Exploring the effects of invasive algae on the persistence of gorgonian populations
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The effects of invasive species on native fauna are understudied in marine ecosystems, even though the impact of introduced species must be taken into consideration for proper conservation and management. The main aim of this study was to investigate the effects of invasive algae on one of the highest diversity assemblages in the Mediterranean, the coralligenous. Two manipulative experiments were designed to study the effects of overgrowth of invasive species on juvenile colonies of the Mediterranean gorgonian Paramuricea clavata, an important structural species in coralligenous assemblages. The experiments were conducted in two French marine protected areas, Port-Cros National Park and Scandola Regional Park, which are separated by hundreds of km and are invaded by Caulerpa racemosa and Womersleyella setacea, respectively. Both invasive species had a strong and consistently negative effect on different components of fitness. This finding was demonstrated by lower survivorship, higher necrosis rates and lower biomass in P. clavata colonies exposed to invasive algal overgrowth compared to the controls. Because invasive species affected the viability of juvenile colonies of P. clavata, we conclude that the persistence of P. clavata populations affected by invasion can be jeopardized, especially in the case of populations already impacted by mass mortalities associated with climate warming.