Port construction increases community variability without directional change in adjacent rocky intertidal...

Coastal infrastructure such as ports and breakwaters can substantially alter hydrodynamic conditions, yet their long-term ecological consequences for adjacent natural habitats remain poorly understood. Here, we used a long-term before-after-control-impact (BACI) design to evaluate the effects of port construction on rocky intertidal communities along the Pacific coast of eastern Hokkaido, Japan. We analyzed changes in abundance, diversity metrics (Hill's numbers), and community structure across four major functional groups using a 10-year dataset spanning five-year period before construction and a five-year period beginning eight years after port construction across one impact and four control shores. Results showed that port construction did not induce a consistent directional shift in community composition, but instead increased variability in community structure at the impacted shore. This change was associated with a pronounced, direction-specific reduction in wave exposure caused by the offshore breakwater. At the functional-group level, responses were limited, with significant declines detected only in herbivorous molluscs, whereas species-level responses were highly heterogeneous within and among groups. Diversity responses were group-specific and primarily reflected changes in evenness and dominance rather than species richness. Wave exposure explained a significant but modest proportion of variation in community structure with species-specific effects. Together, these results indicate that port construction weakens wave-driven environmental filtering, leading to increased variability rather than deterministic shifts in community structure. Our findings emphasize that evaluating the ecological impacts of coastal infrastructure requires moving beyond mean responses to explicitly account for changes in variability and community predictability.