Nutrient availability is a key abiotic factor that determines plant performance in a heterogeneous environment.  Depending on the life history or developmental stage of the plant, resources will be allocated to optimize growth and/or reproduction. This optimization is negatively affected by insect herbivores. Herbivores have direct negative effects on plant performance by feeding on photosynthetic tissues and destroying conducting tissue, which can deter plant performance and decrease survival within a competitive environment.  However, top predators can limit the negative effect that herbivores have on the plants. The objective of the experiment reported here was to experimentally assess the effect that resources (nitrogen level) and microhabitat (valley vs. ridge) growing conditions have on the performance of the native Cirsium altissimum (tall thistle), the number of insect herbivores, and the number of spiders within a tallgrass prairie ecosystem. In summer 2007, we set up a two-way factorial experiment with two levels of nitrogen (ambient [N_O] vs. augmented [N₊]) under two growing conditions (ridge vs. valley) using two different developmental stages (juvenile rosettes vs. flowering plants) of tall thistle.   

Results/Conclusions

Flowering plants with augmented nitrogen had significantly more biomass compared to plants in ambient conditions.  In regards to growing conditions, the biomass was marginally significant when analyzing valley versus ridge. When comparing the treatment combinations (ridge:N_O vs. valley:N₊) for number of insect herbivores and spiders for flowering plants, there were significantly more herbivores and spiders in the valley:N₊ treatment compared to the ridge:N_O treatment. Rosette plants with N₊ had significantly more leaves than the N_O treatment in the ridge, but there was no significant difference between nitrogen treatments in the valley. When comparing the treatment combinations for number of insect herbivores and spiders for rosette plants, there are significantly more herbivores in the valley:N₊ treatment compared to the ridge:N_O.  For the juvenile rosettes, there is no difference in spider number when comparing any treatment.  This study provides further insight on how nutrient availability affects community interactions within a grassland ecosystem and how developmental stage of a plant might affect these interactions.