Invasive species is a term used to refer to a non-native animal, plant or micro-organism whose introduction is causing negative environmental impacts. Most invasive species possess common characteristics which allow them to proliferate and dominate a new ecosystem. They usually lack natural predators, pests, diseases and competitors. Nearly 1000 non-native species have been identified in the Mediterranean Sea. Many of these species have established themselves and have become invasive, threatening marine biodiversity and damaging local ecosystems.

Invasive species are considered one of the major drivers of global biodiversity loss. They pose a threat to native species by preying on them, introducing new diseases, competing for resources and space and altering the ecosystem. For example, the Calulerpa sp., an invasive alga found In the Mediterranean Sea, possesses a toxin which kills native organisms which feed on it such as the gilt-head bream (Sparus aurata). Humans have facilitated the dispersal of marine invasive species, through the release of pets from aquariums, fish from aquaculture and marine organisms from the water ballast of ships. The invasive blue crab (Callinectes sapidus) was accidently introduced in the Mediterranean through the ballast water of ships. This species has no natural predators, has a fast reproductive rate and competes with native organisms for food and resources. 

Climate change also increases the likelihood of species moving across countries to find a more suitable climate possibly causing a new introduced species to become invasive. For example, the lionfish (Pterois volitans), has migrated from the Red Sea to the Mediterranean Sea. This species has no natural predators in the Mediterranean Sea and is therefore able to reproduce rapidly, prey and compete with native species. Climate change may also stress native species, reducing their resilience to the impacts of invasive non-native species.

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