Host-Manipulation by Trophically Transmitted Parasites: The Switcher-Paradigm.

Host-manipulation by trophically transmitted parasites is thought to always predispose the intermediate hosts to enhanced predation by definitive hosts ('enhancement'). However, theory predicts that enhancement can disrupt stable, bottom-heavy predator-prey ratios, leading to fluctuation-driven extinction of intermediate hosts and parasites. How then can enhancement persist in nature despite this apparent instability? We address this paradox and conceptualize the 'switcher-paradigm', a novel framework incorporating sequential phases of reduced predation ('suppression') followed by enhancement. Theoretical models within the framework that consider 'switching' from suppression to enhancement indicate that switching likely increases parasite persistence and, in some circumstances, cancels out the effects of strong enhancement, leading to bottom-heavy predator-prey ratios. The switcher-paradigm confronts interdisciplinary research challenges, linking ecological processes across scales from within-host to community-wide dynamics.