The joint influence of environmental and anthropogenic factors on the invasion of two alien caulerpae in northwestern Mediterranean

Many factors are known to influence the invasion of alien species depending on the spatial scales considered. Caulerpa taxifolia and Caulerpa cylindracea are two green macroalgae that are classified as among the most invasive exotic macroalgae in the world. Their spread into the Mediterranean Sea seems to be favored by the concomitant decline of Posidonia oceanica . We studied the effect of four anthropogenic pressures and four environmental factors, and the conservation statuts of P. oceanica seagrass , on the presence/absence and abundance of these caulerpae along 336 km of French Mediterranean coastline (159,691 ha of seabed) using generalized linear model (GLM). Our results showed that the presence of caulerpae increased as the conservation status of the P. oceanica seagrass decreased and as boat anchoring intensity is high. Also, the increase in turbidity values significantly reduced the presence of caulerpae independently of variations in other environmental factors. Urban effluents were the human factor that interacted the most by disturbing all environmental variables. These results contribute to understanding the large-scale factors which have promoted caulerpae invasion and highlight the necessity to improve marine habitat's conservation for prevent biological invasions. Graphic abstract