Coastal ecosystems are suffering severe impacts worldwide due to excessive human pressure. Habitat destruction, pollution, eutrophication, species introduction, overfishing and global warming, which often act synergistically, are affecting species, ecosystems and their ability to provide ecosystem services (Halpern et al. 2008). For example, the Canary Islands are a “hot spot” of marine biodiversity in the North Atlantic (Sansón et al. 2001), which is threatened by human impacts, e.g. pollution, overfishing, occupation of the coast and progressive tropicalization (Riera et al. 2015).

Along rocky shores of temperate and subtropical areas, large canopy-forming brown algae, in particular kelps (Laminariales, Phaeophyceae, Ochrophyta) and fucoids (Fucales, Phaeophyceae, Ochrophyta), are the dominant species in pristine environments (Schiel and Foster 2006). These large perennial macroalgae are considered as “engineering species” (Jones et al. 1994), because their three-dimensional structure dramatically alters the physical, chemical and biological environment. These forests provide shelter, food, habitat and nurseries for a multiplicity of species (Cheminée et al. 2013). The decline of kelps and fucoids is a global phenomenon due, directly or indirectly, to human-mediated activities (Wernberg et al. 2011, Lamela-Silvarrey et al. 2012, Franco et al. 2015). Some species have even been driven to regional and local extinction (Thibaut et al. 2005, Franco et al. 2015, Thibaut et al. 2016a). The loss of these well-structured and diverse ecosystems facilitates the appearance of less complex habitats, such as filamentous algal turfs, ephemeral seaweed assemblages and barren grounds dominated by encrusting algae and sea urchins (Benedetti-Cecchi et al. 2001, Ling et al. 2015).

The genus Cystoseira C. Agardh (Fucales, Phaeophyta) is distributed in temperate and subtropical coasts around the world, although 80% of the species live in the Mediterranean Sea (Oliveras and Gómez 1989). In the Mediterranean and the adjacent Atlantic Ocean, species of the genus Cystoseira are the main group of habitat-forming macroalgae, from the littoral to the lower limit of the euphotic zone (Giaccone et al. 1994, García-Fernández and Bárbara 2016). Losses of Cystoseira forests have been reported all around the Mediterranean and attributed to habitat destruction, eutrophication and overgrazing by herbivores (Thibaut et al. 2005, Iveša et al. 2016, Blanfuné et al. 2016a). Due to their high sensitivity to anthropogenic impacts, several species of Cystoseira indicate high quality waters and facilitate the implementation of the EU Water Framework Directive (2000/06/EC) (Ballesteros et al. 2007, Blanfuné et al. 2016b, 2017). All the Mediterranean species of the genus Cystoseira, except C. compressa, have been protected under the Annex II of the Barcelona Convention (2010). Five species, Cystoseira amentacea, Cystoseira mediterranea, Cystoseira sedoides, Cystoseira spinosa and Cystoseira zosteroids, are protected under the Berne Convention (Annex I, 1979). In addition, all Mediterranean Cystoseira species are under surveillance by international organizations, such as IUCN, RAP/ASP and MedPan (Thibaut et al. 2014). All species of Cystoseira are “habitat-forming” and are therefore considered EU habitats of interest (Micheli et al. 2013).

The brown alga Cystoseira abies-marina (S. G. Gemelin) C. Agardh has been considered the most abundant fucoid species on rocky shores of the Canarian Archipelago (Wildpret et al. 1987, Tuya and Haroun 2006), and its populations typically form extensive stands in both the eulittoral and shallow sublittoral, mainly on rocky wave-exposed zones (Wildpret et al. 1987, Medina and Haroun 1993). C. abies-marina is a caespitose plant with large numbers of erect branches, up to 50 cm long. Similar to other species in the genera Cystoseira, this species undergoes an annual thallus loss at the end of summer, when a high proportion of the fronds break down at the base. The holdfasts overwinter and regrow the next year. Therefore, although individuals are perennial, the thalli are annual (Buonomo et al. 2017). However, the plant never goes through a total rest phase: during unfavourable months, branches from different seasons coexist (González-Rodríguez and Afonso-Carrillo 1990). This alga spreads through both vegetative propagation and sexual reproduction (Medina 1997). Similar to other species of the genus, thalli are negatively buoyant and propagules normally settle at <20-40 cm from the source population (Mangialajo et al. 2012), which gives the species a low-dispersal ability (Bulleri et al. 2002). This is one of the most productive macroalgae in the Canary Islands (Johnston 1969), and at the end of summer, after the maximum reproductive peak, it is possible to find a large amount of wrack on beaches from nearby forests (Portillo-Hahnefeld 2008).

In the last few decades, Cystoseira abies-marina forests have declined significantly at certain points of the Canaries (Medina and Haroun 1993, Rodríguez et al. 2008). In order to analyse the long-term patterns in the distribution and extension of C. abies-marina along the entire coastal perimeter of the island of Gran Canaria, we collected all available data to reconstruct historical distributions. The aims were: (i) to provide an up-to-date assessment of the current distribution and extent of C. abies-marina, (ii) to facilitate a comparison with historical data, including populations from certain sites, and (iii) to evaluate the influence of local anthropogenic pressures, as drivers of regression.

Changes in the distribution and extent of Cystoseira abies-marina on the island of Gran Canaria over the last few decades are evident and dramatic. In the late 1980s, C. abies-marina occupied 928 ha (12.84% of the rocky bottoms) and now it only covers 7.4 ha (0.1%). Existing C. abies-marina populations have been reduced to narrow belts in the lower eulittoral, i.e. as scattered patches with underdeveloped branches. Our results are in agreement with those found for other Cystoseira species from the Mediterranean Sea (Thibaut et al. 2005, Mangialajo et al. 2008, Sales et al. 2011), for other fucoids from the Canary Islands (Rodríguez et al. 2008, Riera et al. 2015) and, in general, for habitat-forming brown algae worldwide (Wahl et al. 2015). Our data show a similar trend to that observed for C. brachyccarpa var. brachycarpa, a species having the same depth range and ecological function as C. abies-marina, including a massive decline from the sublittoral to a narrow fringe immediately below the surface (Thibaut et al. 2015, 2016b).

It is plausible that the area occupied by Cystoseira abies-marina in the 1980s is not entirely accurate, because of the lack of technical procedures to accurately trace communities at this time. The map of Wildpret et al. (1987) only reached 10 m depth, so they may even have underestimated the area occupied by C. abies-marina. Even assuming these inaccuracies, the regression of C. abies-marina is acute, in particular because all sublittoral populations have been lost.

In our study, we found no direct relationship between local levels of anthropogenic pressures and the magnitude of local regression; the decay has been dramatic from almost pristine environments to highly altered coasts. This result contrasts with the disappearance of some Cystoseira species only from highly artificialized areas (harbours, marinas, piers, etc.) and waters severely polluted in the Mediterranean (Thibaut et al. 2014, 2015, Iveša et al. 2016). However, a similar decline has been observed in the pristine environments of the National Park of Port-Cros in France (Thibaut et al. 2016b). In a similar study, the temporal decay in the vitality of the seagrass Cymodocea nodosa in Gran Canaria was connected with an increase in local anthropogenic impacts (Tuya et al. 2014).

The decline of C. abies-marina in Gran Canaria has occurred in a period of pronounced urban and tourism development and, therefore, of many local impacts (Tuya et al. 2014, Ferrer-Valero et al. 2017). Today, the population, urbanization and infrastructure are heavily concentrated on the coast of the island, particularly in the northeast, east and south (Fig. 1). Gran Canaria currently has 847830 inhabitants and a very high population density (543 inhabitants km^–2) (ISTAC 2015); 87% of the population is located along the littoral perimeter, giving a coastal population density of 3142 ind km^–1. In addition, about 2 million tourists visit the island every year (e.g. 1805058 tourists in 2015, ISTAC 2015). This has led to the occupation and degradation of most coastal areas (Ferrer-Valero et al. 2017). Importantly, however, populations of C. abies-marina in poorly impacted areas have also suffered significant regressions. Hence, it remains elusive to unravel the reasons for the loss of C. abies-marina.

The possible causes of the decline may be multiple and cumulative, as happens around the world (Thibaut et al. 2005, Wahl et al. 2015, Franco et al. 2015). Potentially, both local and global stressors are interacting to explain the severe regression of Cystoseira abies-marina in Gran Canaria, as is the case with the disappearance of other fucoids from the study region (Riera at al. 2015). In the Canary Islands, sea surface temperature has increased about 1°C in recent decades (Lima and Wethey 2012, Riera et al. 2015). Global warming is a key factor in the ongoing decline of fucoids and their displacement to colder waters (Wernberg et al. 2011). There is recent regional evidence of the adverse effect of warming on species of both brown and red macroalgae (Sansón et al. 2013). The decrease in the size of thalli of these seaweeds, and in their reproductive success (Zhang et al. 2009), have also been correlated with the warming of the Canarian waters (Sansón et al. 2013, Riera et al. 2015). Furthermore, Cystoseira are low-dispersal species whose propagules do not have a planktonic stage, and reproductive drifting thalli in floating rafts are the main mechanism of connectivity between populations (Susini et al. 2007). Therefore, if connectivity is limited, the subsequent smaller population gene pools and sizes render populations more vulnerable to threats (Buonomo et al. 2017).

The regime shift from marine forests to barren grounds devoid of erect macroalgae is generally linked to overexploitation of predatory fishes (Ling et al. 2015, Thibaut et al. 2015, 2016a). In the Canary Islands, the long-spined sea urchin Diadema africana controls the transition from rocky bottoms dominated by erect macroalgae to barren grounds (Tuya et al. 2004, Sangil et al. 2014). This sea urchin may consume thalli of C. abies-marina at rates of 1-2 mg of algae per day and individual (Tuya et al. 2001). In addition, it is plausible that the sea urchin Paracentrotus lividus and herbivorous fishes (Sparisoma cretense, Sarpa salpa and Diplodus spp.) can contribute to the consumption of C. abies-marina, as in the Mediterranean for other Cystoseira spp. (Verges et al. 2009).

This study highlights the urgent need to monitor remaining Cystoseira abies-marina populations of the Canary Islands, and compare this data with other Macaronesian islands. It is also necessary to promote urgent actions to conserve current populations, including restoration programmes. Currently, C. abies-marina is regionally protected within the framework of the Canary Islands Catalogue of Protected Species (Law 4/2010, of 4 June 2010). This highlights the uselessness of legislation if it is not enforced. Furthermore, in the last update of this catalogue, the species lost the category of “vulnerable”: it now belongs to a recently created category called “species of interest for the Canarian ecosystems”, which only protects the species within zones of the Natura 2000 network. Our results clearly do not support this legislative change.

Cystoseira abies-marina has not yet been assessed for the IUCN Red List, i.e. it is “Not Evaluated” (IUCN 2017), nor is species included in the Catalogue of Life (Roskov et al. 2016). We are aware that C. abies-marina is undergoing a very important decline throughout the Canaries (Rodríguez et al. 2008) and also on Madeira and the Azores (Ballesteros pers. com.), but more data are needed to verify the magnitude of this decline. Nevertheless, with current data and evidence of the regional decline, we propose that C. abies-marina should be classified as “Critically Endangered” under the IUCN criteria CR A2ac. There are only very few algal species in the world whose conservation status has been properly assessed (Blanfuné et al. 2016a), due to lack of historical data. Information provided here could be used as a basis for improving the evaluation of the conservation status of C. abies-marina, an ecologically important species.