THE EFFECTS OF PREY HETEROGENEITY AND CONSUMER IDENTITY ON THE LIMITATION OF TROPHIC-LEVEL BIOMASS

Abstract

The effects of prey heterogeneity and consumer identity on the strength of predator limitation of prey biomass were explored experimentally under controlled laboratory conditions. In this study, I utilized a model aquatic community composed of zooplankton as top predators, algae as prey, and nutrients as basal resources. To examine the effects of prey heterogeneity, I created a food chain initially composed of a single edible prey and a food web initially composed of a diverse assemblage of algae. These two prey treatments were then fully crossed with two predator treatments (a large-bodied zooplankter, Daphnia pulex, and a small-bodied species, Ceriodaphnia quadrangula), and two levels of productivity. Prey heterogeneity had clear effects on the ability of predators to limit overall prey biomass. In food chains, predators had strong negative effects on algae, and algal biomass exhibited a narrow response to enrichment. In contrast, predator limitation was weak in food webs with the consequence that predator and prey biomass both showed positive increases with productivity. The prey community in food webs also exhibited a striking increase in the relative abundance of large inedible algae with enrichment, in keeping with model predictions. These results indicate that prey heterogeneity can have substantial effects on predator–prey dynamics and trophic structure and can serve to shift systems from strong top-down control to ones in which prey are colimited by predators and resources. Comparisons between top predators showed that Daphnia, compared to Ceriodaphnia, more strongly limited the biomass of large algae in food webs at high productivity and total algal biomass in all nutrient-enriched treatments (both chains and webs). Thus, consumer identity and ecological context (productivity and heterogeneity of prey communities) may mediate the strength of zooplankton–algae interactions and the efficacy of trophic cascades.