Nutrient limitation is canonically thought to arise via macronutrients, especially nitrogen and phosphorus, in terrestrial plant and animal communities. Humans, though, have been increasing the amounts of N and P available terrestrially for decades. Mounting correlational evidence has suggested that plant and animal species may become limited by micronutrients (Ca, K, or Na) when macronutrients are abundant, or has suggested co-limitation by macro- and micro-nutrients. Few experiments, however, have actually manipulated both macro- and micronutrients in concert. We manipulated macro- (N and P) and micronutrients (Ca, K, and Na) in every possible combination (16 total treatments, 128 plots total) using a large fertilization experiment (30m x 30m plots) in a coastal tallgrass prairie in Texas to determine the role of micronutrients in structuring plant and insect herbivore communities, and whether this role changed with abundant macronutrients.
We found evidence that micronutrients are important in structuring both plant and insect communities, but that the role of micronutrients and the importance of macronutrients to this role depended on plant functional group and varied among insect taxonomic groups. Major insect herbivores, grasshoppers, were responsive to micronutrients, and their response did not vary with macronutrients. Other insect groups, however, seemed to be co-limited by macronutrients and one or more micronutrients. The response of plants was very dependent upon species-specific differences to soil characteristics and different nutrient requirements. These results argue for a necessity to consider limitation by micronutrients or co-limitation by macro- and micronutrients when thinking about how plant and insect communities are structured in prairie ecosystems.