Can manipulative parasites modify host-mediated trophic effects? Experimental evidence from <em>Schistocephalus solidus</em> and three-spined sticklebacks

Parasites can alter host traits, thereby reshaping host interactions and modifying density- and trait-mediated effects in trophic cascades. But despite increasing research in parasite ecology, the cascading effects of parasitism from individual hosts to population and community levels remain underexplored. Here, we aim to fill this gap by conducting a large-scale mesocosm experiment replicating key phases of the parasite’s development to test our hypothesis that the trophically transmitted tapeworm Schistocephalus solidus weakens stickleback-driven trophic cascades by altering host performance and feeding behaviour. We quantified parasite effects on three-spined sticklebacks (Gasterosteus aculeatus) and their prey communities. Schistocephalus solidus infection reduces stickleback body condition and shifts zooplankton biomass, transiently dampening the likelihood and strength of stickleback-driven trophic cascades at specific time points of infection. Using structural equation models, we show that parasite-driven trait-mediated indirect effects (TMIEs) are equivalent to density-mediated indirect effects (DMIEs) on trophic cascades. Strikingly, our results suggest that infection increases per capita zooplankton consumption of fish and amplifies parasite-driven TMIEs, counterbalancing the effects of DMIEs due to fish mortality.Overall, our findings highlight the complexity of host-parasite interactions and their community and ecosystem-level consequences.