Sublethal effects of parasitism on ruminants can have cascading consequences for ecosystems

Significance We found that pervasive parasitic infections reduce herbivory rates and can trigger trophic cascades. Lethal parasites clearly have cascading impacts on ecosystems, but whether common sublethal infections have similar effects is largely unknown. Using a mathematical model, we probed how parasites that reduce host survival, fecundity, or feeding rates can indirectly alter producer biomass in a helminth–ruminant system. We found that both lethal and sublethal infections triggered trophic cascades by altering the biomass of ruminant herbivore hosts and their resources. However, a global meta-analysis revealed that helminths tend to have pervasive sublethal effects on free-living ruminants, including by reducing host feeding rates. Our findings suggest there are widespread, but overlooked, ecological consequences of sublethal infections in natural ecosystems.