Parasite-driven cascades or hydra effects: susceptibility and foraging depression shape parasite-host-resource interactions

Abstract

When epidemics kill hosts and increase their resources, should the density of hosts decrease (with a resource increase, this constitutes a trophic cascade) or increase (a hydra effect)? Seeking answers, we integrate trait measurements, a resource-host-parasite model, and experimental epidemics with plankton. This combination reveals how a spectrum from cascades to hydra effects can arise. It 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. Epidemics release resources while suppressing hosts (creating a cascade). In contrast, when hosts are less susceptible and parasites depress their foraging, a resource feedback can elevate host density during epidemics (creating a hydra effect), particularly at higher carrying capacity of resources. This combination elevates primary production relative to per-host consumption of resources (two key determinants of host density). We test these predictions of the qualitative effects of host traits and resource carrying capacity with trait measurements and a mesocosm experiment. Trait measurements show clonal lines of zooplankton hosts differ in their foraging depression and susceptibility. We seeded resource-host-parasite mesocosms with different host genotypes and provided different nutrient supplies to test model predictions. Hydra effects and trophic cascades arose under different conditions, as predicted by the model. Hence, tension between trait-mediated and density-mediated effects of parasites governs the fate of host density during epidemics, from cascades to hydra effects, via feedbacks with resources.

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

“…All data and theoretical code required to produce the results are available in Dryad Digital Repository https://doi.org/10.5061/dryad.mw6m905zg (Walsman et al., 2022).…”