Abstract
Agriculture can dramatically alter community composition, food webs, and resource availability, yet the impacts of agriculture on many ecosystems and their constituents remain unstudied. Here, we examined the effect of agriculture on the isotopic ecology of Horned Larks (Eremophila alpestris) in the Colorado Desert. Over the last century, the Imperial Valley in southeastern California has transformed from desert scrub into agricultural land, while breeding Horned Larks have persisted in the area. We compared the isotopic composition of nitrogen (δ15N), carbon (δ13C), and hydrogen (δ2H) from feathers of lark specimens from two time periods in the Imperial Valley: a historical series collected between 1916 and 1923 and more recent vouchers collected between 1984 and 2014. We also quantified δ15N and δ13C values from plants and soil samples in contemporary agricultural land and adjacent desert scrub to estimate isotopic baseline changes associated with agricultural intensification. Contemporary larks had lower δ15N and δ13C ratios compared to historical larks, while plant and soil δ15N ratios were higher at agricultural sites compared to adjacent desert sites. These opposing trends suggest temporal change in isotope values that cannot be explained by shifting baselines alone. Furthermore, isotopic niche breadth (‰2; δ15N and δ13C) was lower among contemporary larks. Thus, contemporary larks in the Imperial Valley may capitalize on seeds and phytophagous insects that are abundant among crops, possibly representing an opportunistic change in diet over time in response to agriculture. Finally, δ2H values did not differ between historical and contemporary larks, but contemporary larks had δ2H values that differed from historical ground water, suggesting intake of water introduced by irrigational canals. Collectively, these findings highlight the power of stable isotope analyses combined with natural history collections to examine ecological change in the Anthropocene.
Zusammenfassung
Die Landwirtschaft kann die Artenzusammensetzung, Nahrungsnetze und Ressourcenverfügbarkeit dramatisch verändern, doch sind die Einflüsse der Landwirtschaft auf viele Ökosysteme und ihre Bestandteile unerforscht. Hier untersuchten wir die Auswirkungen der Landwirtschaft auf die Isotopenökologie der Ohrenlerche (Eremophila alpestris) in der Colorado-Wüste. Im Laufe des letzten Jahrhunderts hat sich das Tal Imperial Valley im Südosten Kaliforniens von einem Wüstengestrüpp in landwirtschaftlich genutzte Flächen verwandelt, während die Ohrenlerche unverändert in dieser Landschaft überdauert hat. Wir verglichen die Isotopenzusammensetzung von Stickstoff (δ15N), Kohlenstoff (δ13C) und Wasserstoff (δ2H) aus Federn von Lerchenbälgen aus zwei Zeitabschnitten im Imperial Valley: eine historische Serie, die zwischen 1916–1923 gesammelt wurde, und neuere Belege, die zwischen 1984–2014 gesammelt wurden. Weiterhin quantifizierten wir die δ15N- und δ13C-Werte aus Pflanzen- und Bodenproben auf heutigen landwirtschaftlichen Flächen und im angrenzenden Wüstengestrüpp, um die mit der landwirtschaftlichen Intensivierung verbundenen Basislinienverschiebung der Isotope abzuschätzen. Jüngere Lerchenbälge zeigten im Vergleich zu den historischen Lerchenbälgen niedrigere δ15N- und δ13C-Verhältnisse, während der δ15N-Anteil in den Pflanzen- und Bodenproben von landwirtschaftlichen Flächen höher war als vom anliegenden Wüstengestrüpp. Diese gegenläufigen Trends deuten auf eine zeitliche Veränderung der Isotopenwerte hin, die sich nicht allein durch die Verschiebung der Basislinien erklären lässt. Darüber hinaus war das Isotopenspektrum (‰2; δ15N und δ13C) bei jüngeren Lerchenbälgen geringer. Daher könnten heutige Ohrenlerchen im Imperial Valley von Sämereien und phytophagen Insekten profitieren, die unter den Nutzpflanzen reichlich vorhanden sind. Dies könnte eine opportunistische Veränderung der Ernährung im Laufe der Zeit als Anpassung auf die Landwirtschaft darstellen. Schließlich fanden wir zwischen den historischen und jüngeren Lerchenbälgen keine Unterschiede in den δ2H-Werten. Jedoch unterschieden sich die δ2H-Werte der jüngeren Bälge vom historischen Grundwasser, was auf die Aufnahme von Wasser aus eingeleiteten Bewässerungskanälen hindeutet. Insgesamt verdeutlichen diese Befunde die Leistungsfähigkeit der Stabilen Isotopenanalyse in Kombination mit naturkundlichen Sammlungen zur Untersuchung ökologischer Veränderungen im Anthropozän.
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taken from agricultural land and Creosote desert scrub, respectively. Horned Lark image is reproduced with permission from Lynx Edicions. Satellite imagery was generated via Google Maps with the ggmap package (Khale and Wickham 2013)
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Data accessibility
All data and code used to conduct the analyses presented in this manuscript are available via GitHub (https://github.com/mason-lab/IVLarkIsotopesJoO).
Change history
03 April 2021
A Correction to this paper has been published: https://doi.org/10.1007/s10336-021-01880-2
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Acknowledgements
We would like to thank K. Sparks for assistance in running stable isotope analyses at the Cornell University Stable Isotope Laboratory (COIL). We adhered to all relevant ethics guidelines and protocols to conduct this study. NAM conceived the study. NAM and PU collected specimens and sampled feathers. NAM conducted stable isotope analyses with assistance from JPS. NAM conducted statistical analyses. NAM wrote the manuscript with input from PU and JPS. Six anonymous peer reviewers provided extremely valuable feedback on different versions of this manuscript—some on multiple versions—and we thank them for their efforts. Funding for this project was provided in part by the Atkinson Center for Sustainable Biodiversity and the Frances M. Peacock Scholarship for Native Bird Habitat. NAM was supported by an EPA STAR Fellowship (#91768701-0) and an NSF Postdoctoral Research Fellowship in Biology (#1710739).
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Mason, N.A., Unitt, P. & Sparks, J.P. Agriculture induces isotopic shifts and niche contraction in Horned Larks (Eremophila alpestris) of the Colorado Desert. J Ornithol 162, 381–393 (2021). https://doi.org/10.1007/s10336-020-01834-0
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DOI: https://doi.org/10.1007/s10336-020-01834-0