Carnivores can have positive as well as negative effects on populations. Positive effects may be direct or they may involve other species in the food web. How nutrients that affect production might interact with a carnivore's impact on lower trophic levels has not been investigated. Experiments are proposed in which four food webs will be exposed to up to 5 levels of nutrients and 5 levels of predation by newts in an array of 75 outdoor mesocosms. The four food webs will differ in the occurrence of Rana larvae and Daphnia. Community development and production of zooplankton, phytoplankton, littoral crustaceans and periphyton will be monitored. It is predicted that the responses of these organisms to predation and nutrient levels will depend strongly on the structure of food webs. This will be the first experiment to test such predictions. These investigations should provide critical insights for understanding natural variation in the abundance of organisms and in predicting the responses of communities to eutrophication and changes in predation intensity.
