Intertidal mussels as ecosystem engineers: maintenance of invertebrate assemblages amid intertidal stress gradients

Species that form complex biogenic structures may increase niche space for many others and are assumed to be paramount for whole‐assemblage stability and ecosystem functioning. Because they often ameliorate physical conditions, ecosystem engineering species are also predicted to become more important with environmental stress. Testing this hypothesis, however, has proven difficult because isolating facilitation effects along the entire stress gradient is challenging. Here we addressed motile invertebrate assemblages associated to the mid‐intertidal chthamalid barnacle cover at four rocky shores, either with (‘facilitated') or without (‘control') a secondary natural cover of small mussels (Mytilaster solisianus and Brachidontes darwinianus). Following a factorial balanced design, replicate samples were taken at three tidal heights in all sites, encompassing a clear gradient of thermal stress and desiccation potential along the vertical range of the mussel‐enriched zone. While observations on general abundance were mostly consistent with stress‐independent facilitation, results for richness, diversity and assemblage structure indicated overarching stress‐dependent responses. All these later three response variables steadily declined from the low to the high level in the control barnacle habitat but remained unaltered in the mussel‐facilitated habitat. Increased facilitation higher on the shore is mostly due to retention of stress‐vulnerable groups such as polychaetes, flatworms and large nematodes which virtually collapse in the higher control habitat, further indicating that mechanisms of mussel facilitation involve relief of environmental stress rather than protection from higher‐order consumers. Our results suggest that mussel ecosystem engineering was fully preserved during the summer season when sampling took place. Ongoing research simulating heat waves compatible to climate‐change scenarios will test whether mussel facilitation would hold over the coming decades.