Coastal areas host half of the world’s cities and more than half of the world’s population, with population density near the coast three times higher than the global average (Creel, 2003; Small and Nicholls, 2003; Airoldi and Beck, 2007). Shoreline urbanization combined with the increase of tourist, recreational and commercial activities, has resulted in the introduction and proliferation of artificial structures in marine coastal habitats worldwide (Bulleri and Chapman, 2010; Firth et al., 2016). The human impact associated with these structures has been reported to affect biological processes on all spatial and temporal scales (Airoldi et al., 2005). Among them, fragmentation of natural habitats by urbanization processes can cause a reduction in population size and connectivity of species, and thus increase their risk of extinction in the face of certain permanent disturbances (Aguilera et al., 2020; Chase et al., 2020).

Limpets are considered keystone species in rocky coastal ecosystems as they maintain the composition and structure of the biological community and, when they disappear from their habitats, cause a cascading effect on them (Raffaelli and Hawkins, 1996). Therefore, limpets may be the most important group of grazers in temperate rocky intertidal zones, because their feeding activity limits the growth of macroalgae and controls barnacle numbers (Branch, 1981).

Among limpets, the larger species are frequently the most ecologically relevant, being defined as ‘giant limpets’ (shell length greater than 100 mm). With this classification a total of 14 species in the world can be found, three of them catalogued as threatened (Espinosa and Rivera-Ingraham, 2017). Patella ferruginea Gmelin, 1791 (Gastropoda: Patellidae) belongs to this classification, as it achieves a maximum recorded shell length of 115 mm ( Supplementary Figure 1-S ). It is endemic to the Western Mediterranean Sea and it is considered the most endangered marine invertebrate (Ramos, 1998), together with Pinna nobilis, within the European coastline (Vázquez-Luis et al., 2017; Lattos et al., 2021).

Like other limpet species, P. ferruginea is a broadcast spawner with external fertilization so that the urban development of coastal areas has dramatically fragmented its habitat (Airoldi et al., 2005; Firth et al., 2016). Moreover, the conspicuous star‐shaped shell and large sizes that P. ferruginea can reach has made this species the target of human collection since the Quaternary (Espinosa, 2006; Espinosa et al., 2009; Rivera-Ingraham et al., 2011a; Guallart et al., 2013a).

On account of these anthropogenic impacts, its distribution range has progressively been contracted, and P. ferruginea has nearly disappeared from European continental coasts, except for the small and fragmented populations present in Andalusia Southern Spain, Liguria (Italy) and Southern France (Cottalorda et al., 2004; Espinosa et al., 2014b; Luque et al., 2018; Ferranti et al., 2019; CMAOT, 2020). The remaining most notable populations are found in scattered North African gated communities: Ceuta, Melilla, Chafarinas Islands, Habibas Islands, and the island of Zembra (Espinosa et al., 2014b; Luque et al., 2018). In this regard, it has recently been identified that possibly the population located in Ceuta (Strait of Gibraltar, North Africa) plays the role of source population in the area, maintaining nearby sink populations in the south of the Iberian Peninsula (Espinosa et al., 2018) according to the “source-sink population model” (see Pulliam, 1988; Pulliam, 1996).

This study focuses on the population dynamics of P. ferruginea in Ceuta and the environmental factors that shape the population size structure. Consequently, the objectives of the present study are i) to determine the current status of this P. ferruginea population, ii) track the any change in the population in the last 10 years, iii) to confirm the result of the ¨population viability analysis¨ (PVA) prediction method performed by Espinosa et al. (2018) of the P. ferruginea population in Ceuta and iv) explore the environmental factors affecting the populations of P. ferruginea.

The results have shown that from 2011 to 2021 the population or P. ferruginea increased by about 200%. Currently, the population numbers 168,464 individuals, of which 131,776 are adults. In about a decade, the population has grown from 55,902 to 168,464 individuals. Recent studies have shown that there has been recolonization and repopulation of P. ferruginea in some localities of the Andalusia, Melilla, Alboran and Chafarinas island (Casado et al., 2017; Maestre et al., 2018; CMAOT, 2020). Therefore, Ceuta’s population could be an important contributor to the recolonization of these areas, especially when considering that nearly 80% of its individuals are adults. Nevertheless, the situation of P. ferruginea is still critical because the individuals settled in Andalucía are the last stronghold of continental Europe, depending heavily on recruits from North African populations (CMAOT, 2020). These results show that the area of Ceuta is a ¨stronghold¨ for this endangered species and support the hypothesis that this population has the role of a ¨source population¨ of new recruits for other nearby ¨sink populations¨ in the metapopulation of the Alboran Sea supporting the hypothesis proposed by Espinosa et al., 2018.

These conclusions can be supported by different oceanographic and biological studies carried out in this biogeographic area. In this sense, it is known that all P. ferruginea populations from the Alboran Sea and North Africa up to Tunisia belong to the same genetic cluster (Casu et al., 2011). Dispersal studies showed that the directionality of the surface currents of the Alboran Sea, especially those near the Strait of Gibraltar in its first loop go from south to north (therefore, the potential larval input should go from south to north and not the other way around, at least from the coast of Ceuta) (see Periáñez, 2007). Likewise, dispersal studies of P. ferruginea larvae in the Alboran Sea show simulated regional trajectories for virtual larvae released from Ceuta with strong prevailing westerly wind at high and low spring tide conditions with a Pelagic Larval Duration of 14 days during January 2016 can cross and complete the first loop of the Alboran Sea (Stephen Warr unpublished data). The settlement ability of larvae of this species is reached 3-4 days after fertilization, depending on water temperature, and crawling and swimming pediveliger larvae have a phase of 7-32 days (probably up to 40 days) until metamorphosis to juvenile occurs (Ferranti et al., 2022), which would argue that the population studied in Ceuta should be classified as a “source population” according to the “source-sink population model” (see Pulliam, 1988; Pulliam, 1996).

Although the sampling effort was larger than previous studies, in light of the results a review of the existing literature, reveals that the population in Ceuta is the largest within the species’ natural distribution (see Table 4 ).

The mean density of the population has increased from 2.33 ind/m in 2010 (Rivera-Ingraham et al., 2011b) to 7.39 ind/m in 2020. This is in agreement with the output of the population viability analysis (PVA) model carried out by Espinosa et al. (2018), and based on monitoring during a ten-year period. This model indicates the unlikeliness of the population in Ceuta facing extinction within the next 50 years. This confirms the effectiveness of the control and temporary monitoring programs of the population proposed in Spain’s National Conservation Strategy for P. ferruginea, established in 2008 by the Spanish Ministry of Environment (MMAMRM, 2008).

The increase of Ceuta’s population could be due to two different factors. Firstly, there are some littoral zones with difficult or restricted access, such as sectors H and I, which are a natural cliffs, and sectors K, L and M, which are access-restricted areas in the commercial harbor of artificial structure type riprap (great heterogeneity, macroscale and microscale). These sectors would be therefore protected from anthropogenic impact, which has been identified as the main factor promoting the decline of the species (Ostalé-Valriberas et al., 2022 and references therein). Secondly, there has been an improvement in social awareness of the conservation status of this endangered species, promoted by dissemination activities performed by NGOs and the environmental administration, and increased enforcement by local authorities. In this sense, many projects and news have been published in social media publications (Mar, 2020), giving notoriety to P. ferruginea and turning it into a flagship species. In this sense, flagship species attract funding, increase awareness, and promote the conservation of habitats, acting as umbrella species (Smith and Sutton, 2008; Home et al., 2009).

An analysis of the evolution of the population of P. ferruginea over the last two decades in a biggest area distribution, the Alboran Sea, shows an increase in the number of individuals (Maestre et al., 2018; CMAOT, 2022). The control by the local authorities (environmental administration and security forces) in the application of the strict legislation that protects the species (community, national and regional regulations), in addition to the increase in the awareness of the local population through environmental education, together with the advances in research on the species give an impression of good results.

Statistical modeling has shown that P. ferruginea adult density increases with coastal heterogeneity (macroscale in meters) and substratum roughness (microscale in cm), and decreases with slope, chlorophyll-a concentration and anthropogenic impact. Among the different environmental factors, the coastal heterogeneity (m) has the most impact on the adult density. A one-point increase in the macroscale value favors adult density by 10.52 while a one-point increase in the microscale value (cm) favors it only by 1.27. Therefore, coastal and substratum heterogeneity at multiple scales must be taken into consideration for designing sites for further protection (Marine protected areas or Area-based conservation measures such as Artificial marine micro-reserves), to establish critical areas for its conservation or for future translocation procedures for reinforcement and/or reintroduction, or due to destruction of the original habitat (see MMAMRM, 2008; García-Gómez et al., 2011; García-Gómez et al., 2015; IUCN-WCPA, 2019; Agung et al., 2022; Ostalé-Valriberas et al., 2022).

The vertical slope of the coastline negatively influences the density of P. ferruginea adults. Within vertical substrata such as cliffs or seawalls, a smaller surface (area) of settlement for planktonic larvae is available, which explains the result.

A practical example of these results has been shown in the different abundances between the subpopulation settled inside the harbor (sectors L and L’). Yet still sharing the same environment, the subsector of dolomitic rip-rap (L) and the vertical concrete wall (L’) show the most different results according to the population structure (number of individuals in each size class) (see Figures 5 , 6 ).

Analyzing the results obtained for the Chl-a concentration in each sector, it is observed that there are other environmental factors that more strongly affect the species.

According to the results of previous studies, the accessibility and visitation of the coastal areas negatively affect limpet populations of P. ferruginea, biasing the populations structure via harvesting of the greatest sizes (mostly females) and therefore affecting the adult density (see Figure 7 ; Table 2-S ) (Espinosa et al., 2009; Ostalé-Valriberas et al., 2022). In fact, the results of the present study show that a one-point decrease in the value of accessibility increases the adult density by 0.96.

The results indicate that Ceuta is the main population of this endangered species in its entire range, being a source population on the Southern Iberian Peninsula that should be conserved as a priority. According to the Spanish National Conservation Strategy, sectors H, I, K, L and M have the highest number of adults and should therefore be categorized and managed as Critical Areas (MMAMRM, 2008; see Supplementary Figure 2-S ). It could benefit many species of conservation concern also settled in the same habitat (“umbrella effect”), as demonstrated by Ostalé-Valriberas et al. (2022).

Previous studies have shown that 90% of the populations of a metapopulation are usually “sink populations” and only 10% are “source populations” (Pulliam, 1988), so this population is of great ecological relevance for this species.

The results also support the concept of “Artificial Marine Micro-Reserves” as a new area-based management tool for marine conservation, establishing a network of these source populations that promote gene flow between populations with eventual recolonization of their original distribution (García-Gómez et al., 2011; Ostalé-Valriberas et al., 2022) ( Supplementary Figure 2-S ).

Finally, for future translocation procedures, the reported results could serve as a guide to select the most suitable receptor sites. Such areas should have high values of macro- or microscale heterogeneity and, similarly, should avoid vertical substrata and areas with anthropogenic impact.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding author.

EO-V, JS-V, FE, G-GJ conceptualization of the work and development the methodology. EO-V, FE application of statistical, mathematical, computational, or other formal techniques to analyze or synthesize study data. EO-V, AL, AA-A, JS-V, AP-P: Conducting a research and investigation process, specifically performing the experiments, or data/evidence collection. EO-V writhing the original draft, visualization work and supervision. AP-P, JS-V, FE, G-GJ writing -review & editing. EO-V, AL Management and coordination responsibility for the research activity planning and execution. EO-V, G-GJ acquisition of the financial support for the project leading to this publication. All authors contributed to the article and approved the submitted version.