Boom-bust cycles constrain host-parasite dynamics, suppress parasite spread, and drive parasites extinct

Host-parasite theory is rooted in equilibrium dynamics. However, many host species exhibit "boom-bust" life histories or range expansions characterized by population booms and severe bottlenecks. The dynamic host density in boom-bust systems may disrupt the interactions between density-dependent processes, like parasite transmission and birth, resulting in unique dynamics compared to a host population at equilibrium. We subjected a simple compartment model to recurring host bottlenecks to approximate a boom-bust life history. We found that recurring bottlenecks suppressed disease spread by giving the host population an opportunity post-bottleneck to expand faster than the disease could spread. As bottlenecks became more frequent and/or severe, disease spread was suppressed to such low levels that parasite extinction was virtually guaranteed. We found that our model was conservative and presented a near best case scenario for the parasite. Our results indicate that the dynamic host density of boom-bust systems creates new system behaviors that are not seen in equilibrium models. Additionally, we argue that our results generalize to any horizontally-transmitted symbiont, including mutualists and commensals. We conclude that boom-bust dynamics must be explicitly modeled to accurately predict disease spread and the resulting evolutionary dynamics in hosts and their symbionts.