LAUSR

Abstract Predation is considered an important structuring process in ecology; however, the effect size attributed to predation can vary across manipulative experiments. Complex interactions between predators of different sizes and trophic levels can confound observations in field-based experimental studies. Excluding large (macro) predators has demonstrated their potential to affect prey assemblage dynamics and structure. However, successful manipulations of smaller (meso) predators are limited due to the difficulty of excluding them in field studies while avoiding procedural artefacts. Here, we aim to manipulate both macro and mesopredators through the combination of exclusion cages (for macropredators) and a chemical deterrent (for mesopredators), avoiding the use of fine mesh and associated experimental artefacts. Using a novel chemical deterrent technique, we successfully manipulated predatory flatworm abundance in sessile hard substrate assemblages with no significant artefacts. Combined with the use of large cages to exclude fish, we tested the individual and interactive effects of macro and mesopredators on target prey species within sessile hard substrate assemblages. Flatworms reduced live barnacle abundances in the absence of fish, although this effect was small and only evident in the later successional stages. The effect of flatworms on barnacles was reduced in the presence of fish, resulting in more live barnacles persisting under the multiple predator scenario. Here, predatory interactions between different trophic levels were antagonistic, leading to reduced predation pressure on the common prey item. Considering mesopredator predation in predator exclusion experiments is important for correctly attributing effect sizes of larger bodied predators; failing to do so may confound experimental interpretations.