Patterns of wide‐scale substitution within meadows of the seagrass Posidonia oceanica in NW Mediterranean Sea: invaders are stronger than natives

• 1. The endemic seagrass Posidonia oceanica meadows in the Mediterranean Sea are presently facing invasion by alien algal species, especially where they are already showing signs of regression as a result of other stresses. Regressed meadows may be recolonized by native or alien macrophytes that take the place of P. oceanica.
• 2. The pattern and degree of substitution by either native (Cymodocea nodosa and Caulerpa prolifera) or alien (Caulerpa taxifolia and C. racemosa) species in 17 P. oceanica meadows along the coast of Liguria (Italy, NW Mediterranean) were investigated. The invaders were found to be stronger colonizers, in terms of habitat occupancy within meadows, than the natives.
• 3. A set of indices was used to quantify biological invasion within the seagrass ecosystem and to explore the relationship between the meadow's state of health and the replacement by substitutes. The ecological status of the receiving habitat was shown to play a major role in modulating the spread of substitutes.
• 4. Substitutes are thought likely to act simply as passengers of habitat degradation in deep meadows, where previous regression has increased the likelihood of their colonization, but could be the drivers for P. oceanica loss in shallow ones, where they settled even in healthy meadows thus starting the process of regression. Long term monitoring and sustained maintenance of healthy P. oceanica meadows appear to be the most effective management tools to contain the risk of ecological substitution. Copyright © 2010 John Wiley & Sons, Ltd.