IF YOU CAN’T BEAT THEM, JOIN THEM: ENEMY SHELLS AS REFUGIA FROM GRAZING & COMPETITION PRESSURES

Ecosystem engineers have profound effects on ecosystem structure and functioning. While habitat-forming ecosystem engineers like trees and corals are acknowledged for their habitat-modifying function, habitat-modifying mobile organisms are not typically recognised for their habitat-forming function. Limpets are appreciated as important modifiers of benthic assemblages through their grazing activity, but little is known about the potential role of their shells in providing habitat for either their competitors or the species they consume. We show that in the Northeast Atlantic, limpets are simultaneous consumers and facilitators of algae on rocky shores, with species-specific outcomes. When limpets are abundant, only the most grazing-resistant algae persist on the rocks; however, when one particular species (Patella ulyssiponensis) is present, rich algal assemblages develop on their shells. This facilitatory role is less pronounced in other species (Patella vulgata, Patella depressa). Manipulative experiments suggest that aggressive behaviour in P. ulyssiponensis moderates mutual grazing on shells, providing an important associational refuge for algae from consumption. Patella vulgata shells provide an associational refuge for small individuals of other limpets from competition on primary rock substrata. Limpets not only modify the surrounding environment but also constitute whole microcosms supporting diverse and dense epibiotic communities. Not only is this the case in natural habitats, but even more so in artificial environments where smooth engineered habitats like sea walls favour limpet dominance. This can lead to the emergence of ‘limpet barrens’ (akin to subtidal urchin barrens), where high limpet densities coupled with maximal grazing efficiency prevent recruitment of other taxa to the surrounding substratum. These results are not just relevant for theoretical ecology but also for practical wildlife management in a changing world.