An official website of the United States government
Abstract Animals often consume resources from multiple energy channels, thereby connecting food webs and driving trophic structure. Such ‘multichannel feeding’ can dictate ecosystem function and stability, but tools to quantify this process are lacking. Stable isotope ‘fingerprints’ are unique patterns in essential amino acid (EAA) δ13C values that vary consistently between energy channels like primary production and detritus, and they have emerged as a tool to trace energy flow in wild systems. Because animals cannot synthesize EAAs de novo and must acquire them from dietary proteins, ecologists often assume δ13C fingerprints travel through food webs unaltered. Numerous studies have used this approach to quantify energy flow and multichannel feeding in animals, but δ13C fingerprinting has never been experimentally tested in a vertebrate consumer.We tested the efficacy of δ13C fingerprinting using captive deer micePeromyscus maniculatusraised on diets containing bacterial, fungal and plant protein, as well as a combination of all three sources. We measured the transfer of δ13C fingerprints from diet to consumer liver, muscle and bone collagen, and we used linear discriminant analysis (LDA) and isotopic mixing models to estimate dietary proportions compared to known contributions. Lastly, we tested the use of published δ13C source fingerprints previously used to estimate energy flow and multichannel feeding by consumers.We found that EAA δ13C values exhibit significant isotopic (i.e. trophic) fractionation between consumer tissues and diets. Nevertheless, LDA revealed that δ13C fingerprints are consistently routed and assimilated into consumer tissues, regardless of isotopic incorporation rate. Isotopic mixing models accurately estimated the proportional diets of consumers, but all models overestimated plant‐based protein contributions, likely due to the digestive efficiencies of protein sources. Lastly, we found that δ13C source fingerprints from published literature can lead to erroneous diet reconstruction.We show that δ13C fingerprints accurately measure energy flow to vertebrate consumers across tissues with different isotopic incorporation rates, thereby enabling the estimation of multichannel feeding at various temporal scales. Our findings illustrate the power of δ13C fingerprinting for quantifying food web dynamics, but also reveal that careful selection of dietary source data is critical to the accuracy of this emerging technique.
more »
« less
Calibration chain transformation improves the comparability of organic hydrogen and oxygen stable isotope data
Abstract Stable hydrogen and oxygen isotopic compositions (δ2H and δ18O, respectively) of animal tissues have been used to infer geographical origin or mobility based on the premise that the isotopic composition of tissue is systematically related to that of local water sources. Isotopic data for known‐origin samples are required to quantify these tissue–environment relationships. Although many of such data have been published and could be reused by researchers, differences in the standards used for calibration and analytical procedures for different datasets limit the comparability of these data.We develop an algorithm that uses results from comparative analysis of secondary standards to transform data among reference scales and estimate the uncertainty inherent in these transformations. We apply the algorithm to a compilation of known‐origin keratin data published over the past ~20 years.We show that transformation improves the comparability of data from different laboratories, and that the transformed data suggest ecophysiologically meaningful differences in keratin–water relationships among different animal groups and taxa.The compiled data and algorithms are freely available in the ASSIGNRr‐package to support geographical provenance research, and more generally offer a methodology overcoming several challenges in geochemical data integration and reuse.
more »
« less
- Award ID(s):
- 1759730
- PAR ID:
- 10452462
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Methods in Ecology and Evolution
- Volume:
- 12
- Issue:
- 4
- ISSN:
- 2041-210X
- Page Range / eLocation ID:
- p. 732-747
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Summary While plant δ15N values have been applied to understand nitrogen (N) dynamics, uncertainties regarding intraspecific and temporal variability currently limit their application. We used a 28 yr record of δ15N values from two Mojave Desert populations ofEncelia farinosato clarify sources of population‐level variability.We leveraged > 3500 foliar δ15N observations collected alongside structural, physiological, and climatic data to identify plant and environmental contributors to δ15N values. Additional sampling of soils, roots, stems, and leaves enabled assessment of the distribution of soil N content and δ15N, intra‐plant fractionations, and relationships between soil and plant δ15N values.We observed extensive within‐population variability in foliar δ15N values and found plant age and foliar %N to be the strongest predictors of individual δ15N values. There were consistent differences between root, stem, and leaf δ15N values (spanningc. 3‰), but plant and bulk soil δ15N values were unrelated.Plant‐level variables played a strong role in influencing foliar δ15N values, and interannual relationships between climate and δ15N values were counter to previously recognized spatial patterns. This long‐term record provides insights regarding the interpretation of δ15N values that were not available from previous large‐scale syntheses, broadly enabling more effective application of foliar δ15N values.more » « less
-
Abstract Ectomycorrhizal (EM) effects on forest ecosystem carbon (C) and nitrogen (N) cycling are highly variable, which may be due to underappreciated functional differences among EM‐associating trees. We hypothesise that differences in functional traits among EM tree genera will correspond to differences in soil organic matter (SOM) dynamics.We explored how differences among three genera of angiosperm EM trees (Quercus,Carya, andTilia) in functional traits associated with leaf litter quality, resource use and allocation patterns, and microbiome assembly related to overall soil biogeochemical properties.We found consistent differences among EM tree genera in functional traits.Quercustrees had lower litter quality, lower δ13C in SOM, higher δ15N in leaf tissues, greater oxidative extracellular enzyme activities, and higher EM fungal diversity thanTiliatrees, whileCaryatrees were often intermediary. These functional traits corresponded to overall SOM‐C and N dynamics and soil fungal and bacterial community composition.Our findings suggest that trait variation among EM‐associating tree species should be an important consideration in assessing plant–soil relationships such that EM trees cannot be categorised as a unified functional guild. Read the freePlain Language Summaryfor this article on the Journal blog.more » « less
-
Summary Rising atmospheric carbon dioxide concentrations (CO2) and atmospheric nitrogen (N) deposition have contrasting effects on ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) symbioses, potentially mediating forest responses to environmental change.In this study, we evaluated the cumulative effects of historical environmental change on N concentrations and δ15N values in AM plants, EM plants, EM fungi, and saprotrophic fungi using herbarium specimens collected in Minnesota, USA from 1871 to 2016. To better understand mycorrhizal mediation of foliar δ15N, we also analyzed a subset of previously published foliar δ15N values from across the United States to parse the effects of N deposition and CO2rise.Over the last century in Minnesota, N concentrations declined among all groups except saprotrophic fungi. δ15N also declined among all groups of plants and fungi; however, foliar δ15N declined less in EM plants than in AM plants. In the analysis of previously published foliar δ15N values, this slope difference between EM and AM plants was better explained by nitrogen deposition than by CO2rise.Mycorrhizal type did not explain trajectories of plant N concentrations. Instead, plants and EM fungi exhibited similar declines in N concentrations, consistent with declining forest N status despite moderate levels of N deposition.more » « less
-
Abstract The fundamental tradeoff between carbon gain and water loss has long been predicted as an evolutionary driver of plant strategies across environments. Nonetheless, challenges in measuring carbon gain and water loss in ways that integrate over leaf lifetime have limited our understanding of the variation in and mechanistic bases of this tradeoff. Furthermore, the microevolution of plant traits within species versus the macroevolution of strategies among closely related species may not be the same, and accordingly, the latter must be addressed using comparative phylogenetic analyses.Here we introduce the concept of ‘integrated metabolic strategy’ (IMS) to describe the ratio between carbon isotope composition (δ13C) and oxygen isotope composition above source water (Δ18O) of leaf cellulose. IMS is a measure of leaf‐level conditions that integrate several mechanisms contributing to carbon gain (δ13C) and water loss (Δ18O) over leaf lifespan, with larger values reflecting higher metabolic efficiency and hence less of a tradeoff. We tested how IMS evolves among closely related yet ecologically diverse milkweed species, and subsequently addressed phenotypic plasticity in response to water availability in species with divergent IMS.Integrated metabolic strategy varied strongly among 20Asclepiasspecies when grown under controlled conditions, and phylogenetic analyses demonstrate species‐specific tradeoffs between carbon gain and water loss. Larger IMS values were associated with species from dry habitats, with larger carboxylation capacity, smaller stomatal conductance and smaller leaves; smaller IMS was associated with wet habitats, smaller carboxylation capacity, larger stomatal conductance and larger leaves. The evolution of IMS was dominated by changes in species’ demand for carbon (δ13C) more so than water conservation (Δ18O). Although some individual physiological traits showed phylogenetic signal, IMS did not.In response to experimental decreases in soil moisture, three species maintained similar IMS across levels of water availability because of proportional increases inδ13C and Δ18O (or little change in either), while one species increased IMS due to disproportional changes inδ13C relative to Δ18O.Synthesis.IMS is a broadly applicable mechanistic tool; IMS variation among and within species may shed light on unresolved questions relating to the evolution and ecology of plant ecophysiological strategies.more » « less
