Does vegetation complexity affect host plant chemistry, and thus multitrophic interactions, in a human-altered landscape?
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Anthropogenic land use may shape vegetation composition and affect trophic interactions by altering concentrations of host plant metabolites. Here, we investigated the hypotheses that: (1) plant N and defensive secondary metabolite contents of the herb Plantago lanceolata are affected by land use intensity (LUI) and the surrounding vegetation composition (=plant species richness and P. lanceolata density), and that (2) changes in plant chemistry affect abundances of the herbivorous weevils Mecinus pascuorum and Mecinus labilis, as well as their larval parasitoid Mesopolobus incultus, in the field. We determined plant species richness, P. lanceolata density, and abundances of the herbivores and the parasitoid in 77 grassland plots differing in LUI index in three regions across Germany. We also measured the N and secondary metabolite [the iridoid glycosides (IGs) aucubin and catalpol] contents of P. lanceolata leaves. Mixed-model analysis revealed that: (1) concentrations of leaf IGs were positively correlated with plant species richness; leaf N content was positively correlated with the LUI index. Furthermore: (2) herbivore abundance was not related to IG concentrations, but correlated negatively with leaf N content. Parasitoid abundance correlated positively only with host abundance over the three regions. Structural equation models revealed a positive impact of IG concentrations on parasitoid abundance in one region. We conclude that changes in plant chemistry due to land use and/or vegetation composition may affect higher trophic levels and that the manifestation of these effects may depend on local biotic or abiotic features of the landscape.
KeywordsPlantago lanceolata Land use intensity index Iridoid glycosides Herbivore abundance Parasitoid abundance
We thank the managers of the three Biodiversity Exploratories, Swen Renner, Sonja Gockel, Andreas Hemp, Martin Gorke, and Simone Pfeiffer for their work in maintaining the plot and project infrastructure, and Markus Fischer, the late Elisabeth K. V. Kalko, K. Eduard Linsenmair, Dominik Hessenmöller, Jens Nieschulze, Daniel Prati, Ingo Schöning, François Buscot, Ernst-Detlef Schulze and Wolfgang W. Weisser for their role in setting up the Biodiversity Exploratories project. Furthermore, we thank Manfred Forstreuter for technical help with the C/N analyser, Frank Müller for technical assistance with the HPLC, and Caroline Müller for the donation of aucubin und catalpol. Philipp Braun, Katharina Fraunhofer, Ivonne Halboth, Judith Escher, Sabina Reschke and Christoph Rothenwöhrer are appreciated for assistance in the field and in the laboratory. We thank Peter Sprick and Stefan Vidal for identifying weevils and parasitoids. We thank Spencer T. Behmer, Ivo Beyaert, Kazumi Miura and two anonymous reviewers for comments on earlier versions of the manuscript. We thank Martin McLean for English revisions. The work has been funded by the Deutsche Forschungsgemeinschaft Priority Program 1374 Infrastructure-Biodiversity-Exploratories (ME 1810/5-1, OB 185/2-1). Fieldwork permits were issued by the state environmental offices of Baden-Württemberg, Thuringia, and Brandenburg, Germany (according to §72 BbgNatSchG).
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