Meso-predators: A confounding variable in consumer exclusion studies

• We experimentally manipulated predation on subtidal hard-substrate communities.
• Meso-predator abundance varied between caged and control assemblages.
• Predation had a strong influence on community composition.

Large consumers are important in determining community structure and have been extensively researched. The impact of predation on hard-substrate marine invertebrate communities tends to be mixed. Numerous exclusion studies fail to incorporate appropriate procedural controls or account for meso-predators. An experimental exclusion study was performed to investigate the effects of large predators on the development of hard-substrate invertebrate assemblages, whilst simultaneously quantifying changes in the abundances of meso-predators. Settlement plates were deployed subtidally and were either caged (to exclude large predators), placed in cage controls or left exposed to predation. Assemblages were collected and censused after 9, 14 and 19 weeks. The overall structure of the communities differed due to predation, and this effect was consistent through time. Encrusting bryozoans and hydroids occupied more space on exposed surfaces. Amphipod tube cover was greater in caged treatments, with the magnitude of the difference increasing through time, whilst solitary ascidian cover was greater on caged treatments in week 19 only. Meso-predators varied in their responses to large predators, with whelks more abundant in the absence of predators, whereas flatworm abundance was greater where predators were present. Our study demonstrated that strong effects of predators on particular taxa was able to alter the overall community composition, with the cascading effects of large predators on meso-predator abundances potentially contributing to this outcome.