ABIOTIC CONTROLS OF TROPHIC CASCADES IN A SIMPLE GRASSLAND FOOD-CHAIN

I experimentally examined the relative importance of a top predator in a three trophic level food chain in which the abiotic conditions were varied. Food chains consisting of plants, grasshoppers and wolf spide rs were established in field cages placed over natural prairie. Grassh opper resource intake was manipulated by altering the abiotic environm ent (i.e., shading during peak grasshopper feeding times to reduce tem perature and radiation), and thus the time they had available for feed ing. Grasshoppers in control (unshaded) food chains compensated for sp ider predation by increased per capita survival and food consumption b y those remaining. In this scenario, plant biomass was unaffected by t he presence of spiders (i.e., no trophic cascade). However, when the g rasshoppers were shaded for the morning hours, reducing their time ava ilable for feeding, spiders reduced grasshopper density, and the abund ance of plants increased relative to when spiders were absent (i.e., a trophic cascade). This experiment showed that by simply altering the abiotic environment such that grasshopper foraging ability changed, pr ofound differences in the impacts of top predators resulted.