Invasive alien species, what are they and how do they do it?

First of all, we must define certain concepts. Species that exist within their natural range and dispersal area are called native or autochthonous, while exotic or allochthonous species are those introduced outside their natural range and potential dispersal area, which they would not have been able to occupy without direct or indirect introduction, or without human care.

Of all the exotic species that can be found in ecosystems, only a very low percentage become established and stabilized in the long term (established species), and only a small percentage of these become invasive. Therefore, an invasive alien species is one that is introduced or established in an ecosystem, and is an agent of change and threat to native biological diversity. 

A very current example of an invasive alien species on the Spanish coast is the seaweed Rugulopteryx okamurae. It is a brown algae first detected in Ceuta in 2015, where more than 5000 tons were collected that same year. Since then it has spread throughout the Atlantic and the Mediterranean reaching the coast of Almeria. It has covered more than 70% of the seabed between 5 and 30 m depth (García-Gómez et al. 2019). The greatest threat of this species is that it displaces local algae and key species for the marine system, including Posidonia (Posidonia oceanica), a fact that has been observed in areas such as Estepona, Maro or Almería.

Beach of Tarifa (Cádiz). Maria Altamirano.

The most likely is that, like many other allochthonous species, this species arrived with the water from the fill of large ships that bring part of it to the ports of destination, such as Tangier or Algesires.

In most cases related to invasive species, but especially in cases such as the Rugulopteryx okamurae algae, in which the expansion is occurring at dizzying rates, citizen participation is very important, since the data they provide are of great importance to detect their advance and be able to fight against them (García-Gómez et al. 2021).

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