Unravelling the effects of multiple cross-ecosystem subsidies on food webs.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Adjacent ecosystems are connected by the exchange of resources across ecosystem boundaries, also known as cross-ecosystem subsidies, which can directly influence consumers and indirectly influence food-web interactions, such as bottom-up propagation of energy and top-down trophic cascades. The trophic level at which a subsidy enters a recipient ecosystem has the potential to alter consumer dynamics and mechanistic drivers of food-web interactions. Here I examined the effects of subsidy trophic level on bottom-up propagation of energy through food webs, top-down trophic cascades on primary producers, and reciprocal subsidies back to donor ecosystems. Results of a meta-analysis revealed results from past studies investigating effects of resource subsidies to primary producers, primary consumers, and predators on bottom-up energy propagation and top-down trophic cascades were inconsistent. The inconsistencies were likely due to differences in study duration or how subsidies were manipulated. To address some of the knowledge gaps revealed from the metaanalysis, I ran an 18-month pond mesocosm experiment, manipulating subsidies to primary consumers (terrestrial leaf subsidies), subsidies to predators (terrestrial insect subsidies), and presence of top predators (fish). Subsidy trophic level significantly altered food-web structure, with terrestrial insect subsidies increasing biomass of intermediate and top predators, while terrestrial leaf subsidies increased biomass of intermediate predators, increased isotopic niche widths of primary consumers, and altered community composition of invertebrates. However, although subsidies altered biomass and composition within trophic levels, subsidy frequency and strong top-down control had overriding influences on food-web interactions. Subsidy effects on trophic cascade strength varied through time and with subsidy trophic level; insect subsidies only increased trophic cascades early in the experiment, whereas leaf subsidy effects on trophic cascade strength increased with experiment duration. Interestingly, insect and leaf subsidies not only altered top-down interactions, but also influenced temporal and spatial variation of primary consumer and primary producer biomass. The input of subsidies to predators increased the total biomass and rate of aquatic insect emergence, compared to dampened emergence with subsidies to primary consumers, thereby creating sources and sinks of terrestrial energy. Overall, results suggest that altering the flow of energy subsidies entering at different trophic levels, not only alters bottom-up and top-down interactions within the recipient food web, but can affect the extent to which exchanges of resources are reciprocal.