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-d own'' (from the trophic level above) vs. ''bottom-up'' (from the troph ic level below) effects in ecological communities. This study compares the strength of top-down effects of parasites and bottom-up effects o f 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). Hi gh levels of parasitism were obtained by adding high densities of para sitized galls, and high levels of host-plant nitrogen were obtained by adding fertilizer. We established three replicates of each of the fol lowing treatments: low parasitism, low nitrogen; low parasitism, high nitrogen; high parasitism, low nitrogen; and high parasitism, high nit rogen. There was a significant interaction of parasitism and plant nit rogen: parasitism was important only on high-nitrogen plants where gal ls were abundant. Thus, our results contribute new insights into top-d own vs. bottom-up effects: bottom-up forces set the stage on which top -down forces may act.