Experimental Manipulations of Top-Down and Bottom-Up Factors in a Tri-Trophic System

There is much interest in assessing the relative importance of “top-down” (from the trophic level above) vs. “bottom-up” (from the trophic level below) effects in ecological communities. This study compares the strength of top-down effects of parasites and bottom-up effects of plants on herbivores. We established populations of sea-oxeye daisy, Borrichia frutescens, and gall-making flies, Asphondylia borrichiae, on 12 separate off-shore islands in Pinellas County, Florida (USA). High levels of parasitism were obtained by adding high densities of parasitized galls, and high levels of host-plant nitrogen were obtained by adding fertilizer. We established three replicates of each of the following treatments: low parasitism, low nitrogen; low parasitism, high nitrogen; high parasitism, low nitrogen; and high parasitism, high nitrogen. There was a significant interaction of parasitism and plant nitrogen: parasitism was important only on high-nitrogen plants where galls were abundant. Thus, our results contribute new insights into top-down vs. bottom-up effects: bottom-up forces set the stage on which top-down forces may act.