite infection, host FAs composition and anthropization. Here, we analysed 165 juvenile birds either infected by Plasmodium or uninfected, captured at 15 localities grouped in triplets containing urban, rural and natural habitats. The total level of FAs was higher in birds from urban than from rural habitats, suggesting a greater availability of fat-rich foods sources. Furthermore, Plasmodium infected birds had higher relative levels of ω-3 polyunsaturated fatty acids (PUFAs) but lower of ω-6 PUFAs than uninfected birds. In concordance, the ω-6/ω-3 ratio was also lower in infected than in uninfected birds, but only from natural habitats, likely driven by the slightly higher ω-3 PUFAs in infected birds from natural habitats. Birds from anthropized environments may metabolize the ω-3 PUFAs to promote anti-inflammatory responses against stressors, which would result in lower ω-3 affecting their response against Plasmodium. Alternatively, lower ω-6 PUFAs may influence birds susceptibility to infection due to a weaker pro-inflammatory response. These descriptive results do not allow us to identify the causality of these associations but highlight the need to further investigate the relevance of FAs for birds to fight infectious diseases in habitats with different degree of urbanization.

Ecological Impacts of Altered Stream Hydrogeomorphic Characteristics Extend Beyond the Channel Boundary: Evidence From Urban Streams of Columbus, OH, United States

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 13C and 15N 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 δ13C 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.

Ecology and extent of freshwater browning - What we know and what should be studied next in the context of global change

Water browning or brownification refers to increasing water color, often related to increasing dissolved organic matter (DOM) and carbon (DOC) content in freshwaters. Browning has been recognized as a significant physicochemical phenomenon altering boreal lakes, but our understanding of its ecological consequences in different freshwater habitats and regions is limited. Here, we review the consequences of browning on different freshwater habitats, food webs and aquatic-terrestrial habitat coupling. We examine global trends of browning and DOM/DOC, and the use of remote sensing as a tool to investigate browning from local to global scales. Studies have focused on lakes and rivers while seldom addressing effects at the catchment scale. Other freshwater habitats such as small and temporary waterbodies have been overlooked, making the study of the entire network of the catchment incomplete. While past research investigated the response of primary producers, aquatic invertebrates and fishes, the effects of browning on macrophytes, invasive species, and food webs have been understudied. Research has focused on freshwater habitats without considering the fluxes between aquatic and terrestrial habitats. We highlight the importance of understanding how the changes in one habitat may cascade to another. Browning is a broader phenomenon than the heretofore concentration on the boreal region. Overall, we propose that future studies improve the ecological understanding of browning through the following research actions: 1) increasing our knowledge of ecological processes of browning in other wetland types than lakes and rivers, 2) assessing the impact of browning on aquatic food webs at multiple scales, 3) examining the effects of browning on aquatic-terrestrial habitat coupling, 4) expanding our knowledge of browning from the local to global scale, and 5) using remote sensing to examine browning and its ecological consequences.

Biofilm and invertebrate consumption by western sandpipers (Calidris mauri) and dunlin (Calidris alpina) during spring migratory stopover: insights from tissue and breath CO2 isotopic (δ 13C, δ 15N) analyses

Shorebirds use key migratory stopover habitats in spring and fall where body proteins are replenished and lipids stored as fuel for the remaining journey. The Fraser River estuary, British Columbia, Canada, is a critical spring stopover site for hundreds of thousands of migrating western sandpiper, Calidris mauri, and dunlin, Calidris alpina. Intertidal biofilm in spring is an important nutritional source for western sandpiper, with previous isotopic research predicting 45-59% of total diet and 50% of total energy needs. However, these studies relied on isotopic mixing models that did not consider metabolic routing of key dietary macromolecules. Complexity arises due to the mixed macromolecular composition of biofilm that is difficult to characterize isotopically. We expanded on these earlier findings by considering a protein pathway from diet to the body protein pool represented by liver tissue, using a Bayesian mixing model based on δ ¹³C and δ ¹⁵N. We used δ ¹³C measurements of adipose tissue and breath CO₂ to provide an estimate of the carbohydrate and protein δ ¹³C values of microphytobenthos and used these derived values to better inform the isotopic mixing models. Our results reinforce earlier estimates of the importance of biofilm to staging shorebirds in predicting that assimilated nutrients from biofilm contribute ~35% of the protein budgets for staging western sandpipers (n = 13) and dunlin (n = 11) and at least 41% of the energy budget of western sandpiper (n = 69). Dunlin's ingestion of biofilm appeared higher than anticipated given their expected reliance on invertebrate prey compared to western sandpiper, a biofilm specialist. Isotopic analyses of bulk tissues that consider metabolic routing and that make use of breath CO₂ and adipose lipid assays can provide new insights into avian physiology. We advocate further isotopic research to better understand biofilm use by migratory shorebirds in general and as a critical requirement for more effective conservation.

Nonideal nest box selection by tree swallows breeding in farmlands: Evidence for an ecological trap?

Animals are expected to select a breeding habitat using cues that should reflect, directly or not, the fitness outcome of the different habitat options. However, human-induced environmental changes can alter the relationships between habitat characteristics and their fitness consequences, leading to maladaptive habitat choices. The most severe case of such nonideal habitat selection is the ecological trap, which occurs when individuals prefer to settle in poor-quality habitats while better ones are available. Here, we studied the adaptiveness of nest box selection in a tree swallow (Tachycineta bicolor) population breeding over a 10-year period in a network of 400 nest boxes distributed along a gradient of agricultural intensification in southern Québec, Canada. We first examined the effects of multiple environmental and social habitat characteristics on nest box preference to identify potential settlement cues. We then assessed the links between those cues and habitat quality as defined by the reproductive performance of individuals that settled early or late in nest boxes. We found that tree swallows preferred nesting in open habitats with high cover of perennial forage crops, high spring insect biomass, and high density of house sparrows (Passer domesticus), their main competitors for nest sites. They also preferred nesting where the density of breeders and their mean number of fledglings during the previous year were high. However, we detected mismatches between preference and habitat quality for several environmental variables. The density of competitors and conspecific social information showed severe mismatches, as their relationships to preference and breeding success went in opposite direction under certain circumstances. Spring food availability and agricultural landscape context, while related to preferences, were not related to breeding success. Overall, our study emphasizes the complexity of habitat selection behavior and provides evidence that multiple mechanisms may potentially lead to an ecological trap in farmlands.

Temperature and land use influence tree swallow individual health

Aerial insectivorous bird populations have declined precipitously in both North America and Europe. We assessed the effects of insect prey availability, climate and shifts in water quality associated with urbanization on haematocrit, haemoglobin concentration and heterophil-lymphocyte (H/L) ratios among ~13-day-old tree swallow (Tachycineta bicolor) nestlings in the Columbus, Ohio area. Higher mean temperature and increased frequency of extreme heat days during the early breeding period (May-June) were linked to reduced nestling physiological condition as evidenced by lower concentrations of haemoglobin and haematocrit, potentially due to increased heat stress, shifts in insect prey availability or altered parental provisioning efforts. Urbanization and the size and density of emergent aquatic insects were associated with elevated physiological stress, whereas higher mean temperatures and terrestrial insect size were related to lower stress as measured by H/L ratios. Overall, these findings highlight the complex environmental conditions driving nestling health, which may be indicative of post-fledging survival and, consequently, population growth. Our results underscore the need for conservation approaches that adequately address the interrelated effects of changes in climate, land use and food resources on aerial insectivorous birds.

Warming Decreases Bioconversion of Polyunsaturated Fatty Acids in Chironomid Larvae Maintained on Cyanobacterium Microcystis

Cyanobacteria dominance and warming have been suggested to decrease the production of polyunsaturated fatty acids (PUFA) in freshwater ecosystems. Physiological adaptations of poikilothermic animals to higher temperatures may further decrease PUFA levels in aquatic food webs. We conducted diet manipulation experiments to investigate the combined effects of dietary PUFA and warming on the proportions of eicosapentaenoic acid (EPA) and arachidonic acid (ARA) in Chironomus riparius. The experimental diet consisted of a nontoxic cyanobacterium Microcystis, which contained C₂₀ PUFA: 20:3n-3, 20:4n-3, and 20:3n-6, but no EPA or ARA. Additionally, we used TetraMin^® fish flakes as a control treatment. A temperature increase from 20 °C to 25 °C decreased the proportion of n-3 C₂₀ PUFA and the n-3/n-6 ratio in Microcystis. Diet manipulation experiments indicated that Chironomus desaturated dietary C₂₀ precursors to EPA and ARA, but warming decreased this bioconversion and resulted in lower levels of EPA and ARA in Chironomus. Warming did not alter the proportions of EPA and ARA in Chironomus larvae if these PUFA were readily available in the diet (TetraMin^® control treatment). In conclusion, warming and cyanobacteria dominance may decrease the production and trophic transfer of physiologically important PUFA in freshwaters by (1) decreasing the n-3/n-6 ratio and the abundance of n-3 C₂₀ precursors in Microcystis, and (2) decreasing the bioconversion of n-3 and n-6 C₂₀ precursors to EPA and ARA in chironomids. These changes may have cascading effects throughout the food web and decrease the content of EPA in fish, potentially affecting its availability to humans.

Use of Fatty Acids From Aquatic Prey Varies With Foraging Strategy

Across ecosystems, resources vary in their nutritional composition and thus their dietary value to consumers. Animals can either access organic compounds, such as fatty acids, directly from diet or through internal biosynthesis, and the extent to which they use these two alternatives likely varies based on the availability of such compounds across the nutritional landscape. Cross-ecosystem subsidies of important dietary nutrients, like omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA), may provide consumers with the opportunity to relax the demands of synthesis and rely upon dietary flexibility rather than internal metabolic processes. Here, we examined how dietary flexibility and distance from a lake influenced the degree to which generalist insectivores relied upon dietary n-3 LC-PUFA from emergent aquatic insects versus n-3 LC-PUFA synthesized from precursor compounds found in terrestrial insects. We used bulk and compound-specific stable isotope analyses to understand spider and insectivorous bird (Blue Tit; Cyanistes caeruleus) reliance on aquatic and terrestrial resources, including dietary PUFA sources, along a riparian to upland gradient from a lake. We simultaneously investigated n-3 LC-PUFA synthesis ability in nestlings using 13C fatty acid labeling. We found that riparian spiders took advantage of emergent aquatic insect subsidies, deriving their overall diet and their n-3 PUFA from aquatic resources whereas nestling birds at all distances and upland spiders relied upon terrestrial resources, including PUFA. Our 13C labeling experiment demonstrated that nestling tits were able to synthesize the n-3 LC-PUFA docosahexaenoic acid from the dietary precursor α-linolenic acid, suggesting that they are not limited by aquatic resources to satisfy their LC-PUFA requirements. Overall, this study suggests that habitat generalist insectivores vary in the degree to which they can shift diet to take advantage of high-quality aquatic resources depending upon both their foraging flexibility and internal synthesis capacity.

Longitudinal variation in the nutritional quality of basal food sources and its effect on invertebrates and fish in subalpine rivers

There is growing recognition of the importance of food quality over quantity for aquatic consumers. In streams and rivers, most previous studies considered this primarily in terms of the quality of terrestrial leaf litter and importance of microbial conditioning. However, many recent studies suggest that algae are a more nutritional food source for riverine consumers than leaf litter. To date, few studies have quantified longitudinal shifts in the nutritional quality of basal food resources in river ecosystems and how these may affect consumers. We conducted a field investigation in a subalpine river ecosystem in Austria to investigate longitudinal variations in diet quality of basal food sources (submerged leaves and periphyton) and diet source dependence of stream consumers (invertebrate grazers, shredders, filterers and predators, and fish). Fatty acid (FA) profiles of basal food sources and their consumers were measured. Our results indicate systematic differences between the FA profiles of terrestrial leaves and aquatic biota, that is periphyton, invertebrates and fish. Submerged leaves contained very low proportions of long-chain polyunsaturated fatty acids (LC-PUFAs), which were conversely rich in aquatic biota. While the FA composition of submerged leaves remained similar among sites, the LC-PUFAs of periphyton increased longitudinally, which was associated with increasing nutrients from upstream to downstream. Longitudinal variations in periphyton LC-PUFAs were reflected in the LC-PUFAs of invertebrate grazers and shredders, and further tracked by invertebrate predators and fish. However, brown trout Salmo trutta contained a large proportion of docosahexaenoic acid (DHA, 22:6ω3), a LC-PUFA almost entirely missing in basal sources and invertebrates. The fish accumulated eicosapentaenoic acid (EPA, 20:5ω3) from invertebrate prey and may use this FA to synthesize DHA. Our results provide a nutritional perspective for river food web studies, emphasizing the importance of algal resources to consumer somatic growth and the need to account for the longitudinal shifts in the quality of these basal resources.

Natural variation in yolk fatty acids, but not androgens, predicts offspring fitness in a wild bird

BACKGROUND

In egg-laying animals, mothers can influence the developmental environment and thus the phenotype of their offspring by secreting various substances into the egg yolk. In birds, recent studies have demonstrated that different yolk substances can interactively affect offspring phenotype, but the implications of such effects for offspring fitness and phenotype in natural populations have remained unclear. We measured natural variation in the content of 31 yolk components known to shape offspring phenotypes including steroid hormones, antioxidants and fatty acids in eggs of free-living great tits (Parus major) during two breeding seasons. We tested for relationships between yolk component groupings and offspring fitness and phenotypes.

RESULTS

Variation in hatchling and fledgling numbers was primarily explained by yolk fatty acids (including saturated, mono- and polyunsaturated fatty acids) - but not by androgen hormones and carotenoids, components previously considered to be major determinants of offspring phenotype. Fatty acids were also better predictors of variation in nestling oxidative status and size than androgens and carotenoids.

CONCLUSIONS

Our results suggest that fatty acids are important yolk substances that contribute to shaping offspring fitness and phenotype in free-living populations. Since polyunsaturated fatty acids cannot be produced de novo by the mother, but have to be obtained from the diet, these findings highlight potential mechanisms (e.g., weather, habitat quality, foraging ability) through which environmental variation may shape maternal effects and consequences for offspring. Our study represents an important first step towards unraveling interactive effects of multiple yolk substances on offspring fitness and phenotypes in free-living populations. It provides the basis for future experiments that will establish the pathways by which yolk components, singly and/or interactively, mediate maternal effects in natural populations.