The ecological condition of vermetid platforms affects the cover of the alien seaweed Caulerpa cylindracea

The seaweed Caulerpa cylindracea Sender is one of the most important invaders on Mediterranean rocky shores. However, many driving pressures affecting its spread are poorly understood and seem to involve the interactions between abiotic and biotic factors. We studied the invasiveness of C. cylindracea on two shallow vermetid platforms with a contrasting ecological status on which C. cylindracea was first detected simultaneously 15 years ago. The cover values of C. cylindracea and the other macroalgal species were assessed for one year, embracing the whole platform width. Caulerpa cylindracea cover was higher on the platform that had a low ecological status, especially during warmer months at the outer seaward margin. The ecological status of the overstory of native species seems to be a key point conditioning the success of C. cylindracea invasiveness on these platforms.