A tale of two invaders: divergent spreading kinetics of the alien green algae Caulerpa taxifolia and Caulerpa cylindracea

The two alien green algae Caulerpa taxifolia and Caulerpa cylindracea (formerly Caulerpa racemosa var. cylindracea) are among the most notorious and threatening invasive species in the Mediterranean Sea. From the beginning of their introduction, C. taxifolia aroused a great concern due to its initial exponential spread and impacts on native ecosystems, whilst C. cylindracea received lower attention. In this paper, the distribution of the two aliens over the last 30 years in the Italian side of the Ligurian Sea (NW Mediterranean Sea) is reviewed and the kinetics of their spreading is compared. We combined available data on habitat occupancy with data on substratum cover obtained during underwater surveys to compute the average annual spreading rates for both aliens. C. taxifolia had an impressive expansion phase from 1984 to 2000 but then, its dispersal rate showed lower than that predicted and the species did not persist in areas formerly colonized. Today, abundance of this species is strongly declined as it disappeared from most of the attained areas. On the contrary, C. cylindracea exhibited an impressive and constant expansion from the beginning of its first appearance in the Ligurian Sea and, today, it is still increasing its range and habitat occupancy. Its abundance results 1.5-times greater than that reached by C. taxifolia during the period of its maximum expansion. The divergent kinetics of spreading of the two aliens prevents any possibility of generalization of the future behaviour of invasive species in the Mediterranean Sea: continuous and periodic surveys are thus mandatory to understand the kinetics of expansion and to define species-specific models.