Constant and temporally variable spatial subsidies and the strength of trophic cascades

Natural ecosystems are open to flows of energy, materials, and organisms. These subsidies are ubiquitous and influence ecosystem structure and functioning at local and regional extents. Subsidized consumers can attain higher biomass and abundance than unsubsidized consumers but the indirect, cascading effects generated from subsidized consumers are not well understood. I derive ecosystem models to investigate the relationship between subsidies and trophic cascades. I show that the ratio of subsidy to equivalent in situ prey may not be the best predictor of consumer response to subsidies, particularly when subsidies are temporally variable. I predict strong generalist consumer responses to subsidies in ecosystems with high in situ prey and relatively frequent subsidies. Next, I use response ratios to quantify the relative effect of predator regulation of herbivores and consumer-mediated recycling on producers stocks. I observe that predator regulation of herbivores has a larger, positive, effect on producer stocks than consumer-mediated recycling, however, consumer-mediated recycling can influence producer biomass in many cases. The relative contribution of both mechanisms to cascading trophic interactions depends on feeding relationships between predator and prey, nutrient turnover rates and the rate of external nutrient loading.;Trophic cascade theory and experiments to date, have been conducted in closed ecosystems but recent evidence suggests that predators can have cascading effects across ecosystem boundaries. I derive a model of a recipient ecosystem and demonstrate that ecosystems with higher rates of constant subsidies experience stronger trophic cascades because these subsidies facilitate higher secondary production and consumption. I extend this previous model to meta-ecosystem extents in order to consider the effects of reciprocal pulsed subsidies on ecosystem functioning. I show that reciprocal pulsed subsidies can be reinforcing, particularly when the flows occur with a low level of asynchrony. These positive feedbacks can dampen the strength of local trophic cascades but increase the strength of trophic cascades across local ecosystems. Overall, my thesis provides a suite of novel predictions on the relationship between subsidies and trophic cascades. We must consider ecosystems as open to spatial flows if we wish to understand and manage natural ecosystems.