Can secondary species maintain a primary role? Consistent inter-regional effects of understory algae on diversity

Loss of habitat is a global threat to biodiversity. Habitat-forming species in particular are shifting their distribution at local and regional scales, changing habitat aspect globally. In temperate intertidal rocky shores, this poleward shift of canopy-forming seaweeds is leading to a shift to mat- or turf-dominated communities. These changes and their consequences are poorly understood and can vary globally. Most studies, however, have focused on local and regional processes. Using rocky low intertidal algal-dominated systems, this study aims to understand the independent role of a rich understory vegetation layer formed by cespitose algae as community drivers. Moreover, the study aimed to understand whether the observed patterns can be consistent over spatial regions, thus three distinct regional zones of the North Atlantic (Eastern shores of Nova Scotia, Canada; the Poitou-Charentes coast, France; the Eastern shores of Northern Ireland, UK) were sampled as part of this study. From surveys, results showed that in the intertidal zone with low desiccation potential, canopies generally do not drive understory cespitose species richness or distribution. Cespitose algae on the other hand positively influenced richness in all regions. Community composition was also influenced by cespitose algal species, which increased the number of mobile epifauna species, while decreasing the number of encrusting epifauna species. This has wide implications: (i) understory species that are often overlooked may play a primary role as habitat formers; (ii) understory species may help maintain biodiversity in a changing world where primary species distribution is shifting.