Trophic interactions among invertebrates in termitaria in the African savanna: a stable isotope approach

Abstract 1. Termites (Isoptera) in tropical savannas are known as ecosystem engineers, affecting the spatial and temporal distribution of water, carbon, cations, and nutrients through their mound structures. Their mounds, however, also offer habitation to diverse taxa and feeding guilds of other invertebrates; a keystone role that has not been properly quantified. 2. The aim of this study was to explore the ecosystem role of termitaria in determining invertebrate diversity and their potential trophic interactions. We used stable isotopes to distinguish termite‐feeding invertebrates from invertebrates merely living in termite mounds under field conditions. 3. The results suggest that inquiline spiders (Arachnida) do not feed on termites directly, but on other invertebrates within the termitaria that are termitophagous, elevating the spiders three trophic levels higher than the termites. 4. This study is the first to demonstrate food web interactions among inquiline invertebrates with a stable isotope approach. It provides evidence that termites play a keystone role in the system by providing habitat for various, trophically interacting invertebrates. These results illustrate a rather unexplored ecosystem property of savanna termites.     

Dual‐tracer‐based isotope turnover rates in a highly invasive mysid Limnomysis benedeni from Lake Constance

Understanding the ecological patterns of invasive species and their habitats require an understanding of the species’ foraging ecology. Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values provide useful information into the study of animal ecology and evolution, since the isotope ratios of consumers reflect consumer's dietary patterns. Nevertheless, the lack of species‐ and element‐specific laboratory‐derived turnover rates could limit their application. Using a laboratory‐based dual stable isotope tracer approach (Na¹⁵NO₃ and NaH¹³CO₃), we evaluated the δ¹⁵N and δ¹³C isotope turnover rates in full‐grown adult invasive Limnomysis benedeni from Lake Constance. We provide δ¹⁵N and δ¹³C turnover rates based on nonlinear least‐squares regression and posterior linear regression models. Model precisions and fit were evaluated using Akaike's information criterion. Within a couple of days, the δ¹⁵N and δ¹³C of mysids began to change. Nevertheless, after about 14 days, L. benedeni did not reach equilibrium with their new isotope values. Since the experiment was conducted on adult subjects, it is evident that turnover was mainly influenced by metabolism (in contrast to growth). Unlike traditional dietary shifts, our laboratory‐based dual stable isotope tracer approach does not shift the experimental organisms into a new diet and avoids dietary effects on isotope values. Results confirm the application of isotopic tracers to label mysid subpopulations and could be used to reflect assimilation and turnover from the labeled dietary sources. Field‐based stable isotope studies often use isotopic mixing models commonly assuming diet‐tissue steady state. Unfortunately, in cases where the isotopic composition of the animal is not in equilibrium with its diet, this can lead to highly misleading conclusions. Thus, our laboratory‐based isotopic incorporation rates assist interpretation of the isotopic values from the field and provide a foundation for future research into using isotopic tracers to investigate invasion ecology.     

Trophic structure of a coastal fish community determined with diet and stable isotope analyses

A combination of dietary guild analysis and nitrogen (δ¹⁵N) and carbon (δ¹³C) stable‐isotope analysis was used to assess the trophic structure of the fish community in Rhode Island and Block Island Sounds, an area off southern New England identified for offshore wind energy development. In the autumn of 2009, 2010 and 2011, stomach and tissue samples were taken from 20 fish and invertebrate species for analysis of diet composition and δ¹⁵N and δ¹³C signatures. The food chain in Rhode Island and Block Island Sounds comprises approximately four trophic levels within which the fish community is divided into distinct dietary guilds, including planktivores, benthivores, crustacivores and piscivores. Within these guilds, inter‐species isotopic and dietary overlap is high, suggesting that resource partitioning or competitive interactions play a major role in structuring the fish community. Carbon isotopes indicate that most fishes are supported by pelagic phytoplankton, although there is evidence that benthic production also plays a role, particularly for obligate benthivores such as skates Leucoraja spp. This type of analysis is useful for developing an ecosystem‐based approach to management, as it identifies species that act as direct links to basal resources as well as species groups that share trophic roles.     

Feeding ecology of granivorous carabid larvae: a stable isotope analysis

Although most carabids are primarily carnivorous, some carabid species are omnivorous, with mainly granivorous feeding habits during the larval and/or adult stages (granivorous carabids). This feeding habit has been established based on laboratory and field experiments; however, our knowledge of the feeding ecology of these beetles in the field is limited owing to the lack of an appropriate methodology. In this study, we tested the utility of stable isotope analysis in investigations of the feeding ecology of granivorous carabids in the field, using two closely related syntopic species, Amara chalcites and Amara congrua. We addressed two issues concerning the feeding ecology of granivorous carabids: food niche differentiation between related syntopic species during the larval stage and the effect on adult body size of supplementing seeds with an animal diet during the larval stage. To investigate larval feeding habits, we analysed newly emerged adults, most somatic tissues of which are considered of larval origin. In the two populations examined, both δ¹⁵N and δ¹³C were significantly higher in A. chalcites than A. congrua, suggesting that the two species differentiate food niches, with A. chalcites larvae being more carnivorous than A. congrua larvae. The two isotope ratios of A. chalcites samples from one locality were positively correlated with body size, suggesting that more carnivorous larvae become larger adults. However, this relationship was not detected in other species/locality groups. Thus, our results were inconclusive on the issue of diet supplementation. Nevertheless, overall, these results are comparable with those of previous laboratory‐rearing experiments and demonstrate the potential utility of stable isotope analysis in field studies on the feeding ecology of granivorous carabids.     

Diverse diet compositions among harpaline ground beetle species revealed by mixing model analyses of stable isotope ratios

1. Species diversities of some insect lineages have been attributed to differentiation of feeding habits among species. Our objective was to determine variation in diet composition among harpaline ground beetle species occurring in a riverside grassland. 2. We examined the diet compositions of 14 species from six genera in the spring and 10 species from two genera in the autumn. We performed measurements of nitrogen and carbon stable isotope ratios in consumers and in their potential food items, and estimated relative contributions of different food items with two mixing models, IsoSource and MixSIR. 3. IsoSource and MixSIR software gave similar results, but IsoSource tended to calculate higher contributions of principal food items and smaller percentile ranges than MixSIR. Among harparine beetle species, there were diverse food utilisation patterns among four food categories (detritivorous invertebrates, herbivorous invertebrates, C3 plants, and C4 plants). Detritivores comprised the main diets of abundant harpaline species in the spring, whereas abundant harpaline species in the autumn were primarily herbivores feeding on C4 plants, or omnivores feeding on herbivorous invertebrates and C3 plants. Seasonal changes in food use were related to seasonal changes in the abundance of each food resource. 4. Mixing model analysis of stable isotope ratios is a convenient and effective method for roughly estimating diets of many species with diverse food habits (such as ground beetles). This method can contribute to determining the trophic relationships of related insects in one ecosystem.     

Carbon (δ13C) and nitrogen (δ15N) stable isotope composition in plant and soil in Southern Patagonia's native forests

Stable isotope natural abundance measurements integrate across several biogeochemical processes in ecosystem N and C dynamics. Here, we report trends in natural isotope abundance (δ¹³C and δ¹⁵N in plant and soil) along a climosequence of 33 Nothofagus forest stands located within Patagonia, Southern Argentina. We measured 28 different abiotic variables (both climatic variables and soil properties) to characterize environmental conditions at each of the 33 sites. Foliar δ¹³C values ranged from ?35.4‰ to ?27.7‰, and correlated positively with foliar δ¹⁵N values, ranging from ?3.7‰ to 5.2‰. Soil δ¹³C and δ¹⁵N values reflected the isotopic trends of the foliar tissues and ranged from ?29.8‰ to ?25.3‰, and ?4.8‰ to 6.4‰, respectively, with no significant differences between Nothofagus species (Nothofagus pumilio, Nothofagus antarctica, Nothofagus betuloides). Principal component analysis and multiple regressions suggested that mainly water availability variables (mean annual precipitation), but not soil properties, explained between 42% and 79% of the variations in foliar and soil δ¹³C and δ¹⁵N natural abundance, which declined with increased moisture supply. We conclude that a decline in water use efficiency at wetter sites promotes both the depletion of heavy C and N isotopes in soil and plant biomass. Soil δ¹³C values were higher than those of the plant tissues and this difference increased as annual precipitation increased. No such differences were apparent when δ¹⁵N values in soil and plant were compared, which indicates that climatic differences contributed more to the overall C balance than to the overall N balance in these forest ecosystems.     

Omnivory,vertical food‐web structure and system productivity: stable isotope analysis of freshwater planktonic food webs

1. The energetic hypothesis proposes that the vertical structure of food webs should increase in height with increasing system productivity. I measured the trophic positions and extent of trophic separation between the invertebrate planktivores Mysis relicta and Chaoborus spp. and their putative zooplankton prey along a gradient of lake productivity with the use of stable nitrogen isotopes. 2. In lakes of low productivity, these planktivores were found to be herbivorous, becoming omnivorous at intermediate lake productivities, and only able to be truly zooplanktivorous as lakes approached mesotrophy. A subsequent secondary analysis of literature data revealed that the strength of top‐down trophic cascades among these organisms increased with lake productivity as reflected by relationships between the abundance of planktivores and that of phytoplankton. 3. Increased omnivory under conditions of low productivity, effectively shortening the vertical structure of food webs as predicted by the energetic hypothesis, may produce increased community stability.     

Nitrogen and carbon isotope variability in the green‐algal lichen Xanthoria parietina and their implications on mycobiont–photobiont interactions

Stable isotope patterns in lichens are known to vary largely, but effects of substrate on carbon and nitrogen stable isotope signatures of lichens were previously not investigated systematically. N and C contents and stable isotope (δ¹⁵N, δ¹³C) patterns have been measured in 92 lichen specimens of Xanthoria parietina from southern Bavaria growing on different substrates (bark and stone). Photobiont and mycobiont were isolated from selected populations and isotopically analyzed. Molecular investigations of the internal transcribed spacer of the nuclear ribosomal DNA (ITS nrDNA) region have been conducted on a subset of the specimens of X. parietina. Phylogenetic analysis showed no correlation between the symbionts X. parietina and Trebouxia decolorans and the substrate, isotope composition, or geographic origin. Instead specimens grown on organic substrate significantly differ in isotope values from those on minerogenic substrate. This study documents that the lichens growing on bark use additional or different N sources than the lichens growing on stone. δ¹⁵N variation of X. parietina apparently is controlled predominantly by the mass fraction of the mycobiont and its nitrogen isotope composition. In contrast with mycobionts, photobionts of X. parietina are much more ¹⁵N‐depleted and show less isotopic variability than mycobionts, probably indicating a mycobiont‐independent nitrogen acquisition by uptake of atmospheric ammonia.     

Tracking long‐distance migration of marine fishes using compound‐specific stable isotope analysis of amino acids

The long‐distance migrations by marine fishes are difficult to track by field observation. Here, we propose a new method to track such migrations using stable nitrogen isotopic composition at the base of the food web (δ¹⁵N_Base), which can be estimated by using compound‐specific isotope analysis. δ¹⁵N_Base exclusively reflects the δ¹⁵N of nitrate in the ocean at a regional scale and is not affected by the trophic position of sampled organisms. In other words, δ¹⁵N_Base allows for direct comparison of isotope ratios between proxy organisms of the isoscape and the target migratory animal. We initially constructed a δ¹⁵N_Base isoscape in the northern North Pacific by bulk and compound‐specific isotope analyses of copepods (n = 360 and 24, respectively), and then we determined retrospective δ¹⁵N_Base values of spawning chum salmon (Oncorhynchus keta) from their vertebral centra (10 sections from each of two salmon). We then estimated the migration routes of chum salmon during their skeletal growth by using a state‐space model. Our isotope tracking method successfully reproduced a known chum salmon migration route between the Okhotsk and Bering seas, and our findings suggest the presence of a new migration route to the Bering Sea Shelf during a later growth stage.     

Multiyear dual nitrate isotope signatures suggest that N‐saturated subtropical forested catchments can act as robust N sinks

In forests of the humid subtropics of China, chronically elevated nitrogen (N) deposition, predominantly as ammonium (NH₄⁺), causes significant nitrate (NO₃^?) leaching from well‐drained acid forest soils on hill slopes (HS), whereas significant retention of NO₃^? occurs in near‐stream environments (groundwater discharge zones, GDZ). To aid our understanding of N transformations on the catchment level, we studied spatial and temporal variabilities of concentration and natural abundance (δ¹⁵N and δ¹⁸O) of nitrate (NO₃^?) in soil pore water along a hydrological continuum in the N‐saturated Tieshanping (TSP) catchment, southwest China. Our data show that effective removal of atmogenic NH₄⁺ and production of NO₃^? in soils on HS were associated with a significant decrease in δ¹⁵N‐NO₃^?, suggesting efficient nitrification despite low soil pH. The concentration of NO₃^? declined sharply along the hydrological flow path in the GDZ. This decline was associated with a significant increase in both δ¹⁵N and δ¹⁸O of residual NO₃^?, providing evidence that the GDZ acts as an N sink due to denitrification. The observed apparent ¹⁵N enrichment factor (ε) of NO₃^? of about ?5‰ in the GDZ is similar to values previously reported for efficient denitrification in riparian and groundwater systems. Episode studies in the summers of 2009, 2010 and 2013 revealed that the spatial pattern of δ¹⁵N and δ¹⁸O‐NO₃^? in soil water was remarkably similar from year to year. The importance of denitrification as a major N sink was also seen at the catchment scale, as largest δ¹⁵N‐NO₃^? values in stream water were observed at lowest discharge, confirming the importance of the relatively small GDZ for N removal under base flow conditions. This study, explicitly recognizing hydrologically connected landscape elements, reveals an overlooked but robust N sink in N‐saturated, subtropical forests with important implications for regional N budgets.     

Estimating tissue‐specific discrimination factors and turnover rates of stable isotopes of nitrogen and carbon in the smallnose fanskate Sympterygia bonapartii (Rajidae)

This study aimed to estimate trophic discrimination factors (TDFs) and metabolic turnover rates of nitrogen and carbon stable isotopes in blood and muscle of the smallnose fanskate Sympterygia bonapartii by feeding six adult individuals, maintained in captivity, with a constant diet for 365 days. TDFs were estimated as the difference between δ¹³C or δ¹⁵N values of the food and the tissues of S. bonapartii after they had reached equilibrium with their diet. The duration of the experiment was enough to reach the equilibrium condition in blood for both elements (estimated time to reach 95% of turnover: C t95%_blood = 150 days, N t95%_blood = 290 days), whilst turnover rates could not be estimated for muscle because of variation among samples. Estimates of Δ¹³C and Δ¹⁵N values in blood and muscle using all individuals were Δ¹³C_blood = 1·7‰, Δ¹³C_muscle = 1·3‰, Δ¹⁵N_blood = 2·5‰ and Δ¹⁵N_muscle = 1·5‰, but there was evidence of differences of c.0·4‰ in the Δ¹³C values between sexes. The present values for TDFs and turnover rates constitute the first evidence for dietary switching in batoids based on long‐term controlled feeding experiments. Overall, the results showed that S. bonapartii has relatively low turnover rates and isotopic measurements would not track seasonal movements adequately. The estimated Δ¹³C values in S. bonapartii blood and muscle were similar to previous estimations for elasmobranchs and to generally accepted values in bony fishes (Δ¹³C = 1·5‰). For Δ¹⁵N, the results were similar to published reports for blood but smaller than reports for muscle and notably smaller than the typical values used to estimate trophic position (Δ¹⁵N c. 3·4‰). Thus, trophic position estimations for elasmobranchs based on typical Δ¹⁵N values could lead to underestimates of actual trophic positions. Finally, the evidence of differences in TDFs between sexes reveals a need for more targeted research.     

Stable isotope composition and parasitic infections of harbor seal young‐of‐the‐year used as prey‐based diet indicators

The stable isotope composition (δ¹³C and δ¹⁵N values) of harbor seals (Phoca vitulina) is influenced by their diet. Young‐of‐the‐year during lactation and postweaning fast are expected be enriched in ¹⁵N compared to foraging seals. We studied the temporal variation of stable isotope composition of young‐of‐the‐year and adults to determine from which point in time the young‐of‐the‐year tissues (i.e., muscles and vibrissae) are influenced by independent foraging only. These results were compared with the development of trophically transmitted parasitic infections. The δ¹⁵N values in young‐of‐the‐year muscles decreased from June (20.3‰ ± 0.5‰) to October (18.5‰ ± 0.4‰), while those of foraging seals were all year long below 19.2‰. This decrease coincides with the increase of parasitic infections in young‐of‐the‐year, reflecting a shift to fish diet. Together these results suggest that the muscles of the young‐of‐the‐year older than 5–6 mo reflect independent foraging and that they can therefore be used in community diet studies. The nursing signal in vibrissae was unclear, as the δ¹⁵N values of young‐of‐the‐year were stable over time, whereas those of adults varied seasonally. However, δ¹⁵N values of nursing pups were significantly higher than those of adults in May and June, maybe due to their reliance on milk.     

Effects of decomposition and storage conditions on the δ13C and δ15N isotope values of killer whale (Orcinus orca) skin and blubber tissues

Several different factors in the collection and preservation of whale skin and blubber samples were examined to determine their effect on the results obtained by stable nitrogen and carbon isotope (δ¹⁵N and δ¹³C) analysis. Samples of wet killer whale skin retained their original stable isotope values for up to 14 d at 4°C or lower. However, decomposition significantly changed the δ¹⁵N value within 3 d at 20°C. Storage at ?20°C was as effective as ?80°C for the preservation of skin and blubber samples for stable isotope analysis for at least a year. By contrast, once a skin sample had been freeze‐dried and lipid extracted, the stable isotope values did not change significantly when it was stored dry at room temperature for at least 12 mo. Preservation of whale skin samples for a month in DMSO‐salt solution, frozen or at room temperature, did not significantly change the δ¹⁵N and δ¹³C values of lipid extracted tissues, although the slight changes seen could influence results of a study if only small changes are expected.     

Resource use by salmonids in riverine,lacustrine and marine environments: Evidence from stable isotope analysis

Synopsis We measured stable isotope ratios (δ¹³C and δ¹⁵N) of invertebrates, Atlantic salmon, Salmo salar, and brook trout, Salvelinus fontinalis, in three distinct freshwater environments (headwater tributary, ultra-oligotrophic lake, and main-stem river) in the Western Brook system, Newfoundland, Canada. Large differences in the stable carbon signatures of invertebrates allowed the identification of organic matter assimilation from each environment by resident parr and migrating smolts. Brook trout captured in the headwater tributary in June had a carbon signature characteristic of the tributary, while those collected in August had enriched ¹³C (maximum = −15.6‰) and ¹⁵N (maximum = 12.8‰) values. These enriched carbon and nitrogen signatures were indicative of foraging at sea. There was a low correlation between δ¹³C and δ¹⁵N (r² = 0.198) for individual fish that was likely due to the confounding influence of trout feeding in the lake and the lower main-stem of the river, where δ¹³C of food sources was high but δ¹⁵N was low. Smolts emigrating from Western Brook Pond where they had been foraging (based on lacustrine carbon signatures) were significantly larger than those emigrating from a nursery brook and the main river in the same basin, despite having the same median age. These results suggest better growth opportunities in the lake environment. Trout fork length was positively correlated with δ¹³C and δ¹⁵N, demonstrating that larger individuals had been feeding outside the brook. These results support previous studies that found increased growth potential for salmonids in lacustrine and marine environments, and further, indicate possible adaptive advantages for salmonid movement away from natal brooks.     

The nutritional significance of a winter‐flowering succulent for opportunistic avian nectarivores

The winter‐flowering succulent Aloe marlothii provides nectar for many opportunistic avian nectarivores in southern African savannas. We assessed the importance of A. marlothii nectar sugar for opportunistic nectarivores by analysing temporal changes in stable carbon isotope ratios (δ¹³C) in the tissues of birds in Suikerbosrand Nature Reserve, South Africa. The blood of the 11 most common non‐granivorous opportunistic nectarivores at our site was enriched in ¹³C by 3.4 ± 1.5‰ during the flowering period of A. marlothii, reflecting the enriched crassulacean acid metabolism (CAM) isotopic signature of nectar (?12.6 ± 0.5‰). This relatively small contribution of A. marlothii nectar to assimilated carbon in whole blood contrasted with that of exhaled CO₂ in African Red‐eyed Bulbuls Pycnonotus nigricans and Cape White‐eyes Zosterops capensis. In both these species, the δ¹³C of breath samples was significantly enriched compared with blood and feathers, and closely resembled that of the nectar, revealing combustion of ingested nectar rather than assimilation. Although our analysis was complicated by the presence of C₄ grasses, whose δ¹³C values are similar to those of CAM photosynthesizers, when considered with previously published feeding observations our data reveal that opportunistic nectarivores feeding on A. marlothii nectar obtain a relatively small fraction of their assimilated carbon, but most of their metabolized carbon, from this seasonally available carbohydrate food resource. Because the δ¹³C values of insects associated with C₃ plants also became enriched during the flowering season, some insect‐eating opportunistic nectarivores may have assimilated A. marlothii carbon indirectly from insects. This study highlights the importance of understanding isotopic routing when assessing the nutritional significance of specific dietary items to consumer communities.     

Marine moult migration of the freshwater Scaly‐sided Merganser Mergus squamatus revealed by stable isotopes and geolocators

Scaly‐sided Mergansers Mergus squamatus breed on freshwater rivers in far eastern Russia, Korea and China, wintering in similar habitats in China and Korea, but nothing was known of their moulting habitat. To investigate the moult strategies of this species, we combined wing feather stable isotope ratios (males and females) with geolocator data (nesting females) to establish major habitat types (freshwater, brackish or saltwater) used by both sexes during wing moult. Although most Scaly‐sided Mergansers of both sexes probably moult on freshwater, some males and non‐breeding and failed breeding females appeared to undertake moult migration to brackish and marine waters. Given the previous lack of any surveys of coastal or estuarine waters for this species during the moult period, these findings suggest important survey needs for the effective conservation of the species during the flightless moult period.     

Stable Isotope Analysis in Primatology: A Critical Review

Stable isotope analysis has become an important tool in ecology over the last 25 years. A wealth of ecological information is stored in animal tissues in the relative abundances of the stable isotopes of several elements, particularly carbon and nitrogen, because these isotopes navigate through ecological processes in predictable ways. Stable carbon and nitrogen isotopes have been measured in most primate taxonomic groups and have yielded information about dietary content, dietary variability, and habitat use. Stable isotopes have recently proven useful for addressing more fine‐grained questions about niche dynamics and anthropogenic effects on feeding ecology. Here, we discuss stable carbon and nitrogen isotope systematics and critically review the published stable carbon and nitrogen isotope data for modern primates with a focus on the problems and prospects for future stable isotope applications in primatology. Am. J. Primatol. 74:969‐989, 2012. © 2012 Wiley Periodicals, Inc.