Sal’s Endemic Flora: Botanical Survival on a Desert Island

Islands as Evolutionary Laboratories

Islands occupy just 5% of the Earth’s land surface, yet they harbour more than 20% of all plant species. This remarkable concentration of biodiversity becomes even more striking when we consider that many of these species exist nowhere else. Sal Island, in the Cape Verde archipelago, exemplifies this phenomenon under the most extreme conditions imaginable — a natural experiment in how far plant life can push against environmental limits.

The Cape Verde archipelago supports 92 endemic plant species within its total vascular flora of approximately 740 species. Yet these endemics cluster unevenly across the islands. The mountainous western islands of Santo Antão and Santiago, with their varied topography and altitudinal gradients, support rich endemic communities. Sal presents an entirely different scenario: a flat, desiccated landscape where endemic plants shouldn’t logically exist, yet somehow do.

What makes Sal particularly fascinating for botanical study is precisely its inhospitable nature. On this salty island, evolution has operated under constraints so severe that each surviving endemic species represents a masterclass in adaptation. These aren’t simply plants that, by accident, happen to grow here. They’re organisms that have solved biological problems most plants never encounter.

Geological Foundation and Environmental Extremes

Sal’s botanical story begins with geology. The island emerged through submarine volcanic activity millions of years ago, making it among the archipelago’s oldest landmasses. Time has not been kind. Where other Cape Verdean islands retain dramatic topography — Fogo’s active volcano reaches 2,829 meters — erosion has reduced Sal to near-planarity. Monte Grande, the island’s highest point at 406 meters, barely qualifies as a hill.

This topographical monotony eliminates the elevation gradients that generate diverse microclimates elsewhere in the archipelago. There are no cloud forests here, no mist-shrouded peaks, no rain shadows creating moisture differentials. Instead, Sal presents 216 square kilometres of relatively uniform aridity.

The climate data read like a plant’s nightmare. Annual precipitation averages below 100 millimetres, arriving unpredictably between August and October — if it arrives at all. The northeast trade winds blow relentlessly, carrying Atlantic salt spray that deposits marine salts across every surface. Dig anywhere on the island, and you encounter brackish groundwater, that problematic mixture of fresh and saline that creates osmotic chaos for most plants. The botanical literature notes, with characteristic understatement, that:

“A number of xerophilous plants grow in the brackish subsoil of Maio, Sal, and Boa Vista.”

Phoenix atlantica, aka Tamareira

No discussion of Sal’s endemic flora can proceed without extensive consideration of Phoenix atlantica, the Cape Verde Island date palm. Known locally as “tamareira,” this species is one of only two tree species endemic to the entire archipelago — a fact that underscores both its rarity and significance.

Phoenix atlantica confounds expectations about palm architecture. Rather than the single-stemmed growth typical of most palm species, it produces multiple trunks from a common base. Such clusters can reach heights of 5-15 meters in their growth pattern, providing mutual protection against desiccating winds while facilitating the sharing of resources among connected stems.

The species’ root system demonstrates remarkable physiological adaptation. These roots penetrate deeply into brackish water tables, which would induce fatal osmotic stress in most plants. Genetic analysis has confirmed that:

“P. atlantica is clearly distinct from its close relatives and that its closest relative is likely to be its nearest geographical neighbour, P. dactylifera.”

Yet that cultivated relative would perish within days under Sal’s conditions.

Recent molecular work has uncovered unique genetic markers within Phoenix atlantica populations. It includes fixed alleles absent from all other Phoenix species. Research indicates that:

“Wild samples from certain populations in Boavista and Sal are best suited for establishing a seed and/or germplasm bank for replantation efforts.”

This genetic distinctiveness, combined with the palm’s ability to thrive (without irrigation or chemical inputs) in coastal areas, positions the Cape Verdean date palm as a potentially valuable genetic resource for developing salt-tolerant crops.

Succulents

Water storage through succulence represents a good solution to Sal’s aridity, and several endemic species have independently evolved this strategy.

Euphorbia tuckeyana: succulent with toxic latex

Euphorbia tuckeyana, endemic to Cape Verde, exemplifies this approach while adding a chemical defence dimension. The plant’s swollen stems store precious water, but when they break, caustic white latex immediately emerges. This latex contains complex mixtures of diterpenoids and triterpenoids that deter herbivores while possessing demonstrated cytotoxic properties — a dual-purpose adaptation that addresses both water scarcity and herbivore pressure.

Aeonium gorgoneum: rosette succulent

Aeonium gorgoneum, another Cape Verdean endemic succulent, takes a different approach. This member of the Crassulaceae forms rosettes of thick, waxy leaves that minimise water loss while maximising photosynthetic surface area during favourable conditions. The species can survive months without rainfall by closing its stomata and relying on stored water, essentially shutting down until conditions improve.

Kalanchoe porphyrocalyx: reproductive specialist

Kalanchoe porphyrocalyx, restricted to Cape Verde’s arid zones, including the island of Sal, demonstrates the reproductive flexibility common among island succulents. The plant produces plantlets along its leaf margins — tiny clones that drop off and establish independently. This amazing vegetative reproduction ensures that the population persists even when other reproduction ways fail due to pollinator scarcity or unsuitable conditions.

Masters of Salt Tolerance

The halophytic endemics of Sal have evolved sophisticated mechanisms for managing salt stress that would prove lethal to most plants.

Limonium sundingii, Limonium braunii & Limonium jovi-barba: salt excreters

Limonium sundingii and Limonium braunii are both endemic to Cape Verde and possess a fascinating desalting system. They have specialised salt glands that actively excrete excess sodium chloride through their leaves. If you walk among these plants after morning dew, you’ll see salt crystals glittering on leaf surfaces — visible evidence of this remarkable physiological adaptation.

Limonium jovi-barba, another endemic sea lavender, colonises hypersaline coastal depressions where salt concentrations exceed those of seawater during dry periods. The species has evolved not just to tolerate but to require high salt levels, occupying an ecological niche where competition from less specialised species doesn’t exist.

Frankenia ericifolia Caboverdeana: triple-threat survivor

Frankenia ericifolia subsp. Caboverdeana, endemic to the eastern Cape Verde islands, combines salt tolerance with extreme drought resistance. This small shrub’s needle-like leaves reduce surface area while thick cuticles minimise water loss. The plant can survive salt spray, drought, and high temperatures simultaneously — a triple threat that would eliminate most competitors.

Nitrogen Fixers and Soil Builders

In the nutrient-poor volcanic soils of Sal Island, the ability to fix atmospheric nitrogen provides a significant advantage.

Lotus jacobaeus and Lotus purpureus: Nitrogen Fixers

Lotus jacobaeus, a Cape Verde endemic legume, has perfected this strategy. The species forms root nodules that harbour Rhizobium bacteria, which convert atmospheric nitrogen into forms that are bioavailable. But L. jacobaeus goes beyond simple nitrogen fixation—its deep taproot, sometimes extending several meters underground, mines water and minerals from depths other plants cannot reach while providing anchorage against fierce winds.

Lotus purpureus, another endemic legume found in Sal’s interior plains, demonstrates how nitrogen-fixing plants can act as ecosystem engineers. By enriching soil nitrogen content, these plants create fertility islands that facilitate the establishment of other species, potentially playing a crucial role in plant community assembly in nutrient-limited environments.

Indigofera pedicellata: pioneer species

Indigofera pedicellata, endemic to the Cape Verde Islands, adds another dimension to the nitrogen-fixation story. This species colonises disturbed areas and recent volcanic deposits, serving as a pioneer that begins the slow process of soil development. The traditional use of this plant for producing blue dye demonstrates how endemic species can provide both ecological and cultural services.

Drought Survivors

Some of Sal’s endemics have abandoned succulence and salt excretion in favour of more extreme drought-avoidance strategies.

Tornabenea insularis and Tornabenea humilis: extreme drought specialists

Tornabenea insularis and Tornabenea humilis, both Cape Verde endemic members of the carrot family, have reduced their leaves to mere scales. These plants concentrate their biomass underground in deep taproots that store water and nutrients, emerging above ground only to flower during brief favourable periods.

Nauplius smithii: developmental plasticity

Nauplius smithii (formerly Asteriscus smithii), restricted to Cape Verde’s driest zones, produces two distinct leaf types depending on water availability. During drought, small, heavily pubescent leaves minimise water loss. Following rain, the plant rapidly produces larger, thinner leaves to maximise photosynthesis. This developmental plasticity allows a rapid response to unpredictable precipitation.

Erysimum caboverdeanum: rock crevice specialist

Erysimum caboverdeanum, an endemic wallflower, survives in rocky crevices where soil barely exists. The species has evolved an extensive root system that infiltrates microscopic rock fissures, extracting moisture from stone that appears completely dry. Dense leaf pubescence reflects excess radiation while creating a boundary layer that reduces transpiration.

Coastal and Transition Zone Inhabitants

The boundary between the marine and terrestrial environments favours the formation of unique endemic communities.

Suaeda vermiculata Caboverdeana: tidal zone endemic

Suaeda vermiculata subsp. Caboverdeana thrives in salt marshes and coastal flats, where tidal flooding creates extreme fluctuations in salinity. This endemic subspecies has evolved enhanced salt-accumulation capacity compared to its widespread relatives, sequestering sodium in specialised cells that eventually die and shed, removing excess salt from the plant.

Polycarpaea gayi: transition zone species

Polycarpaea gayi, a Cape Verde endemic in the carnation family, occupies the transition between coastal halophytic zones and inland xerophytic communities. The species produces tiny leaves covered in silvery scales that reflect radiation while trapping moisture from ocean fogs — a rare water source in Sal’s arid climate.

Sideroxylon marginatum: broad tolerance shrub

Sideroxylon marginatum, an endemic shrub in the sapodilla family, demonstrates remarkable ecological amplitude. The species spans areas ranging from near-coastal regions receiving salt spray to interior valleys where aridity is the primary stress. This broad tolerance suggests either exceptional physiological plasticity or cryptic genetic differentiation among populations — a question deserving further investigation.

Reproductive Innovations in Challenging Climate

The reproductive strategies of Sal’s endemics reflect the constant challenges of unpredictable resources and limited pollinator diversity.

Campanula bravensis: flexible flowering

Campanula bravensis, the endemic Cape Verdean white bellflower, exhibits extended and flexible flowering periods. Rather than blooming on a fixed schedule, the species flowers opportunistically whenever conditions permit, sometimes producing flowers continuously for months if moisture remains available.

Echium stenosiphon: specialised pollination

Echium stenosiphon, endemic to the Cape Verde Islands, has evolved flowers with exceptionally long corolla tubes that restrict access to a limited suite of long-tongued pollinators. This specialisation may seem risky on a species-poor island, but it ensures efficient pollination when the correct pollinator is present, while reducing pollen waste from ineffective visitors.

Lobularia canariensis fruticosa: bet-hedging seeds

Lobularia canariensis subsp. fruticosa, endemic to Cape Verde despite its species name suggesting Canarian origin, demonstrates a bet-hedging reproductive strategy. The plant produces two types of seeds: some that germinate immediately if conditions permit, and others that remain dormant in the soil seed bank. Their germination occurs only after receiving specific environmental signals from outside. This temporal dispersion of germination risk ensures population survival across years.

Chemical Ecology and Medicinal Properties

The chemistry of secondary metabolites of the endemic Sal flora reveals complex evolutionary responses to multiple selective pressures.

Artemisia gorgonum: medicinal compounds

Artemisia gorgonum, endemic to Cape Verde, produces unique artemisinin derivatives distinct from those in related species. These compounds serve multiple functions, such as deterring herbivores, inhibiting the growth of competing plants through allelopathy, and providing antimicrobial protection. Because of this, traditional medicine has long recognised the value of this species in treating fever and digestive problems.

Lavandula rotundifolia: unique terpenes of Cape Verdean lavender

Lavandula rotundifolia is Cape Verde’s endemic lavender that synthesises a distinctive suite of monoterpenes and sesquiterpenes not found in any other continental lavender species. Gas chromatography analysis reveals over 40 distinct compounds, many of which are unique to this species. These chemicals create a complex olfactory signature that may facilitate pollinator recognition while simultaneously repelling herbivores.

Cynanchum daltonii: cardiac glycosides of Cape Verdean Milkweed Wine

Cynanchum daltonii, an endemic milkweed vine, produces cardiac glycosides similar to but distinct from those in African and Mediterranean relatives. These toxic compounds provide defence against most herbivores while potentially serving as sequesterable toxins for specialised insects, though the coevolutionary relationships remain unstudied in Cape Verde.

Globularia amygdalifolia: anti-inflammatory iridoids

Globularia amygdalifolia, endemic to Cape Verde, contains iridoid glycosides with demonstrated anti-inflammatory properties. Traditional healers have used this species for centuries to treat rheumatism and skin conditions. Modern phytochemical analysis confirms the presence of bioactive compounds, validating traditional uses while revealing potential for pharmaceutical development.

Importance of Genetic Diversity

The genetics of Sal’s endemic plant populations show their long and complex history of isolation, adaptation, and vulnerability. Their recent studies, using microsatellite markers, have revealed unexpected patterns such as the fact that, despite restricted ranges, some populations maintain surprisingly high genetic diversity. Scientists discovered that:

“Comparable levels of genetic diversity were found in insular P. atlantica and continental P. dactylifera despite the large difference in geographic range size.”

This paradox may result from several factors, including long generation times that preserve genetic variation, occasional long-distance gene flow between islands, or historical population sizes that were much larger than present. Whatever the mechanism, this genetic diversity represents irreplaceable adaptive potential.

Population genetic structure analysis reveals significant differentiation among islands and even within populations on the same islands. Phoenix atlantica populations on Sal harbour unique alleles not found elsewhere in the archipelago. These private alleles may represent local adaptation to Sal’s specific conditions — genetic solutions to environmental problems that evolved over thousands of generations.

The implications for conservation are profound. Each population potentially contains unique adaptive variants. Losing any population means losing evolutionary potential that cannot be recovered. This argues for protecting multiple populations across each species’ range rather than focusing on single large reserves.

Conservation Status

The numbers paint a grim picture: 78% of the endemic plants are threatened, with 29.3% classified as Critically Endangered, 41.3% as Endangered, and 7.6% as Vulnerable. For Sal’s endemic flora, with its restricted habitat availability and ongoing development pressure, the situation is particularly acute.

Habitat loss from tourism development represents the most immediate threat. Sal has transformed from a salt-production backwater to Cape Verde’s premier tourist destination in just two decades. The international airport, expanded resort complexes around Santa Maria, and associated infrastructure have eliminated or fragmented habitats that supported endemic plant populations for millennia.

Invasive species compound habitat loss.

• Prosopis juliflora (mesquite), introduced from the Americas, forms dense monospecific stands that exclude native vegetation.
• Nicotiana glauca (tobacco tree) colonises disturbed areas before natives can establish.
• Parkinsonia aculeata (Mexican palo verde) competes directly with native legumes for limited water and soil nutrients.

Each successful invasion reduces the habitat available for endemic species.

Overgrazing by free-ranging goats prevents regeneration of woody endemics. Young Phoenix atlantica rarely survive to maturity in areas with high goat density. The selective browsing pressure favours unpalatable or toxic species, potentially altering plant community composition over time.

Climate change intensifies all already existing threats. Sal’s endemic flora evolved under harsh, arid conditions; however, projected temperature increases and altered precipitation patterns may exceed species’ tolerance limits. Rising sea levels threaten coastal endemic populations, while increased storm intensity could eliminate small plant populations in more exposed locations.

Conservation Strategies

Effective conservation of Sal’s endemic flora requires integrated strategies that address multiple threats simultaneously. In situ conservation through the protected areas provides the foundation and basic insurance against extinction. However, the passive protection alone won’t suffice. Active management — invasive species control, grazing regulation, habitat restoration — is essential here. But it’s also much more expensive.

Seed banking is effective for species with orthodox seeds that can tolerate drying and freezing. However, some endemic species produce recalcitrant seeds that die if dried. For these, alternative approaches like living collections or cryopreservation of embryonic axes may be necessary.

Research priorities should focus on filling critical knowledge gaps. Basic biological information, including pollination biology, seed dispersal mechanisms, and mycorrhizal associations, remains unknown for most endemic species. Understanding these fundamental aspects is a prerequisite to developing effective conservation strategies.

The physiological mechanisms underlying stress tolerance deserve particular attention. How Phoenix atlantica manages salt stress, what allows Tornabenea species to survive extreme drought, or how Limonium species excrete salt — understanding these mechanisms could inform both conservation and potential biotechnological applications.

Community engagement proves essential for long-term conservation success. Traditional ecological knowledge about the uses, management, and distribution of endemic plants provides many invaluable insights. Educational programs that highlight the uniqueness and value of endemic species can foster local support for conservation initiatives.

International cooperation isn’t optional but essential. As researchers have noted:

“Wealthier, more developed nations especially need to support those island nations that have many endemic plant species.”

This support should extend beyond funding to include technical expertise, research collaboration, and capacity building.

Lessons from the Edge

Sal’s endemic flora demonstrates that biological richness cannot be measured solely in species numbers. Each endemic plant on this wind-scoured island represents millions of years of evolutionary refinement — successful experiments in pushing life’s boundaries. These aren’t merely botanical curiosities, but libraries of genetic information that encode solutions to environmental challenges increasingly relevant in our changing world.

The plants that colonised Sal faced seemingly impossible challenges: minimal rainfall, constant salt stress, nutrient-poor soils, and geographic isolation. Over time, they transformed these constraints into opportunities, developing novel adaptations that enabled not only survival but also success. Phoenix atlantica’s salt tolerance, Euphorbia tuckeyana’s water storage, Lotus jacobaeus’s nitrogen fixation — each represents a different solution to Sal’s environmental puzzle.

Yet these evolutionary triumphs now face extinction within decades. The same isolation that permitted unique adaptations to evolve now prevents rapid adaptation to human-caused changes. The endemic flora that survived volcanic origins, millennia of drought, and centuries of human presence might not survive the current century of development and climate change.

The question isn’t whether Sal’s endemic plants deserve conservation. The question is whether we will recognise these species’ value before they disappear. On this small Atlantic island, some of the world’s most resilient plants await our decision. Their persistence or extinction will reflect not their failure to adapt, but our inability to value what their evolution has achieved.

Bibliography & Further Reading

• Diversity of Useful Plants in Cabo Verde Islands: A Biogeographic and Conservation Perspective, Maria Cristina Duarte, Isildo Gomes, Silvia Catarino, Miguel Brilhante, Samuel Gomes, Aline Rendall, Ângela Moreno, Arlindo Rodrigues Fortes, Vladmir Silves Ferreira, Isaurinda Baptista, Herculano Dinis, Maria Manuel Romeiras, Plants (Basel). 2022 May 15;11(10):1313, PMC PubMed Central, National Library of Medicine, National Center of Biotechnology Information;
• Patterns of genetic diversity in three plant lineages endemic to the Cape Verde Islands, Maria M Romeiras, Filipa Monteiro, M Cristina Duarte, Hanno Schaefer, Mark Carine, AoB Plants. 2015 May 15:7:plv051, PubMed, National Library of Medicine, National Center of Biotechnology Information; 
• Preservation and knowledge of medicinal plants in Cape Verde Islands: diversity and uses of exotic species since the colonisation of this archipelago, Romeiras M.M., Torrão M.M., .Joao Tavares, Luis Catarino, Maria Cristina Duarte, Conference: XIXth AETFAT Congress, via ResearchGate; 
• IUCN Red List assessment of the Cape Verde endemic flora: towards a global strategy for plant conservation in Macaronesia, Maria M. Romeiras, Silvia Catarino, Isildo Gomes, Claudia Fernandes, Jose C. Costa, Juli Caujapé-Castells, Maria Cristina Duarte, Botanical Journal of the Linnean Society, Volume 180, Issue 3, March 2016, Pages 413–425;
• Sobre la biota de la isla de Sal (Cabo Verde), Juan José Bacallado, Leopoldo Moro, Alberto Brito, Jesús Ángel Ortea Rato, MAKARONESIA, N. 18-19, 2017, via ResearchGate; 
• A global assessment of endemism and species richness across island and mainland regions, Gerold Kier, Holger Kreft hkreft@ucsd.edu, Tien Ming Lee, Walter Jetz, Pierre L. Ibisch, Christoph Nowicki, Jens Mutke and Wilhelm Barthlott, June 9, 2009, PNAS – Proceedings of the National Academy of Sciences of the United States of America;
• Cape Verde Islands Dry Forests, Emma Martin and Neil Burgess, One Earth; 
• Flora of Cape Verde and Wildlife of Cape Verde, Wikipedia;