Water stress in grasslands: dynamic responses of plants and insect herbivores
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2014 The Authors. Global climate change is altering precipitation patterns. The effect of water stress on plant-herbivore interactions is poorly understood even though this is a primary ecological interaction that will be altered by climate change. This is especially true for grasslands where water is often limiting. In this study we manipulated water inputs in open grassland plots (1 m2) during a severe drought and assessed plant and insect herbivore responses. There were two watering treatments: ambient and supplemented. Supplemented plots received water weekly in amounts that mimicked average seasonal rainfall. For plants, we were interested in how water input affected protein and digestible carbohydrate content; previous studies predicted water stress would increase the concentration of these two nutrients. Grasshoppers are the dominant insect herbivores in grasslands and we assessed their responses to water inputs by measuring abundance and diversity. Previous studies suggested grasshoppers would prefer water-stressed plots. Protein and carbohydrate content in bulk grass and forb samples, plus plant biomass and diversity, were measured monthly (May-August). Immediately prior to harvesting plant tissue, we counted and identified individual grasshoppers in each plot. Grass biomass was reduced with water stress, but macronutrient content and species diversity were unaffected. After three months water-stressed forbs were less protein biased, and diverse, relative to watered forbs; forb biomass was indistinguishable between treatments. Grasshopper abundance and diversity were lower in water-stressed plots as the season progressed. However, grasshopper-feeding biology mattered: densities of mixed-feeders and grass-feeders, but not forb-specialists, decreased over time in water-stressed plots, but not in water supplemented plots. Our results demonstrate the importance of focusing on plant and insect herbivore functional groups and provide valuable new data that can be incorporated into models to explore the effects of global climate change in greater detail.
