RESOURCE EDIBILITY AND TROPHIC EXPLOITATION IN AN OLD-FIELD FOOD-WEB

I tested a food web model that predicts how environmental productivity (nutrient supply) and tap carnivores should mediate interactions amon g herbivores, edible plants, and plants that are resistant to herbivor y because they possess anti-herbivore defenses. Feeding trials with th e dominant grasshopper herbivore at the study site confirmed that cert ain plant species were resistant to herbivory because of protection by pubescent leaves and stems. Experimental food webs with various numbe rs of trophic levels composed of edible and resistant plants, grasshop pers, and hunting spiders were assembled in enclosure cages. I randoml y assigned half of the cages to a nutrient-enrichment treatment and ha lf remained as a control. Nutrient supply directly enhanced primary pr oductivity and plant and herbivore biomass. Experimentally changing sp ider abundance caused a classic ''trophic cascade'' in which herbivore biomass increased and edible plant biomass decreased. Resistant plant biomass increased. These results matched predictions of the model wit h one exception. A trophic cascade was not observed under enriched con ditions. The study nevertheless shows that a simple model attempting t o explain heterogeneous interactions in food webs may give considerabl e insight into the dynamics of natural systems.