Conservation theory has long highlighted the need for ensuring connectivity between protected areas, but the focus has been mainly on highly mobile taxa, particularly among vertebrates (Magris et al., 2014; Balbar and Metaxas, 2019). Despite their potential keystone role in ecosystems, such large mobile vertebrates are a small fraction of all biodiversity. Most populations along species ranges are passively connected, often by abiotic factors, with distance scales that are insufficiently understood. This is particularly striking for marine species, the majority of which are dispersed by planktonic stages that differ greatly in longevity, ranging from a few hours to hundreds of days (Shanks et al., 2003; Treml et al., 2012). The duration of planktonic dispersal can exert a major influence on connectivity and consequently, on whether populations function and should be managed as either, isolated units or a metapopulation with sources and stepping stone regions allowing a regionally larger biodiversity pool to persist (D’Aloia et al., 2017; Assis et al., 2021). To ensure conservation of regional pools of biodiversity, marine protected areas (MPAs) must function as networks with sufficient continuity of suitable habitats, allowing passive connectivity by ensuring stepping stone migration nodes for all species.

Maintaining effective connectivity is particularly relevant in the scope of offsetting impacts of climate change and other localized disturbances, such as overexploitation or pollution. Well-connected networks of MPAs can reduce and reverse disturbances through the replenishment of the impacted populations from external connected sources, thus, enhancing resilience and decreasing the risk of local extinctions (Magris et al., 2014; Fung et al., 2017). For instance, climate-induced mass mortality events (e.g., extreme conditions during heatwaves; Garrabou et al., 2009) can be reversed by replenishing impacted populations from well-connected sources (Bonin et al., 2016). Even extremely low connectivity achieved only through sporadic or rare events, can be sufficient to bridge distinct oceanographic regions shaped by strong oceanographic barriers and displaying distinct communities across taxa (Spalding et al., 2007). Evidence of sporadic gene flow events across such major barriers have been reported for different taxa (Lessios et al., 1998; Waters and Roy, 2004; Baus et al., 2005; Assis et al., 2018).

The importance of maximizing connectivity in marine management and conservation has been stressed in the Convention on Biological Diversity target 11 (also known as the Aichi Target 11), mandating the establishment of effectively managed and well-connected MPAs (or other spatial conservation measures). The post-2020 Global Biodiversity Framework (CBD; Convention on Biological Diversity (2021), currently being discussed, which aims to expand protection cover to at least 30% of sea areas, also places emphasis on the implementation of well-connected systems of protected areas. However, in practice, connectivity has rarely been considered in management planning (Balbar and Metaxas, 2019), often leaving connectivity gaps that generate isolated subnetworks, far from optimized for effective conservation. For instance, the European network of no-take MPAs has numerous isolated areas, which can compromise effective connectivity of biodiversity, despite being in a global “hotspot” of MPAs (Assis et al., 2021). In general, the extent to which MPA networks are able to provide the ecological benefits of connected metapopulations is unknown. Evaluating their potential connectivity status and identifying potential gaps is thus important for better planning and application of resources in management and conservation (Andrello et al., 2017; Lindegren et al., 2018). This is even more urgent for regions still harboring high but threatened biodiversity, currently undergoing great pressures (e.g., climate change or overexploitation).

The MPAs of the Western coast of Africa between Mauritania and Sierra Leone represent a case of important commercial (e.g., fisheries resources) and ecosystem-supporting species (e.g., seagrasses, macroalgae, corals and other invertebrates) under pressure (Gascuel et al., 2007; Durand et al., 2013). These MPAs were initially delineated to protect large and threatened vertebrates but have progressively been considering also the protection of fisheries resources and traditional means of human subsistence. However, most of the marine biodiversity of these coastlines has not been considered in this planning, including ecological groups that form structural ecosystems upon which other species depend for feeding and nursery habitats. The number of MPAs in the region has expanded greatly with the creation of the Regional Marine and Coastal Program (PRCM), from 35 in 2003 to 88 in 2018, with coverage augmenting from 17,000 to 27,000 km² (Failler et al., 2020c). The creation of the Network for West African MPAs (Réseau Régional d’Aires Marine Protegées en Afrique de l’Ouest; RAMPAO) in 2007, aimed to enhance the coherence of MPAs and ensure that they were fully represented at the political and public policy levels, but cross-border connectivity was neglected then, with MPAs being implemented at regional or national levels. Soon after the RAMPAO implementation, connectivity between MPAs was recognized as decisive to ensure overall coherence of the MPAs network (Weigel et al., 2007). However, the RAMPAO was mainly a demand drive process initiated by both the countries and the communities and thus not used to ensure suitable migratory corridors between MPAs. An overall assessment of biodiversity connectivity across all countries in the RAMPAO is thus important to integrate management and conservation actions of the network as a whole.

This study aims to address this knowledge gap by inferring marine connectivity between MPAs across borders along the West African network of MPAs (from Mauritania to Sierra-Leone, inc., Cape Verde archipelago) for major ecological groups of marine biodiversity with distinct planktonic dispersal traits mediated by oceanographic currents. These are inferred using a bio-physical modeling framework and network analyses, to estimate and illustrate the different connectivity levels across passively dispersed marine taxa as a function of their propagule duration (PD) and to identify key areas acting as connectivity corridors essential for preventing network fragmentation. Compared to other approaches aimed to estimate connectivity (e.g., genetic and chemical element analyses; Hellberg et al., 2002; Gillanders, 2005), which generally address single species or ecological groups (Pujolar et al., 2013; Johansson et al., 2015; Assis et al., 2018), and despite some potential limitations linked to unknown biological parameters (e.g., behavior and competence of larvae), bio-physical modeling allows covering multiple taxa through PD parametrization, as well as broad spatial and temporal scales. When incorporating high-resolution oceanographic and geophysical data, this line of modeling increases the realism of connectivity processes, and recurrently produces estimates matching those from empirical data (Pujolar et al., 2013; Assis et al., 2015, 2018; Buonomo et al., 2016; Cunha et al., 2017). This study provides the first connectivity assessment of West African MPAs, filling an important gap of scientific information needed for the planning of the overall coherence of the regional MPA network.

The bio-physical modeling approach based on high-resolution data provided key insights about the potential connectivity in the west African MPA network for species dispersed by ocean currents, which comprise most marine biodiversity. The results revealed that connectivity differs sharply among distinct ecological groups, from highly connected to predominantly isolated. Remarkably, regardless of their dispersal duration, all ecological groups have in common a major connectivity gap in the Bijagós region of Guinea-Bissau, which coincided with a well-known biogeographic transition zone. This novel information has strong implications for biodiversity conservation and management (Magris et al., 2014; Assis et al., 2021), revealing the extreme importance of conserving the important and rare connectivity nodes persisting in the Bijagós islands, upon which most north-south links depend along this global region.

Using the range of planktonic dispersal durations (PD period) for all types of macroscopic marine species, provided a global overview of connectivity across major ecological groups. This approach revealed how ecological groups with distinct dispersal potential were affected by the MPA network spatial distribution and aggregation, as previously shown for the European context (Assis et al., 2021) and for particular ecological groups (e.g., Selkoe and Toonen, 2011). As observed elsewhere, discrete connections between adjacent MPAs, for taxa with poor dispersal capacity, can increase in number and expand in distance (from tens to hundreds of km) for increasing PD periods (Shanks et al., 2003; Krueck et al., 2017; Assis et al., 2021). The size of MPA can further play an important role in network connectivity (Assis et al., 2021).

The reduced connectivity inferred for ecosystem structuring species (i.e., corals, tunicates, sponges, macroalgae, and seagrasses) with short PD, resulted in multiple distinct MPA aggregations (i.e., clusters), higher probability of MPA isolation and reduced number of stepping-stone corridors. Conversely, species with longer PD (i.e., fish and crustaceans), potentially connect more MPAs across greater distances. Such a relationship is in line with previous studies (e.g., Treml et al., 2012) and corroborates the expectations of genetic diversity across ecological groups. Corals (Pilczynska et al., 2019), sponges (Duran et al., 2004), and marine forests of macroalgae (Johansson et al., 2015; Assis et al., 2018) and seagrasses (Alberto et al., 2008) recurrently exhibit well-defined genetic structure at short spatial scales, while in contrast, fish (Klein et al., 2016), crustaceans (Heras et al., 2019) and echinoderms (Maltagliati et al., 2010) tend to display more homogeneous genetic patterns, although not completely panmictic across all distributional ranges (Schunter et al., 2011). Taxa with planktonic dispersed stages considered in our study represent the large majority of macroscopic marine species. Even for mobile species, able to swim actively while performing migrations or ontogenetic shifts in habitat use (Abecasis et al., 2009; Almpanidou et al., 2019), PD alone can explain more than 50% of the variation in genetic dispersal estimates (Kinlan and Gaines, 2003). Taxa without planktonic dispersal stages might also benefit from some aid in transportation by oceanographic currents, as suggested for some taxa like green turtles (e.g., Jensen et al., 2020), but not others (e.g., Briscoe et al., 2016).

The low network connectivity predicted for ecosystem structuring species may undermine conservation efforts if retention is not sufficient to maintain population sizes. This information was unknown until now and may provide useful guidelines for future management of the West Africa network of MPA (Balbar and Metaxas, 2019). In particular, stepping-stone reserves for ecosystem structuring species are particularly relevant to ensure biodiversity persistence under future environmental changes and overexploitation, and increase the network resilience (Fung et al., 2017). For instance, massive mortality events (e.g., Garrabou et al., 2009) can be reversed through the replenishment of impacted populations from well-connected neighboring locations (Bonin et al., 2016). Particular MPAs of Cape Verde, Guinea, Senegal, and Sierra Leone with higher betweenness values (Figure 3) may play a key role toward connectivity in the face of future environmental changes, by acting as stepping-stones for the dispersal of propagules between different islands or regions/countries. Importantly, the few data describing population structure along the West African coastal waters and its network of MPAs already reports losses at the ecosystem level (seagrass, rocky bottom communities and coral reefs; Failler et al., 2020a; Chefaoui et al., 2021). However, betweenness centrality is strongly dependent on network configuration, and one MPA can be particularly important in promoting stepping-stone connectivity for a given ecological group, but not for others. For instance, the aforementioned region of Cape Verde contains a set of MPAs which are central for ecosystem structuring species (G1 and G2), but of lower importance for species with increased dispersal potential (G3 and G4). As connections increase, so does the size of clusters, and consequently the MPAs with higher betweenness centrality change to those strictly ensuring within-cluster connectivity between large water masses (e.g., Dakar in Senegal linking continual coastlines with Cape Verde archipelago).

The higher network connectivity predicted for the groups with higher dispersal potential (Bryozoa, Mollusca, Polychaeta, Pisces, Crustacea, and Echinodermata) may be encouraging for resource management, particularly for commercially targeted species (e.g., fish). However, the persistence of better connected populations relies on habitat traits, for feeding and reproduction, that are often dependent on ecosystem structuring species (Vassallo et al., 2013; Lefcheck et al., 2019) with poor network connectivity. Thus, habitat structuring species should be prioritized as management targets for marine connectivity both inside and outside the MPAs of the West African network, given their ecological role for other groups that despite being more coherently protected could still become limited in terms of suitable habitats. Not ensuring the future protection of essential habitats for commercial species might result in a stressor adding on to the already observed pattern of overexploitation, as recently reported for West African fish species (Gascuel et al., 2007; Atkinson et al., 2011; Durand et al., 2013). It must be highlighted that more MPAs do not resolve all main threats to connectivity of such essential structural taxa. Worldwide, ecosystems of sessile invertebrates and sea vegetation are frequently threatened by terrestrial disturbances caused by land management, particularly the consequences of removal of natural terrestrial vegetation cover that cause massive erosion, turbidity and sedimentation, destroying marine nursery and feeding habitats for fisheries, that cannot be prevented solely by MPAs.

A major connectivity gap was identified for all biodiversity in the central Guinea-Bissau area. This region lacks stepping-stone MPAs, and coincides with the transition zone (Supplementary Information 6) between the biogeographical eco-regions West Africa Transition and Gulf of Guinea West (Spalding et al., 2007; Costello et al., 2017), an oceanographic boundary zone. Transition zones have naturally low connectivity and increased genetic differentiation (Gourene et al., 1993; Pouyaud and Agnèse, 1995; Chikhi et al., 1998; Durand et al., 2005, 2013). In this scenario, the few connectivity events taking place across this region of Guinea-Bissau, which includes the very rich an unique Bijagós islands, can have strong implications on the effectiveness of the entire network of West Africa MPAs. This can be particularly important in mediating climate-induced range shifts, as conserving the natural (low) connectivity across transition zones provides protection of natural corridors for species to disperse tracking suitable habitat within scales of climate velocity. Thus, the implementation of more stepping-stone MPAs in the Bijagós islands is expected to enhance population resilience to climate change and to increase the overall effectiveness of the entire West African MPA network.

In contrast with Guinea-Bissau, the large connectivity gap identified between the Cape Verde Islands and continental Africa for taxa with limited dispersal capacities, is caused by the large distances lacking coastline habitat, and therefore additional stepping stone MPA planning is not possible. This natural isolation is responsible for the rich, high endemicity of Cape Verde biodiversity, but leaves the archipelago and its MPAs more vulnerable due to total dependence on local retention. Large-scale raft events that could replenish potentially affected populations in Cape Verde islands are unlikely, and thus a higher protection regime, to increase the islands’ MPA efficiency (Zupan et al., 2018), would be particularly important to minimize their vulnerability. Notably, isolation was still found in regions with high MPA coverage (e.g., Cape Verde and Senegal), possibly linked to the small size of MPAs, since MPA size is a major determinant of connectivity (Supplementary Information 5; Assis et al., 2021). Therefore, increasing the size of existing MPAs is an alternative and/or complementary measure to the implementation of new MPAs, which could help conservation of connectivity and restore ecological processes and population sizes (Claudet et al., 2008; McLeod et al., 2009; Cheng et al., 2019). So far, the implementation of MPAs has been done at a national level with the aim to preserve and manage biodiversity. A novel approach at a multinational scale is now decisive to achieve an effective network of MPAs following the goals of RAMPAO. Setting-up a transformative agenda is thus required to ensure the development of a network that is both based on national inspiration and regional connectivity. As RAMPAO aims to federate MPAs in the region through a common vision oriented toward maintenance of biodiversity and ecosystem services, RAMPAO can play a leading role ensuring that connectivity information is used to enhance the consistency of its network.

The bio-physical modeling approach considered a key ecological trait (i.e., duration of propagules; Assis et al., 2021) and oceanographic processes (e.g., fronts and eddies; Chassignet et al., 2007), as well as the actual shape, size and spatial structure of the MPAs to realistically estimate biodiversity connectivity (Assis et al., 2021). Additional drivers like the behavior (e.g., diel migration; Romero et al., 2012), competency and settlement success of larvae, as well as marine debris and translocations, could also play an important role in connectivity (Anadón et al., 2013; Faillettaz et al., 2018; Manel et al., 2019). However, their incorporation into modeling is still challenging, particularly because some of these drivers may vary between species, between individuals of the same species or between development stages (e.g., competency; Endo et al., 2019). In addition, connectivity of coastal areas at short distances might be strongly determined by regional patterns of tidal flows and near-shore circulation (Findlay and Allen, 2002; Cowen and Sponaugle, 2009) that might not be completely represented in HYCOM, nor in additional circulation models (Chassignet et al., 2006). Thus, only a comprehensive assessment of population genetic differentiation across distinct biodiversity groups or a validation of the HYCOM model for the study area (e.g., by passive drifters) could bring to light such limitations. Nevertheless, previous studies across the globe, comparing independent genetic and demographic data with our bio-physical model, systematically showed the central role of PD alone and oceanographic transport and barriers structuring population connectivity, and maintaining or diluting the genetic signatures of past changes (Buonomo et al., 2016; Lourenço et al., 2017; Pascual et al., 2017; Assis et al., 2018; Nicastro et al., 2019; Ntuli et al., 2020).

Our results show that connectivity in the West Africa network of MPAs differs sharply between major ecological groups, and provide unique insights for conservation and management actions, highlighting regions requiring additional protection (e.g., Bijagós region of Guinea-Bissau) to ensure functional connectivity corridors between major oceanographic regions (in line with terrestrial migration corridors; Ward et al., 2020). Initially dedicated to the protection of key threatened species such as birds in Mauritania and Senegal, manatees in Senegalese and Gambian estuaries, seals in the north of Mauritania, turtles in Cape Verde and Bijagos archipelagos, the MPAs of West Africa have progressively been implemented for the protection of other valuable biodiversity (Leeney and Poncelet, 2015; Laudisoit et al., 2017) and are continuing to work toward global improvement. As observed globally, the performance of the West African countries regarding the Aichi target 11 is overall poor (Failler et al., 2020a) yet, the new Global Biodiversity Framework pursues positive trends in biodiversity, while structured to focus on ecosystems and processes (Phang et al., 2020) and promoting involvement of stakeholders, at various levels. Management of the West African MPA network does not yet fully consider the potential biodiversity impacts coming from human activities and climate change (Failler et al., 2020b), and measures are predominantly aimed at preventing overfishing and terrestrial overexploitation (Failler et al., 2020a). However, our results show that MPA management in the West Africa should strongly focus on ecosystem structuring species (e.g., corals, sponges, macroalgae, and seagrasses), which are the essential nursery and feeding habitats of numerous commercial and essential subsistence species, besides threatened species, and might suffer the impacts of global climate change (Assis et al., 2017), as well as additional local stressors like marine invasions (Giakoumi and Pey, 2017), bottom trawling (Fragkopoulou et al., 2021) and water pollution (Abessa et al., 2018). These results provide a solid basis that will help coordinate efforts between the different countries/stakeholders toward the implementation of an efficient network of MPAs in West Africa.

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

JA, PF, EF, DA, GT-G, AR, ES, HD, and EAS conceived the ideas. JA, EF, and DA analyzed the data. JA, EF, DA, and EAS led the writing. All authors reviewed the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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