The Canary Islands: A Hub for Marine Alien Species Invasion.

When Is a Species Considered Alien?

Alien or non-native species are species that are found outside their original range due to voluntary or involuntary introduction by humans. Non-native species may or may not become invasive. For an alien species to be invasive, it must survive the journey, adapt to its new environment, and reproduce on its own without human help (Fig. 1).Consequently, invasive alien species are those alien species that threaten biodiversity and the associated ecosystem services, therefore negatively affecting them by altering them and significantly impacting human health and economic activities.

The introduction of non-indigenous species (NIS), along with other threats such as habitat destruction, pollution, overfishing, and global climate change, is compromising biodiversity, ecosystem services, and ecosystem functionality on a global scale. As a human-driven phenomenon, NIS invasions not only threaten ecosystems but also have severe economic and social consequences, impacting human well-being (Castro et al., 2024; Tricarico et al., 2018).

Figure 1: Invasion process of an alien species.

The Canary Islands, strategically located in the Atlantic, are a crossroads for international maritime traffic, making them particularly vulnerable to the introduction of non-native species. The main ports of the archipelago, located in Las Palmas de Gran Canaria and Santa Cruz de Tenerife, play a key role in this context, acting as entry points for many alien species through two predominant vectors:

Ballast water: used to stabilize ships during voyages, carries marine organisms that, once released into ports, may find favorable conditions to proliferate. Unfortunately, despite the 2004 Ballast Water Management Convention, which sets rules to control this, the problem persists (International Maritime Organization, n.d.).
Biofouling: occurs when algae, molluscs, and other organisms attach to the surfaces of ships and are transported between regions, aiding the spread of species across different geographical areas.

These processes are further amplified by artificial port infrastructures, such as docks and piers, which provide optimal substrates for the settlement of alien species.

Macroalgaes, in particular, emerge as one of the most significant groups of NIS in the Canary Islands, due to their ability to quickly colonize artificial surfaces and adapt to diverse environmental conditions. In Tenerife, the largest and most populated island of the archipelago, the risk of macroalgae spread is especially high, disrupting local ecological balances (Castro et al., 2022).

A Hotspot for Non-Native Species

Specifically, Tenerife provides a favorable environment for H. incrassata, as it is less affected by the substrate mobility and wave energy that characterize the coasts of the other Canary Islands (Sangil et al., 2018).

This scenario differs significantly from what has been observed in the Mediterranean, where the species has shown a faster spread, establishing persistent and sometimes invasive communities. In the Canary Islands, by contrast, its presence, while significant, does not yet appear to have a negative impact on local communities (Sangil et al., 2018). However, the potential ecological impacts, such as the displacement of native species like Cymodocea nodosa, cannot be ignored, especially under future conditions of climate change and tropicalization.

Moreover, the case of H. incrassata is part of a broader trend of ecosystem changes driven by the presence of other alien species in the archipelago, a growing concern for local ecological balances. In fact, macroalgae like Penicillus capitatus and Caulerpa prolifera have previously demonstrated a rapid expansion in similar environments, facilitated by warming waters and other environmental changes (Sangil et al., 2018).

These observations highlight the critical importance of implementing comprehensive and continuous monitoring of the spread of Halimeda incrassata and other alien species across the Canary Islands. The ongoing impacts of climate change, including rising sea temperatures, tropicalization, and the degradation of marine habitats, could create increasingly favorable conditions for the expansion of these species. Such changes may disrupt the delicate balance of local marine biodiversity, potentially leading to shifts in species composition, declines in native populations, and alterations in the structure and functionality of coastal ecosystems. Proactive measures are essential to mitigate these risks and to safeguard the ecological integrity and resilience of these vital habitats (Sangil et al., 2018; Vaquer-Sunyer et al., 2020; Mateo-Ramirez et al., 2022).

Source: Innoceana

Figure 2. Halimeda incrassata (J.Ellis) J.V.Lamouroux.

As mentioned above, the Canary Islands represent a hotspot for the introduction of marine alien species. Among these, Halimeda incrassata (Fig. 2), a tropical green algae, was recently detected in the archipelago’s waters, colonizing deep sandy seabeds (5-55 meters). Although its current distribution is limited, its potential as a habitat-forming species raises concerns due to the ecological disruptions it may cause.