Scientists have long been fascinated by the unique nature of remote oceanic islands, especially after Charles Darwin’s study of the Galapagos Islands. Young oceanic islands of volcanic origin represent the summits of large volcanoes that have risen from the ocean floor. They are isolated from other landmasses by deep seas and have never been connected to continental landmasses, making their environments unique products of trans-oceanic dispersal and subsequent speciation (Wace and Holdgate 1976, Cowie and Holland 2006). Islands therefore represent natural laboratories that simplify the complexities of the natural world (Whittaker and Fernández-Palacios 2007). However, the introduction of non-native species poses a constant threat to island biota. It is widely recognized that warmer temperatures can enhance the likelihood of alien invasive species settling, reproducing and successfully outcompeting with the native biota, leading to significant biodiversity losses (Gritti et al. 2006, Burgier and Muir 2010).

There are numerous islands and archipelagos in the Atlantic Ocean (Hansom 2010): including the archipelagos of the Azores and Madeira within the territorial waters of Portugal; the archipelago of the Canary Islands (Spain); the archipelagos of Cape Verde and São Tomé and Príncipe (Africa); Fernando de Noronha, Trindade and Martin Vaz (Brazil); and the islands of St Helena, Ascension and the archipelago of Tristan da Cunha (England) (Fig. 1).

The bryofloras of the ten islands and archipelagos were inventoried: Azores, Madeira, São Tomé and Príncipe, Cape Verde, and Canary islands (Northern Atlantic) and Trindade, Fernando de Noronha, Ascension, Saint Helena and Tristan da Cunha (Southern Atlantic) to: (1) provide an overview of their bryofloras; (2) describe the species richness of mosses and liverworts and incorporate new reports of bryophyte taxa; (3) analyze the distribution patterns and endemism of those species; (4) analyze the similarities amongst northern and southern islands; and (5) establish a diversity baseline for comparisons. We describe here the diversity, endemism, and similarities of mosses and liverworts on ten islands and archipelagos located in the Atlantic Ocean.

Studied islands and archipelagos (physical structures and bryofloras)

Methods

Diversity of the ten islands and archipelagos of the Atlantic Ocean (alpha, beta and gamma diversity). A total of 1,498 taxa (gamma diversity) were listed for the ten islands and archipelagos in the Northern and Southern Atlantic Ocean, including 874 taxa of mosses and 624 taxa of liverworts and hornworts. Of these, 201 taxa are endemic (73 taxa of liverworts and hornworts and 128 taxa of mosses), representing 13% of all taxa surveyed (Suppl. material 2); only 10 taxa were shared by all five archipelagos. The number of endemic taxa in the five North Atlantic Ocean archipelagos (107 spp.) was higher than the five islands and archipelagos of the South Atlantic Ocean (91 spp.), highlighting their importance to the conservation of bryophyte biodiversity on Atlantic Ocean islands. This is especially critical given that these islands are likely to experience the consequences of climate change, compounded by ongoing pressures such as tourism, pollution, population growth, the introduction of alien species and habitat destruction (Fernández-Palacios et al. 2021). Bryophyte richness and diversity differed significantly amongst study sites illustrating their high degrees of heterogeneity and reflected the importance of habitat differences for high diversity.

with the inclusion of taxa from the South Atlantic Ocean islands and archipelagos (Fernando de Noronha, Trindade, Saint Helena, Ascension, and Tristan da Cunha) for analysis and comparison with those of the North Atlantic Ocean (gamma diversity), the total number of taxa increased to 1,498 taxa (24 hornworts, 600 liverworts, and 874 mosses).

Species richness varied strongly amongst the ten southern Atlantic islands and archipelagos, ranging from 29 taxa on Fernando de Noronha, to 586 on Madeira (Table 3). The bryophyte diversity on the ten islands and archipelagos studied here (gamma diversity) was high when compared to Africa, Tropical America, and the world (Wigginton 2018, O’Shea 2006, Shaw and Goffinet 2000, Gradstein et al. 2001), (Fig. 2).

Phytogeographical patterns and endemism. The islands and archipelagos of North and South Atlantic Oceans were analyzed separately and together with the results presented in Suppl. materials 3, 4. The phytogeographical patterns of the North Atlantic Ocean islands and archipelagos were categorized into ten groups: 1) Global (107 taxa; 21 liverworts and 86 mosses); 2) Africa (182 taxa; 99 liverworts and 83 mosses); 3) Endemic (107 taxa; 38 liverworts and 69 mosses); 4) Europe (40 taxa of mosses); 5) North America Europe, Asia (46 taxa; 26 liverworts and 20 mosses); 6) Europe, Macaronesia, Africa (8 taxa; 7 liverworts and 1 moss); 7) Europe, Macaronesia, Asia (16 taxa; 8 liverworts and 8 mosses); 8) Europe and Macaronesia (14 taxa; 5 liverworts and 9 mosses); 9) South America and Africa (5 taxa of liverworts); 10) others (597 taxa; 215 liverworts and 382 mosses). The African, Endemic and Global elements were the largest, comprising 35% of the total taxa (Suppl. material 3).

Distinct phytogeographical patterns were observed for liverworts and mosses on the North Atlantic Ocean islands (Suppl. material 3). Liverworts exhibited low numbers of European and Macaronesia taxa (5 taxa), but the highest numbers of African elements (99), followed by endemic taxa (38) and global elements (21). In contrast, mosses exhibited a high number of global elements (86), followed by African and endemic elements (83 and 69, respectively). The phytogeographical patterns of the South Atlantic Ocean islands and archipelagos were classified into seven categories for this analysis: 1) Global (50 taxa; 15 liverworts and 35 mosses); 2) Pantropical (20 taxa; 10 liverworts and 10 mosses); 3) Tropical America (21 taxa; 11 liverworts and 10 mosses); 4) Africa (19 taxa; 6 liverworts and 13 mosses); 5) South America (91 taxa; 75 liverworts and 16 mosses); 6) Endemic (91 taxa; 36 liverworts and 55 mosses); 7) others (171 taxa; 86 liverworts and 85 mosses). Endemic elements represented the largest category, comprising approximately 37% of all taxa (Suppl. material 4).

Liverworts and mosses exhibited distinct phytogeographical patterns on the South Atlantic Ocean islands and archipelagos (Suppl. material 4). Liverworts had a high number of taxa with South American distributions (75 taxa), followed by endemic taxa (36), but low number of taxa with global distributions (15); mosses had high number of endemic taxa (55) and taxa with global distributions (35).

A notable feature of the bryofloras of the North Atlantic Ocean archipelagos is that species richness was concentrated in a few families (Table 2, Fig. 3). Amongst mosses, the families Brachytheciaceae (44 taxa), Bryaceae (47), Fissidentaceae (58), and Pottiaceae (135) stood out, collectively accounting for 284 taxa – fully 31% of the total of moss taxa (901) recorded for the five archipelagos. For liverworts, just three families (Lejeuneaceae [121 taxa], Plagiochilaceae [33], and Ricciaceae [34]) represented 30% of the total liverwort species (629) recorded for these archipelagos. Thus, the richness of bryophyte taxa was concentrated in seven families that collectively accounted for 40% of the total taxa (1,161).

The taxa richness of the bryofloras of the South Atlantic Ocean islands and archipelagos was concentrated in six families (Table 2, Fig. 3), Lejeuneaceae (38 taxa), Lophocoleaceae (40), Bryaceae (25), Fissidentaceae (28), Grimmiaceae (11), and Pottiaceae (25), which together totalled 167 taxa, representing 27% of the total.

There were differences amongst the islands and archipelagos with unique families, or amongst those which had significantly different numbers of taxa (beta diversity) (Tables 1, 3). The Brazilian archipelagos in the South Atlantic Ocean (Trindade and Fernando de Noronha) illustrated how habit destruction by humans can influence bryophyte species diversity. These archipelagos had only 35 and 29 taxa, respectively, the lowest diversity of bryophyte taxa amongst the ten islands and archipelagos studied and neither contained any endemic taxon. This situation contrasts with the two English islands and one archipelago (Ascension, Saint Helena, and Tristan da Cunha), which have between 84 and 303 bryophytes taxa, including 15 to 51 endemic taxa – demonstrating their higher degrees of preservation. The African archipelagos of Cape Verde and São Tomé and Príncipe in the North Atlantic Ocean have between 207 and 292 bryophyte taxa, including 8 to 19 endemic taxa. The Portuguese archipelagos of Azores and Madeira and the Spanish Canary Islands archipelago evidenced the highest diversity and shared more taxa, with the Canary Islands being the most preserved (with a total of 522 bryophyte taxa, 47 of which are endemic).

AZORES. Three bryophyte families occur exclusively in this archipelago. The family Jamesoniellaceae is represented here by a species that is otherwise restricted to the Neotropics: Syzygiella rubricaulis (Nees) Steph. The family Sphagnaceae is also notable in this archipelago, with 17 taxa (Corley et al. 1981, Grolle 1983, Sjögren 2001, Gabriel et al. 2010, Hodgetts and Lockhart 2020). The Azores share the highest number of taxa with the Madeira archipelago (372 taxa, including 37 endemic species).

MADEIRA. Three families occur exclusively on this archipelago: Blasiaceae (Liverworts), Aongstroemiaceae, and Flexitrichaceae (Mosses). Additionally, the families Brachytheciaceae (34 species) and Grimmiaceae (27) are particularly prominent (Fontinha et al. 2020, Hodgetts and Lockhart 2020, Sérgio et al. 2006, Sérgio et al. 2008). As noted earlier, Madeira shares the highest number of taxa with the Azores.

SÃO TOMÉ and PRÍNCIPE. This archipelago has the highest number of restricted families. It is the only archipelago where the hornwort family Dendrocerotaceae is present, alongside moisture-indicating families such as Hypopterygiaceae and Meteoriaceae. São Tomé and Príncipe share 26 taxa with Cape Verde, 24 with the Canary Islands, 24 with the Azores and 26 with the Madeira.

CAPE VERDE. Two bryophyte families occur exclusively in this archipelago. A total of 69 exclusive taxa are found here, with seven being endemic. Cape Verde is the most unique archipelago amongst the five studied in the North Atlantic Ocean, with the smallest total number of taxa and endemic taxa. However, it shares a significant number of taxa with the Canary Islands (125 taxa).

CANARY ISLANDS. Three bryophyte families occur exclusively in this archipelago. The Canary Islands share the most taxa with the Azores and Madeira (298 taxa, 37 of which are endemic) and with Cape Verde (125 taxa). Together with Madeira, the Canary Islands have the best-documented bryofloras, which have been extensively detailed in numerous publications (Table 3).

TRINDADE. No bryophyte family occurs exclusively on this island. Its native vegetation has been almost completely destroyed through the grazing of introduced animals. E. Knight visited the island in 1881 and 1889 and reported its already degraded vegetation and a large waterfall on the west coast. Vegetation restoration efforts were initiated in 1990 through the elimination of goats (Silva and Alves 2011). Since then, the vegetation has slowly recovered, and the creeks now have more water. The vegetation above 400 m is considered a “Giant Fern Forest" and is dominated by a single species, Cyathea delgadii Sternb.; it is the most diverse region for bryophytes. Most of the taxa are Neotropical, followed by cosmopolitan taxa; there are no endemic taxa (Costa and Rezende 2022). The growth of Campylopus introflexus (Hedw.) Brid. was probably facilitated by human impacts on the original vegetation (Gama et al. 2016). This archipelago shares four taxa with Ascension Island and four taxa with the Fernando de Noronha archipelago.

FERNANDO DE NORONHA. Only one moss family, Stereophyllaceae, occurs exclusively on this island. Charles Darwin visited the island and made the first botanical collections there. A later expedition by Ridley (1890) reported the predominance of herbaceous vegetation due to the intensive cutting of tall trees that could be used to build rafts and aid prisoner escapes. Fissidens, with seven taxa, is the richest genus. Cosmopolitan taxa compose approximately 26% of vegetation; there are only one endemic taxa, Fissidens noronhensis Teixeria, Bordin and Carv.-Silv. This archipelago shares seven taxa with Ascension Island (Costa and Rezende 2022, Costa et al. 2021, Pressel et al. 2017, Yano and Mello 2016, Pereira and Câmara 2015).

ASCENSION. No bryophyte family occurs exclusively on this island. The island harbours over 200 introduced taxa ranging from large trees to shrubs and herbs; many native plants, such as the endemic fern Pitsana purpurascens (de Vriese) Murdock, are becoming severely outcompeted. Campylopus, with seven taxa, is the richest genus. There is a notable absence of cosmopolitan taxa (Ceratodon purpureus (Hedw.) Brid., Tortula muralis Hedw., Octoblepharum albidum Hedw., etc.), (Pressel et al. 2017). There are no representatives of Grimmiales, despite the highly suitable volcanic rock substrate. The island has ten endemic bryophyte taxa. A handful of taxa are probable human introductions, but, despite the small size of the island and the destruction of its natural habitats, relatively few bryophytes are threatened with extinction. This island shares 24 taxa with Saint Helena Island, three of which are endemic (Pressel et al. 2017, Wigginton 2012).

SAINT HELENA. No bryophyte family occurs exclusively on this island, but there is a notable absence of cosmopolitan taxa, Ceratodon purpureus, Tortula muralis, Octoblepharum albidum, etc. There are no representatives of Grimmiales despite the highly suitable volcanic rock substrate there. Fissidens, with 11 taxa, is the richest genus. The destruction of the natural vegetation began with the introduction of goats and the establishment of permanent settlements there introduced exotic vegetation. Native trees were cut for different purposes (house building, fuel etc.) and many endemic taxa are now extinct or critically endangered (Wigginton 2012). This archipelago shares 17 taxa with the Tristan da Cunha archipelago, two of which are endemic (Table 3).

TRISTAN DA CUNHA. Eleven bryophyte families occur only on this archipelago (Table 2). There are currently very few introduced taxa on the archipelago. The family Orthotrichaceae is represented by several endemic taxa, and Dicranoloma (Renauld) Renauld by three endemic taxa (Váňa and Engel 2013, Wace and ​​​​​​​Dickson 1965, Wace 1961, Dixon 1960). This archipelago shares 35 taxa with the Azores archipelago and 32 with Madeira, none are endemic. There are strong indications of a close connection with South America, as mosses, liverworts and hornworts can be dispersed by the western winds – ‘West Wind Drift’ – and connections with South America are conspicuous in all groups of non-vascular plants, except the algae (Muñoz et al. 2004, Galloway 1996).

Sunding (1979) considered Cape Verde to be different from the other archipelagos of Macaronesia, which was subsequently corroborated by Vanderpoorten et al. (2007). According to these authors, Macaronesia (comprising the archipelagos of Azores, Madeira, Canary Islands and Cape Verde) is considered a single biogeographical unit, with their floras being relics of a widely distributed Tertiary subtropical flora. These authors rejected the concept of Macaronesia sensu lato and viewed the Cape Verde archipelago as being more associated with tropical Africa. Liverworts in the other Macaronesian archipelagos support an Azores-Madeira-Canary clade (Macaronesia sensu stricto), while mosses support the Canary Islands as being related to North Africa, rejecting the concept of Macaronesia s.s. for the group. The exchange of taxa with neighbouring continental areas better explains the relationships between the cryptogamic flora of Cape Verde and the mossy flora of the Canary Islands. In contrast, the relic flora is consistent with a monophyletic Macaronesia s.s. group of liverworts. These congruent patterns, however, can hide a complex mix of relict distributions and more recent speciation and dispersal events. According to Mouton et al. (2023), the differences in beta diversity patterns, caused by different responses of Macaronesian land plant lineages to the main factors shaping their community composition, explain their different biogeographic affinities.

Similarities amongst the ten islands and archipelagos (beta diversity). The results of our study, when comparing the bryofloras of the ten Atlantic Ocean islands and archipelagos, demonstrated that their bryofloras were very dissimilar, with low floristic affinities (Fig. 4). Amongst the ten islands and archipelagos, the ones demonstrating the greatest similarity were the archipelagos of Macaronesia – Azores, Madeira and the Canary Islands – which share 278 taxa (Fig. 4A–C). Our study also suggests, based on the compilation of bryophytes, Macaronesia as being formed by the Azores, Madeira, and the Canary Islands archipelagos, with Cape Verde outside the clade when the islands and archipelagos of the North Atlantic Ocean were analyzed separately or together with the islands and archipelagos of the South Atlantic Ocean (Fig. 4A, C). As expected, the Azores, Madeira, and Canary Islands archipelagos (Macaronesia s.s.) share several taxa and evidenced the highest similarity. The archipelago of São Tomé and Príncipe was found to be highly dissimilar to the others, sharing few taxa with any of the North Atlantic Ocean archipelagos (Fig. 4A, C).

The results for the South Atlantic Ocean also show low similarity amongst most islands and archipelagos. However, Saint Helena and Ascension demonstrated the highest similarity and formed a distinct clade (with Tristan da Cunha outside of it). Saint Helena and Ascension share several taxa. The archipelagos of Trindade and Fernando de Noronha also formed a separate clade, despite sharing few taxa; this can be attributed to the several ecological degradations suffered by these two archipelagos since their discovery and occupation (Fig. 4B, C).

When we analyzed all ten Atlantic Ocean islands and archipelagos together, their similarities were very low, except among Madeira, the Azores, and the Canary Islands (Marcaronesia s.s.), each of which demonstrates a largely unique bryoflora. Our results also corroborate the exclusion of Cape Verde from Macaronesia (Fig. 4C).

Conservation. The ten islands and archipelagos studied here have proven to be rich areas for bryophytes (with the exception of Fernando de Noronha and Trindade). Their bryophyte diversity represents approximately 40% of all taxa known from Africa (3,800 species), 38% of all bryophyte taxa known from tropical America (4,000 species) and 8% of all species globally (Wigginton 2018, O’Shea 2003, O’Shea 2006, Gradstein et al. 2001). Our study indicated that the vegetation on the islands and archipelagos must be preserved and protected, as 200 taxa are only known from them. However, when analyzed separately, the Brazilian islands are not particularly rich in bryophyte diversity – this observation holds true only when considering all ten islands together. Additionally, more botanical collections should be undertaken on Cape Verde, Tristan da Cunha and São Tomé and Príncipe. Our results emphasize the importance of the bryofloras of the Atlantic Ocean islands and archipelagos for bryophyte conservation, as they contain high percentages of the total bryophyte diversity of Africa and tropical America, as well as the taxa diversity known for the Macaronesia region, despite their small areas.

The main threats to biodiversity on all ten islands and archipelagos include species introductions, pollution (from maritime traffic, tourism and fishing), the introduction of invasive species with their potential to alter native habitats and compete with native species (as observed on Saint Helena, where New Zealand flax (Phromium tenax J.R. Forst. & G. Forst., a monocotyledon from the Asphodelaceae family) and the moss Pseudoscleropodium purum (Hedw.) M. Fleisch., from the Brachytheciaceae family, pose significant threats) and rising ocean levels due to climate change (Thomas et al. 2020, Soares 2018). According to Wigginton (2012), Pseudoscleropodium purum has been used as packing material for imported plants and should be treated as an introduced species in Saint Helena.

Five of the North Atlantic Ocean islands and archipelagos studied here had 240 taxa classified as being threatened to some degree (2 EX – extinct [According to Sim-Sim et al. (2019a), Sim-Sim et al. (2019b), both from Madeira island, Fissidens microstictus Dixon & Luisier and Nobregaea latinervis Hedenäs]; 15 CR – critically endangered; 72 EN – endangered; 42–145 VU – vulnerable; 6 LC – least concern) – fully 20% of the total number of taxa (Hodgetts 2019). This study confirms the significance of these islands and archipelagos as key sites of bryophyte diversity for Macaronesia and Africa. However, there is a lack of adequate protection for endemic and/or endangered taxa on these islands and archipelagos. Addressing this should be a priority, as global warming is expected to accelerate the destruction of these sensitive environments worldwide.

The bryophytes of the Macaronesian laurel forests (Laurisilva) are also at considerable risk due to climate change (Patiño–Llorente et al. 2016, Patiño–Llorente and Vanderpoorten 2018, Carvalho et al. 2022). Many areas are predicted to become water-deficient and forest fires are becoming more frequent – with projections indicating significantly increased risks to important endemic taxa (Hodgetts 2019).

Many other bryophyte taxa are under conservation threat, with tropical species often known solely from type specimens or small private collections. For example, approximately 45 bryophyte taxa reported for São Tomé and Príncipe have not been collected again since the 19^th century (Costa et al. 2024, Garcia et al. 2012, Sérgio and Garcia 2011). The ecology, habitat specificity, and distribution patterns of many bryophyte taxa remain poorly understood, while threats to forest habitats in São Tomé and Príncipe continue to escalate (including habitat destruction and competition from invasive taxa (for example Figueiredo et al. 2011) can affect the survival of many bryophyte taxa (Garcia et al. 2022).

This study represents the first comprehensive survey of bryophyte diversity across ten Atlantic Ocean islands and archipelagos. The findings have significantly enhanced our understanding of the bryofloras in island habitats, introducing numerous novelties and new records. These data are crucial for the conservation of bryophyte diversity on Atlantic Ocean islands and archipelagos and can contribute to achieving the targets of the Global Strategy for Plant Conservation (GSPC). The survey provides a deeper understanding of the floristic compositions of these islands and archipelagos, increasing our estimates of taxa richness and distributions. However, this information remains incomplete, highlighting the need for additional inventories in unexplored regions of the Cape Verde, Tristan da Cunha, and São Tomé and Príncipe archipelagos (see references and Table 3). Bryophyte richness and diversity differed significantly amongst study sites (alpha diversity), and their low similarities (beta diversity) illustrated their high degrees of heterogeneity and reflected the importance of habitat differences for high diversity. Understanding the factors influencing taxa richness and endemism on oceanic islands is vital for effective conservation efforts and biodiversity management, particularly in the context of global climate change (Grabherr et al. 2010). Climate change is expected to result in strong impacts on oceanic islands and archipelagos, with habitat losses and declines and extinction risks, as isolated bryophyte taxa in these regions have no access to refuges. The complexity of island ecosystems, characterized by high diversity and endemism, requires researchers to generate primary data and integrate geological and climatic information into diversity models. Further studies are essential for developing proactive conservation plans, for these environments harbour a significant portion of the world’s biodiversity. The main threats to taxa diversity, forest structure, and habitats include urbanization, habitat destruction and fragmentation, the introduction of invasive taxa that can affect the survival of native taxa, uncontrolled tourism and rising oceans (which will mainly affect islands smaller than 1,000 ha - Médail (2017)). Our findings establish baselines for bryophyte diversity and endemism across ten Atlantic Ocean islands and archipelagos, providing benchmarks for future surveys. These data can help determine whether taxa can adapt or face extinction in response to climate change. The unique characteristics of those oceanic islands and archipelagos necessitate conservation strategies and address their biological and environmental diversities while recognizing their importance as protected areas.

The authors would like to thank CNPq for the PDE grant (no. 200858/2022-0), also to University of Lisbon, National Museum of Natural History and Science for providing all the infrastructure for the development of the work. Finally, the collaboration of the colleagues César A. Garcia, Fátima Sales, Ireneia Melo, Jorge Paiva, José Cardoso, and Maria Cristina Duarte, and that collected and sending samples from different islands. Also César A. Garcia that reviewed the manuscript offering suggestions for its improvements.

University of Lisbon, National Museum of Natural History and Science

Denise Pinheiro Costa developed the idea of this publication, both authors contribute to the writing and editing draft and gave approval for its publication.

The authors have declared that no competing interests exist.

Disclaimer: This article is (co-)authored by any of the Editors-in-Chief, Managing Editors or their deputies in this journal.