Plant diversity effects on pollinating and herbivorous insects can be linked to plant stoichiometry

Changes in plant diversity have consequences for higher trophic levels, e.g., higher plant diversity can enhance the reproduction and fitness of plant-associated insects. This response of higher trophic levels potentially depends on diversity-related changes in both resource quantity (abundance) and quality (nutritional content). The availability of elemental nutrients in plant resources is one aspect of nutritional quality, but has rarely been addressed as a pathway relating plant diversity to associated insects. Using the experimental plant diversity gradient of a large biodiversity grassland project, the Jena-Experiment, we analysed the %C, %N and %P and the molar ratios of those elements (C:N, C:P and N:P) in a pollinating bee, Chelostoma distinctum, and an herbivorous grasshopper, Chorthippus parallelus, reared on plots of different plant diversity. Insects showed higher content of C, N and P (% dry mass), and lower C:N and C:P ratios than plants. C:N ratios were significantly higher in grasshoppers than in bees and higher in females than in males of both species. Increasing plant species richness increased the C:N ratio of male bees and female grasshoppers. In both groups, stoichiometry was positively related to plant stoichiometry (male bees: C:P and N:P; grasshoppers: C:N and N:P). Path analysis revealed that diversity-driven changes in plant elemental composition can have consequences for abundance and chemical composition of higher trophic levels, with different responses of the two functional groups. Die Artenvielfalt von Pflanzen strukturiert die Gesellschaften höherer trophischer Ebenen und verbessert die Fortpflanzung und Fitness von Pflanzen-assoziierten Insekten. Dieser Einfluss auf höhere trophische Ebenen kann von der Quantität (Verfügbarkeit) und Qualität (Zusammensetzung) der Ressourcen abhängen. Wir untersuchten die Stöchiometrie von Interaktionen zwischen Pflanzen und zwei funktionellen Insektengruppen (Herbivoren, Bestäuber) entlang des experimentellen Pflanzendiversitätsgradienten im „Jena-Experiment“. Wir analysierten den prozentuellen Anteil von Kohlenstoff (C), Stickstoff (N) und Phosphat (P), sowie die entsprechenden molaren Verhältnisse dieser Elemente (C:N, C:P, N:P) bei einer bestäubenden Bienenart (Chelostoma distinctum) und einer herbivoren Heuschreckenart (Chorthippus parallelus), welche wir auf Graslandflächen verschiedener Diversitätsstufen gezüchtet haben. Insekten zeigten höhere prozentuale Anteile in der Trockenmasse