Parasite‐driven cascades or hydra effects: Susceptibility and foraging depression shape parasite–host–resource interactions

Abstract Parasites kill hosts but also can indirectly increase the abundance of their resources. Given this resource feedback, how much will parasites decrease host density? Can they increase host density? Seeking answers, we integrate trait measurements, a resource–host–parasite model and experimental epidemics with plankton. This combination reveals how parasites may decrease or increase host density. This spectrum of outcomes reflects tension between parasite‐driven mortality (a density‐mediated effect) and foraging depression upon contact with parasite propagules (a trait‐mediated one). In the model, mortality rises when higher susceptibility to infection increases infection prevalence. These large epidemics release resources while suppressing hosts (creating a trophic cascade). In contrast, when hosts are less susceptible and parasites depress host foraging, a resource feedback can elevate host density during epidemics (creating a hydra effect), particularly at higher carrying capacity of resources. This combination creates the hydra effect because it elevates primary production relative to per‐host consumption of resources (two key determinants of host density). We test these predictions qualitatively with trait measurements and a mesocosm experiment. Clonal lines of zooplankton hosts differed in their foraging depression and susceptibility. Then, with these different host genotypes, we created epidemics in mesocosms supplied with either low or high nutrients (to manipulate carrying capacity). Hydra effects and trophic cascades both arose and in the trait–nutrient combinations predicted by the model. Hence, we show how tension between trait‐ and density‐mediated effects of parasites can govern the fate of host density during epidemics—from trophic cascades to hydra effects. Read the free Plain Language Summary for this article on the Journal blog.