Determining trophic niche width: an experimental test of the stable isotope approach

Determining the trophic niche width of an animal population and the relative degree to which a generalist population consists of dietary specialists are long‐standing problems of ecology. It has been proposed that the variance of stable isotope values in consumer tissues could be used to quantify trophic niche width of consumer populations. However, this promising idea has not yet been rigorously tested. By conducting controlled laboratory experiments using model consumer populations (Daphnia sp., Crustacea) with controlled diets, we investigated the effect of individual‐ and population‐level specialisation and generalism on consumer δ¹³C mean and variance values. While our experimental data follow general expectations, we extend current qualitative models to quantitative predictions of the dependence of isotopic variance on dietary correlation time, a measure for the typical time over which a consumer changes its diet. This quantitative approach allows us to pinpoint possible procedural pitfalls and critical sources of measurement uncertainty. Our results show that the stable isotope approach represents a powerful method for estimating trophic niche widths, especially when taking the quantitative concept of dietary correlation time into account.     

Seasonal variation in the trophic structure of a spatial prey subsidy linking aquatic and terrestrial food webs: adult aquatic insects

Research over the past decade has established spatial resource subsidies as important determinants of food web dynamics. However, most empirical studies have considered the role of subsidies only in terms of magnitude, ignoring an important property of subsidies that may affect their impact in recipient food webs: the trophic structure of the subsidy relative to in situ resources. This may be especially important when subsidies are composed of organisms, as opposed to nutrient subsidies, because the trophic position of subsidy organisms may differ from in situ prey. I explored the relative magnitude and trophic structure of a cross-habitat prey subsidy, adult aquatic insects, in terrestrial habitats along three streams in the south–central United States. Overall, adult aquatic insects contributed more than one-third of potential insect prey abundance and biomass to the terrestrial habitat. This contribution peaked along a permanent spring stream, reaching as high as 94% of abundance and 86% of biomass in winter. Trophic structure of adult aquatic and terrestrial insects differed. Nearly all adult aquatic insects were non-consumers as adults, whereas all but one taxon of terrestrial insects were consumers. Such a difference created a strong relationship between the relative contribution of the prey subsidy and the trophic structure of the prey assemblage: as the proportion of adult aquatic insects increased, the proportion of consumers in the prey assemblage declined. Specific effects varied seasonally and with distance from the stream as the taxonomic composition of the subsidy changed, but general patterns were consistent. These findings show that adult aquatic insect subsidies to riparian food webs not only elevate prey availability, but also alter the trophic structure of the entire winged insect prey assemblage.     

Niche width and niche overlap: a method based on type-2 fuzzy sets

Complicated ecosystems and the high non-linearity of evolution has made biology more adaptable to a variety of environments. The relationship between life and environment demands a dynamic definition of niche and its measurement. In this paper we propose a model of niche with dynamic character based on the “broad band” effect in type-2 fuzzy sets. The niche in this definition is an interval in each ecological dimension which is dynamic in character and depends on the actual environment. We also give formulas for niche width and niche overlap. We compute the niche width and overlap for plants and animals and then compare these results with previous results. The results for niche width in this paper reflect the diversity of resources used by species or communities. The results for niche overlap demonstrate overlap under different environmental conditions. The results are, moreover, intervals, which could provide more information. The model in this paper could therefore be used to describe the state of every resource comprehensively, reflecting the interaction between species and environment.     

Fifty years later: trophic ecology and niche overlap of a native and non-indigenous fish species in the western basin of Lake Erie

Since the introduction of white perch (Morone americana) into Lake Erie over 50 years ago, the population size of native yellow perch (Perca flavescens) has decreased up to 79 % and significant changes to the ecosystem have occurred. We examined long-term population estimates and used stable isotopes of carbon (δ ¹³C) and nitrogen (δ ¹⁵N) paired with stomach content analysis to quantify the trophic ecology and niche overlap of adult yellow perch and white perch in the western basin of Lake Erie. We found that changes in yellow perch abundance since 1979 appeared to be better correlated with changes in fishery exploitation rates than with food competition effects from white perch. At the time of this study, yellow perch were found to have higher δ ¹³C values, indicating greater utilization of benthic food resources than white perch, and white perch occupied higher trophic positions based on δ ¹⁵N. The diets of both species varied spatially and seasonally based on stable isotopes and stomach contents, likely driven by changes in prey abundance. Comparison of niche widths using stable isotope population metrics and Schoener diet similarity index suggested a low to moderate degree of niche overlap between species. Isotopic niches of white perch were generally larger than those of yellow perch demonstrating broader resource utilization by this non-indigenous species. We submit that isotopic niche overlap comparisons are more appropriate for studies seeking to understand interactions among populations over course temporal scales, while diet overlap indices, such as the Schoener index provide a means to study fine-scale interactions such as ontogenetic and seasonal diet shifts.     

Habitat overlap/niche segregation in two Umbilicaria lichens: a possible mechanism

Summary Umbilicaria muhlenbergii and Umbilicaria papulosa are two similar lichens which show extensive habitat overlap. Current theory predicts that for those resources that are limited, and of critical importance to both species, segregation is demand for or use of those resources ought to be present. Controlled experiments showed that the upper and lower cortical surfaces of the two species functioned in very different ways so that U. muhlenbergii is wetted most rapidly by runoff and mist-like precipitation whereas U. papulosa is wetted most rapidly by large raindrops. An analysis of the meteorological conditions in the natural habitat for the two lichens shows that the four months providing significant productivity in U. papulosa are the same months with a very high frequency of thunderstorms (which produce large drops). Conversely, the months which produce the greatest production in U. muhlenbergii are spread out over most of the year. When water is supplied after periods of dryness, the two species absorb it at different times and from different sources. This difference supports current ideas about niche segregation in plants but represents the first time it has been demonstrated for lichens.     

Spatial variation in the strength of mutualism between a jumping spider and a terrestrial bromeliad: Evidence from the stable isotope N

Psecas chapoda, a neotropical jumping spider strictly associated with the terrestrial bromeliad Bromelia balansae in cerrados and semi-deciduous forests in South America, effectively contributes to plant nutrition and growth. In this study, our goal was to investigate if spider density caused spatial variations in the strength of this spider–plant mutualism. We found a positive significant relationship between spider density and δ¹⁵N values for bromeliad leaves in different forest fragments. Open grassland Bromeliads were associated with spiders and had higher δ¹⁵N values compared to forest bromeliads. Although forest bromeliads had no association with spiders their total N concentrations were higher. These results suggest that bromeliad nutrition is likely more litter-based in forests and more spider-based in open grasslands. This study is one of the few to show nutrient provisioning and conditionality in a spider–plant system.     

Trophic niche width,offspring condition and immunity in a raptor species

Strategies developed by organisms to maximize foraging efficiency have a strong influence on fitness. The way in which the range of food resources is exploited has served to classify species, populations and individuals from more specialist (narrow trophic niche) to more generalist (broad trophic niche). Recent studies have provided evidence that many of the considered generalist species/populations are actually composed of different specialist individuals (individual specialization). Even the existence of generalism as an adaptive strategy has been questioned. In this study, we investigated the relationship between trophic niche width, individual quality and offspring viability in a population of common kestrel Falco tinnunculus during 4 years. We showed that the diet of kestrels varied significantly among years and that individuals of better quality fed their offspring with a higher diversity of prey species and a higher amount of food. Moreover, body condition and immune response of nestlings were positively correlated with diversity of prey delivered by parents. Our study suggests that generalism has the potential to increase fitness and that broadening the trophic niche may be an adaptive strategy in unpredictable environments.     

Patterns of resource use and isotopic niche overlap among three species of sharks occurring within a protected subtropical estuary

Predation is one of the most fundamental and unifying concepts in ecology, and we are beginning to obtain a more complete understanding of how predators drive community structure and ecosystem function through their impacts on prey. We know considerably less about how predators affect each other through intraguild interactions, which is surprising considering predators often occur simultaneously and may compete for resources while avoiding being killed themselves. In the present study, we examined aspects of inter- and intra-specific resource use among three species of large-bodied predatory sharks (blacktip, bull, lemon) co-occurring within a subtropical, protected bay in the southeastern USA. Specifically, we inferred relative trophic position, isotopic niche overlap, and patterns of resource use of sharks using stable isotope analysis of carbon-13 and nitrogen-15 from blood and fin cartilage samples. We also combined these approaches with estimates of abundance and occurrence from empirical shark surveys to consider whether these species may exhibit resource partitioning in space and time. We found that all three species overlapped in space, and there was some isotopic niche overlap between the species. We also found evidence of temporal isotopic niche stability, suggesting that co-occurring shark species may compete for available prey resources, but individuals of those species may have similar patterns of resource use over time. We discuss our findings as they relate to the ecologies of the species in question and how sound conservation and management of ecosystems can allow for predator diversity, sympatry, and stable use of resources at the top of the food chain.     

Food webs in sub-Antarctic lakes: a stable isotope approach

In order to improve the understanding of food-web interactions in sub-Antarctic freshwater systems, we complemented earlier experimental studies with analyses of differences in stable isotopes (¹⁵N and ¹³C) among organisms in two lakes with contrasting productivity. The distribution of the stable isotopes showed that the small copepod Boeckella michaelseni feeds mainly on pelagic POM (particulate organic material), whereas the larger copepod species B. poppei also feeds on benthic algae. Furthermore, the predatory copepod Parabroteas sarsi seems to mainly feed on B. michaelseni, but also on B. poppei and the benthic cladoceran, Alona weineckii. Moreover, stable-isotope data suggest that the diving beetle, Lancetes angusticollis, is not only feeding on B. poppei as indicated from experimental studies, but also on the benthic cladoceran A. weineckii. Although the food webs of the two lakes are very similar, they show considerable differences in the distribution of stable isotopes. We conclude that monitoring, experiments and stable-isotope analysis in combination give a reasonably clear picture of sub-Antarctic freshwater food webs.     

Resting breathing frequency in aquatic birds: a comparative analysis with terrestrial species

Several studies have indicated that in birds breathing frequency ( f , breaths min⁻¹) scales to the −1/3 of body weight ( W , kg); this is different from the −1/4 of mammals. We wondered if this discrepancy was due to the peculiar scaling pattern of aquatic birds, as is the case of aquatic mammals. In fact, we had noted previously that the allometric scaling of f differs considerably between aquatic and terrestrial mammals, respectively, W ^−0.42 and W ^−0.25. Measurements of f were obtained in 48 aquatic birds of 22 species and in 35 terrestrial birds of 27 species, during resting conditions on land. Additional data from 11 aquatic and 14 terrestrial species, different from the ones measured, were obtained from the literature. The allometric curve of all species combined (terrestrial and aquatic, n =74) was f =13.3 W ^−0.36, similar to what is reported in previous studies. However, the allometric curve of the aquatic species ( n =33, f =14.5 W ^−0.56) differed greatly ( P <0.001) from that of the terrestrial species ( n =41, f =13.4 W ^−0.26). On average, f of aquatic birds of the 3–5 kg range was 63%, and that of birds of larger size was 57%, of the values of terrestrial birds of similar W . We conclude that, as in mammals, also in terrestrial birds f scales to the −1/4 exponent of W . The similarity of the scaling patterns of f between aquatic birds and mammals suggests a common breathing adaptation to life in the aquatic environment irrespective of phylogenetic relations.     

Assessing trophic interactions in a guild of primary parasitoids and facultative hyperparasitoids: stable isotope analysis

Facultative hyperparasitism is likely to be the most common form of intraguild predation among parasitoids. However, difficulties associated with studying facultative hyperparasitoids in the field have hampered a thorough understanding of their trophic ecology. In this study, we used a combination of stable isotope analysis and published natural history information to infer trophic interactions in a guild of field-collected primary parasitoids and facultative hyperparasitoids that attack a gall-making midge on Baccharis pilularis. Our three a priori hypotheses were: (1) stable isotope values should increase incrementally from the host plant to higher trophic levels; (2) the two species of ectoparasitoids should exhibit higher stable isotope signatures than the two endoparasitoids, and; (3) the two facultative hyperparasitoids should exhibit stable isotope signatures that fall between zero and one trophic level steps above that observed for the primary parasitoids. Food webs inferred from stable isotope data generally agreed with previously published accounts of community structure. As expected, both δ¹³C and δ¹⁵N were progressively enriched in the heavy isotope from the host plant to the herbivorous midge to the parasitic wasps. Multivariate analysis of stable isotope data revealed that the two primary ectoparasitoids occupied a similar trophic niche, but were significantly different from the primary endoparasitoids. We attribute this result to “coincidental intraguild predation” by ectoparasitoids that develop on already-parasitized midge larvae. One of the facultative hyperparasitoids, Zatropis capitis, exhibited a stable isotope signature approximately one trophic step above the primary parasitoids. Unexpectedly, the second facultative hyperparasitoid, Mesopolobus sp., appeared to be developing as a primary parasitoid at all sites. Coupled with independent assessments of community structure, stable isotope analysis validated trophic links constructed by previous researchers and identified potential taxon-specific differences in trophic interactions for two facultative hyperparasitoids in the B. pilularis gall community.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Differences in above-ground resource acquisition and niche overlap between a model invader (Phragmites australis) and resident plant species: measuring the role of fitness and niche differences in the field

Biological Invasions - Identifying the mechanisms that result in a “high impact” invasive species can be difficult. Coexistence theory suggests that detrimental invasive species can be...     

Seasonal variation in the composition of aquatic organic matter in some Nova Scotian brownwaters: a nuclear magnetic resonance approach

Using a novel concentration technique (reverse osmosis and freeze-drying) as well as a standard analytical technique little used with limnological samples (solid state ¹³C nuclear magnetic resonance), we studied the chemical structure of aquatic organic matter from four closely located freshwater sites in Nova Scotia. The main conclusions drawn from the data are that: (a) the aromatic C fraction which is assumed to be refractory remains at less than 10% of the total, with a slight increase in relative importance in the fall, (b) less structurally complex aliphatic carbon decreases from winter to spring and remains at lower levels into later fall, (c) carbohydrates are at a maximum during the summer, (d) the carboxylic C fraction is at a minimum in the summer and maximum in the fall and winter. Results show roughly the same annual patterns of C composition for the two running water sites. Our data suggest that the hydrological processes which carry terrestrial and wetland DOM to streams and lakes allow a selective adsorption process of DOM to occur in soils. We compare our data to those from other freshwater studies and suggest that the importance of aromatic carbon in freshwaters has probably been overestimated in the past, with a corresponding underestimate of the more biologically labile carbohydrate and aliphatic material.Department of Chemistry, McMaster Universtiy     

Niche overlap and trophic resource partitioning of two sympatric batoids co‐inhabiting an estuarine system in southeast Australia

Elasmobranchs play an important role within the trophic structure of marine ecosystems, but there are relatively few studies published on the feeding ecology of these species. Reported herein is the feeding ecology and trophic resource partitioning of two sympatric batoid species, Urolophus cruciatus and Narcine tasmaniensis from southeast Australia. The diet of males and females of both species was similar, suggesting no sex‐specific dietary preferences. Ontogenetic changes in diet were observed from the diets of both species: as the body size increased, the proportion consumed of crustacea to polychaeta decreased. A relatively high degree of niche overlap (70%) was detected between the trophic resources of the two species. The way in which the predators partitioned the resources, however, was significantly different. U. cruciatus fed predominately on small benthic crustaceans (amphipods and decapods), while N. tasmaniensis displayed a preference towards Maldanidae polychaetes. Therefore, although U. cruciatus and N. tasmaniensis both feed predominately on benthic invertebrates, they specialise on different taxa. This trophic resource partitioning contributes to the biodiversity of the region by facilitating the coexistence of these sympatric species.     

Seasonal variation of depression and other moods: a longitudinal approach

The present study examined the effect of season of the year on depression and other moods. Previous work, primarily cross sectional or retrospective in design and involving clinically depressed or seasonally affective disordered samples, has suggested that mood changes as a function of season. However, the literature also shows conflicting and/or inconsistent findings about the extent and nature of this relationship. Importantly, these prior studies have not adequately answered the question of whether there is a seasonal effect in nondepressed people. The present study employed a longitudinal design and a large sample drawn from a normal population. The results, based on those participants for whom mood measures were collected in each season, demonstrated strong seasonal effects. Beck Depression Inventory (BDI) scores were highest in winter and lowest in summer. Ratings on scales of hostility, anger, irritability, and anxiety also showed very strong seasonal effects. Further analyses revealed that seasonal variation in BDI scores differed for females and males. Females had higher BDI scores that showed strong seasonal variation, whereas males had lower BDI scores that did not vary significantly across season of the year.     

Inherent variation in stable isotope values and discrimination factors in two life stages of green turtles

Abstract We examine inherent variation in carbon and nitrogen stable isotope values of multiple soft tissues from a population of captive green turtles Chelonia mydas to determine the extent of isotopic variation due to individual differences in physiology. We compare the measured inherent variation in the captive population with the isotopic variation observed in a wild population of juvenile green turtles. Additionally, we measure diet-tissue discrimination factors to determine the offset that occurs between isotope values of the food source and four green turtle tissues. Tissue samples (epidermis, dermis, serum, and red blood cells) were collected from captive green turtles in two life stages (40 large juveniles and 30 adults) at the Cayman Turtle Farm, Grand Cayman, and analyzed for carbon and nitrogen stable isotopes. Multivariate normal models were fit to the isotope data, and the Bayesian Information Criterion was used for model selection. Inherent variation and discrimination factors differed among tissues and life stages. Inherent variation was found to make up a small portion of the isotopic variation measured in a wild population. Discrimination factors not only are tissue and life stage dependent but also appear to vary with diet and sea turtle species, thus highlighting the need for appropriate discrimination factors in dietary reconstructions and trophic-level estimations. Our measures of inherent variation will also be informative in field studies employing stable isotope analysis so that differences in diet or habitat are more accurately identified.     

Alteration of trophic interactions between periphyton and invertebrates in an acidified stream: a stable carbon isotope study

The hypothesis according to which proliferation of periphytic algae under acid conditions results from a release of grazing pressure is tested. Stable carbon isotope analysis is used to investigate the autochthonous/allochthonous balance of invertebrate feeding in streamside artificial channels that were experimentally acidified. We find that the relative contribution of autochthonous food sources (epilithon) to total invertebrate biomass was slightly lower (after 1 mo of acidification) or not altered (after 2 mo) under acidified conditions when compared with a control. Feeding shifts were exhibited by some invertebrate taxa and provided evidence that acidification modifies trophic interactions between attached algae and primary consumers. Cross-treatment calculations showed that reduction of grazing pressure after the first month of acidification was an effect rather than the cause of periphyton proliferation. Our approach using stable carbon isotope analysis and biomass measurements of macroinvertebrates allows the quantification of the trophic base of lotic secondary producer communities under both experimental and natural conditions.