Consumer-resource cycles present a beguiling theoretical challenge to ecologists. Populations often readily cycle in simple models, yet why don’t they cycle more in nature? Or do they cycle more than they should? Consumer-resource systems might be more stable than we expect because consumers more often have a type III functional response than we anticipate; because stage structure (i.e., competition between juvenile and adult stages for shared resources) can dampen oscillation size; and/or virulent parasites can prevent over-exploitation of resources by consumer-hosts. Here, we use a suite of models, parameterized for plankton system, to show how interplay between functional responses of consumer-hosts, stage-structure, and disease epidemics can destabilize otherwise stable dynamics, but in turn could stabilize otherwise stable dynamics. We make our case using systems of parameterized differential equations, bifurcation diagrams, and analysis of feedback loops.
Results/Conclusions
First, we revisit and expand upon how oscillating dynamics of consumer-hosts and resources can be stabilized by disease. Type II functional responses of consumer-hosts can destabilize dynamics by generating positive density dependence in prey (through ‘safety in numbers’), a low level positive feedback. Disease can suppress that lower level one. However, through positive feedbacks generated in more connected upper level loops, disease can then trigger oscillations in the dynamics that they had stabilized (particular for parasites with more castration-inducing life histories). Alternatively, type III functional responses in consumer-hosts (e.g., Daphnia) ordinarily confer stability to consumer-resource dynamics by squelching that lower-level positive feedback. However, parasites, through upper level loops, can generate new types of positive feedback that then destabilizes those dynamics. This result applies to unstructured consumer-hosts and those with stage-structure. While making this argument, we point out similarities to models with predators, and highlight opportunities for parasite-mediated hydra effects to arise. Hence, an explicit focus on upper-level feedback loops with parasites helps why populations might cycle more or less than otherwise anticipated with their resources.