Reliability of δ13C and δ15N in faeces for reconstructing savanna herbivore diet

We tested the reliability of herbivore faecal δ¹³C and δ¹⁵N values for reconstructing diet through review of an extensive database derived from a 3-year study of ungulates in South Africa's Kruger National Park. Faeces are a useful material for stable isotope studies of diet because they record dietary turnover at very short time scales, and because sampling is non-invasive. However, the validity of faecal isotope proxies may be questioned because they represent only undigested food remains. Results from Kruger Park confirm that free-ranging browsers have faecal δ¹³C consistent with C₃ feeding, grazer faeces are C₄, and mixed-feeder faeces intermediate. Although the respective ranges do not overlap, there is significant variation in faecal δ¹³C of browsers and grazers (～2.0–4.0‰) across space and through time. We demonstrate that most (～70%) of this variation can be ascribed to corresponding patterns of variation in the δ¹³C of C₃ and C₄ plants, respectively, re-enforcing the fidelity of faecal isotope proxies for diet but highlighting a need for mixing models that control for variations in plant δ¹³C in order to achieve accurate diet reconstructions. Predictions for the effects of climate (rainfall) and ecophysiology on ¹⁵N-abundance variations in mammals do not persist in faeces. Rather, faecal δ¹⁵N tracks changes in plant δ¹⁵N, with further fractionation occurring primarily due to variations in dietary protein (reflected by %N). Controlling for these effects, we show that a dual-isotope multiple source mixing model (Isosource) can extend diet reconstructions for African savanna herbivores beyond simplified C₃/C₄ distinctions, although further understanding of variations in mammal δ¹⁵N are needed for greater confidence in this approach.     

Environmental information from guano palynology of insectivorous bats of the central part of the United States of America

Bat droppings accumulate in caves, and the resultant guano contains a stratigraphic record of the environment analogous to the record from lake sediment and peat. The bats forage at night for insects; they return to the cave during the day to sleep and care for their young. They attach themselves to suitable perches in the cave ceiling, and their excrement accumulates on the floor below. Flying requires a lot of energy, and bats of temperate regions consume large numbers of night-flying insects. In some situations the guano can reach a depth of meters in hundreds to thousands of years, and it has a valuable chronostratigraphy. The bat scats occur as small pellets that represent the non-digestible portion of the animal's diet in the preceding few hours; hence the diet provides information about the time of the year the feeding occurred. Bat guano contains, among other things, insect fragments, hair, pollen, and mineral matter. Night-flying insects do not normally visit flowers for the pollen; many species do not eat during the flying phase of their life cycle, and those that do generally are nectar feeders. Although the insects are not after the pollen, they do fly through a pollen-laden environment, and the pollen and dust adheres to their bodies. The insects essentially act as living traps for airborne debris. The bats also are furry pollen traps; during grooming they ingest pollen and dust enmeshed in their fur, and this also is excreted. The pollen in an individual scat contains a record of the atmospheric pollen during a single day in the past. This kind of detail is rarely available from lake sediment. Chemical analysis of individual bat scats in a time series can chart the changing environment caused by agriculture, industry, volcanic dust, and a host of other details that depend only on the cleverness of the researcher. Careful ¹⁴C analysis can isolate the times when bats did not use the cave, and that may be useful in interpreting past conditions. If the insect types in the guano change over time, that may provide evidence of changing climate. Pollen was analyzed from guano samples taken from Tumbling Creek Cave near Protem, Missouri, USA. The cave contains a maternal colony of the Grey Bat (Myotis grisescens) that occupies the cave for a short time each year. Scats collected from the base of a 70 cm thick cone of guano yielded an AMS ¹⁴C date of 2810 ± 40 yr BP. The fecal material has a crumbly structure below the surface; it was of mahogany color (7.5 YR 2/1 to 3/2) and had no noticeable odor. Guano can be processed like normal sediment, but simple washing in a weak detergent solution followed by acetolysis appears adequate.     

Asymmetric Contribution of Isotopically Contrasting Food Sources to Vertebrate Consumers in a Subtropical Semi‐arid Ecosystem

Habitat heterogeneity is a primary ecological factor that is particularly pronounced in arid ecosystems. The Tehuacán valley is a subtropical semi‐arid ecosystem in which several species of columnar cacti and agave (i.e., CAM plants) constitute the dominant elements accompanied by patches of trees and shrubs (i.e., C₃ plants). Vegetation in Tehuacán is isotopically heterogenous because CAM plants have less depleted δ¹³C values than C₃ plants. Fruits and flowers of cactus and agaves offer abundant food to vertebrates, but their leaves might be less attractive to insects than the leaves of C₃ plants. Therefore, we use carbon and nitrogen stable isotope analysis to test the hypothesis that C₃ and CAM food would contribute asymmetrically to different guilds of birds and bats. We predict that granivorous and frugivorous birds and nectarivorous and frugivorous bats will consume a CAM diet, whereas insectivorous birds and bats will consume a C₃ diet. Due to omnivory of bird and bat consumers, we predict that the importance of CAM food will decrease as the trophic level of the animal increases. Our results showed that CAM food predominated in plant‐eating birds and in some flower‐visiting bats, whereas C₃ food predominated in insect‐eating bats and birds and frugivorous bats. Habitat heterogeneity in Tehuacán is important for conservation due to the asymmetric role of CAM and C₃ food in the nutrition of different feeding guilds of vertebrates. Our study provides basic information to evaluate the potential impact of habitat loss on functional groups of consumers in a semi‐arid ecosystem.     

Iron age to medieval entomogamous vegetation and Rhinolophus hipposideros roost in South-Eastern Wales (UK)

Karst cave systems are well developed in Wales (UK) and, in some instances, constitute important bat roosts. Ogof Draenen, near Blaenavon in south-east Wales, is the most recent major cave discovery (1994) with already > 70 km of passages explored spanning a vertical range of 148 m.With the exception of one small chamber (Siambre Ddu) located directly above the main Ogof Draenen system, very few bats have been noticed inside. Extensive accumulations of guano, attributable to Rhinolophus hipposideros, are however found in parts of the Ogof Draenen system. In places covering many square metres and sometimes building heaps > 0.5 m thick, these represent volumes not yet found in any other cave system in the British Isles.Although the date of the abandonment of the main Ogof Draenen system as a bat roost remains unknown, six radiocarbon dates on guano from Ogof Draenen place the occupation in the Iron Age to Medieval period at least. Palynological analysis was undertaken on ten samples distributed through the cave. Comparisons were made with a moss polster and a lake mud sample from the area to provide a first approximation of the regional modern pollen rain and with two modern guano samples, one from Siambre Ddu and one from Agen Allwedd cave (5 km to the north-west) to provide a temporal comparison with the fossil guano. Agen Allwedd cave currently is one of the largest active roosts for Lesser Horseshoe bats in Britain and lies close to the present northern limit of this endangered species in Europe.The main results are that (1) the cave appears to have been used both as a summer and a winter roost; (2) most of the Ogof Draenen guano is formed within c. 1600 ¹⁴C years and, if the largest heap is continuous, it has accumulated within 750 ¹⁴C years, i. e. 0.16 mm yr^− 1; 3) the fossil guano samples reflect a relatively closed oak forest with more abundant ivy (Hedera) and holly (Ilex) than at present; (4) insect-pollinated plants such as Ilex, Acer, Hedera and Impatiens glandulifera are over-represented in the guano samples; (5) in addition to the usual causes of bat roost decline (pesticides, pollution), in the case of Ogof Draenen, we may add entrance blocked by rock collapse and decline of the local forest cover as well as change in its composition.     

Legume and Non-legume Trees Increase Soil Carbon Sequestration in Savanna

Savanna ecosystems are increasingly pressured by climate and land-use changes, especially around populous areas such as the Mt. Kilimanjaro region. Savanna vegetation consists of grassland with isolated trees or tree groups and is therefore characterized by high spatial variation and patchiness of canopy cover and aboveground biomass. Both are major regulators for soil ecological properties and soil-atmospheric trace gas exchange (CO₂, N₂O, CH₄), especially in water-limited environments. Our objectives were to determine spatial trends in soil properties and trace gas fluxes during the dry season and to relate above- and belowground processes and attributes. We selected a Savanna plain with vertic soil properties, south east of Mt. Kilimanjaro. Three trees were chosen from each of the two most dominant species: the legume Acacia nilotica and the non-legume Balanites aegyptiaca. For each tree, we selected one transect with nine sampling points, up to a distance of 4 times the crown radius from the stem. At each sampling point, we measured carbon (C) and nitrogen (N) content, δ¹³C of soil (0–10, 10–30 cm depth) and in plant biomass, soil C and N pools, water content, available nutrients, cation exchange capacity (CEC), temperature, pH, as well as root biomass and greenhouse-gas exchange. Tree species had no effect on soil parameters and gas fluxes under the crown. CEC, C, and N pools decreased up to 50% outside the crown-covered area. Tree leaf litter had a far lower C:N ratio than litter of the C₄ grasses. δ¹³C in soil under the crown shifted about 15% in the direction of tree leaf litter δ¹³C compared to soil in open area reflecting the tree litter contribution to soil organic matter. The microbial C:N ratio and CO₂ efflux were about 30% higher in the open area and strongly dependent on mineral N availability. This indicates N limitation and low microbial C use efficiency in the soil of open grassland areas. We conclude that the spatial structure of aboveground biomass in savanna ecosystems leads to a spatial redistribution of nutrients and thus C mineralization and sequestration. Therefore, the capability of savanna ecosystems to act as C sinks is both directly and indirectly dependent on the abundance of trees, regardless of their N-fixing status.     

Stable isotope enrichment in laboratory ant colonies: effects of colony age,metamorphosis, diet,and fat storage

Ecologists use stable isotopes to infer diets and trophic levels of animals in food webs, yet some assumptions underlying these inferences have not been thoroughly tested. We used laboratory‐reared colonies of Solenopsis invicta Buren (Hymenoptera: Formicidae: Solenopsidini) to test the effects of metamorphosis, diet, and lipid storage on carbon and nitrogen stable isotope ratios. Effects of metamorphosis were examined in ant colonies maintained on a control diet of domestic crickets and sucrose solution. Effects of a diet shift were evaluated by adding a tuna supplement to select colonies. Effects of lipid content on stable isotopes were tested by treating worker ants with polar and non‐polar solvents. δ¹³C and δ¹⁵N values of larvae, pupae, and workers were measured by mass spectrometry on whole‐animal preparations. We found a significant effect of colony age on δ¹³C, but not δ¹⁵N; larvae, pupae, and workers collected at 75 days were slightly depleted in ¹³C relative to collections at 15 days (Δδ¹³C = ?0.27‰). Metamorphosis had a significant effect on δ¹⁵N, but not δ¹³C; tissues of each successive developmental stage were increasingly enriched in ¹⁵N (pupae, +0.5‰; workers, +1.4‰). Availability of tuna resulted in further shifts of about +0.6‰ in isotope ratios for all developmental stages. Removing fat with organic solvents had no effect on δ¹³C, but treatment with a non‐polar solvent resulted in enriched δ¹⁵N values of +0.37‰. Identifying regular patterns of isotopic enrichment as described here should improve the utility of stable isotopes in diet studies of insects. Our study suggests that researchers using ¹⁵N enrichment to assess trophic levels of an organism at different sites need to take care not to standardize with immature insect herbivores or predators at one site and mature ones at another. Similar problems may also exist when standardizing with holometabolous insects at one site and spiders or hemimetabolous insects at another site.     

Stable forest–savanna mosaic in north‐western Tanzania: local‐scale evidence from δ13C signatures and 14C ages of soil fractions

Aim The spatio‐temporal dynamics of dry evergreen forest patches in the savanna biome of the Kagera region (north‐western Tanzania) are largely unknown owing to a lack of pollen and macrofossil evidence. Our aims were to reconstruct local‐scale shifts of the forest–savanna boundary in order to determine whether the forests have been expanding or retreating on a centennial and millennial time‐scale. Location The Kagera region of north‐western Tanzania, East Africa. Methods The vegetation reconstruction was based on analysing δ¹³C signatures in soils along a transect spanning both C₄ open savanna and C₃ forest vegetation. Furthermore, we fractionated soil organic matter (SOM) according to density and chemical stability to analyse δ¹³C values of soil fractions with distinct radiocarbon ages. Results We found sharp changes in δ¹³C signatures in bulk SOM from the forest to the savanna, within a few metres along the transect. The forest soil profiles carried a persistent C₃‐dominated signature. Radiocarbon dating of the oldest, most recalcitrant forest soil fraction yielded a mean age of 5500 cal. yr bp , demonstrating that the forest has existed since at least the mid‐Holocene. The savanna sites showed a typical C₄ isotopic signature in SOM of topsoils, but subsoils and more recalcitrant SOM fractions also contained signals of C₃ plants. The dense soil fraction (ρ > 1.6 g cm^?3) carrying a pure C₄ label had a mean age of c. 1200 cal. yr bp , indicating the minimum duration of the dominance of grass vegetation on the savanna site. At the forest edge, the older C₄ grass signature of SOM has steadily been replaced by the more negative δ¹³C fingerprint of the forest trees. As this replacement has occurred mainly in the 10‐m‐wide forest–savanna ecotone over the last c. 1200 years, the forest expansion must be very slow and is very likely less than 15 m century^?1. Main conclusions Our results suggest that forest patches in the Kagera savanna landscape are very stable vegetation formations which have persisted for millennia. During the last millennium, they have been expanding very slowly into the surrounding savanna at a rate of less than 15 m century^?1.     

Does the proportion of arthropods versus fruit in the diet influence overwintering condition of an omnivorous songbird?

Many migratory songbirds switch from a primarily insectivorous diet during the breeding season to either a mixed diet or fruit diet during the non‐breeding season. However, for species with mixed diets, arthropods may be superior food items because of their higher protein content and easier digestibility. We tested this hypothesis by analyzing the diet and body condition of omnivorous Wood Thrushes (Hylocichla mustelina) at a non‐breeding site in tropical forest in Belize, Central America. We used analysis of stable isotopes δ¹⁵N and δ¹³C in the blood to measure diet. Our objective was to determine if a higher dietary proportion of arthropods relative to fruit (i.e., higher δ¹⁵N and δ¹³C) was associated with better body condition. We also examined the possible effect of age, sex, and habitat type on Wood Thrush diets, as well as any changes in diet through the overwintering period. We used a hierarchical Bayesian mixing model (MixSIAR) to estimate the proportion of different prey items in the diet of overwintering Wood Thrushes overall, in each habitat type, and over time during the non‐breeding period. From January to April, we found a significant decline in δ¹⁵N in forest habitats, whereas δ¹⁵N increased in scrub habitat. There was no significant seasonal change in δ¹³C. Birds with higher δ¹⁵N or δ¹³C values were not in better body condition. Females in dry‐scrub habitat consumed more fruit than males, but this did not affect body condition. Mixing model results indicated that most Wood Thrushes at our study sites consumed primarily arthropods, even during the driest times of the non‐breeding season and in the driest habitat. Overall, our results suggest that the diet of Wood Thrushes varies with habitat and during the overwintering period, but diet alone was not a predictor of body condition. Wood Thrushes, and possibly other omnivorous migratory songbirds, are apparently flexibly able to meet their wintering and pre‐migration nutritional demands with a variety of diets.     

Effects of Bromus tectorum invasion on microbial carbon and nitrogen cycling in two adjacent undisturbed arid grassland communities

Soil nitrogen (N) is an important component in maintaining ecosystem stability, and the introduction of non-native plants can alter N cycling by changing litter quality and quantity, nutrient uptake patterns, and soil food webs. Our goal was to determine the effects of Bromus tectorum (C₃) invasion on soil microbial N cycling in adjacent non-invaded and invaded C₃ and C₄ native arid grasslands. We monitored resin-extractable N, plant and soil δ¹³C and δ¹⁵N, gross rates of inorganic N mineralization and consumption, and the quantity and isotopic composition of microbial phospholipid biomarkers. In invaded C₃ communities, labile soil organic N and gross and net rates of soil N transformations increased, indicating an increase in overall microbial N cycling. In invaded C₄ communities labile soil N stayed constant, but gross N flux rates increased. The δ¹³C of phospholipid biomarkers in invaded C₄ communities showed that some portion of the soil bacterial population preferentially decomposed invader C₃-derived litter over that from the native C₄ species. Invasion in C₄ grasslands also significantly decreased the proportion of fungal to bacterial phospholipid biomarkers. Different processes are occurring in response to B. tectorum invasion in each of these two native grasslands that: 1) alter the size of soil N pools, and/or 2) the activity of the microbial community. Both processes provide mechanisms for altering long-term N dynamics in these ecosystems and highlight how multiple mechanisms can lead to similar effects on ecosystem function, which may be important for the construction of future biogeochemical process models.     

Within trophic level shifts in collagen–carbonate stable carbon isotope spacing are propagated by diet and digestive physiology in large mammal herbivores

Stable carbon isotope analyses of vertebrate hard tissues such as bones, teeth, and tusks provide information about animal diets in ecological, archeological, and paleontological contexts. There is debate about how carbon isotope compositions of collagen and apatite carbonate differ in terms of their relationship to diet, and to each other. We evaluated relationships between δ¹³C_collagen and δ¹³C_carbonate among free‐ranging southern African mammals to test predictions about the influences of dietary and physiological differences between species. Whereas the slopes of δ¹³C_collagen–δ¹³C_carbonate relationships among carnivores are ≤1, herbivore δ¹³C_collagen increases with increasing dietary δ¹³C at a slower rate than does δ¹³C_carbonate, resulting in regression slopes >1. This outcome is consistent with predictions that herbivore δ¹³C_collagen is biased against low protein diet components (¹³C‐enriched C₄ grasses in these environments), and δ¹³C_carbonate is ¹³C‐enriched due to release of ¹³C‐depleted methane as a by‐product of microbial fermentation in the digestive tract. As methane emission is constrained by plant secondary metabolites in browse, the latter effect becomes more pronounced with higher levels of C₄ grass in the diet. Increases in δ¹³C_carbonate are also larger in ruminants than nonruminants. Accordingly, we show that Δ¹³C_collagen‐_carbonate spacing is not constant within herbivores, but increases by up to 5 ‰ across species with different diets and physiologies. Such large variation, often assumed to be negligible within trophic levels, clearly cannot be ignored in carbon isotope‐based diet reconstructions.     

Tracing prey origins,proportions and feeding periods for predatory beetles from agricultural systems using carbon and nitrogen stable isotope analyses

Predatory beetles are an important component of the natural enemy complex that preys on insect pests such as aphids within agroecosystems. Tracing diet origins and movement of natural enemies aids understanding their role in the food web and informs strategies for their effective conservation. Field sampling and laboratory experiments were carried out to examine the changes of carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) among crops (cotton and maize), pests (cotton and maize aphids), and between wing and abdomen of predatory beetles, Propylea japonica, and to test the hypothesis that prey origins, proportions and feeding periods of the predatory beetles can be deduced by this stable isotope analysis. Results showed that the δ¹³C values both in wing and abdomen of adult P. japonica were changing from a C₃- to a C₄-based diet of aphids reared on maize or cotton, respectively; the isotope ratio of their new C₄ substrates were detectable within 7 days and the δ¹⁵N values began to reflect their new C₄ substrates within 3 days. The relationship between δ¹³C and δ¹⁵N values of P. japonica adults in wing or abdomen and diets of aphids from a C₃-based resource transitioning to a C₄-based resource were described best in linear or quadratic equations. Results suggest that integrative analysis of δ¹³C and δ¹⁵N values can be regarded as a useful method for quantifying to trace prey origins, proportions of diets and feeding periods of natural enemies. The results can provide quantifying techniques for habitat management of natural enemies.     

An elemental and stable isotope assessment of water strider feeding ecology and lipid dynamics: synthesis of laboratory and field studies

1. Despite the ubiquity and abundance of water striders (Hemiptera: Gerridae) in temperate streams and rivers and their potential usefulness as sentinels in contaminant studies, little is known about their feeding ecology and lipid dynamics. 2. In this study we used stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) and elemental carbon to nitrogen ratios (C/N) to assess dietary habits and lipid content, respectively, for water striders. 3. To determine diet‐tissue fractionation factors, nymphs of the most common species in New Brunswick, Canada, Aquarius remigis were reared in the laboratory for 73 days and exhibited rapid isotopic turnover in response to a switch in diet (C half‐life = 1.5 days, N half‐life = 7.8 days). Their lipid content increased towards the end of the growing season and resulted in lower δ¹³C values. Diet‐tissue fractionation factors were established after correction of δ¹³C data for the confounding effect of de novo lipid synthesis (strider δ¹³C_adj– diet δ¹³C_adj = 0.1‰, strider δ¹⁵N – diet δ¹⁵N = 2.7‰). 4. Water striders from the majority of 45 stream sites (83%) in New Brunswick had less than 50% contribution of aquatic carbon to their diets but showed a gradual increase in the contribution of this carbon source to their diet with increasing stream size. 5. These data indicate that striders exhibit a strong connection to terrestrial carbon sources, making them important users of energy subsidies to streams from the surrounding catchment. However, this dependence on terrestrial organic matter may limit their utility as indicators of contamination of aquatic systems by heavy metals and other pollutants.     

Ammonia volatilization in a Mexican bat cave ecosystem

Direct measurements of gaseous ammonia in the atmosphere of a dry bat cave containing several million insectivorous bats revealed a peak concentration of 1779 ppm (0.96 mg/dm³). Observations indicate that the origin of the gaseous ammonia is rapid microbial decomposition of bat urea, not chitinous guano. Modelling of ammonia distribution and diffusion indicates that ammonia production at the Cueva del Tigre is 257 g NH₃/day, equivalent to the decomposition of 454 g urea/day. Ammonia production is also characterized by significant isotopic fractionation favoring isotopically light (¹⁵N depleted) ammonia.     

Understanding the ecology of mammalian carnivorans and herbivores from Valdegoba cave (Burgos, northern Spain) through stable isotope analysis

Stable carbon and oxygen isotope analyses of mammalian carnivoran and herbivore species from the late Pleistocene Valdegoba cave site in northern Spain imply competition and partitioning in resource use. In general, the data support the previously recognized ecology for the analyzed species. δ¹³C values show that the ecosystem around the cave was dominated by C₃ plants. The observed δ¹⁸O values are similar to what is found in modern environments. The analyzed bovids, Bos primigenius, Capra pyrenaica, and Rupicapra rupicapra, showed the most positive δ¹³C values. Bos primigenius had the most positive mean carbon isotope value and is suggested to feed on grasses in open environments. Values for Capra pyrenaica primarily indicate grass feeding, while Rupicapra rupicapra had the widest diet for the analyzed species, likely including grass and browse. Cervus elaphus, Equus ferus, Equus hydruntinus, and Stephanorhinus hemitoechus displayed more negative δ¹³C values indicating the use of similar resources. The smallest species analyzed, Castor fiber, displayed the most negative δ¹³C and δ¹⁸O values, implying a preference for eating C₃ plants and being semi-aquatic. The canids, Canis lupus and Vulpes vulpes, displayed the most positive δ¹³C and δ¹⁸O values, and overlap many of the sampled ungulate species. Positive δ¹⁸O values in canids implies that this group obtains much of its water from its prey, uses a different water source, or has physiological differences from the other carnivorans that influence oxygen isotope values. Lynx pardinus had values similar to the canids. Crocuta crocuta had δ¹³C values more negative than expected for a generalist predator. These values are likely due to concentration of diet on taxa from more forested environments. The most negative δ¹³C values are observed in the bears, Ursus arctos and Ursus spelaeus. These values are the likely result of hibernation and the inclusion of significant vegetation in bear diets.     

Interpretation of soil δ13C as an indicator of vegetation change in African savannas

Question: The relationship between carbon‐13 in soil organic matter and C₃ and C₄ plant abundance is complicated because of differential productivity, litter fall and decomposition. As a result, applying a mass balance equation to δ¹³C data from soils cannot be used to infer past C₃ and C₄ plant abundance; only the proportion of carbon derived from C₃ and C₄ plants can be estimated. In this paper, we compare δ¹³C of surface soil samples with vegetation data, in order to establish whether the ratio of C₃:C₄ plants (rather than the proportion of carbon from C₃ and C₄ plants) can be inferred from soil δ¹³C. Location: The Tsavo National Park, in southeastern Kenya. Methods: We compare vegetation data with δ¹³C of organic matter in surface soil samples and derive regression equations relating the δ¹³C of soil organic matter to C₃:C₄ plant abundance. We use these equations to interpret δ¹³C data from soil profiles in terms of changes in inferred C₃:C₄ plant ratio. We compare our method of interpretation with that derived from a mass balance approach. Results: There was a statistically significant, linear relationship between the δ¹³C of organic matter in surface soil samples and the natural logarithm of the ratio of C₃:C₄ plants in the 100m² surrounding the soil sample. Conclusions: We suggest that interpretation of δ¹³C data from organic matter in soil profiles can be improved by comparing vegetation surveys with δ¹³C of organic matter in surface soil samples. Our results suggest that past C₃ plant abundance might be under‐estimated if a mass balance approach is used.     

Trophic level delineation and resource partitioning in a South African afromontane forest bird community using carbon and nitrogen stable isotopes

Southern African forests are naturally fragmented yet hold a disproportionately high number of bird species. Carbon and nitrogen stable isotopes were measured in feathers from birds captured at Woodbush (n = 27 species), a large afromontane forest in the eastern escarpment of Limpopo province, South Africa. The δ¹³C signatures of a range of forest plants were measured to categorise the food base. Most plants sampled, including two of five grass species, had δ¹³C signatures typical of a C₃ photosynthetic pathway (?29.5 ± 1.9‰). Three grass species had a C₄ signature (?12.0 ± 0.6‰). Most bird species had δ¹³C values representing a predominantly C₃‐based diet (?24.8‰ to ?20.7‰). δ¹⁵N values were as expected, with higher levels of enrichment associated with a greater proportion of dietary animal matter. The cohesive isotopic niche defining most species (n = 22), where the ranges for δ¹³C and δ¹⁵N were 2.4‰ and 3.4‰, respectively, highlight the difficulties in understanding diets of birds in a predominantly C₃‐based ecosystem using carbon and nitrogen stable isotopes. However, variation in isotopic values between and within species provides insight into possible niche width and the use of resources by different birds within a forest environment.     

Isotopic marking of natural enemies fed on C4 honey for habitat management studies in agroecosystems

1. To improve biological control and habitat management, how pest predators spread from natural habitats to crops must be understood. We studied whether intrinsic differences in stable-isotopic ratios of C and N from an artificial C₄ (sugar-cane) or C₃ (muti-flower honey) diet could help mark and track predators that feed on them.
2. Two aphid predators solely feeding on sugar resources as adults were used: the green lacewing Chrysoperla carnea s.l., and the parasitic wasp Aphidius colemani. δ¹³C and δ1⁵N values from wild individuals helped distinguish them from marked ones and determine the habitat resources they used.
3. Green lacewings fed on C₄ showed significantly higher δ¹³C values than those on C3 and wild individuals. However, parasitoid values were unaffected, with no mark acquired. Logistic regression was fitted to assess the probability of green lacewings having either diet with a probability of 0.93. Marks were acquired after 5 days and were detectable 20 days after switching diets with a probability of 0.67. Similar δ¹⁵N values for both wild populations indicated both predators had similar prey, but different vegetal resources (different δ¹³C values).
4. Sugar-cane honey is a natural and reliable marker for tracking lacewing populations in the field, but not for parasitoids.

     

Vegetation dynamics in a Quercus‐Juniperus savanna: An isotopic assessment

Abstract. Woody plants are increasing in many grassland and savanna ecosystems around the world. As a case in point, the Edwards Plateau of Texas, USA, is a vast region (93 000 km²) in which rapid woody encroachment appears to be occurring. The native vegetation (prior to the Anglo‐European settlement 150–200 yr ago) and the biogeochemical consequences of woody encroachment in this region, however, are poorly understood. To assess these matters we measured plant and soil δ¹³C, soil organic C and soil N content from grasslands and two important woody patch types (mature Quercus virginiana clusters and Juniperus ashei woodlands) in this region. Soil δ¹³C values showed that relative productivity of C₃ species has increased in grassland and both woody habitats in recent times. δ¹³C of SOC in grasslands and Q. virginiana clusters increased with depth from the litter layer to 30 cm (grasslands =?21 to ?13‰Q. virginiana clusters =?27 to ?17‰) and were significantly different between habitats at all depths, indicating that Q. virginiana has been a long‐term component of the landscape. In J. ashei woodlands, soil δ¹³C values (at 20–30 cm depth) near the woodland edge (‐13‰) converged with those of an adjacent grassland (‐13‰) while those from the woodland interior (‐15‰) remained distinct, indicating that the woodland has been present for many years but has recently expanded. Concentrations and densities of SOC and total N were generally greater in woody patches than in grasslands. However, differences in the amount of SOC and N stored beneath the two woody patch types indicates that C and N sequestration potentials are species dependent.     

Root exclusion through trenching does not affect the isotopic composition of soil CO2 efflux

Disentangling the autotrophic and heterotrophic components of soil CO₂ efflux is critical to understanding the role of soil system in terrestrial carbon (C) cycling. In this study, we combined a stable C-isotope natural abundance approach with the trenched plot method to determine if root exclusion significantly affected the isotopic composition (δ¹³C) of soil CO₂ efflux (R_S). This study was performed in different forest ecosystems: a tropical rainforest and two temperate broadleaved forests, where trenched plots had previously been installed. At each site, R_S and its δ¹³C (δ¹³C_Rs) tended to be lower in trenched plots than in control plots. Contrary to R_S, δ¹³C_Rs differences were not significant. This observation is consistent with the small differences in δ¹³C measured on organic matter from root, litter and soil. The lack of an effect on δ¹³C_Rs by root exclusion could be from the small difference in δ¹³C between autotrophic and heterotrophic soil respirations, but further investigations are needed because of potential artefacts associated with the root exclusion technique.     

Factors influencing stable nitrogen isotope ratios in wing membranes of insectivorous bat species: A field study

Nitrogen stable isotope analysis studies have become a useful tool in dietary studies to quantify differences in diet composition among species or individuals. However, laboratory studies have revealed several factors that influence the nitrogen isotope ratios (δ¹⁵N) in animal tissues (e.g. habitat, amount of ingested food, and metabolic rate). The influence of these factors has not been validated for free-ranging bat species, which is important for interpreting nitrogen isotope data in field studies. In this study, we measured δ¹⁵N from wing membranes (δ¹⁵N_w) and habitat samples (δ¹⁵N_h; soil and leaf, or sediment) to test for effects of diet, age, and individual energy demand in three free-living bat species (Myotis daubentonii, Nyctalus noctula and Nyctalus leisleri). We hypothesised that based on differences in diet composition δ¹⁵N_w values should increase in the sequence M. daubentonii, N. leisleri, and highest N. noctula. Furthermore, juveniles should have significantly lower δ¹⁵N_w compared to their mothers. Thirdly, the δ¹⁵N_w values of reproductively active individuals should be significantly higher than those of non-reproductive individuals. Values of δ¹⁵N_w were significantly influenced by differences in feeding ecology among species and individual reproductive status, but not by animal age. Our results show that species feeding on primary consumers have lower δ¹⁵N_w values than species feeding on secondary and tertiary consumers. Further, independent young did not differ in their δ¹⁵N_w values from adults suggesting that their tissues already reflect the insect diet. The effect of reproductive status varied among species which might reflect intra-specific differences in foraging behaviour of generalist vs. specialist species. In this study, we demonstrated that δ¹⁵N can provide information on the dietary range of bats, but researchers should account for variation owing to reproductive status, habitat, and species.