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.

Functional response of Franklinothrips vespiformis (Thysanoptera: Aeolothripidae) to eggs and nymphs of Bemisia tabaci (Hemiptera: Aleyrodidae).

The Middle East Asia Minor 1 biotype of Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) is a greenhouse and field crop pest of global significance. The objective of this study was to assess the potential of the generalist predatory thrips, Franklinothrips vespiformis Crawford (Thysanoptera: Aeolothripidae), as a biological control agent for B. tabaci. This was achieved by determining the functional responses of F. vespiformis larvae and adults to the egg and nymphal stages of B. tabaci under laboratory conditions. Analyses consisted of 10 replicates of each predator and prey stage combination on bean leaf discs for a 24-h period. Following logistic regression analyses to determine the functional response type exhibited, response parameters were estimated with nonlinear least squares regression using Roger's equation. Results showed that F. vespiformis larvae and adults exhibited a Type II functional response when feeding on immature B. tabaci. The handling times (Th) of F. vespiformis larvae and adults were magnitudes higher for B. tabaci nymphs than they were for eggs, which were in part driven by the higher attack rates (a) observed on eggs. The maximum attack rate (T/Th) for B. tabaci eggs and nymphs exhibited by first-stage larvae, second-stage larvae, and adult F. vespiformis increased with increasing predator age. Results from this study suggest that F. vespiformis larvae and particularly adults are promising biological control agents for B. tabaci and are efficient predators at both low and high prey densities.

Diet of the critically endangered blackchin guitarfish Glaucostegus cemiculus revealed using DNA metabarcoding.

We present the first assessment of the diet of the blackchin guitarfish Glaucostegus cemiculus (Geoffroy Saint-Hilaire, 1817) for West Africa using DNA metabarcoding on stomach contents of individuals captured in the Bijagós Archipelago, Guinea-Bissau. The diet was dominated by crustaceans, particularly caramote prawn Penaeus kerathurus (frequency of occurrence [FO] = 74%, numerical frequency [NF] = 54%) and fiddler crab Afruca tangeri (FO = 74%, NF = 12%). Bony fishes were present in 30% of the stomachs. We highlight the importance of conservation action for intertidal habitats and their associated benthic invertebrates for the survival of the critically endangered blackchin guitarfish.

Ecological variation in invasive brown trout (Salmo trutta) within a remote coastal river catchment in northern Patagonia complicates estimates of invasion impact.

Salmonids were first introduced into the Chilean fresh waters in the 1880s, and c. 140 years later, they are ubiquitous across Chilean rivers, especially in the southern pristine fresh waters. This study examined the brown trout (Salmo trutta) and native taxa ecology in two adjacent but contrasting rivers of Chilean Patagonia. During spring 2016 and spring-fall 2017 we examined the variation in benthic macroinvertebrate and fish community composition and characterized fish size structure, stomach contents, and stable isotopes (δ13 C and δ15 N) to understand population structure, fish diet, and trophic interactions between S. trutta and native taxa. The native Galaxias maculatus (puye) dominated the fish community (74% of abundance). S. trutta was less abundant (16% of survey catch) but dominated the fish community (over 53%) in terms of biomass. S. trutta showed distinct diets (stomach content analysis) in the two rivers, and individuals from the larger river were notably more piscivorous, consuming native fish with a relatively small body size (<100-mm total length). Native fishes were isotopically distinct from S. trutta, which showed a wider isotopic niche in the smaller river, indicating that their trophic role was more variable than in the larger river (piscivorous). This study provides data from the unstudied pristine coastal rivers in Patagonia and reveals that interactions between native and introduced species can vary at very local spatial scales.

Temperature, productivity, and habitat characteristics collectively drive lake food web structure.

While many efforts have been devoted to understand variations in food web structure among terrestrial and aquatic ecosystems, the environmental factors influencing food web structure at large spatial scales remain hardly explored. Here, we compiled biodiversity inventories to infer food web structure of 67 French lakes using an allometric niche-based model and tested how environmental variables (temperature, productivity, and habitat) influence them. By applying a multivariate analysis on 20 metrics of food web topology, we found that food web structural variations are represented by two distinct complementary and independent structural descriptors. The first is related to the overall trophic diversity, whereas the second is related to the vertical structure. Interestingly, the trophic diversity descriptor was mostly explained by habitat size (26.7% of total deviance explained) and habitat complexity (20.1%) followed by productivity (dissolved organic carbon: 16.4%; nitrate: 9.1%) and thermal variations (10.7%). Regarding the vertical structure descriptor, it was mostly explained by water thermal seasonality (39.0% of total deviance explained) and habitat depth (31.9%) followed by habitat complexity (8.5%) and size (5.5%) as well as annual mean temperature (5.6%). Overall, we found that temperature, productivity, and habitat characteristics collectively shape lake food web structure. We also found that intermediate levels of productivity, high levels of temperature (mean and seasonality), as well as large habitats are associated with the largest and most complex food webs. Our findings, therefore, highlight the importance of focusing on these three components especially in the context of global change, as significant structural changes in aquatic food webs could be expected under increased temperature, pollution, and habitat alterations.

Rapid evolution of diet choice in an introduced population of Trinidadian guppies.

Eco-evolutionary theory has brought an interest in the rapid evolution of functional traits. Among them, diet is an important determinant of ecosystem structure, affecting food web dynamics and nutrient cycling. However, it is largely unknown whether diet, or diet preference, has a hereditary basis and can evolve on contemporary timescales. Here, we study the diet preferences of Trinidadian guppies Poecilia reticulata collected from directly below an introduction site of fish transplanted from a high-predation environment into a low predation site where their densities and competition increased. Behavioural assays on F2 common garden descendants of the ancestral and derived populations showed that diet preference has rapidly evolved in the introduced population in only 12 years (approx. 36 generations). Specifically, we show that the preference for high-quality food generally found in high-predation guppies is lost in the newly derived low-predation population, who show an inertia toward the first encountered food. This result is predicted by theory stating that organisms should evolve less selective diets under higher competition. Demonstrating that diet preference can show rapid and adaptive evolution is important to our understanding of eco-evolutionary feedbacks and the role of evolution in ecosystem dynamics.

Colour-specific diet specialization is associated with differences in owlet weight in a polymorphic owl: influence of the trophic quality variation.

The niche divergence hypothesis proposes that the evolution and maintenance of colour polymorphism is based on a mechanism of disruptive selection. In a trophic context, the hypothesis predicts that individuals differing in colour vary in their trophic niche, either because they differ in foraging efficiency or feed in different habitats. A major evolutionary conundrum is how these expectations are affected by variation in trophic quality. Using an owl species with colour plumage polymorphism, the Eurasian scops owl Otus scops, we examined diet and habitat segregation during reproduction in relation to plumage colouration and trophic quality. Intensive sampling revealed that trophic quality for scops owls (i.e. abundance of grasshoppers and locusts) varied more among territories than between years, but scops owls did not segregate among territories of different quality by their colouration. However, we found that sex, plumage colouration and territory differences in trophic quality explained differences in the degree of dietary specialization. Brownish males delivered a higher diversity of prey to the nest than greyish ones in high trophic quality territories. We also found that the more diverse the diet provided by males, the heavier the owlets at fledging. Our study provides evidence for a different sensitivity to trophic quality of the colour morphs with potential fitness consequences in scops owls. We highlight the importance of studying the mechanisms leading to the persistence of colour polymorphism in patchy environments, since segregation may pass otherwise unnoticed if only habitats or years with similar conditions are considered.

Zinc isotopes in Late Pleistocene fossil teeth from a Southeast Asian cave setting preserve paleodietary information.

Stable carbon and nitrogen isotope ratios of collagen from bone and dentin have frequently been used for dietary reconstruction, but this method is limited by protein preservation. Isotopes of the trace element zinc (Zn) in bioapatite constitute a promising proxy to infer dietary information from extant and extinct vertebrates. The 66Zn/64Zn ratio (expressed as δ66Zn value) shows an enrichment of the heavy isotope in mammals along each trophic step. However, preservation of diet-related δ66Zn values in fossil teeth has not been assessed yet. Here, we analyzed enamel of fossil teeth from the Late Pleistocene (38.4-13.5 ka) mammalian assemblage of the Tam Hay Marklot (THM) cave in northeastern Laos, to reconstruct the food web and assess the preservation of original δ66Zn values. Distinct enamel δ66Zn values of the fossil taxa (δ66Zncarnivore < δ66Znomnivore < δ66Znherbivore) according to their expected feeding habits were observed, with a trophic carnivore-herbivore spacing of +0.60‰ and omnivores having intermediate values. Zn and trace element concentration profiles similar to those of modern teeth also indicate minimal impact of diagenesis on the enamel. While further work is needed to explore preservation for settings with different taphonomic conditions, the diet-related δ66Zn values in fossil enamel from THM cave suggest an excellent long-term preservation potential, even under tropical conditions that are well known to be adverse for collagen preservation. Zinc isotopes could thus provide a new tool to assess the diet of fossil hominins and associated fauna, as well as trophic relationships in past food webs.

Mercury contamination in the tropical seabird community from Clipperton Island, eastern Pacific Ocean.

Mercury (Hg) pollution is a global problem affecting remote areas of the open ocean, but the bioaccumulation of this neurotoxic pollutant in tropical top predators remains poorly documented. The objective of this study was to determine Hg contamination of the seabird community nesting on Clipperton Island using blood and feathers to investigate short and longer-term contamination, respectively. We examined the significance of various factors (species, sex, feeding habitat [δ13C] and trophic position [δ15N]) on Hg concentrations in six seabird species. Among species, Great Frigatebirds had the highest Hg concentrations in blood and feathers, boobies had intermediate values, and Brown Noddies and Sooty Terns the lowest. At the interspecific level, although δ13C values segregated boobies from frigatebirds and noddies/terns, Hg concentrations were explained by neither δ13C nor δ15N values. At the intraspecific level, both Hg concentrations in blood and feathers show relatively small variations (16-32 and 26-74%, respectively), suggesting that feeding ecology had low seasonal variation among individuals. Despite most species being sexually dimorphic, differences in Hg contamination according to sex was detected only in Brown Boobies during the breeding period. Indeed, female Brown Boobies feed at a higher trophic level and in a different area than males during this period, resulting in higher blood Hg concentrations. The present study also shows that most of the seabirds sampled at Clipperton Island had little or no exposure to Hg toxicity, with 30% in the no risk category and 70% in the low risk category.

Spatiotemporal variability in diet composition of Greenland halibut (Reinhardtius hippoglossoides) from the eastern Canadian Arctic.

Greenland halibut (Reinhardtius hippoglossoides) sustain one of the most lucrative fisheries in the eastern Canadian Arctic and Labrador Sea. This species also plays an important role in food web connectivity and benthic-pelagic coupling. Despite the relatively rich knowledge of this species, R. hippoglossoides ecology in these specific areas remains poorly understood. The main aim of this study was to characterize the diet of this deepwater fish in the Labrador Sea and Davis and Hudson Straits and characterize the predator-prey relationship with northern shrimp (Pandalus borealis), another commercially important species in the region. Stomach contents analyses were conducted on 1199 fish captured from 2018 to 2020. Small specimens (<20 cm) fed on invertebrates, whereas larger individuals (>60 cm) fed primarily on fish, indicative of size-related changes in diet composition. The relative abundance of Pandalus shrimp species in the environment was reflected in the diet. Location appeared to be the most influential variable on feeding patterns. Distinct oceanographic conditions among areas, resulting in differences in prey availability, could explain these results. Arctic cod (Boreogadus saida) and redfish (Sebastes sp.) were selected in locations where fish prey were the most abundant. These results shed light on the opportunistic nature of R. hippoglossoides and its preference for fish at large size. With the rapidly changing oceanographic conditions of Arctic waters, a distributional change in the biomass of shrimp is expected. Results suggest that an increase in abundance of predatory groundfish species in the system (e.g., Sebastes sp.) could lead to acute predation on shrimp and competition with R. hippoglossoides. By revealing key trophic links within the demersal ecosystem, this work provides valuable information on the development of ecosystem approaches to fisheries management for the region.

Quantifying spatio-temporal consistency in the trophic ecology of two sympatric flatfishes.

Sympatric flatfish predators may partition their resources in coastal environments to reduce competition and maximise foraging efficiency. However, the degree of spatial and temporal consistency in their trophic ecology is not well understood because dietary studies tend to overlook the heterogeneity of consumed prey. Increasing the spatial and temporal scale of dietary analyses can thus help to resolve predator resource use. We applied a stomach content and multi-tissue (liver and muscle) stable isotope (δ13 C, δ15 N and δ34 S) approach to investigate the feeding habits of two co-occurring flatfish predators, common dab (Limanda limanda) and European plaice (Pleuronectes platessa), across four bays on the Northumberland coast (UK) over short (hours), medium (days) and long (months) temporal scales. Stomach content analyses showed spatial consistencies in predator resource use, whereas stable isotope mixing models revealed considerable inter-bay diet variability. Stomach contents also indicated high dietary overlap between L. limanda and P. platessa, while the stable isotope data yielded low to moderate levels of overlap, with cases of complete niche separation. Furthermore, individual specialisation metrics indicated consistently low levels of specialisation among conspecifics over time. We document changes in resource partitioning in space and time, reflecting diet switching in response to local and temporal fluctuations of patchily distributed prey. This study highlights how trophic tracers integrated at multiple temporal and spatial scales (within tens of kilometres) provide a more integrative approach for assessing the trophic ecology of sympatric predators in dynamic environments.

Effects of preservation by ethanol on δ13 C and δ15 N of three tissues of the critically endangered European eel Anguilla anguilla.

The temporal effects of ethanol preservation on the δ13 C and δ15 N values of tissues excised from European eel Anguilla anguilla were assessed. Preservation significantly enriched 13 C values of fin and mucus but not dorsal muscle. The 13 C enrichment occurred in the initial 15 days of preservation and was independent of initial eel mass. Tissue preservation effects on δ15 N values were negligible. These tissue-specific isotopic shifts should be considered when ethanol-preserved eel samples are used.

Rapid Range Expansion of a Marine Ectotherm Reveals the Demographic and Ecological Consequences of Short-Term Variability in Seawater Temperature and Dissolved Oxygen.

AbstractThe distributions of marine ectotherms are governed by physiological sensitivities to long-term trends in seawater temperature and dissolved oxygen. Short-term variability in these parameters has the potential to facilitate rapid range expansions, and the resulting ecological and socioeconomic consequences may portend those of future marine communities. Here, we combine physiological experiments with ecological and demographic surveys to assess the causes and consequences of sudden but temporary poleward range expansions of a marine ectotherm with considerable life history plasticity (California market squid, Doryteuthis opalescens). We show that sequential factors related to resource accessibility in the core range-the buildup of large populations as a result of competitive release and climate-associated temperature increase and oxygen loss that constrain aerobic activity-may drive these expansions. We also reveal that poleward range expansion alters the body size-and therefore trophic role-of invading populations, with potential negative implications for socioeconomically valuable resident species. To help forecast rapid range expansions of marine ectotherms, we advocate that research efforts focus on factors impacting resource accessibility in core ranges. Determining how environmental conditions in receiving ecosystems affect body size and how body size is related to trophic role will help refine estimates of the impacts of future marine communities.

DNA metabarcoding reveals the importance of gelatinous zooplankton in the diet of Pandalus borealis, a keystone species in the Arctic.

Information about the dietary composition of a species is crucial to understanding their position and role in the food web. Increasingly, molecular approaches such as DNA metabarcoding are used in studying trophic relationships, not least because they may alleviate problems such as low taxonomic resolution or underestimation of digestible taxa in the diet. Here, we used DNA metabarcoding with universal primers for cytochrome c oxidase I (COI) to study the diet composition of the northern shrimp (Pandalus borealis), an Arctic keystone species with large socio-economic importance. Across locations, jellyfish and chaetognaths were the most important components in the diet of P. borealis, jointly accounting for 40%-60% of the total read abundance. This dietary importance of gelatinous zooplankton contrasts sharply with published results based on stomach content analysis. At the same time, diet composition differed between fjord and shelf locations, pointing to different food webs supporting P. borealis in these two systems. Our study underlines the potential of molecular approaches to provide new insights into the diet of marine invertebrates that are difficult to obtain with traditional methods, and calls for a revision of the role of gelatinous zooplankton in the diet of the key Arctic species P. borealis, and in extension, Arctic food webs.

Reconstructing Hominin Diets with Stable Isotope Analysis of Amino Acids: New Perspectives and Future Directions.

Stable isotope analysis of teeth and bones is regularly applied by archeologists and paleoanthropologists seeking to reconstruct diets, ecologies, and environments of past hominin populations. Moving beyond the now prevalent study of stable isotope ratios from bulk materials, researchers are increasingly turning to stable isotope ratios of individual amino acids to obtain more detailed and robust insights into trophic level and resource use. In the present article, we provide a guide on how to best use amino acid stable isotope ratios to determine hominin dietary behaviors and ecologies, past and present. We highlight existing uncertainties of interpretation and the methodological developments required to ensure good practice. In doing so, we hope to make this promising approach more broadly accessible to researchers at a variety of career stages and from a variety of methodological and academic backgrounds who seek to delve into new depths in the study of dietary composition.

EcoDiet: A hierarchical Bayesian model to combine stomach, biotracer, and literature data into diet matrix estimation.

Although quantifying trophic interactions is a critical path to understanding and forecasting ecosystem functioning, fitting trophic models to field data remains challenging. It requires flexible statistical tools to combine different sources of information from the literature and fieldwork samples. We present EcoDiet, a hierarchical Bayesian modeling framework to simultaneously estimate food-web topology and diet composition of all consumers in the food web, by combining (1) a priori knowledge from the literature on both food-web topology and diet proportions; (2) stomach content analyses, with frequencies of prey occurrence used as the primary source of data to update the prior knowledge on the topological food-web structure; (3) and biotracers data through a mixing model (MM). Inferences are derived in a Bayesian probabilistic rationale that provides a formal way to incorporate prior information and quantifies uncertainty around both the topological structure of the food web and the dietary proportions. EcoDiet was implemented as an open-source R package, providing a user-friendly interface to execute the model, as well as examples and guidelines to familiarize with its use. We used simulated data to demonstrate the benefits of EcoDiet and how the framework can improve inferences on diet matrix by comparison with classical network MM. We applied EcoDiet to the Celtic Sea ecosystem, and showed how combining multiple data types within an integrated approach provides a more robust and holistic picture of the food-web topology and diet matrices than the literature or classical MM approach alone. EcoDiet has the potential to become a reference method for building diet matrices as a preliminary step of ecosystem modeling and to improve our understanding of prey-predator interactions.

Stable isotopes unravel the feeding mode-trophic position relationship in trematode parasites.

Stable isotopes have been sporadically used over the last two decades to characterise host-parasite trophic relationships. The main reason for this scarcity is the lack of an obvious pattern in the ratio of nitrogen stable isotope values (δ15 N) of parasites in comparison to their host tissues, which would be key to understand any host-parasite system dynamics. To circumvent this, we focused on a single snail host, Zeacumantus subcarinatus, and three of its trematode parasites. We used stable isotopes to investigate each host-trematode trophic relationship and shed light on the mechanisms utilised by the parasite to reroute its hosts' biomass. All our trematodes were found to be 15 N-enriched compared to their host, with their δ15 N values strongly related to their feeding behaviours: passive versus active. It was possible to 'rank' these parasite species and assess their 'relative' trophic position using δ15 N values. We also demonstrated that including a broader range of samples (e.g. host food and faeces, multiple parasite life stages) helped understand the metabolic mechanisms used by the various participants, and that using carbon stable isotope values and C:N ratios allowed to identify an important lipid requirement of these trematode parasites. Finally, we show how critical it is to not ignore parasitic infections as they can have a great influence on their host's trophic position. We have shown that by focussing on a single host species and a single taxonomic group of parasites, we can remove a certain amount of variation recorded by broader isotope studies. We hope that these data will ultimately improve our ability to place parasites in food webs, and thus improve our understanding of the connections and interactions that dictate food web dynamics.

Riparian buffers maintain aquatic trophic structure in agricultural landscapes.

Local and regional habitat conditions associated with agricultural activity can fundamentally alter aquatic ecosystems. Increased nutrient inputs, channelization and reduced riparian habitat both upstream and locally contribute to the degradation of stream ecosystems and their function. Here, we examine stream food webs in watersheds that feed into Lake Erie to determine the effects of agricultural land cover on major food web energy pathways and trophic structure. Given that higher agricultural intensity can increase nutrient runoff and reduce the riparian zone and litter in-fall into streams, we predicted that generalist fish would derive less energy from the terrestrial pathway and become more omnivorous. Consistent with these predictions, we show that both mean terrestrial energy use and trophic position of the resident top consumer, creek chub (Semotilus atromaculatus), decrease with local agricultural intensity but not with watershed-level agriculture intensity. These findings suggest that local riparian buffers can maintain trophic structure even in the face of high whole-watershed agricultural intensity.

Isotopic niche partitioning and individual specialization in an Arctic raptor guild.

Intra- and inter-specific resource partitioning within predator communities is a fundamental component of trophic ecology, and one proposed mechanism for how populations partition resources is through individual niche variation. The Niche Variation Hypothesis (NVH) predicts that inter-individual trait variation leads to functional trade-offs in foraging efficiency, resulting in populations composed of individual dietary specialists. The degree to which niche specialization persists within a population is plastic and responsive to fluctuating resource availability. We quantified niche overlap and tested the NVH within an Arctic raptor guild, focusing on three species that employ different foraging strategies: golden eagles (generalists); gyrfalcons (facultative specialists); and rough-legged hawks (specialists). Tundra ecosystems exhibit cyclic populations of arvicoline rodents (lemmings and voles), providing a unique system in which to examine predator diet in response to interannual fluctuations in resource availability. Using blood δ13C and δ15N values from 189 raptor nestlings on Alaska's Seward Peninsula (2014-2019), we calculated isotopic niche width and used Bayesian stable isotope mixing models (BSIMMs) to characterize individual specialization and test the NVH. Nest-level specialization estimated from stable isotopes was strongly correlated with indices of specialization based on camera trap data. We observed a high degree of isotopic niche overlap between the three species and gyrfalcons displayed a positive relationship between individual specialization and population niche width on an interannual basis consistent with the NVH. Our findings suggest plasticity in niche specialization may reduce intra- and inter-specific resource competition under dynamic ecological conditions.

Non-lethal sampling for the stable isotope analysis of the critically endangered European eel Anguilla anguilla: how fin and mucus compare to dorsal muscle.

Ecological studies on the critically endangered European eel Anguilla anguilla often incorporate stable isotope analysis that typically uses dorsal muscle sampled from euthanised eels. To minimise the lethal sampling of imperilled populations, fin tissue and/or epidermal mucus can provide non-lethal alternatives to muscle. The results here indicate that δ13 C and δ15 N values of both eel fin and mucus are not significantly different from those of muscle and can be applied directly in comparative SI studies.