Print PageTop down and bottom up pressures both influence plant community structure. Nutrient availability can alter structure by promoting vegetative or reproductive growth, while herbivores affect structure directly through consumption and indirectly by stimulating compensatory growth. Because coexistence depends in part on propagule production and dispersal, changes in nutrient deposition and herbivore abundance could influence both local and regional diversity. We tested the relative effects of nutrients and herbivores on plant reproduction in a tallgrass prairie in central Iowa. Full fertilizer (NPK) treatments were crossed factorially with herbivore exclosure treatments.
Results/Conclusions
Reproductive output of grasses and forbs responded differently to these treatments. The number of forb flowering heads increased by an order of magnitude with fertilization, and also interacted with herbivory such that herbivore exclusion further increased reproductive output. Total number of grass flowering heads however were not influenced by nutrient inputs but increased flowering stems by nearly 100% when herbivores were present. These opposing trends in reproductive output between grasses and forbs suggest a potential tradeoff in maximizing reproductive effort under top down or bottom up pressures. This tradeoff could allow for regional coexistence through differential promotion of seed production in landscapes with heterogeneous influences of herbivores and fertilization.
