Tri-Trophic Interactions Influenced by Resource Availability: Predator Effects on Plant Performance Depend on Plant Resources

Abstract

We tested the interactive effects of top-down (mite herbivores and predators) and bottom-up (fertilization) factors on cottonwood sapling performance, measured as leaf area, number of leaves, and height growth rate. Herbivorous spider mites in the absence of predators reduced plant performance on both fertilized and unfertilized plants. Plants responded to fertilization with increased growth only in the absence of herbivores. Without fertilization, plant performance with herbivores and predators present exceeded that of control plants with no herbivore damage. With fertilization, plant performance was intermediate between plants with herbivores and plants without herbivores or predators. By four weeks predators had effectively removed herbivorous mites on fertilized and unfertilized plants. Predator populations were unaffected by fertilization but significantly reduced herbivore populations more on fertilized than unfertilized plants. The results suggest that plant performance depends on the complex interaction between top-down and bottom-up factors.