Subsidy trophic level differentially affects bottom-up and top-down food-web interactions

Amanda J. Klemmer, University of Maine, Orono, ME, Isaac Shepard, School of Biology and Ecology, University of Maine, Orono, ME and Angus R. McIntosh, School of Biological Sciences, University of Canterbury, Christchurch, New Zealand

Background/Question/Methods
The trophic level at which a subsidy enters an ecosystem can influence food-web structure and dynamics. For example, subsidies to predators enter at the top of food webs and can have consequences for biomass at lower trophic levels through top-down processes. In comparison, subsidies to primary consumers can lead to bottom-up increases in consumer biomass which subsequently alter community structure. Although these processes are well documented in isolation, little is known about how these two types of cross-ecosystem resources interact to alter food-web regulation. Our aim was to investigate how subsidies entering at the top and bottom of food webs altered bottom-up changes to consumer biomass and food-web structure, testing the hypothesis that subsidies entering at different trophic levels would have fundamentally different effects on these characteristics. We ran an 18-month, freshwater pond, mesocosm experiment, factorially manipulating input of subsidies to predators (terrestrial insect subsidies), input of subsidies to primary consumers (terrestrial leaf subsidies), and presence of top-predators (fish). We measured top-predator and primary consumer biomass, primary consumer community composition, and stable isotope mixing models of key organisms in the food web.
Results/Conclusions
We found that the trophic level of subsidy entry fundamentally altered food-web composition and biomass, as well as indirectly altering other food-web interactions, such as competition and top-down consumption. Subsidies to primary consumers altered community composition, but did not affect primary consumer biomass. Subsidies to top-predators increased their biomass, which interestingly did not lead to increased top-down control. However, isotope mixing models revealed competition for subsidies between male and female top predators, increasing female omnivory. We also observed competition and facilitation between top- and intermediate-predators with different subsidy inputs. In conclusion, not all subsidies directly increased consumer biomass, but may alter consumer composition. Also, top-down effects of subsidies aren’t as strong as hypothesized, even with extreme changes in biomass at top-predator trophic levels.