Relative Trophic Position Research Articles

Just Hitching a Ride: Stable Isotopes Reveal Non-Feeding Behaviour of Anisakis simplex Within Its Host Fish.

Anisakis simplex larvae, commonly found in marine fish, cause anisakiasis in humans, resulting in gastric to gastro-allergic symptoms. Despite known health risks, the impact of Anisakidae larvae on fish hosts is less understood. This study aimed to investigate this interaction by assessing the feeding strategy of A. simplex. Anisakis larvae were isolated from North Sea Merluccius merluccius tissues (stomach, body cavity, liver and muscle) and were analysed for carbon (δ13C) and nitrogen (δ15N) isotope values. Significant differences in δ13C values were found among host tissues, with the liver differing from muscle and stomach tissues. In contrast, no differences were noted for the associated parasites. Additionally, δ15N values indicated that the host occupied a significantly higher relative trophic position than its parasite. This suggests a lack of direct nutrient transfer from host to parasite, as the parasite would typically exhibit higher stable isotope values than the tissue they feed on. Therefore, A. simplex's stable isotope values might reflect those of its previous host (crustacean and/or small fish), providing insights into diet and movement of the paratenic M. merluccius host. Further research is needed to confirm these findings across different fish species and to explore A. simplex as a proxy for trophic ecology.

Ecological interactions and unique resource partitioning between dolphins in the ultraoligotrophic eastern Mediterranean Sea

This study delves into the eco-dynamics of three dolphin species in the ultra-oligotrophic waters off the southern Israeli Mediterranean coast - two neritic: the common bottlenose dolphin (Tursiops truncatus) and the common dolphin (Delphinus delphis) and one pelagic: the striped dolphin (Stenella coeruleoalba). It utilizes compound-specific stable isotope analysis of individual amino acids to investigate carbon and nitrogen source variability and trophic positioning among the three species. Muscle samples from stranded individuals were analyzed for carbon (δ13C) and nitrogen (δ15N) isotopic ratios of amino acids, with Δδ15N (Glutamate-Phenylalanine) acting as an indicator of relative trophic position. The findings reveal minor differences in carbon source signatures and trophic position among species, but distinct nitrogen source signatures. The latter indicate discrete foraging habitats for bottlenose and striped dolphins and suggest that common dolphins transition between shallow benthic and deep pelagic feeding areas during the day and night, respectively, as part of their survival strategy.

Comparative analysis of trophic niche using stable isotopes provides insight into resource use of giant pandas.

Trophic niche is the result of the long-term evolution of species and can reflect the pressures they experience in habitats. The whole-genome sequencing of giant pandas (Ailuropoda melanoleuca) has indicated that populations distributed in Qinling (QIN) and non-QIN probably diverged 300 ky ago. Although many studies regarding foraging strategy, habitat preference, and niche partition have been conducted on these populations, there is still a lack of precise quantification of trophic niches. Here, we calculated and compared isotopic trophic niche widths of giant pandas from Sichuan (SC) and QIN populations by measuring carbon and nitrogen isotopes of their hairs; combined with data from sympatric mammals, we explored the relative trophic positions of giant pandas in the ecosystem, respectively. The Stable Isotope Bayesian Ellipses in R (SIBER) model results showed the trophic niche width of QIN pandas was 3.44‰2 , which was significantly bigger than those of the SC population (2.03‰2 ), with an overlapping about 1.45‰2 ; and they both occupied a unique position in the context, almost one trophic level lower than herbivores. Then, we determined the isotopic ratios of the main foods from the habitats of these pandas; the results suggested that the isotopic difference between bamboo shoots and other parts plus the various feeding selections of pandas on them accounted for pandas' trophic niche widths. We considered the higher nutrition availability and digestible food resources giving QIN pandas a wider trophic niche than pandas from SC. This conclusion provides a new insight into the resource use and trophic ecology of giant pandas and is important to develop refined management plans for the two populations.

Global investigation of lake habitat coupling by fishes.

Habitat coupling, where consumers acquire resources from different habitats, plays an important role in ecosystem functioning. In this study, we provide a global investigation of lake habitat coupling by freshwater fishes between littoral (nearshore) and pelagic (open water) zones and elucidate the extent to which magnitude of coupling varies according to environmental context and consumer traits. We consider the influence of lake factors (surface area, depth, shoreline complexity, and annual temperature), relative trophic position of consumers, fish community species richness, and fish morphological traits on habitat coupling by fishes. Using a worldwide dataset consisting of fish stable isotope values (δ13C and δ15N), we developed an index of habitat coupling, and used Bayesian hierarchical and non-hierarchical beta regressions to estimate the effects of environmental lake context and morphological traits on habitat coupling by fishes. Our results show high rates of habitat coupling among fishes globally with marked taxonomic differences in the magnitude and variation. Habitat coupling was higher in lower elevation lakes and in regions characterized by relatively colder climates, whereas other environmental context factors had little or no effects on habitat coupling. Furthermore, habitat coupling was associated with several locomotion and feeding traits, but independent from species maximum body length. Overall, we highlight the prevalence of multiple resources supporting fish populations and suggest future research identify implications to ecosystem functioning that may result from alterations to habitat coupling by fishes.

Nile tilapia (Oreochromisniloticus) invasion impacts trophic position and resource use of commercially harvested piscivorous fishes in a large subtropical river

BackgroundAlthough freshwater ecosystems cover less than 1% of the earth's surface, they support extremely high levels of biodiversity and provide vital ecosystem services. However, due to the introduction of non-native fishes, aquatic ecosystem functioning has been altered, and in some cases, declined sharply. Quantifying the impacts of invasive species has proven problematic. In this study, we examined the relative trophic position of native piscivorous fishes to estimate the effects of invasive Nile tilapia on food webs in the downstream sections of an invaded large subtropical river, the Pearl River, China. Furthermore, we quantified how native piscivorous fish diets changed as the Nile tilapia invasion progressed.ResultsThe trophic position of the widely distributed and locally important economically harvested piscivorous culter fish (Culterrecurviceps), mandarinfish (Sinipercakneri), and catfish (Pelteobagrusfulvidraco) lowered significantly in the invaded Dongjiang River compared to an uninvaded reference Beijiang River. The lower trophic position of these piscivorous fishes was reflected by a major reduction in the proportion of prey fish biomass in their diets following the Nile tilapia invasion. Small fishes in the diet of culter fish from the reference river (33% small fishes, 17% zooplankton) shifted to lower trophic level zooplankton prey in the invaded river (36% zooplankton, 25% small fish), possibly due to the presence of Nile tilapia. Additionally, small fishes in the diet of mandarinfish in the reference river (46% small fishes, 11% aquatic insects) declined in the invaded river (20% aquatic insects, 30% small fishes). Similarly, the diet of catfish from the reference river shifted from fish eggs (25% fish eggs, 25% aquatic insects) to aquatic insects in the invaded river (44% aquatic insects, 5% fish eggs).ConclusionsThe results of this study contributed to a growing body of evidence, suggesting that Nile tilapia can modify trophic interactions in invaded ecosystems. It is crucial to understand the processes outlined in this study in order to better assess non-native aquatic species, conserve the stability of freshwater ecosystems, and improve current conservation strategies in reaches of the Pearl River and other similar rivers that have experienced invasions of non-native species.

Evaluating impacts of non‐native submerged aquatic vegetation on native nekton

AbstractThe introduction and spread of non‐native species restructure native ecosystems and can be particularly impactful when invaders are ecosystem engineers or habitat‐forming species. In coastal, estuarine, and marine systems, submerged aquatic vegetation (SAV), like macroalgae and seagrasses, form key habitats for nekton, serving as nurseries, foraging grounds, and reproduction sites. If non‐native ecosystem engineers can provide sufficient structure and/or resources, they may exert a neutral or positive effect on organisms occupying higher trophic positions. As such, we hypothesized that nekton response to non‐native SAV species may be neutral or positive. We performed a quantitative meta‐analysis to quantify impacts of non‐native SAV on native crabs, fishes, and shrimps. We extracted data from 35 studies and evaluated 11 response metrics related to facilitation (e.g., habitat use and foraging), restricting our analysis to studies that compared at least one of these metrics in nekton from co‐occurring native and non‐native SAV habitats in marine, coastal, or estuarine systems. We found that nekton abundance, species richness, and biomass were the most assessed metrics of nekton performance. Our pooled data revealed differential results among response metrics, with nekton growth and reproduction enhanced in non‐native habitats and species richness enhanced in a native setting. The mean effect sizes for all other nekton response metrics, including abundance, had 95% CIs that overlapped zero, indicating no difference in response between native and non‐native SAV. For many endpoints, limited sample sizes prevented robust inferences, but they also highlighted areas where more research is needed in future studies. Non‐native species have the potential to restructure the systems they invade. Our results lend support to the relative trophic position hypothesis, indicating that non‐native habitat formers may facilitate native organisms in higher trophic levels. We identify research gaps that may guide future studies and allow for a more comprehensive understanding of responses to non‐native ecosystem engineers.

Testing the predictive value of functional traits in diverse ant communities.

Associating morphological features with ecological traits is essential for understanding the connection between organisms and their roles in the environment. If applied successfully, functional trait approaches link form and function in an organism. However, functional trait data not associated with natural history information provide an incomplete picture of an organism's role in the ecosystem. Using data on the relative trophic position of 592 ant (Formicidae) samples comprising 393 species from 11 subfamilies and 19 widely distributed communities, we tested the extent to which commonly used functional proxies (i.e., morphometric traits) predict diet/trophic position as estimated from stable isotopes (δ15N). We chose ants as a group due to their ubiquity and abundance, as well as the wealth of available data on species traits and trophic levels. We measured 12 traits that have previously been identified as functionally significant, and corrected trait values for size and evolutionary history by using phylogenetically corrected trait residuals. Estimated trophic positions varied from 0.9 to 4.8 or roughly 4 trophic levels. Morphological data spanned nearly the entire size range seen in ants from the smallest (e.g., Strumigenys mitis total length 1.1 mm) to the largest species (e.g., Dinoponera australis total length 28.3 mm). We found overall body size, relative eye position, and scape length to be informative for predicting diet/trophic position in these communities, albeit with relatively weak predictive values. Specifically, trophic position was negatively correlated with body size and positively correlated with sensory traits (higher eye position and scape length). Our results suggest that functional trait-based approaches can be informative but should be used with caution unless clear links between form and function have been established.

Stomach content and stable isotopes illustrate large spatial similarity in the Wadden Sea fish food-web structure

Spatial variability in the Wadden Sea fish food-web structure was studied by comparing stomach content and bulk stable isotopes of fish species caught simultaneously in the Ems and Marsdiep basins during 2012-2014. Almost all 31 fish species caught were generalist feeders. In both basins, similar predator-prey relationships were found in which a few key prey species fuelled the fish food web. Copepods and brown shrimp were the most important prey species in both basins, mysid shrimp were more important as prey in the Ems basin, while shore crab and herring were more important prey species in the Marsdiep basin. The observed spatial variability in prey preferences was most likely the result of local differences in predator and prey abundances. Published absolute trophic positions based on compound-specific stable isotopes were available for some fish species and indicated low variability between the basins. Estimated absolute trophic positions based on stomach content and on bulk stable isotopes could not be used for the analysis of spatial variability due to sensitivity to sampling procedure (stomach content) and sampling size and baseline (bulk stable isotopes). Although estimates based on bulk stable isotopes underestimated absolute trophic levels in both basins, they can be used for the analysis of relative trophic positions of fish species. Relative trophic positions showed a significant correlation for most fish between the Ems and Marsdiep basins, also indicating a large spatial similarity in trophic structure.

Interpreting past trophic ecology of a threatened alpine parrot, kea Nestor notabilis, from museum specimens.

When ecosystems are under severe pressure or environments change, trophic position and intraspecific niche width may decrease or narrow, signalling that conservation action is required. In New Zealand, alpine and subalpine ecosystems have been extensively modified through farming since 19th-century European settlement, with consequences for indigenous species such as the kea Nestor notabilis. We investigated feather stable isotope values in the kea and predicted a lower trophic position in modern kea populations, to reflect reduced lowland habitat and a mixed diet with more plant material. We predicted that size and sex would influence trophic values in this sexually dimorphic species, with larger birds more likely to have a high protein diet. We examined potential dietary changes in 68 museum collected kea from 1880s to 2000s, first recording accession details including provenance and sex and measuring culmen length. We used bulk carbon and nitrogen stable isotope analyses (BSIAs) of feathers and a further feather subset using compound-specific stable isotope analyses of amino acids (CSIA-AA) to obtain isotopic values and estimate trophic position. BSIA showed δ15 N values in kea feathers declined through time and could indicate that early century kea were highly omnivorous, with δ15 N values on average higher than in modern kea. Variance in δ15 N values was greater after 1950, driven by a few individuals. Few differences between males and females were evident, although females in the south region had lower δ15 N values. There was a tendency for large male birds to have higher trophic values, perhaps reflecting dominant male bird behaviour noted in historical records. Nonetheless, CSIA-AA performed on a subset of the data suggested that variation in BSIA is likely due to baseline changes rather than relative trophic position which may be more homogenous than these data indicate. Although there was more variability in modern kea, we suggest caution in interpretation. Stable isotope data, particularly CSIA-AA, from museum specimens can reveal potential change in ecological networks as well as sexually dimorphic feeding patterns within species. The data can reveal temporal and regional variation in species trophic position and changes in ecosystem integrity to inform conservation decision-making.

Phoretic sharksuckers Echeneis naucrates associated with an elasmobranch host occupy higher relative trophic positions

The relationship between phoretic diskfishes and their hosts is a classic example of marine symbiosis, yet surprisingly few studies have quantified this trophic relationship. We investigated the hypothesis that by consuming host parasites and prey scraps phoretic diskfishes (Echeneidae) feed at a higher relative trophic position than free-living individuals through expanded foraging opportunities. We used carbon and nitrogen stable isotope analysis of muscle tissue from both free-living and commensal sharksuckers Echeneis naucrates and their hosts, supplemented with gut-content analysis, to investigate this hypothesis. Our analysis revealed that commensal sharksuckers likely occupy higher relative trophic positions than free-living sharksuckers. The importance of scavenging host prey decreased ontogenetically as sharksuckers shifted from symbiotic phoresis to free-swimming behavior, leading to an ontogenetic change in which obligately commensal juveniles occupy higher trophic positions than free-living adults which are only facultatively commensal. Relative differences in δ13C and δ15N among individual host-commensal pairs varied among host species, suggesting potential differences in foraging opportunities among host taxa.

Diet reveals potential for competition and coexistence among coyotes (Canis latrans), red foxes (Vulpes vulpes), and gray foxes (Urocyon cinereoargenteus)

Species can alleviate competition by reducing diet overlap. Non-native coyotes (Canis latrans Say, 1823) and historically native gray foxes (Urocyon cinereoargenteus (Schreber, 1775)) have expanded their ranges and may compete with native red foxes (Vulpes vulpes (Linnaeus, 1758)). To examine potential competition among canids in Maine, USA, we compared δ13C and δ15N from muscle and hair samples to assess relative resource use, and we compared frequency of occurrence of prey items from stomach contents to assess diets. For these species, red foxes consumed anthropogenically based foods the most in fall and early winter, gray foxes consumed anthropogenically based foods the most in summer, and coyotes consumed anthropogenically based foods the least in all seasons. Coyotes held the highest relative trophic position in fall and early winter, red foxes held the highest relative trophic position in summer, and gray foxes held the lowest relative trophic position. Based on stomach contents, gray foxes had the broadest diet and consumed the most plants, and coyotes had the narrowest diet. Red foxes were the only species to show isotopic niche overlap with both potential competitors across seasons. Thus, they may be most susceptible to competitive exclusion among these canids, with implications for community dynamics as ranges shift due to human activity.

Compound-specific isotope analyses of harp seal teeth: tools for trophic ecology reconstruction

As sentinels of ecosystem health, high trophic level predators integrate information through all levels of the food web. Their tissues can be used to investigate spatiotemporal variability in foraging behaviour, and with the appropriate analytical methods and tools, archived samples can be used to reconstruct past trophic interactions. Harp seal Pagophilus groenlandicus teeth collected in the 1990s from the Northwest Atlantic were analysed for bulk stable carbon and nitrogen isotopes (δ13Cbulk and δ15Nbulk), and compound-specific stable nitrogen isotopes of amino acids (δ15NAA) for the first time. We developed a fine-scale, annual growth layer group (GLG) dentine sub-sampling method corresponding to the second and third year of life. In accordance with previous diet studies, while there was individual variability in δ15Nbulk, δ13Cbulk and δ15NAA measurements, we did not detect significant differences in isotopic niche widths between males and females, or between GLGs. Relative trophic position was calculated as the baseline-corrected δ15NAA values using trophic (glutamic acid) and source (phenylalanine and glycine) amino acids. Variability was measured between individuals in their relative trophic position, but within-individual variability was low, suggesting that they fed at the same trophic level over these 2 yr of life. These novel δ15NAA data may therefore suggest individual, specialist harp seal foraging behaviour in sub-adults. Our findings show that compound-specific stable isotope signatures of archived, inert predator tissues can be used as tools for the retrospective reconstruction of trophic interactions on broad spatiotemporal scales.

Distinguishing zooplankton fecal pellets as a component of the biological pump using compound‐specific isotope analysis of amino acids

AbstractZooplankton contribute a major component of the vertical flux of particulate organic matter to the ocean interior by packaging consumed food and waste into large, dense fecal pellets that sink quickly. Existing methods for quantifying the contribution of fecal pellets to particulate organic matter use either visual identification or lipid biomarkers, but these methods may exclude fecal material that is not morphologically distinct, or may include zooplankton carcasses in addition to fecal pellets. Based on results from seven pairs of wild‐caught zooplankton and their fecal pellets, we assess the ability of compound‐specific isotope analysis of amino acids (CSIA‐AA) to chemically distinguish fecal pellets as an end‐member material within particulate organic matter. Nitrogen CSIA‐AA is an improvement on previous uses of bulk stable isotope ratios, which cannot distinguish between differences in baseline isotope ratios and fractionation due to metabolic processing. We suggest that the relative trophic position of zooplankton and their fecal pellets, as calculated using CSIA‐AA, can provide a metric for estimating the dietary absorption efficiency of zooplankton. Using this metric, the zooplankton examined here had widely ranging dietary absorption efficiencies, where lower dietary absorption may equate to higher proportions of fecal packaging of undigested material. The nitrogen isotope ratios of threonine and alanine statistically distinguished the zooplankton fecal pellets from literature‐derived examples of phytoplankton, zooplankton biomass, and microbially degraded organic matter. We suggest that δ15N values of threonine and alanine could be used in mixing models to quantify the contribution of fecal pellets to particulate organic matter.

Seasonal rainfall affects occurrence of organohalogen contaminants in tropical marine fishes and prawns from Zanzibar, Tanzania

Seasonal differences in precipitation may affect contaminant dynamics in tropical coastal regions due to terrestrial runoff of contaminants to the marine environment after the rainy seasons. To assess the effect of seasonal rainfall on occurrence of organohalogen contaminants in a coastal ecosystem, marine fishes and prawns were collected off the coast of Zanzibar, Tanzania in January and August 2018, representing pre- and post-rainy season, respectively. Samples were analyzed for organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), brominated flame retardants (BFRs), including polybrominated diphenyl ethers (PBDEs) and emerging BFRs, as well as the dietary descriptors stable isotopes of carbon (δ13C) and nitrogen (δ15N). Across all species and seasons, mean contaminant concentrations ranged from below limit of detection (LOD) to 129 ng/g lipid weight (lw) ΣPCBs; 5.6–336 ng/g lw ΣOCPs; and < LOD –22.1 ng/g lw ΣPBDEs. Most of the emerging BFRs were below LOD. Contaminant concentrations generally increased with higher pelagic carbon signal (δ13C) and higher relative trophic position (δ15N). The ratio of DDE/ΣDDTs in fishes and prawns was lower in August than in January, suggesting runoff of non-degraded DDT into the marine system during or after the seasonal rainfall. Contaminant patterns of OCPs and PCBs, and concentrations of BFRs, differed between seasons in all species. A higher relative concentration-increase in lower halogenated, more mobile PCB and PBDE congeners, compared to higher halogenated congeners with lower mobility, between January and August aligns with a signal and effect of terrestrial runoff following the rainy season.

Individual‐level data reveal high prevalence of positive size‐trophic position relationships for vertebrates in temperate streams

Abstract A positive relationship between body size and trophic position is often assumed in ecology, but efforts to confirm the generality of this relationship for freshwater vertebrates have produced mixed results. Some authors have tested for among‐species relationships, using species‐level estimates of average size and trophic position. Others have used individual‐level body size and trophic position data to test for within‐species relationships. However, no study has yet estimated prevalence, defined here as the fraction of total standing stock biomass within a given ecosystem that consists of taxa exhibiting positive size versus trophic position relationships. Individual‐level estimates of body size and relative trophic position (inferred from bulk‐tissue nitrogen stable isotopes) were collected for vertebrates in six temperate streams. In each stream, all locally occurring vertebrate taxa were collected and closed population depletion samples were used to obtain standing stock biomass estimates. Ordinary least squares regression was used with the individual‐level data to test for positive relationships between body size and relative trophic position (r‐STPRs). A separate r‐STPR model was tested for each vertebrate species collected at the six study sites. Linear mixed‐effects modelling was then used to test for differing r‐STPRs among species. Prevalence of the r‐STPR was calculated for each site by summing the standing stock biomass of all taxa that exhibited a statistically significant, positive STPR at a given site, then dividing this number by the total standing stock biomass of vertebrates at the site. Significant, positive r‐STPRs were detected in 15 of 22 species × site regression models. Prevalence of species with positive r‐STPRs ranged from 91 to 100% of the total vertebrate biomass in western streams and from 45 to 66% in eastern streams. Results confirm that positive r‐STPRs are characteristic of many of the vertebrate taxa considered in this study. Furthermore, species that exhibit positive r‐STPRs comprise a clear majority of the standing stock biomass in five of six study streams. By using individual‐level data to account for prevalence, a more complete understanding of size‐dependent trophic dynamics should be possible in freshwater ecosystems.

Fluid-preserved fishes are one solution for assessing historical change in fish trophic level.

There are few resources available for assessing historical change in fish trophic dynamics, but specimens held in natural history collections could serve as this resource. In contemporary trophic ecology studies, trophic and source information can be obtained from compound‐specific stable isotope analysis of amino acids of nitrogen (CSIA‐AA‐N).We subjected whole Sebastes ruberrimus and Clupea pallasii to formalin fixation and 70% ethanol preservation. We extracted tissue samples from each fish pre‐fixation, after each chemical change, and then in doubling time for 32–64 days once placed in the final preservative. All samples were subjected to CSIA‐AA‐N, and their glutamic acid and phenylalanine profiles and associated trophic position were examined for differences over time by species.Glutamic acid and phenylalanine values were inconsistent in direction and magnitude, particularly during formalin fixation, but stabilized similarly (in 70% ethanol) among conspecifics. In some cases, the amino acid values of our final samples were significantly different than our initial pre‐preservation samples. Nonetheless, significant differences in glutamic acid, phenylalanine, and estimated trophic position were not detected among samples that were in 70% ethanol for >24 hr.Our results suggest that the relative trophic position of fluid‐preserved specimens can be estimated using CSIA‐AA‐N, and CSIA‐AA‐N estimates for fluid‐preserved specimens should only be reported as relative differences. Timelines of trophic position change can be developed by comparing specimens collected at different points in time, revealing trophic information of the past and cryptic ecosystem responses.

Earlier and slower or later and faster: Spring migration pace linked to departure time in a Neotropical migrant songbird.

Migratory birds travel vast distances and the timing of migratory flights can affect survival and the ability to reproduce. For Neotropical migrant songbirds, early spring departure from wintering sites, early arrival to the breeding grounds and higher reproductive success have been related to the use of suitable habitats and environmental conditions during the non-breeding season. However, how migratory strategies are shaped by winter habitat choice is largely unknown due to the general inability to track birds from specific wintering habitats to stopovers or breeding destinations. We assessed how winter habitat (native forest vs. shade-grown coffee plantations) relates to spring departure date and migration pace in Swainson's Thrush Catharus ustulatus. We also determined the effect of departure date and total migration duration on the arrival date of birds detected near or within their breeding range. We used a novel application of Motus radiotelemetry arrays to track individuals from their wintering grounds in the Andes of South America along their migratory journey to North America. We found variation in migratory strategies between habitats, with birds wintering in native forest departing later than birds in coffee. We present isotopic evidence for native forest being of higher quality than shade-coffee for Swainson's Thrush and hypothesize that moister conditions in forest, as shown by stable isotope (δ13 C) analysis of thrush whole blood, provides favourable pre-migratory conditions allowing birds to delay departure from wintering grounds. Habitat, between-site and -year variation in departure date, suggests that birds made facultative adjustments to winter habitat quality and environmental conditions. Independent of habitat, birds that departed later migrated faster and this pattern was maintained along the migration route (n=44). Migrating earlier and slower or later and faster was unlikely to result in significant differences in arrival time to breeding destinations. Our findings reveal underappreciated complexity in migratory decisions by long-distance migrants that contrast with the current paradigm of earlier departures and arrival from optimal habitats. The next step is to understand the relative fitness benefits of early versus late schedules or whether each strategy is an equally good response to experienced conditions.

Stable isotope analysis indicates positive effects of river restoration on aquatic-terrestrial linkages

Abstract Hydromorphological river restoration can significantly alter habitat configuration and modify invertebrate assemblages of rivers and floodplains. However, the consequences of these changes for ecosystem functioning and aquatic-terrestrial interactions are not known. As a restored shoreline has a more heterogeneous structure compared to a straightened river, restoration is likely to impact aquatic-terrestrial linkages in multiple ways, which might be captured based on biomarker indicators to characterize changes in food web functioning. We conducted a large scale comparative study targeting eleven river restoration projects in central and northern Europe to assess effects of river restoration on trophic patterns across the aquatic-terrestrial interface. We investigated the isotopic composition (δ13C, δ15N) of prey and of invertebrate consumers stratifying between the aquatic, riparian and terrestrial zones. The isotopic distance of riparian arthropods to instream macroinvertebrates and terrestrial arthropods was used as a measure of trophic linkage, and its variation with riparian habitat composition was quantified. Restoration enhanced aquatic-terrestrial linkages, indicated especially by differentiation in the δ15N isotopic signatures between aquatic, riparian and terrestrial consumers, rather than by δ13C signatures. The δ15N isotopic signatures of riparian arthropods revealed a higher relative trophic position in restored sections (δ15NRestored: 8.64‰, n = 11) as compared to non-restored sections (δ15NDegraded: 8.05‰, n = 11), lending support to the conjecture that restoration increased the proportion of more highly enriched aquatic prey (δ15NRestored: 10.01‰; δ15NDegraded: 10.38‰) while simultaneously reducing the share of lower enriched terrestrial prey (δ15NRestored: 4.88‰; δ15NDegraded: 5.53‰). Riparian habitat diversity and the share of exposed sand and gravel bars were positively related to the strength of aquatic-terrestrial linkages (R2 = 0.28 and R2 = 0.31, respectively), pointing to the importance of habitat diversification in the riparian zone in promoting trophic linkages between river and floodplain. These findings expand our understanding of the multifaceted outcomes of hydromorphological restoration, beyond biodiversity in the aquatic environment. It highlights the need to expand our current set of indicators in order to mechanistic understand restoration effects on ecological networks spanning across boundaries. This knowledge is highly relevant for the large restoration efforts driven by legislative frameworks such as the Water Framework Directive in Europe.

Mercury and selenium concentrations, and selenium:mercury molar ratios in small cetaceans taken off St. Vincent, West Indies

This study measured the concentration of total mercury (THg) and selenium (Se), and calculated the Se:Hg molar ratios in the muscle, blubber, liver, and kidney of small cetaceans (false killer whale, Pseudorca crassidens; killer whale, Orcinus orca; Risso's dolphin, Grampus griseus; short-finned pilot whale, Globicephala macrorhynchus; and dolphins of the genus Stenella) taken for human consumption off St. Vincent, West Indies. Overall, 122 samples were analyzed; mean THg concentrations (μg/g dry weight) were highest in the liver (730), followed by the kidney (274), muscle (76.4), and blubber (4.57). To explain variability in muscle THg concentrations, carbon (δ13C) and nitrogen (δ15N) stable isotope ratios were analyzed to explore differences in dietary carbon source and relative trophic position, respectively, among species. There was no relationship between δ15N and THg concentration, but there was a positive relationship between δ13C and THg concentration. On average for each species, the Se:Hg molar ratios were >1 in blubber and <1 in muscle. All liver samples and the majority of kidney, muscle, and blubber samples exceeded the FAO/WHO human consumption advisory level of 1 μg/g wet weight. Based on our estimations, consuming only 6.6 g of muscle a week would exceed the MeHg provisional tolerable weekly intake of 1.6 μg MeHg/kg body weight/week for a 60 kg person. Given the high THg concentration in these cetaceans and the frequency at which these tissues are consumed, this is a potential human health issue that warrants further investigation.

Trophic status of a non-native crayfish in an oligotrophic lake: bottom-up view of a mixed warmwater and coldwater sport fishery food web

Loffredo JR, Moore BC, Lee RW, Katz SL, Cross BK. 2019. Trophic status of a non-native crayfish in an oligotrophic lake: Bottom-up view of a mixed warmwater and coldwater sport fishery food web. Lake Reserv Manage. 35:396–414.Numerous studies in recent decades have observed variable trophic status for freshwater crayfish among endemic and exotic populations in lentic or lotic habitats. In this study, we used stable isotope analyses (SIA) to explore trophic status of an introduced population of northern crayfish (Faxonius virilis) in Buffalo Lake, Washington. Northern crayfish are an important prey item for introduced warmwater and coldwater fish species in this lentic system and may buffer competitive interactions between largemouth bass (Micropterus salmoides), rainbow trout (Oncorhynchus mykiss), and kokanee (Oncorhynchus nerka). In 2017, the δ13C and δ15N values of liver samples from 208 crayfish informed proportional diet estimates. Crayfish diets were comprised of 8 littoral sources and were examined for cannibalism, temporal, spatial, and ontogenetic trophic shifts. There was no significant spatial or temporal trophic shift detected in this crayfish population. Seasonal δ15N values among all crayfish size classes mirrored seasonal abundance shifts of littoral macroinvertebrates. A multiple-source stable isotope mixing model and relative trophic positions provided evidence for an ontogenetic trophic shift between juvenile and adult crayfish, from a generalist detritivore to a predatory diet dominated by Chironomidae and Ephemeroptera. The trophic status of Buffalo Lake crayfish may affect salmonid management goals if interspecific competition for food between crayfish and sport fish is biologically significant. Our research is an important first step to understanding crayfish trophic status in a lentic food web, and should inform fish stocking regimes and fishery regulations in management efforts to achieve long-term sustainability of this diverse fishery.