THE EFFECTS OF SALINITY AND NUTRIENTS ON A TRITROPHIC SALT-MARSH SYSTEM

Abstract

In recent years, ecologists have begun to develop a more fine-grained and integrative approach to examining the importance of top-down and bottom-up effects on herbivore populations by investigating how changes in abiotic heterogeneity affect the relative roles of these forces. We performed two factorial field experiments to determine how increasing salinity and nutrient supply affected the relative strengths of top-down and bottom-up forces among the gall-making midge, Asphondylia borrichiae, its host plant Borrichia frutescens, and a suite of parasitoids. Salinity was increased by the addition of salt pellets, and nutrient supply was increased by the addition of fertilizer. In both experiments, parasitism pressure was decreased by trapping hymenopteran parasitoids with yellow sticky traps. In both experiments, bottom-up manipulations had significant effects on gall density. Elevated salinity levels decreased the number of galls per 200 Borrichia stems, and fertilization increased the number of galls. In...