Biogeographic patterns in rocky shore benthic invertebrate communities revealed by citizen science: limited influence of observer expertise

Despite the rapid shifts in species distributions observed on intertidal rocky shores worldwide, which reportedly occur faster than in other ecosystems, the patterns and drivers of these changes remain poorly understood in many regions, notably due to a lack of data. In this context, citizen science (CS) initiatives are valuable tools for addressing data gaps, providing broad spatial, temporal and taxonomic coverage. However, CS data require quality validation. In this study, we used CS data to investigate the biogeographic patterns of rocky shore communities in Brittany, the westernmost region of France (a biogeographic transition zone) while addressing the quality of CS data by quantifying the impact of expertise levels on taxonomic detection and identification, and thus on community distributions observed. Using clustering, we highlight 7 community types matching reported biogeographic patterns in the region. In particular, we identified a biogeographic barrier at the Ushant Front, with an associated ecotone community showing the highest species richness. We then explored the relative influence of environmental variables, spatial predictors and expertise-related descriptors on recorded community composition using variance partitioning. Our results confirm an expertise-related bias in the CS data. However, its influence on community composition was limited, and largely independent of spatial and environmental factors, which emerged as the primary drivers. Both large-scale spatial gradients (latitudinal and longitudinal) and local environmental conditions (bays and estuaries) were found to shape community structure. These findings suggest that CS data can provide valuable insights into the biogeography of rocky shores when variability in volunteer expertise and sampling effort is accounted for.