INTERACTIONS BETWEEN FOOD-WEB STRUCTURE AND NUTRIENTS ON POND ORGANISMS

THE suggestion that the densities of organisms are regulated by interactions involving entire food chains1 has resurfaced into a new debate about the role of population regulation in environments that vary in productivity2-5. Past work has shown that density responses of organisms to enhanced productivity should be strongly affected by the number of trophic levels in the food chain6-11 but more recent theoretical work on this question has suggested that heterogeneity within a trophic level can be just as important12-15. Here we present direct experimental evidence for such an interaction between environmental productivity and the species composition of herbivores. Our results illustrate that differences in food-web structure similar to those documented in natural communities16,17 can dramatically alter how organisms will respond to abiotic factors such as nutrients that regulate primary productivity.