Wed, Aug 04, 2021:On Demand
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.
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.