Tropicalisation of rocky shore gastropod communities along the Baja California Peninsula

Tropicalisation is rapidly restructuring marine communities globally. Despite extensive research in subtidal habitats, the extent of tropicalisation and its impact on rocky intertidal communities remains poorly understood. Here we integrated contemporary field surveys with genetic barcoding, historic surveys, museum collections, and fossil records to document changes in the rocky shore gastropod communities along 8 degrees of latitude (approximately 3,120 km of coastline) of the Baja California Peninsula. We detected poleward range expansions of 30 tropical and subtropical species, and trailing-edge retractions of 13 temperate species, with some range shifts occurring across long-standing biogeographic boundaries. Fossil evidence reveals that several range-extending species previously occupied latitudes beyond their modern distributions during Pleistocene warm periods, suggesting that contemporary expansions echo historical responses to climatic warming, albeit under accelerated rates. Using Lottia gigantea as a case study, we demonstrated early range retractions can be marked by shifts in size-frequency distributions demonstrating declines in abundance, and loss of small individuals suggestive of reduced recruitment. We showed that rocky intertidal systems are sensitive to climate-driven range shifts, and highlighted the importance of integrating historical, palaeontological and contemporary data to detect early ecological and evolutionary consequences of tropicalisation. Continued monitoring combined with molecular approaches is crucial for predicting and managing biodiversity responses under ongoing climate change.