Trophic Habitat Research Articles

The Feeding Behaviour of Gall Midge Larvae and Its Implications for Biocontrol of the Giant Reed: Insights from Stable Isotope Analysis.

The gall midge Lasioptera donacis, whose larval stage interferes with the reed's leaf development, is a potential candidate agent for the biological control of Arundo donax. Reed infestation is always associated with the presence of a saprophytic fungus, Arthrinium arundinis, which is believed to provide food for the larvae. Larvae also interact with a parasitic nematode, Tripius gyraloura, which can be considered its natural enemy. To deepen our knowledge of the plant-fungus-insect trophic interactions and to understand the effects of the nematode on midge larval feeding behaviour, we applied stable isotope analysis, one of the most effective methods for investigating animal feeding preferences in various contexts. The results showed that on average the fungus accounted for 65% of the diet of the midge larvae, which however consumed the reed and the fungus in variable proportions depending on reed quality (expressed as the C:N ratio). No differences in feeding behaviour were observed between parasitised and non-parasitised midge larvae, indicating that nematodes have no effect in this regard. Due to its trophic habits, L. donacis could be an effective control agent of A. donax and these results need to be considered when implementing biological control measures.

Seabirds as Biomonitors of Mercury Bioavailability in the Venice Lagoon

Seabirds accumulate mercury (Hg) due to their long-life span together with their high trophic position. A Hg monitoring in Venice's Lagoon using three seabird species occupying different trophic habitat (Thalasseus sandvicensis, Ichthyaetus melanocephalus, and Chroicocephalus ridibundus) confirmed that fledgelings might effectively be used as sentinels of Hg bioavailability. The significant differences in Hg residues in feathers observed among the species highlighted a possible differential exposure due to different diets, with C. ridibundus accumulating more Hg than the other species. Average residues in feathers were not above the threshold associated with adverse effects on birds (5mg kg- 1). Nevertheless, a large part of the C. ridibundus individuals (58%) exceeded the adverse effect level, underlining the need for strengthening Hg monitoring. Seabirds indeed may provide relevant insight on Hg transfer in food webs and a better picture of the hazards to men when bird species forage on species exploited for human consumption.

Isotopic Niche and Trophic Position of the Invasive Portunid Portunus segnis Forskål, (1775) in Elounda Bay (Crete Island, Eastern Mediterranean)

There is a growing recognition that an advanced understanding of the trophic characteristics of an invasive consumer can provide important information on its ecological impact. In recent years, the blue swimming crab Portunus segnis, one of the earliest Lessepsian invaders, has considerably expanded its distribution range in the Mediterranean Sea, yet, its trophic habits in invaded areas remain scarcely investigated. In this study, we used carbon and nitrogen stable isotopes analysis (SIA) to determine the trophic position and isotopic niche of the crab compared with other representatives of the flora and fauna occurring in Elounda Bay (Crete). P. segnis showed a trophic position of 3.9, higher than the values determined by SIA or conventional gut content analysis in other areas of the Mediterranean Sea or in the native range. Crab specimens showed a high inter-individual variability in both δ13C and δ15N values; further analysis indicated negligible differences in the isotopic niche of adult males and females. Conversely, δ15N values were significantly related to the size of the specimens, ultimately suggesting an ontogenetic dietary shift. This study represents the first attempt to characterize the trophic habits of the blue swimming crab in the context of an invaded food web and may contribute to the implementation of long-term management strategies of control and mitigation of its ecological impact.

The trophic niche of subterranean populations of Speleomantes italicus

The determination of a species trophic niche can clarify its functional role within a food web and how prey resources are used in relation with the spatial and temporal variability of environmental conditions. This information may result particularly useful for the implementation of conservation plans of endangered species having a cryptic behaviour or living in places difficult to be surveyed. Here we present the first long-term study on the trophic niche of the Italian cave salamander Speleomantes italicus, a strictly protected facultative cave species that seasonally exploits surface environments (e.g., forested areas) as well as both natural and artificial subterranean environments. We analysed the diet variation of six populations of S. italicus inhabiting natural caves, surveyed 24 times in a full year. During the surveys, all sampled individuals were subjected to stomach flushing and the ingested prey were identified and enumerated; furthermore, salamanders’ body condition was also evaluated. The results of the analyses provided the first comprehensive, year-round assessment of the diet for a Speleomantes species. Remarkable divergences in terms of trophic niche and body condition were observed between the studied populations. We found a discrepancy in the foraging activity of the populations located in different areas; specifically, the individuals that experienced sub-optimal microclimatic conditions poorly performed in foraging. Furthermore, we found temporal and spatial variability in the body condition of individuals. Our study highlighted a remarkably high spatial and temporal divergence in the trophic habits of conspecific populations, a feature that may represent one of the major factors promoting the variability of multiple population traits.

Diet of Bullfrog Tadpoles Lithobates catesbeianus, Shaw 1802, an Invasive Species from Monte Desert

The competitive effects among tadpoles make the bullfrog a possible agent of native amphibian population declines at its introduction sites. Our aim was to analyze the dietary composition of an invasive population of Lithobates catesbeianus tadpoles and to evaluate the degree of dietary changes among activity months. We hypothesized that bullfrog tadpoles would exhibit temporal changes in diet, consistent with the variation of tadpole size and availability of resources. We collected 94 bullfrog tadpoles and identified 50 taxa in their digestive tracts. The items most frequently consumed were microalgae of the genera Navicula (frequency of occurrence [Fo] = 19) and Cymbella (Fo = 17). The trophic niche breadth was 8.13, which is considered moderate-high. The trophic habits of L. catesbeianus larvae changed seasonally, with the diets during October and December being numerically similar (Morisita's quantitative index = 0.86). The composition of the diet had a similarity of 63% (Jaccard's qualitative index) between August and October. The larvae of L. catesbeianus collected during the different months showed significant differences in their stages of development and body mass. On average, individuals were larger and more developed in the month of December, which was to be expected given that the abundance of prey items is greatest during the summer season. Our results show that L. catesbeianus tadpoles have a wide trophic niche with a diet that is not strongly selective; thus, the aquatic life-history stages play an important role in the structuring of invaded anuran larval communities.

Sympatric otariids increase trophic segregation in response to warming ocean conditions in Peruvian Humboldt Current System.

Determining trophic habits of predator communities is essential to measure interspecific interactions and response to environmental fluctuations. South American fur seals, Arctocephalus australis (SAFS) and sea lions Otaria byronia (SASL), coexist along the coasts of Peru. Recently, ocean warming events (2014–2017) that can decrease and impoverish prey biomass have occurred in the Peruvian Humboldt Current System. In this context, our aim was to assess the effect of warming events on long-term inter- and intra-specific niche segregation. We collected whisker from SAFS (55 females and 21 males) and SASL (14 females and 22 males) in Punta San Juan, Peru. We used δ13C and δ15N values serially archived in otariid whiskers to construct a monthly time series for 2005–2019. From the same period we used sea level anomaly records to determine shifts in the predominant oceanographic conditions using a change point analysis. Ellipse areas (SIBER) estimated niche width of species-sex groups and their overlap. We detected a shift in the environmental conditions marking two distinct periods (P1: January 2005—October 2013; P2: November 2013—December 2019). Reduction in δ15N in all groups during P2 suggests impoverished baseline values with bottom-up effects, a shift towards consuming lower trophic level prey, or both. Reduced overlap between all groups in P2 lends support of a more redundant assemblage during the colder P1 to a more trophically segregated assemblage during warmer P2. SASL females show the largest variation in response to the warming scenario (P2), reducing both ellipse area and δ15N mean values. Plasticity to adapt to changing environments and feeding on a more available food source without fishing pressure can be more advantageous for female SASL, albeit temporary trophic bottom-up effects. This helps explain larger population size of SASL in Peru, in contrast to the smaller and declining SAFS population.

Interspecific and intraspecific comparison of the isotopic niche of shrimps targets of fishing in south-eastern Brazil

AbstractThe study compares the isotopic niche (δ13C–δ15N) of three species of penaeid shrimps (Xiphopenaeus kroyeri, Artemesia longinaris and Litopenaeus schmitti) that are targets of fishing in south-eastern Brazil. The two hypotheses raised are based on the niche theory, in which the coexistence between organisms that have similar dietary demands is possible due to trophic partitioning: (i) shrimp species that share the habitat have segregated isotopic niches; and (ii) stages of maturity and genders vary with the trophic habitat (δ13C) and/or trophic position (δ15N). In multispecific fishing grounds, the isotopic niches of the shrimp species were segregated or had low overlap, indicating trophic partitioning. The intraspecific comparison of δ13C and δ15N showed a similar trend in all species. For δ13C, the values were more enriched in adult individuals than in juveniles, but similar between males and females. For δ15N, adult individuals also had more enriched values, and males were more enriched than females. The results confirm the hypotheses raised, and the assumptions of niche theory apply to these penaeid shrimps.

Trophic hierarchy in a marine community revealed by network analysis on co-occurrence data

Network analysis is employed in biodiversity studies to explore the possible ecological implications of taxa co-occurrences across time or space, but the conceptual robustness of this approach is still under debate. Herein, by focusing our attention on a marine pelagic community previously described by metabarcoding data gathered over a time-series, we explore the usefulness of co-occurrence networks to derive information about food web structures. In our effort we studied a published co-occurrence network by conducting bibliographic research to identify which links were potentially related to ecological interactions. Although biologically plausible links were the minority, we found that they were enough to construct realistic ecological networks. We therefore assembled trophic and non-trophic networks including both putatively biological links and those not explainable based on the literature; we analyzed these networks separately by estimating several indicators chosen among those resulted as informative of ecological processes in previous research. In the trophic networks, we detected some structural properties and ‘trophic hierarchies’ commonly found in food webs with approaches different from co-occurrence networks, such as: i) a positive relationship between body size and trophic level (this latter defined in terms of network topology, i.e., the arrangement of nodes and links in a network); and ii) realistic shifts in the network roles played by co-occurring taxa (i.e., hubs and keystone species), which allowed the persistence of hierarchy despite environmental variability. Our results reveal the power of applying co-occurrence networks in capturing the ecological properties of marine communities, plus possible ecological changes occurring under different environmental conditions. However, such application potential is deeply constrained by the acquiring of fine-scale knowledge of the trophic habits of the taxa in the community of reference.

Biology and ecology of lionfish (Pterois volitans) in the Corales de Profundidad Natural National Park, Colombian Caribbean

Lionfish was studied in the mesophotic environment. 237 specimens were observed, most in the upper mesophotic zone. Males were larger than females and differences between sexes in growth models were found. Sixteen prey items were identified, the most important being the teleost families Acanthuridae and Monacanthidae, and the crustacean Penaeidae. The mean of δ13C was -17.08 ± 0.36 ‰ and δ15N was 8.68 ± 0.46 ‰, with no differences between sexes. Lionfish occupies a less extensive isotopic niche in mesophotic environment than in shallow sectors; there is an isotopic niche overlap between sexes. Likewise, lionfish has specialized trophic habits. All specimens were mature and in females regression phase predominated. In females, condition factor (CF), gonadosomatic index (GSI) and hepatosomatic index (HSI) increased with gonadal development, reaching peak in the active spawning phase, and decreasing in regression. Males had a condition factor similar to spawning females, but IGS and IHS were lower

Distribution of the main taxa and dominant species of macrobentos in the seas of the East Arctic

Macrobenthos of soft bottoms on the shelf of two Arctic seas was investigated in the survey conducted aboard RV TINRO in August-September 2015. The area of 32,800 km2 in the depth range of 15–34 m was researched in the East Siberian Sea where 13 taxonomic groups of bottom fauna were found, mainly Bivalvia (75.9 % by biomass). The area of 22,500 km2 in the depth range of 45–96 m was surveyed in the Laptev Sea, where 20 groups of macrobenthos were identified, with predominance of Polychaeta (34.1 %), Bivalvia (27.7 %), and Sipuncula (21.2 %), by biomass. Significant changes in structure of benthic communities are revealed on the external shelf of the Laptev Sea (below of 70 m), in comparison with the data of previous years, possibly caused by warming of the environments: the previously dominated group of Ophiuroidea was replaced by Sipuncula. Although the dominant groups were widely occurred, their quantitative distribution was neither uniform nor similar. The dominant species had different trophic habits and occupied disjointed zones with domination of sestonophages or detritophages, in dependence on the habitat conditions. The average abundance and biomass of macrobenthos were assessed as 881 ± 174 ind./m2 and 149.2 ± 25.5 g/m2 in the East Siberian Sea, and 251 ± 32 ind./m2 and 71.0 ± 13.0 g/m2 in the Laptev Sea, respectively.

An individual-based dataset of carbon and nitrogen isotopic data of Callinectessapidus in invaded Mediterranean waters.

BackgroundThe characterisation of functional traits of non-indigenous and invasive species is crucial to assess their impact within invaded habitats. Successful biological invasions are often facilitated by the generalist diet of the invaders which can modify their trophic position and adapt to new ecosystems determining changes in their structure and functioning. Invasive crustaceans are an illustrative example of such mechanisms since their trophic habits can determine important ecological impacts on aquatic food webs. The Atlantic blue crab Callinectessapidus is currently established and considered invasive in the Mediterranean Sea where it has been recorded for the first time between 1947 and 1949. In the last decade, the blue crab colonised most of the eastern and central Mediterranean Sea and the Black Sea and it is currently widening its distribution towards the western region of the basin.New informationStable isotope analysis is increasingly used to investigate the trophic habits of invasive marine species. Here, we collated individual measures of the blue crab δ13C and δ15N values and of its potential invertebrate prey into a geo-referenced dataset. The dataset includes 360 records with 236 isotopic values of the blue crab and 224 isotopic data of potential prey collected from five countries and 12 locations between 2014 and 2019. This dataset allows the estimation of the trophic position of the blue crab within a variety of invaded ecosystems, as well as advanced quantitative comparisons of the main features of its isotopic niche.

Knowledge of Tropical Birds through Citizen Science Data: Trophic Habit of the Roadside Hawk (Rupornis magnirostris) (Aves, accipitridae) in the Caatinga And Atlantic Forest, Brazil

Professional ornithologists have been using citizen science data to investigate aspects of the biology of birds around the world. This approach is improving the knowledge of birds in Brazil, including aspects of their feeding ecology. The Roadside Hawk Rupornis magnirostris is widely distributed in the Neotropical region, where it is commonly found in numerous landscapes. Despite this, detailed information on its trophic habit remains scarce and restrict to a few localities. The objective of this study was to examine the trophic habit of the Roadside Hawk in the Caatinga and Atlantic Forest through the use of citizen science data. Searches for photographic records with evidence of feeding activities were done in early 2021 in the WikiAves, the major citizen science project regarding Brazilian birds. Records of Roadside Haws with prey [n = 143] were obtained by citizen scientists along 11 years, and included in this study. Records obtained in the Atlantic Forest [n = 111] were more numerous than those obtained in the Caatinga [n = 32]. Vertebrate prey included animals of four classes: Mammalia, Aves, Reptilia and Amphibia. Invertebrate prey included three classes: Insecta, Arachnida and Clitellata. In both biomes, the number of vertebrate prey was substantially higher than that of invertebrate prey, throughout the year. In the Atlantic Forest, preyed birds and reptiles were significantly more numerous than mammals and amphibians, and invertebrates, in the dry period. A similar pattern was observed in the rainy period, but differences were not significant. In the Caatinga, prey of the classes Aves and Reptilia were the most frequent, followed by those of the classes Amphibia and Mammalia. Invertebrates were uncommon. Therefore, Roadside Hawks feed on similar prey in the Brazilian Atlantic Forest and Caatinga, with a preference for vertebrates, especially birds and reptiles.

The ubiquity of neonicotinoid contamination: Residues in seabirds with different trophic habits

Neonicotinoids are one of the most diffusely used classes of pesticides whose level of danger toward non-target invertebrate and vertebrate species has raised increasing concern in the last decades. Among vertebrates, birds are particularly susceptible to unintentional neonicotinoid poisoning since they can be exposed through different pathways, including ingestion of dressed seeds, sucking of contaminated pollen, ingestion of sprayed insects, predation on contaminated aquatic and terrestrial preys. In the present study, we investigated the possible exposure of seabirds by measuring the residues of five neonicotinoids (acetamiprid, clothianidin, imidacloprid, thiacloprid and thiamethoxam) in samples of pooled feathers collected from fledglings of the strictly piscivorous Sandwich tern (Thalasseus sandvicensis) and the mixotrophic species Mediterranean gull (Ichthyaetus melanocephalus). At least one neonicotinoid was quantified in all the Mediterranean gull samples (n = 11) and 89% of the analysed Sandwich tern samples (n = 36). The active principles with the highest quantification rates were imidacloprid (100% in Mediterranean gulls and 58% in Sandwich terns) and clothianidin (100% in Mediterranean gulls and 61% in Sandwich terns), while thiacloprid was the less frequently detected pesticide (<20% of samples in both species). Mean concentrations ± standard error for imidacloprid, clothianidin and thiamethoxam were 8.8 ± 1.4, 4.5 ± 0.19 and 0.16 ± 0.02 ng g−1 for the Mediterranean gull, and 5.8 ± 0.55, 0.60 ± 0.08 and 0.36 ± 0.03 ng g−1for the Sandwich tern, respectively. Our data evidenced the exposure of seabirds to neonicotinoids and the further need to investigate the extent of neonicotinoid contamination in non-agricultural ecosystems.

Continuous cotton cropping affects soil micro-food web

The resource connection between microflora and microfauna in soil micro-food web is of great importance for soil ecosystem in terms of maintaining its quality, productivity, and sustainability. However, the responses of soil micro-food web to continuous cropping (CC) are still unclear at present. In this study, the responses of soil micro-food web to different duration of CC (0 (the control), 5, 10, 15, and 20 years) were investigated in a long-term experiment in Xinjiang, China. The trophic groups of soil nematodes in 0–10 cm and 11–20 cm soil layers were determined according to the trophic habits, oesophageal structure, and predation pattern. The stability and maturity of soil micro-food web were determined by nematode faunal analysis. The connectance was determined according to the predation relationship between nematodes and microbes. The results showed that the abundance of nematodes in the 10- and 20-year CC treatments were higher than that in the control in the 0–10 cm soil layer, and the abundance of nematodes in the 5- and 20-year CC treatments were also higher than that in the control in the 11–20 cm soil layer. In the same soil layer, the abundance of plant parasites in the 10- and 20-year CC treatments and omnivores-predators in the 5- and 20-year CC treatments were higher than those in the control. CC increased the metabolic footprint of omnivore-predators and structure footprint in both soil layers. The soil micro-food web in the CC treatments received fewer disturbances than that in the control, which increased soil nutrients. Meanwhile, the soil micro-food web became stable and mature. The connectance of omnivore-predators channel in the 5-year CC treatment was higher than that in the control in the 0–10 cm soil layer, and the connectance of bacterial channel in the 20-year CC treatment and omnivores-predators channel in the 5-, 10-, and 15-year CC treatments were also higher than those in the control in the 11–20 cm soil layer. Mantel test showed that soil pH, electrical conductivity, organic carbon, total nitrogen, and C/N ratio were key factors affecting soil nematode communities. Therefore, continuous cropping could effectively enhance the soil micro-food web structure and function through bottom-up effects in the cotton fields.

Titanium in commercial shrimp from anthropized tropical bays and estimated quantities of TiO2

Titanium (Ti) is a metal with low toxicity in organisms; however, Ti in the form of TiO2 nanoparticles is a contaminant of emerging concern. This study determined the concentration of total Ti in the muscle of pink shrimp (Penaeus brasiliensis and P. paulensis) from two anthropized tropical bays (Guanabara and Sepetiba Bays, southeast Brazil) and presented the first estimates of TiO2 in shrimp. Differences in the total Ti concentration were associated with fishing site and season, showing a spatial-temporal trend. The level of anthropization per se did not explain the metal concentration. Both trophic position (δ15N) and trophic habitat (δ13C) of shrimp had negligible influence on the metal concentration. We suggest that Ti point sources into Sepetiba Bay and/or differences regarding fishing sites inside the bays are possible explanations for the results. The estimates of TiO2 in shrimps based on the total Ti concentration are baseline values for future monitoring. Since shrimps are targets of commercial fisheries, monitoring the Ti concentration is a public health interest.

Cave amphipods reveal co‐variation between morphology and trophic niche in a low‐productivity environment

Abstract Identifying the relationships between morphology and trophic niche is at the core of functional morphology. Low resource diversity and fluxes of organic carbon are expected to constrain trophic specialisation of morphological structures because food resources are too scarce to promote trophic differentiation. However, species from low‐productivity habitats often exhibit specialised biological traits such as resistance to starvation and high food‐finding abilities, which may in turn release constraints on trophic and morphological differentiation among species. Groundwaters are food resource‐limited because of the lack of photosynthetic production and limited inputs of organic carbon from surface ecosystems. We tested for co‐variation between morphology and trophic habits in co‐occurring Niphargus amphipods from five groundwater caves of the Dinaric Karst, Europe. We predicted that the size of gnathopods—the accessory feeding appendages—would positively co‐vary with trophic position: species with larger gnathopods should more easily grab and immobilise prey. We quantified gnathopod size and shape by means of morphometric measurements and assessed isotopic niche, trophic position, and carbon signatures using nitrogen (δ15N) and carbon (δ13C) stable isotopes. We tested for correlation between morphological traits and trophic position and δ13C signatures while accounting for phylogenetic relationships among species. All co‐occurring species differed morphologically in at least one gnathopod measurement and all of them differed in their isotopic niches. As predicted, gnathopod size increased with the increasing trophic position. This co‐variation probably reflects differences in detritivorous and predatory habits among species: amphipods with larger gnathopods, hence larger muscle and more powerful grip, could more easily subdue prey. Moreover, we found a significant correlation between gnathopod shape and the normalised δ13C values, indicating that shape of the gnathopods may be related to exploitation of different food resources. We show that low‐productivity subterranean habitat species can exhibit strong trophic specialisation of morphological structures. Gnathopod size and shape of Niphargus amphipods are functional traits that co‐vary with trophic habits. Our findings pave the way for investigating how co‐variation of morphological and trophic traits may control energy flow and species’ coexistence at lower bounds of habitat productivity.

Eye lenses reveal ontogenetic trophic and habitat shifts in an imperiled fish, Clear Lake hitch (Lavinia exilicauda chi)

Stable isotopes recorded in fish eye lenses are an emerging tool to track dietary shifts coincident with use of diverse habitats over the lifetime of individuals. Eye lenses are metabolically inert, sequentially deposited, archival tissues that can open avenues to chronicle contaminant exposures, diet histories, trophic dynamics and migratory histories of individual fishes. In this study, we demonstrated that trophic histories reconstructed using eye lenses can resolve key uncertainties regarding diet and trophic habitat shifts. Clear Lake hitch (Lavinia exilicauda chi), a threatened cyprinid, inhabits a single lake (Clear Lake, Lake County, California) and utilizes tributary streams for reproduction. Bayesian mixing models applied to δ13C and δ15N recorded in eye lenses uncovered ontogenetic diet shifts that corresponded with shifts in occupation of habitats providing spawning (tributary streams), rearing (littoral lake), and growth (pelagic lake) functions. The reconstruction of size-structured trophic and habitat information can provide vital information needed to manage and conserve imperiled species such as the Clear Lake hitch.

From Past Use to Present Effects: Total Mercury in Crustaceans and Fish in the Inner Estuary of Paraíba do Sul River, Southeast Brazil.

Total mercury (Hg) concentration and stable isotope ratios (δ13C and δ15N) were used as chemical tracers in two crustaceans and four fish species that are targets of subsistence fishery in the inner estuary of the Paraíba do Sul River. This fluvial environment in southeast Brazil has a large historical utilization of Hg. δ15N was a better predictor of Hg concentration in the muscle (edible part) than δ13C; i.e., the Hg concentration was more closely related to the trophic position of species than species' trophic habitat. Consumers at higher trophic levels had higher Hg concentrations. The Hg concentration was below the maximum permissible limits established by the Brazilian Government in fishery products, and there are no human risks related to its consumption.

Interannual trophic behaviour of a pelagic fish predator in the western Mediterranean Sea

The exploitation of forage fish species can modify the functioning of marine ecosystems potentially impacting the population status of predators. This may be the case for the western Mediterranean Sea, where a reduction in the biomass of two key pelagic forage fish (European anchovy Engraulis encrasicolus and European sardine Sardina pilchardus) could produce a change in the diet composition of their main predators, which would consume alternative preys or change the size of the prey consumed. Here, we aimed to investigate the potential effect of biomass reduction of sardine and anchovy in the western Mediterranean Sea on the trophic preferences of the little tunny (Euthynnus alletteratus), a medium-sized predator that present a high consumption of these forage fish. We compared its interannual trophic ecology by combining the analysis of stomach contents and stable isotopes. Specifically, we examined if the diet of little tunny changed in its main trophic habits (diet composition, prey size, and trophic niche) during a 6-year period. We found that small pelagic fish, especially clupeiformes, were the most important prey group for the little tunny during the study period. However, we found changes in the body size of anchovy and the relative importance of sardine in recent years, probably reflecting the reported reduction in the biomass and body size of these two forage fish in the study area. In addition to these changes, we found an increase in some demersal and benthopelagic species in the diet of little tunny, which could act as an alternative diet resource.

Spruce girdling decreases abundance of fungivorous soil nematodes in a boreal forest

The relative importance of belowground and aboveground energy inputs for the decomposer communities in soil remains largely unknown. In particular, no research has been done on the significance of root-derived resources for nematode communities in boreal forests. In two spruce stands in the taiga zone, we set up a field experiment in which girdling of spruce trees and clipping of dwarf shrubs was performed. Root-derived resources were hypothesized to be highly important; accordingly, we expected to observe a suppression of the nematode community after experimental manipulations. To obtain information on the nature of changes in the soil food web, nematode community structure indices were applied. In partial confirmation of our hypothesis, spruce girdling decreased mycorrhizal hyphae biomass as assessed via in-growth mesh bags, as well as the abundance of fungivorous nematodes, mostly of the Aphelenchoides and Filenchus genera. The enrichment index (EI) value decreased, indicating reduction of organic matter inputs into the soil food web, whereas nematode channel ratio (NCR) index value increased, indicating a shift towards domination of the bacterial energy channel. Total nematode abundance, genera richness, and abundance of herbivores, omnivores, and predators did not change in response to spruce girdling. Clipping of dwarf shrubs decreased fungal and bacterial PLFA biomarkers, but did not affect nematode communities. Thus, the resources channeled in soil by the roots of canopy trees are of different relative importance for nematodes having different trophic habits. Fungivorous nematodes are at least partly dependent on root-derived resources, suggesting feeding on ectomycorrhizal mycelium. Rhizodeposits of understory vegetation are likely of low importance for nematodes.