Trophic Shift Research Articles

Whole-lake coarse woody habitat addition facilitates ecosystem regime restructuring in an oligotrophic lake

Preul-Stimetz T, Shaw SL, Sass GG. 2024. Whole-lake coarse woody habitat addition facilitates ecosystem regime restructuring in an oligotrophic lake. Lake Reserv Manag. XXX-XXX. The loss of vital littoral zone and riparian habitat due to lakeshore residential development is a pervasive issue across temperate aquatic ecosystems. Oligotrophic glacial lakes may be particularly vulnerable to reductions in coarse woody habitat (CWH) due to their lack of alternative structural habitat. Although influences of CWH on fish populations have been well documented, questions remain regarding CWH influences on lower trophic levels and water quality. We conducted a whole-lake CWH addition experiment within a BACI framework using 2 lakes, Sanford (treatment) and Escanaba (reference), in the Northern Highland Lake District of Wisconsin. Premanipulation monitoring began in 2015, trees were dropped in 2018, and data collection continued until 2023. We monitored limnological, planktonic, and benthic macroinvertebrate parameters to test whether CWH addition initiated trophic shifts and altered the structure of the aquatic food web. We found that CWH addition minimally affected water quality and changed some characteristics of Sanford Lake at lower trophic levels. Chlorophyll a concentrations in Sanford Lake significantly increased but Secchi depth was unaffected. The abundance of macrophytes increased in both systems; that in Sanford Lake increased more and was linked to the CWH addition. Dissolved oxygen significantly increased at multiple depth strata. Water temperature was unaffected. We observed declines in total zooplankton that were not related to CWH addition, but the decline in relative abundance of large grazers was. Our results provide a mechanistic understanding of the effects of habitat enhancement at an ecosystem scale and can help guide stakeholder and manager expectations following CWH addition.

Ontogenetic, Sexual, and Monthly Niche Segregation of Sepia esculenta in the Northern East China Sea Revealed by Stable Carbon and Nitrogen Isotopes.

Golden cuttlefish play a significant role in the food web of the East and Yellow Seas and are a valuable fishery resource in Chinese coastal waters. Samples of golden cuttlefish were obtained from the northern East China Sea between September 2021 and March 2022, and stable isotope methods were utilized in this study to examine the variations in the forage ecology of golden cuttlefish. Our findings reveal dynamic shifts in carbon and nitrogen stable isotopes (δ13C and δ15N), highlighting intricate foraging strategies tailored to growth and environmental changes. A notable trend emerges: an initial growth-linked rise in δ13C and δ15N enrichment, followed by seasonal fluctuations mirroring seasonal food availability. The ontogenetic niche evolution displays striking habitat shifts and trophic level escalation in small mantle length stages, transitioning to niche overlap and subtle trophic shifts later on. Sex-specific differences emerge, with females occupying higher trophic levels than males in most samples. This comprehensive study underscores the complexity and adaptability of golden cuttlefish feeding ecology, inviting further inquiry into their intricate relationships within the marine ecosystem.

Food chain without giants: modelling the trophic impact of bowhead whaling on little auk populations in the Atlantic Arctic.

In the Atlantic Arctic, bowhead whales (Balaena mysticetus) were nearly exterminated by European whalers between the seventeenth and nineteenth centuries. The collapse of the East Greenland-Svalbard-Barents Sea population, from an estimated 50 000 to a few hundred individuals, drastically reduced predation on mesozooplankton. Here, we tested the hypothesis that this event strongly favoured the demography of the little auk (Alle alle), a zooplanktivorous feeder competitor of bowhead whales and the most abundant seabird in the Arctic. To estimate the effect of bowhead whaling on little auk abundance, we modelled the trophic niche overlap between the two species using deterministic simulations of mesozooplankton spatial distribution. We estimated that bowhead whaling could have led to a 70% increase in northeast Atlantic Arctic little auk populations, from 2.8 to 4.8 million breeding pairs. While corresponding to a major population increase, this is far less than predicted by previous studies. Our study illustrates how a trophic shift can result from the near extirpation of a marine megafauna species, and the methodological framework we developed opens up new opportunities for marine trophic modelling.

Flagellar stator genes control a trophic shift from obligate to facultative predation and biofilm formation in a bacterial predator.

The ability of bacteria to form biofilms is a central research theme in biology, medicine, and the environment. We show that cultures of the obligate (host-dependent) "solitary" predatory bacterium Bdellovibrio bacteriovorus, which cannot replicate without prey, can use various genetic routes to spontaneously yield host-independent (H-I) variants that grow axenically (as a single species, in the absence of prey) and exhibit various surface attachment phenotypes, including biofilm formation. These routes include single mutations in flagellar stator genes that affect biofilm formation, provoke motor instability and large motility defects, and disrupt cyclic-di-GMP intracellular signaling. H-I strains also exhibit reduced predatory efficiency in suspension but high efficiency in prey biofilms. These changes override the requirements for prey, enabling a shift from obligate to facultative predation, with potential consequences on community dynamics.

Trophic effects of jellyfish blooms on fish populations in ecosystems of the coastal waters of China

Jellyfish play an important role in the material cycling and energy flow of food webs, and massive aggregations may have deleterious consequences for local fisheries; yet a theoretical framework of the trophic effects of jellyfish blooms on coastal fisheries is unclear. To address this knowledge gap, we assessed the trophic interactions between cooccurring bloom jellyfish and dominant fish groups (omnivorous fish and piscivorous fish) in the coastal waters of China (CWC) via stable isotope analysis; we subsequently discussed how jellyfish blooms may affect energy flow through coastal ecosystems. Our results indicate a considerable degree of trophic overlap (mean ratio > 65 %) between jellyfish and small omnivorous fish (< 10 cm), highlighting a similarity in feeding habits, while the overlap ratio decreased to <55 % of the large omnivorous fish group (> 10 cm). Relatively higher trophic levels and smaller overlaps of large omnivorous fish were found in the ecosystem with high jellyfish biomass, which suggested that they may reinforce the ontogenetic trophic shift pattern to alleviate the potential for resource competition with jellyfish under conditions of jellyfish explosion. The smallest trophic overlap (< 20 %) highlighted the strong trophic differentiation between jellyfish and piscivorous fish. Additionally, our study suggested that a massive aggregation of jellyfish can negatively influence zooplankton but may not transfer energy further up efficiently, implying a weak trophic coupling between jellyfish and upper-trophic levels in CWC ecosystems. Thus, we speculate that jellyfish play an important role in shaping pathways involved in the energy transfer of food webs and that large blooms may negatively affect fisheries through bottom-up control affecting prey availability. In general, these results hold strong potential to further improve the understanding of the trophic interactions between jellyfish and fish populations. Furthermore, this study provides valuable data for predicting the consequences of jellyfish blooms on ecosystems, and is crucial for ecosystem-based management of coastal fisheries.

Can biochemical tracers reveal ontogenetic trophic shift and individual prey selection in white sharks from Guadalupe Island, Northeast Pacific?

Refining the role of apex predators in marine food webs is a necessary step in predicting the consequences of their global decline under the footprint of fishing activities. White sharks (Carcharodon carcharias) are vulnerable predators, performing large migrations and able to forage on a variety of prey in different habitats. In the Northeast Pacific, juvenile and adult white sharks are found seasonally at the same aggregation sites, such as Guadalupe Island off Mexico. While adults are thought to target local pinniped colonies, very few predator-prey interactions have been documented and the diet of juveniles in this area remains poorly understood. Here we used carbon/nitrogen stable isotopes and fatty acids to characterize the trophic ecology of white sharks at Guadalupe Island. In contrast to the ontogenetic trophic shift paradigm, we detected no influence of size on muscle stable isotope and fatty acid composition, revealing no significant dietary variation between juvenile and adult sharks. Stable isotopes did not allow definitive conclusions to be drawn regarding the diet of white sharks at Guadalupe Island, due to significant variability in the contribution of different potential prey depending on the trophic discrimination factors used. However, most sharks were rich in polyunsaturated fatty acids (such as long-chain omega 3), suggesting a local diet of mainly pelagic prey (potentially large fish or cephalopods). A few individuals appeared to show recent consumption of pinnipeds, with higher proportions of saturated and monounsaturated fatty acids. These individual differences in fatty acid composition could reflect an ecological trade-off between consumption of prey rich in fat (marine mammals) versus prey rich in polyunsaturated fatty acids (pelagic prey), respectively meeting the energetic and physiological needs of white sharks. Although ontogenetic trophic changes were not able to be discerned, our results provide new insights into the physiological drivers of predator-prey interactions, which can benefit the definition of conservation strategies in a changing ocean.

Morphometric variations of two patellid limpets between artificial breakwaters and natural reefs

Artificial shorelines often differ from natural reefs in shape, composition, and habitat complexity. They promote higher accumulation of pollutants and increased physiological stress, which lead to changes in species distributions and ecosystem functioning. This can promote trophic shifts and reduced genetic diversity of gastropod populations inhabiting artificial shorelines and might promote morphological changes, which may influence behaviour, vulnerability to predation, feeding efficiency, sex ratios, reproductive development, and overall fitness. This study explores inter- and intraspecific shell morphometric variations in Patella rustica and P. caerulea between breakwaters (ripraps) and natural reefs in three sites of Ceuta (North Africa, Spain), including a physicochemical assessment of the studied locations: shore orientation, inclination and wave exposure, and substratum nature, heterogeneity and roughness. Limpets on artificial substrata had a slightly smaller extra-visceral cavity and flattened shell profile, which suggests that the desiccation and temperature stress driven by the lithological composition and roughness of the studied substrata might have influenced the registered shell shape variation. However, the high morphometric variation registered for both species across sites and substrata suggests that limpets' morphology might be responding to a complex interaction of environmental and ecological factors rather than solely rock type. The findings of this study highlight the complex interplay between rock type and local environmental conditions in shaping patellid limpets’ shell morphology and provide insights into the adaptive mechanisms that drive shell shape variation in limpet populations in the face of coastal sprawl and anthropogenic global change.

Diet analysis of Père David's deer (Elaphurus davidianus) based on stable isotope analysis

Supplemental feed for free‐ranging deer is often advocated by managers to compensate for winter survival. However, providing extra food has great costs so it is important to know how essential supplemental feed is for deer populations. In our study, supplemental feed is supplied year round for free‐ranging Père David's deer Elaphurus davidianus since early 2019 to help them survive food shortages in the Jiangsu Dafeng Milu National Nature Reserve. To understand use of supplemental feed, we quantified the biomass contributions of C3 plants, C4 plants and supplemental feed consumed by free‐ranging deer by assessing their faecal carbon and nitrogen stable isotope values in May, July, October and December. The diet–faeces trophic shift of deer was assessed using semi‐captive animals. Faeces in both areas were collected and faecal δ13C values revealed more variable diets of free‐ranging deer than semi‐captive deer in different months. MixSIAR model showed that C4 plants contributed greatly to deer diet biomass (75.1, 82, 64.3, 79.4%, respectively in May, July, October and December) compared with C3 plants and supplemental feed. We suggest that managers should consider the efficiency of long‐term supplemental feed and consider the selection of C3 and C4 plants during restoration.

Anchovy boom and bust linked to trophic shifts in larval diet

Although massive biomass fluctuations of coastal-pelagic fishes are an iconic example of the impacts of climate variability on marine ecosystems, the mechanisms governing these dynamics are often elusive. We construct a 45-year record of nitrogen stable isotopes measured in larvae of Northern Anchovy (Engraulis mordax) in the California Current Ecosystem to assess patterns in food chain length. Larval trophic efficiency associated with a shortened food chain increased larval survival and produced boom periods of high adult biomass. In contrast, when larval food chain length increased, and energy transfer efficiency decreased, the population crashed. We propose the Trophic Efficiency in Early Life (TEEL) hypothesis, which states that larval fishes must consume prey that confer sufficient energy for survival, to help explain natural boom-bust dynamics of coastal pelagic fishes. Our findings illustrate a potential for trophic indicators to generally inform larval survival and adult population dynamics of coastal-pelagic fishes.

Collagen Contents of 13С and 15N Isotopes in Bones of Small Cave Bear Ursus (Spelaearctos) rossicus Borissiak, 1930 (Mammalia, Carnivora, Ursidae) from Western Siberia.

The 13C and 15N isotope contents in collagen were determined in bones of the small cave bear Ursus (Spelaearctos) rossicus Borissiak, 1930 from three regions of Western Siberia. The bones dated back to marine isotope stage (MIS) 3 and belonged to mature males and females. Some of the samples differed only in δ15N. Bears of all samples were from the same trophic level. Trophic shifts were observed between females and males in one region and between males of two regions. The small cave bear of Western Siberia differed from the small cave bear of the Southern Urals in leading a much more predatory lifestyle. The extent of differences between the Ural and Siberian bears corresponded to different trophic levels.

The oceanic pleuston community as a potentially crucial life-cycle pathway for pelagic fish-infecting parasitic worms

Pleustonic organisms form an important part of pelagic ecosystems by contributing to pelagic trophic chains and supporting connectivity between oceanic habitats. This study systematically analysed the trematode community harboured by pleustonic molluscs and cnidarians from offshore Queensland, Australia. Four mollusc and three cnidarian species were collected from beaches of North Stradbroke Island, Queensland. Two mollusc species and all three cnidarians harboured large numbers of hemiuroid metacercariae (Trematoda: Hemiuroidea). Eight taxa from four hemiuroid families (Accacoeliidae, Didymozoidae, Hemiuridae and Sclerodistomidae) were distinguished via molecular sequencing. Four of those taxa were identified to species. All trematode taxa except one didymozoid were shared by two or more host species; five species occurred in both gastropods and cnidarians. It is hypothesised that the life-cycles of these hemiuroids are highly plastic, involving multiple opportunistic pathways of metacercarial transmission to the definitive hosts. Transmission and the use of pleuston by hemiuroids likely varies with sea surface use and ontogenetic trophic shifts of apex predators. The small number of trematode species found in pleuston is consistent with significant ecological specificity, and the inference that other pelagic trematodes use alternative pathways of transmission that do not involve pleustonic organisms. Such pathways may involve i) pelagic hosts exclusively; ii) benthic or demersal hosts exclusively, consumed by apex predators during their dives; or iii) both benthic and pelagic hosts in transmission chains dependent on vertical migrations of prey. The influence of the connectivity of open-ocean ecosystems on parasite transmission is identified as an area in critical need of research.

Artificial water-level fluctuation modulates trophic niches of benthic fish assemblages in one of the world's largest reservoirs

The impoundment of the Yangtze River and construction of the Three Gorges Dam and Reservoir created an unnatural flow regime with a 30 m water-level fluctuation and four artificial seasons that differ from the river's natural flow-regime timing. Using stable isotopes, we selected 15 common benthic fish species representing five trophic guilds to analyze seasonal variation in production sources, trophic niche width, and niche overlap. We hypothesized that, similarly to a natural flow regime, artificial hydrological seasons effectively influence fish trophic niches. We found consistency in seasonal shifts in the contribution of aquatic and terrestrial producers to fish biomass. δ13C and δ15N depicted seasonal changes in intra-guild trophic niche width and intra- and inter-guild niche overlap. Considering that the Three Gorges Reservoir's flow regime is artificially regulated, our findings suggest that water-level fluctuation is one of the key mechanisms driving such trophic shifts and allowing multiple species to coexist. Given the increased cascaded large reservoir construction and the concomitant creation of unnatural flow regimes in large rivers worldwide, these findings are timely. Our findings contribute to improving water-level management by optimizing rhythmicity while balancing flood control and fish protection.

Sources of intraspecific variation in the isotopic niche of a semi-aquatic predator in a human-modified landscape.

Intraspecific variation modulates patterns of resource use by species, potentially affecting the structure and stability of food webs. In human-modified landscapes, habitat disturbance modifies trophic interactions and intraspecific niche variation, impacting population persistence. Here, we investigated the relationship of sex, ontogeny, and habitat factors with the trophic niche of Caiman crocodilus in an agricultural landscape. We evaluated temporal variation in the trophic niche parameters using carbon and nitrogen stable isotope analysis from different body tissues. We found that caimans exploit the same carbon and nitrogen pools through time, with low isotopic variability between seasons, partly due to the slow isotope turnover rates of tissues in crocodilians. Conversely, the trophic niche of caimans varied across habitats, but with no evidence of a difference between natural and anthropogenic habitats. It apparently results from the influence of habitat suitability, connectivity, and caiman movements during the foraging. Our findings highlight the broader niches of juvenile caimans relative to adults, possibly in response of territorialism and opportunistic foraging strategy. Although using similar resources, females had a larger niche than males, probably associated with foraging strategies during nesting. Considering the sex and body size categories, caimans occupied distinct isotopic regions in some habitats, indicating apparent niche segregation. Ontogenetic trophic shifts in the isotopes (δ13C and δ15N) depended on sex, leading to resource partitioning that can potentially reduce intraspecific competition. Decision-makers and stakeholders should consider the trophic dynamics of sex and body size groups for the sustainable management and conservation of caiman populations, which implies in the maintenance of wetland habitats and landscape heterogeneity in the Formoso River floodplain.

Contrasting intra-individual variation in size-based trophic and habitat shifts for two coastal Arctic fish species

Within and among species variation in trophic and habitat shifts with body size can indicate the potential adaptive capacity of species to ecosystem change. In Arctic coastal ecosystems, which experience dramatic seasonal shifts and are undergoing rapid change, quantifying the trophic flexibility of coastal fishes with different migratory tactics has received limited attention. We examined the relationships among body length and condition (Fulton’s K, phase angle from Bioelectrical Impedance Analysis) with trophic and habitat shifts (differences in δ15N and δ13C between blood tissues with different turnover rates) of two abundant and culturally important species, anadromous Arctic char (Salvelinus alpinus, n = 38) and sedentary Greenland cod (Gadus ogac, n = 65) during summer in coastal marine waters near Ulukhaktok, Northwest Territories, Canada. Habitat shifts (δ13C) increased with length (i.e., pelagic to benthic-littoral) and crossed-equilibrium (zero) at mid-sizes for both species. Seasonal trophic shifts (δ15N) were generally positive (i.e., increasing trophic level) for Arctic char and negative for Greenland cod. As hypothesised, intra-individual variation in size-based trophic shifts (δ15N-length residuals) increased with length for Arctic char. However, there were no trends with length in Greenland cod. Our findings highlight the importance of flexibility through ontogeny and mobility for Arctic char, whereas Greenland cod were generalist to localized prey and habitat across all sizes. The significant effect of body condition (phase angle) on size-based trophic shifts in Arctic char, and size-based habitat shifts in Greenland cod, highlight the potential trade-offs of contrasting life history strategies and capacity for ontogenetic niche plasticity.

Zooplankton biomass, size structure, and associated metabolic fluxes with focus on its roles at the chlorophyll maximum layer during the plankton-contaminant MERITE-HIPPOCAMPE cruise

Recent studies have demonstrated that plankton can be a key pathway for the uptake and transfer of contaminants entering the marine environment up to top predators.The plankton-contaminant MERITE-HIPPOCAMPE cruise was devoted to quantifying contaminants in water and the whole plankton size range (10 size fractions) at 10 stations along a north-south transect in the western Mediterranean Sea from the French to the Tunisian coasts through the Provençal and Algerian basins. Pumping and filtering devices and net sampling have been used for collecting very high amounts of small particles and planktonic organisms in the chlorophyll maximum layer (CML). The present paper characterizes the zooplankton components for which the contaminant measurements were carried out. At each station, a horizontal towed Hydro-Bios net with a 60 μm mesh-size net was used to discriminate 5 size-fractions from 60 μm to a few mm. For each size-fraction, one part of the sample was used for dry weight measurements and the other one for estimating the contribution to biomass of detritus, phytoplankton, and among zooplankton of the major taxonomic groups based on the imagery tools ZOOSCAN and FLOWCAM. In each zooplankton size fraction, metabolic rates were calculated from the size spectrum to estimate trophic and excretion fluxes flowing through this fraction. These observations were compared to a similar analysis of tows in the upper layer (vertical) and the surface layer (horizontal).The total sampled biomass concentration at the CML was higher than in the water column (COL) and much higher than at the surface (SURF) in most of the stations, but in the CML and COL a substantial contribution was due to detritus mostly concentrated in the smallest size-fractions (60–200 μm and 200–500 μm). Absolute values of zooplankton biomass show neither a clear spatial pattern nor a significant difference between strata. The CML layer was dominated by copepods similarly to COL and SURF, but presented a higher contribution of nauplii and a near absence of appendicularians. At some stations, crustaceans and gelatinous plankton could be important contributors to CML. The zooplankton biomass composition of the two smallest fractions (<500 μm) was dominated by nauplii, small copepods and, occasionally, by small miscellaneous organisms (mostly pteropodes). In contrast, clear differences between stations appeared for the largest fractions (>500 μm) due to large crustaceans, gelatinous organisms, and chaetognaths. These changes in biomass composition according to size fractions suggest a progressive trophic shift from dominant herbivory in the smallest fractions to more contrasted trophic structure (including carnivory) in the largest fractions.The daily carbon demand and the N and P excretion of zooplankton were on average higher at the CML but with no significant difference with COL. The zooplankton grazing represented 2.7 to 22.7 % of the phytoplankton stock per day, whereas its excretion represented a daily N and P recycling compared to dissolved inorganic nitrogen and phosphorus stocks ranging respectively from 0.2 to 19 % and from 0 to 21 %. This information should help in the interpretation of the content of various contaminants in zooplankton fractions.

Evidence of ontogenetic partitioning of restored coastal habitat by a generalist sportfish

Coastal and estuarine habitats that provide crucial nursery areas for many economically and ecologically important fish species are in decline. Restoration of benthic habitats can improve fish populations, biomass, and feeding opportunities, but there is limited research on how restoration impacts growth and survival with ontogeny. To address this knowledge gap, here we examine the biometrics (size, biomass, and body condition), recruitment, size structure, and trophic shifts of a sportfish (mangrove snapper, Lutjanus griseus) at restored oyster reefs and stabilized living shorelines to better understand how fish use restored habitats as they grow. Biomass and body condition of L. griseus juveniles and subadults, and post‐settlement recruitment, at restored/stabilized sites was similar, and in some cases greater than natural sites, correlating with benthic habitat, reef location, and lunar phase at oyster reefs. Living shorelines exhibited greater recruitment potential, while oyster reefs supported more juveniles and subadults, as evidenced by differences in fish size and biomass between habitats. Dietary overlap implies subadult L. griseus likely foraged across habitats more than juveniles, while there was greater diet similarity within habitats. Furthermore, ontogenetic shifts also occurred within oyster reef habitats, highlighting the importance of quality habitat to support various sportfish life stages, which can be achieved through restoration. These findings suggest life history attributes can be indicators of habitat restoration success, and specifically provide actionable science to guide the development of more effective strategies for restoring inshore nursery habitats and thus augment production of offshore reef fisheries.

Trophic Ecology of African Dwarf Crocodiles (Osteolaemus spp.) in Perennial and Ephemeral Aquatic Habitats

Crocodilians occupy diverse aquatic and riparian habitats, and through their movements and ontogenetic niche shifts, link the flow of energy and nutrients between aquatic and terrestrial food webs. We analyzed the trophic ecology of African dwarf crocodiles Osteolaemus tetraspis and O. osborni at one site for each species in Cameroon by analyzing carbon and nitrogen stable isotope ratios. We hypothesized that Osteolaemus species inhabiting a perennial river would be primarily associated with aquatic food webs, whereas those inhabiting a small ephemeral stream would assimilate significant fractions of terrestrial-derived material into their biomass. We also hypothesized that Osteolaemus species would undergo ontogenetic trophic shifts, including changes in vertical trophic position (TP). We found that crocodiles in both systems assimilated material from both terrestrial and aquatic-based food chains, and therefore provide a trophic link between habitats. In the perennial river, aquatic gastropods, fish, and amphibians were estimated to have higher maximum feasible contributions to crocodile biomass than terrestrial invertebrates. Prey contributions to crocodile biomass in the ephemeral stream could not be estimated because model results suggested that at least one important prey had not been sampled. Even though isotopic overlap was high between juveniles and adults, and between sexes, ontogenetic shifts in TP were apparent in both Osteolaemus populations.

Systematic changes and random variations: Understanding lake trout (Salvelinus namaycush) growth dynamics in US waters of Lake Huron

Changes in fish length-at-age can be modeled by adding the age, year-class, and year effects together, or tracking cohort-specific growth with time-dependent parameters. We implemented both approaches to better understand lake trout (Salvelinus namaycush) length-at-age variations in US waters of Lake Huron, 1977–2019. We directly compared length-at-age over time using our linear-mixed model, where the expected length-at-age on logscale was represented by the sum of a fixed age effect, a fixed year-class effect (same for all ages of a cohort), and a random year effect (same for all ages within a year). The age, year-class, and year effects together indicated a general decline in length-at-age between the two time-periods of 1977–1997 and 1998–2019. The random year effect had local temporal trends that differed between the two time-periods, while the pattern of decreases in the fixed year-class effect appeared to reflect changes in growth conditions that were represented eventually by the largest prey-biomass decline during 1995–1996 and the final alewife collapse in 2003. We also predicted annual averages of length-at-age by tracking cohort-specific growth histories. We modified the Ford-Walford plot to illustrate how the growth processes are both cohort-specific and time-dependent. The estimates of year-specific asymptotic length indicated the decline in lake trout growth potential between the two time-periods specified above. Our two models differed fundamentally in their statistical assumptions, but they both fitted the data well and explained the growth decline between the two time-periods, which was also consistent with our understanding of the trophic shift in the ecosystem.

Use of an Arboretum and DNA Barcoding for the Detection and Identification of Leaf-Mining Insects on Alien Woody Plants

Arboreta serve as effective tools for identifying alien insect pests and novel trophic associations. In this study, we used an arboretum in Slovenia to survey woody plants and identify both alien and native leaf miners. The leaves and twigs of 50 woody plant species and their cultivars were examined for characteristic damage. We used an integrative approach that combined identification based on leaf mines and DNA barcoding of the larvae and pupae found in the mines. In total, 62 leaf-mining species were identified, including eight alien species, of which the heliozelid Coptodisca lucifluella (Clemens, 1860) and the agromyzid Cerodontha unisetiorbita Zlobin, 1992 were documented for Slovenia for the first time. Additionally, three presumably native Gracillariidae moths Phyllocnistis labyrinthella (Bjerkander, 1790), P. ramulicola Langmaid &amp; Corley, 2007 and P. saligna (Zeller, 1839) represented the first record for Slovenia. Furthermore, we documented 23 novel-to-science trophic associations, 20 of which involved native insects and alien woody plants, primarily from Asia. This study highlights the importance of arboreta and botanical gardens for the interception of invasive alien insects and the early detection of trophic shifts of native insects to alien plants, which can aid in predicting their potential spread.

Grazing intensity drives a trophic shift in the diet of common alpine birds

Large mammalian herbivores (LMH) shape vertebrate communities and structure food webs in many terrestrial ecosystems. However, the mechanisms that underlie the effects of LMH on other vertebrates are poorly understood. In France, domestic LMH have grazed alpine and mountain grasslands for thousands of years, profoundly influencing landscapes and wildlife. As LMH modify habitat structure, favour coprophagous insects and compete with herbivorous insects, the diet of insectivorous alpine birds may be deeply influenced by LMH grazing intensity. To investigate this, we sampled common insectivorous birds faeces (water pipit Anthus spinoletta and wheatear Oenanthe oenanthe) in sites characterized by different levels of grazing intensity by domestic and wild herbivores, in the open landscapes of the southern French Alps and the western Pyrenees. We used isotopic discrimination based on nitrogen stable isotope ratios as an indicator of the trophic level of the arthropods found in bird faeces. From isotopic ratios, we found that bird diets shift from mainly herbivorous arthropods at sites of low grazing intensity, to one mainly composed of other groups, such as predatory, detritivorous or coprophagous arthropods, at sites of higher grazing intensity. This result highlights the strong trophic link between LMH and insectivorous birds in open landscapes, contributing to a better understanding of interactions between domestic grazing, an activity that has historically modelled European landscapes, and biodiversity, with particular relevance for landscape management and bird conservation.