Trophic niche differences between coexisting omnivores silver carp and bighead carp in a pelagic food web

Understanding how omnivorous consumers are affected by their resources and how this is expressed through the food chain is a fundamental issue in ecology. We used stable isotope analysis of archived scales of two pelagic single-chain omnivorous fish species, bighead carp (Hypophthalmichthys nobilis) and silver carp (H. molitrix), to reconstruct historical trophic interactions patterns along a gradient of resources. We found that, although bighead carp and silver carp utilize the similar resources from the pelagic food chain, they can coexist and persist not only by regulating their trophic position and trophic dissimilarity, but also by regulating trophic niche width. Omnivorous fish often exhibit flexible foraging strategies, which is closely related to the availability of ecologic context. We found a positive relationship between trophic dissimilarity and zooplankton density, which may indicate that the competitive interactions induce strong top-down effects on zooplankton, and/or that high zooplankton availability release the between-population trophic interaction through bottom-up effect. The trophic niche width of bighead carp was positively related with zooplankton availability, probably reflecting that the niche of an omnivore at a higher trophic position is more sensitive to high quality resources. Our results indicate how different aspects of the trophic partitioning of coexisting omnivores may be regulated by different ecological contexts. These alternatives are not mutually exclusive and further theoretical work should include both these mechanisms to re-evaluate the effects of omnivory on food web properties.     

Food habits of three carnivores in a mosaic landscape of São Paulo state,Brazil

São Paulo is the most developed state in Brazil, but despite intense land use changes, some mammal species still inhabit remnant fragments of native vegetation of the state. We used fecal analyses to investigate prey diversity, trophic niche breadth, and trophic niche overlap of pumas, ocelots, and maned wolves in protected and non-protected areas in São Paulo state. During 18 months, we inspected more than 10,000 km of roads and trails and collected 233 fecal samples. Maned wolves showed generalist food habit, ocelots were predominantly carnivores, and pumas were strict carnivores. The European hare, an exotic species which had not yet been reported in the study area, was identified in the feces of a puma. Trophic niche breadth was the lowest for maned wolves (BA?=?0.339) and the highest for pumas (BA?=?0.723). Trophic niche overlap was high between maned wolves and ocelots (O_jk?=?0.765) and between pumas and ocelots (O_jk?=?0.639). Significantly different diets of maned wolves and ocelots were observed between dry and rainy seasons. Our findings indicate that these three species may adapt their diets to the disturbed landscapes. This is the second study characterizing food habits of ocelots in Cerrado areas but the first considering a large number of samples.     

Non-linear relationship between body size of terrestrial carnivores and their trophic niche breadth and overlap

Studying food partitioning of mammalian predators is important for understanding trophic structures and interactions between coexisting carnivore species. This is particularly pertinent in the light of expanding ranges of populations of generalist species whose habitat and diet overlap with more specialized species. Here, we tested the resource partitioning hypothesis in terrestrial carnivores, predicting that trophic niche breadth and overlap relate positively to body mass. We used dietary data from 18 terrestrial carnivore taxa in four families (Canidae, Mustelidae, Felidae and Ursidae; body mass 0.1–173.6 kg) in three regions in Central and Eastern Europe, i.e. deciduous forest and forest-steppe region (DFR), temperate deciduous and mixed forest region (MFR) and transitory mixed forest regions (TFR). We ranked carnivores along an axis of trophic niche (breadth and overlap), and analysed the relationship between trophic niche and body mass (or pair-wise difference in body mass). A hierarchical cluster analysis of diet composition divided carnivores into four ecological groups: wild ungulate predators; small-mammal predators; amphibians and small mammal predators and omnivores. The relationship between body mass of predators and both trophic niche breadth and trophic niche overlap were hump-shaped. The trophic niche breadth to body mass ratio was significantly lower in DFR than in TFR and trophic niche overlap was significantly higher in DFR than in MFR and TFR. The predominant food resource is small mammals whose abundance is related to local agricultural and forestry management practices. Modifications of management techniques can affect population dynamics and community composition of carnivore species, especially in the case of small-mammal predators.     

Correspondence of seed traits with niche position in glacier foreland succession

Changes in species composition during succession are driven by biotic and abiotic factors leading to a multitude of niches occupied by distinct species. Gradient analyses of plant communities provide opportunities to approximate the niche position of species along a successional gradient. Several plant traits have been used to explain mechanisms governing successional sequences, but generalising changes in species traits during primary succession is still controversial. This study examined whether the seed mass and the optimum temperature for germination could explain the niche position of several glacier foreland species along a primary successional gradient in the Austrian Central Alps. We hypothesised that pioneer species should possess lighter seeds and a lower optimum temperature for germination than late successional species. We found significant differences in the seed mass between species, but the seed mass did not correspond with the assigned niche position on the successional gradient. Germination responses to temperature also differed significantly between species. Pioneer species performed better at lower temperatures than late successional species, suggesting that the optimum temperature for germination is a driver of niche separation. We discuss the interactions between seed traits and environmental conditions along the primary successional gradient emphasising the importance of temperature requirements for the germination. Differences in the regeneration characteristics are a major cue governing species turnover in glacier foreland succession.     

Heterotrophic succession within dung-inhabiting beetle communities in northern Spain

Successional patterns of beetles inhabiting dung pats were examined during May and July 1993 in a mountain area in northern Spain (Picos de Europa). Beetles belonging to six families were caught during the course of succession (30 d). Coprophagous beetles were more abundant in dung pats than predatory beetles (89 and 11 %, respectively). A trophic sequence was observed in relation to age of the dung, coprophagous beetles occurring earlier in the dung than predatory beetles. The pattern was observed on two occasions during the season, though succession proceeded somewhat faster in July than in May. These results suggest that food availability and microclimatic conditions in dung pats appear to determine the successional occurrence of beetle taxa. On the other hand, coprophagous species (Aphodius) were poorly segregated along the successional axis. Null models failed to support the hypothesis that successional overlap and differences in successional mean occurrence between species could be the result of competition. Successional patterns at the specific level probably reflect differences in behaviour, such as pat location, feeding, mating, egg-laying and larva requirements, rather than competitive replacement.     

Between‐lake variation in the trophic ecology of an invasive crayfish

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Global test of Eltonian niche conservatism of nonnative freshwater fish species between their native and introduced ranges

Despite growing evidence that biotic interactions limit the distribution of species and their potential redistribution under climate change, the recent surge of interest in niche conservatism has predominantly focused on the Grinellian (abiotic) niche, whereas few studies have attempted to quantify potential lability in the Eltonian (biotic or trophic) niche. Here, we test for conservatism in the Eltonian niche of 32 freshwater fish species between their introduced and native ranges from 435 populations across the globe. We used stable isotope data to quantify niche shifts along the horizontal (δ¹³C: indicating the origin of the resources consumed) and vertical (δ¹⁵N: describing the trophic position) dimensions of the isotopic niche, as well as shifts in overall isotopic niche breadth. Using an assemblage centroid standardized isotope vector analysis and controlling for phylogenetic relatedness among species, we demonstrated that introduced freshwater fishes exhibited flexibility in both resource use and trophic position that was beyond levels of natural variability observed in their native ranges. By contrast, niche breadth showed variability only within the limits recorded in native populations and varied independently from shifts in mean isotopic niche positions. Across all species and introduction histories, we found a consistent shift towards more balanced acquisition of resources with mixed origins and at intermediate trophic positions, suggesting a general mechanism by which fish species successfully establish into recipient communities. The mechanisms that promote or inhibit species from shifting their Eltonian niche remains unknown, but trophic flexibility is likely to contribute to both the success and the ecological impacts of invasive species and range shifts of native species under future global change.     

Trophic response to ecological conditions of habitats: Evidence from trophic variability of freshwater fish

1. To adapt to ecological and environmental conditions, species can change their ecological niche (e.g., interactions among species) and function (e.g., prey‐predation, diet competition, and habitat segregation) at the species and guild levels. Stable isotope analysis of bulk carbon and nitrogen of organisms has conventionally been used to evaluate such adaptabilities in the scenopoetic and bionomic views as the isotopic niche width.
2. Compound‐specific stable isotope analysis (CSIA) of nitrogen within amino acids provides trophic information without any disruption of scenopoetic views in the isotope ratios, unlike conventional bulk isotope analysis provides both information and therefore frequently hinders its usefulness for trophic information.
3. We performed CSIA of amino acids to understand the trophic variability of the pike gudgeon Pseudogobio esocinus and largemouth bass Micropterus salmoides as representative specialist and generalist fish species, respectively, from 16 ecologically variable habitats in the four major rivers of Korea.
4. There was little variation (1σ) in the trophic position (TP) among habitats for P. esocinus (± 0.2); however, there was considerably large variation for M. salmoides (± 0.6). The TP of M. salmoides was negatively correlated with the benthic invertebrate indices of the habitats, whereas the TP of P. esocinus showed no significant correlation with any indices. Thus, these two representative fish species have different trophic responses to ecological conditions, which is related to known differences in the trophic niche between specialists (i.e., small niche width) and generalists (i.e., large niche width).
5. Over the past four decades, the conventional bulk isotope analysis has not been capable of deconvoluting “scenopoetic” and “bionomic” information. However, in the present study, we demonstrated that the CSIA of amino acids could isolate trophic niches from the traditional ecological niche composed of trophic and habitat information and evaluated how biological and ecological indices influence the trophic response of specialists and generalists.

     

Generalist trophic ecology in a changing habitat: The case of the four‐striped mouse in a woody‐encroached savannah

Trophic generalists tolerate greater habitat change than specialists; however, few studies explore how generalist trophic ecology is affected. We established how the trophic ecology of an extreme generalist, Rhabdomys pumilio, changed in relation to a directionally changing woody‐encroached savannah in Eastern Cape, South Africa by investigating (a) foraging behaviour, (b) trophic niche and (c) feedback effects. (a) Giving‐up densities showed that R. pumilio preferred foraging in subcanopy microhabitat during the night as a result of lower thermoregulatory costs, but had similar preferences for sub‐ and intercanopy microhabitats during the day. (b) An isotope analysis revealed that the dietary composition and trophic niche occupied by R. pumilio differed among tree canopy cover levels (0%, 30% and 80%), which appeared to be related to changes in C₄ grass material and invertebrate availability. (c) Artificial seed patches suggested that R. pumilio was a potentially important postdispersal seed predator of the woody‐encroaching species, Vachellia karroo. Thus, an increase in tree canopy cover altered the trophic niche of R. pumilio by reducing foraging costs at night and providing alternative food resources in terms of availability and source. These findings demonstrate how an extreme generalist adapted to human‐induced habitat change through changes in its trophic ecology.     

Let’s eat out,there’s crayfish for dinner: American bullfrog niche shifts inside and outside native ranges and the effect of introduced crayfish

In invasion ecology, niche width has been recognized as a crucial factor for the outcome of an invasion. A common characteristic of successful invaders seems to be a broad niche width, and their impact on native communities may increase with increasing niche size. Overall, successful invader predators are predicted to shift their niche width by broadening it from native to invaded conditions. The scarcity of ecological studies examining invasive species in their native ranges prevents researchers from knowing if the prevalence of generalist invaders represents conservatism of broad native-range niches or instead niche shifts as a result of different processes acting in the invaded areas. Here we reviewed literature on trophic niche of the predatory invader American bullfrog (Lithobates catesbeianus) in both native and invaded ranges. We used statistical and graphic tools to analyse possible shifts in dietary niche width and the effect of introduced crayfish on the feeding strategy of L. catesbeianus. Globally, our results indicate that food sources used by the species differed in native and invaded sites, with a narrower trophic niche width in invaded areas. However, this pattern was disrupted by the occurrence of introduced crayfish that represents the major driver of the observed niche-width variation. Our data shed light on possible complications in interpreting and predicting patterns of biological invasions due to the interaction among species from different trophic levels that apparently disrupt general patterns that are likely bound to be idiosyncratic and complex.     

High intraspecific variability in the functional niche of a predator is associated with ontogenetic shift and individual specialization

Investigations on the functional niche of organisms have primarily focused on differences among species and tended to neglect the potential effects of intraspecific variability despite the fact that its potential ecological and evolutionary importance is now widely recognized. In this study, we measured the distribution of functional traits in an entire population of largemouth bass (Micropterus salmoides) to quantify the magnitude of intraspecific variability in functional traits and niche (size, position, and overlap) between age classes. Stable isotope analyses (δ¹³C and δ¹⁵N) were also used to determine the association between individual trophic ecology and intraspecific functional trait variability. We observed that functional traits were highly variable within the population (mean coefficient variation: 15.62% ± 1.78% SE) and predominantly different between age classes. In addition, functional and trophic niche overlap between age classes was extremely low. Differences in functional niche between age classes were associated with strong changes in trophic niche occurring during ontogeny while, within age classes, differences among individuals were likely driven by trophic specialization. Each age class filled only a small portion of the total functional niche of the population and age classes occupied distinct portions in the functional space, indicating the existence of ontogenetic specialists with different functional roles within the population. The high amplitude of intraspecific variability in functional traits and differences in functional niche position among individuals reported here supports the recent claims for an individual‐based approach in functional ecology.     

Rarity in Australian desert lizards

Most species of Australian desert lizards are uncommon. Possible causes of rarity are examined, including body size as measured by snout–vent length (SVL), fecundity, number of sites occupied, habitat niche breadth, microhabitat niche breadth, dietary niche breadth, and average total niche overlap with other species. Rare species tend to be larger with lower fecundities than abundant species and they occur at fewer sites. Many, but not all, uncommon species are specialists, either in habitat, microhabitat, or diet. The niche breadth hypothesis, which states that abundant species should be generalists whereas specialized species should be rare, is tested, but rejected as a general explanation for rarity. Some uncommon species exhibit high overlap with other species suggesting that they may experience diffuse competition. However, no single cause of rarity can be identified, but rather each species has its own idiosyncratic reasons for being uncommon. Multivariate analyses show distinct ecological differences between abundant and uncommon species.     

Modeling co-occurrence between toxic prey and naïve predators in an incipient invasion

Biological invasions can represent important threats to endemic species, including those within the invaders’ food webs. The Asian common toad (Duttaphrynus melanostictus) was introduced to Madagascar in 2011. This introduction presents a potentially dangerous prey item to a relatively naïve, highly diverse endemic carnivore fauna. Using a multivariate niche modeling approach (background test), we assessed the predicted niche overlap between D. melanostictus and six endemic carnivores in eastern Madagascar. The overlap between this potential prey and predators was assessed on four environmental niche axes: temperature, precipitation, vegetation cover and elevation. Our results showed a mixture of niche overlap and divergence between D. melanostictus and the six carnivores for environmental axes tested. There was significant overlap with five of the carnivores on temperature and NDVI axes. On the precipitation axis, there was significant overlap between D. melanostictus with two species. Our results suggested that wide-ranging, locally rare carnivores may overlap extensively with D. melanostictus. The six carnivores that inhabit the eastern rainforest of Madagascar will likely share multiple, niche axes with this novel potential prey item. Species that eat the non-native common toad and are susceptible to its toxins are at conservation risk because their populations may not be robust enough to adapt quickly to this threat. We advocate closely monitoring these emerging interactions and suggest a preemptive conservation strategy for carnivores potentially at risk.     

Morphological and trophic specialization in a subterranean amphipod assemblage

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Microhabitat use by two small benthic stream fish in a 2nd order stream

Direct observation of two benthic fish species — the stone loach (Barbatula barbatula) and the gudgeon (Gobio gobio) — in the field revealed a spatial segregation between the species in a study area with shallow riffle and moderately deeper pool sections. Stone loach generally inhabited shallow, more current-exposed locations and gudgeon preferred deeper, mostly sandy areas. The small individuals of both species were confined to shallow muddy locations and the larger individuals were found in deeper and more current exposed areas. The main factor affecting microhabitat choice was the flow regime:

it was a limiting factor for the stone loach, where the discharge rates controlled the presence of fish in the study area.

distribution patterns of both species were influenced by discharge and by fluctuations in discharge.

The following mechanisms regulating the distribution of stone loach and gudgeon are hypothesized:

1. Gudgeon: They prefer high water depth, low current velocities and sandy substrate, which strongly limits their spatial niche. Mainly relatively small individuals (size class 2; 6—9 cm) changed microhabitat in relation to environmental parameters. Size class 3 (approx. 9—12 cm) may be interpreted as a rather unpredictable transitory period between juvenile and adult stage. Large gudgeon entered the observation area mainly when discharge rates were high and variable. Increasing discharge rates increased the spatial niche of the large gudgeon.
2. Stone loach: At low flow rates, the observation area seemed to be an optimal place for the stone loach. Changes in environmental conditions are reflected in the distribution patterns. The transition between juvenile and adult microhabitat use takes place in size class 2. Under high and/or variable flow regime the species was not found in the observation area.

     

Microhabitat and morphometric variation of two Chaetophoracean (Chaetophorales, Chlorophyta) species in tropical streams of southeastern Brazil

Two populations of Chaetophora elegans (Roth) C. Agardh and two of Stigeoclonium helveticum Vischer were investigated for microhabitat characteristics and morphological variation in streams of São Paulo State, southeastern Brazil. Different patterns of microhabitat distribution were found between species investigated. Populations of C, elegans were distributed under relatively narrow microhabitat conditions (high irradiance, low depth, moderate to high current velocity, rocky substrata and lower values of niche width) and showing little morphometric variation (colony diameter, main axis cell size, and apical branch number), Stigeoclonium helveticum occurred under more diverse microhabitat conditions, revealed by lack of significant difference between sampling units with and without the alga and wider niche width, but also exhibited relatively narrow morphometric variation (plant length, main axis cell and ateral branch cell sizes). The narrow microhabitat conditions and smaller niche width of C. elegans can explain its low abundance (percentage cover) in streams from the area studied as well as in other regions of São Paulo State, In contrast, the wider variation of microhabitat conditions and the higher niche widths of S. helveticum suggest that this green alga is able to grow in a high number of stream ecosystems in the region investigated, ranging from undisturbed to highly disturbed habitats. Thus. the results suggest that S. helveticum is a generalist species.     

Effects of invasive macrophyte on trophic diversity and position of secondary consumers

Invasive species are one of the widespread stressors of aquatic ecosystems. Several studies document food web effects of invasive fish, but little information is available on the effects of invasive macrophytes. We studied differences in food chain length as well as trophic position and trophic diversity of fish and odonates in lakes dominated by native plants or invasive Eurasian watermilfoil. Trophic position and food chain length were determined using baseline-adjusted δ¹⁵N isotope signatures. Trophic diversity, or isotope niche width, was estimated from convex hull area analysis. Results show that trophic position of secondary consumers was not affected by the invasive macrophyte, whereas trophic diversity was greater in watermilfoil-dominated lakes. The direction of isotopic niche expansion was different in fish and odonates, suggesting potential decoupling in predator–prey interactions. This study shows that dominant non-native macrophytes may cause significant changes in food web structure of invaded ecosystems. Trophic diversity may be a more sensitive indicator of environmental stress than trophic position and has the potential to be used for assessment of invasive species impacts and restoration success.     

Exploring the nature of ecological specialization in a coral reef fish community: morphology,diet and foraging microhabitat use

Patterns of ecological specialization offer invaluable information about ecosystems. Yet, specialization is rarely quantified across several ecological niche axes and variables beyond the link between morphological and dietary specialization have received little attention. Here, we provide a quantitative evaluation of ecological specialization in a coral reef fish assemblage (f. Acanthuridae) along one fundamental and two realized niche axes. Specifically, we examined ecological specialization in 10 surgeonfish species with regards to morphology and two realized niche axes associated with diet and foraging microhabitat utilization using a recently developed multidimensional framework. We then investigated the potential relationships between morphological and behavioural specialization. These relationships differed markedly from the traditional ecomorphological paradigm. While morphological specialization showed no relationship with dietary specialization, it exhibited a strong relationship with foraging microhabitat specialization. However, this relationship was inverted: species with specialized morphologies were microhabitat generalists, whereas generalized morphotypes were microhabitat specialists. Interestingly, this mirrors relationships found in plant–pollinator communities and may also be applicable to other ecosystems, highlighting the potential importance of including niche axes beyond dietary specialization into ecomorphological frameworks. On coral reefs, it appears that morphotypes commonly perceived as most generalized may, in fact, be specialized in exploiting flat and easily accessible microhabitats.     

What makes a plant species specialist in mixed broad-leaved deciduous forests?

Knowledge about relationships between specialization degree of species, i.e. the width of their realized niche and functional traits, may have important implications for the assessment of future population developments under environmental change. In this study, we used a recently introduced method to calculate ecological niche widths of plant species in mixed broad-leaved deciduous forests and to investigate the dependence between niche widths of plants and their functional traits and Ellenberg indicator values. The research is based on a dataset of 4556 phytosociological relevés of mixed broad-leaved deciduous forests in Slovenia. We calculated theta indices for 326 species, which ranks them along a continuous gradient of habitat specialization. For 272 species, we compiled 26 functional traits and Ellenberg indicator values. We found some significant correlations between theta indices of species and their functional traits and Ellenberg indicator values; habitat specialists thrive primarily on the highest altitudes, on colder, dry sites and achieve the age of first flowering later than generalists. They also have smaller seed diameter, lower leaf dry matter content, lower mean canopy height and bigger specific leaf area than generalists. Two species groups, chamaephytes and spring green species, are particularly characterized as specialist species. The added value of our work is in complementing the knowledge about the niche differentiating along different environmental gradients and species coexistence in mixed broad-leaved deciduous forests.     

Patterns of infestation by the trombiculid mite Eutrombicula alfreddugesi in four sympatric lizard species (genus Tropidurus) in northeastern Brazil

We studied the parasitism by the chigger mite Eutrombicula alfreddugesi on four sympatric lizard species of the genus Tropidurus in Morro do Chapéu, Bahia state, Brazil: T. hispidus, T. cocorobensis, T. semitaeniatus and T. erythrocephalus. For each species, we investigated the patterns of infestation and analyzed to which extent they varied among the hosts. We calculated the spatial niche breadth of the chigger mite on the body of each host species and the distribution of mites along the hosts' bodies for each Tropidurus species. All four species of Tropidurus at Morro do Chapéu were parasited by the chigger mite, with high (97-100%) prevalences. Host body size significantly explained the intensity of mite infestation for all species, except T. erythrocephalus. The body regions with highest intensity of infestation in the four lizard species were the mite pockets. The spacial niche width of the chigger varied consistently among the four lizards species studied being highest for T. erytrocephalus and lowest for T. cocorobensis. We conclude that the distribution and intensity with which lizards of the genus Tropidurus are infested by Eutrombicula alfreddugesi larvae results from the interaction between aspects of host morphology (such as body size and the occurrence and distribution of mite pockets) and ecology (especially microhabitat use).