What Goes up Must Come down? Nutrient Addition and Predation Pressure on Oak Herbivores

Current research suggests that the strength of top-down forces in communities increases with resource availability to primary producers. We examined the relative impacts of top-down and bottom-up forces on oak–herbivore communities by factorial manipulation of predation pressure and plant growth and quality. Plant growth and quality (phenolics, nitrogen) and avian predation intensity were altered through the addition of fertilizer to and the exclusion of birds from Quercus prinus and Q. rubra saplings. We censused the densities of insect herbivore guilds and predaceous arthropods on experimental and control trees in the summers of 1996 and 1997. We assessed changes in oak foliage quality using chemical assays for nitrogen, gallotannins, proanthocyanidins, and foliar astringency (protein-binding capacity). In general, leaf chewers, phloem feeders, and leaf miners were more frequent on fertilized than on unfertilized trees. Predaceous arthropods were also more frequent on fertilized trees and were positively correlated with herbivore densities on Q. prinus saplings. Bird exclusion by itself had no detectable effects on the presence or absence of herbivores or predaceous arthropods. However, avian predation intensity on phloem feeders on Q. rubra did increase in the presence of nutrient addition to the bottom trophic level in July 1997. Cumulative 1997 yearly densities of herbivores and predaceous arthropods showed similar patterns of increasing avian predation pressure on fertilized Q. rubra saplings. Nonetheless, fertilization and bird exclusion treatments did not alter seasonal defoliation levels. At this early stage in the development of our oak–herbivore community, bottom-up forces appear to influence the impact of top-down forces.