Bottom-up and top-down effects on plant communities: nutrients limit productivity, but insects determine diversity and composition

Top-down effects of herbivores and bottom-up effects of nutrients shape productivity and diversity across ecosystems, yet their single and combined effects on spatial and temporal beta diversity is unknown. We established a field experiment in which the abundance of insect herbivores (top-down) and soil nitrogen (bottom-up) were manipulated over six years in an existing old-field community. We tracked plant and diversity - within plot richness and among plot biodiversity- and aboveground net primary productivity (ANPP) over the course of the experiment. We found that bottom-up factors affected ANPP while top-down factors influenced plant community structure. Across years, while N reduction lowered ANPP by 10%, N reduction did not alter ANPP relative to control plots. Further, N reduction lowered ANPP by 20% relative to N addition plots. On the other hand, the reduction of insect herbivores did not alter plant richness ( diversity) yet consistently promoted Shannon's evenness, relative to plots where insect herbivores were present. Further, insect herbivores promoted spatial-temporal diversity. Overall, we found that the relative importance of top-down and bottom-up controls of plant ANPP, plant diversity, and composition ( diversity) can vary significantly in magnitude and direction. In addition, their effects varied through time, with bottom-up effects influencing ANPP quickly while the effects of top-down factors emerging only late in the experiment to influence plant community composition via shifts in plant dominance.