Land-use effects on plant-herbivore stoichiometry: micro- and macronutrients

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Previous work has shown that land-use intensity strongly alters element concentrations (stoichiometry) of grassland plants (project 'Biocomp'). Here we continue the monitoring of the community biomass quality and will extend this stoichiometric analysis to herbivores, focusing not only on macronutrients but on micronutrients and trace elements as well. Intraspecific variation in stoichiometry across plant and herbivore populations along the land use gradient will give us insights into the extent of plasticity or homeostasis of specialised and generalised species, which may constrain their distribution and performance. Elemental composition of community biomass and single plant species will be related to plant diversity, land use, particularly fertilization and soil resources. We expect a strongly element-dependent response with a higher level of stoichiometric mismatch for single species compared to community biomass and higher variability among species particularly in low productive species-rich grasslands. Similarly, composition of herbivores will be related to variation in their host plants. We hypothesize that apart from secondary metabolites, constraints related to nutritional characteristics of host plants contribute to changes in herbivore performance, host plant choices and herbivore assemblage composition. Based on feeding trials, we compare the consumption by target herbivores (e.g. grasshoppers, caterpillars) and their performance on leaf material sampled along the land use gradient. We expect that those species that predominantly occur in unfertilized grasslands perform less well on plants grown in fertilized sites and vice versa, due to respective nutrient and trace element imbalances in their diet. Additionally, we will analyse the inter-annual variability in plant biomass macronutrient composition across the gradients of land-use intensity, plant species richness and productivity and will thereby address important stoichiometric aspects that will contribute to a better understanding of the biodiversity - ecosystem stability relationship.