Tropical Sea Beaches

Coral Cays, Belize

(Stoddart 1962, 1963; Stoddart et al. 1982)

Supratidal vegetation, as well as the mangroves discussed above, was mapped on coral cays in Belize just before the devastating hurricane of 1961. The interval since the last storm had probably been long enough for the beach vegetation to reach near equilibrium. The more exposed beaches had a sparse, low outpost vegetation of perennial herbs and vines, all well adapted for long-range ocean dispersal. Many are pantropical, including a beach grass, Sporobolus virginicus ; a beach morningglory, Ipomoea pes-caprae ; and sea purslane, Sesuvium portulacastrum . Others are also widespread but only within the Caribbean region, for example, a spider lily, Hymenocallis littoralis , and a bushy spurge, Euphorbia mesembrianthemifolia .

The larger cays had beach ridge thickets of spray-swept shrubs and trees; these are adapted for both sea and bird dispersal. The dominants included some pantropical species, such as Suriana maritima and Sophora tomentosa , but most are neotropical, such as Thrinax radiata, Coccoloba uvifera, Cordia sebestena, Bursera simaruba, Conocarpus erectus, Tournefortia gnaphalodes, Borrichia arborescens . The East Indian almond, Terminalia catappa , was naturalized on some cays. Many had been largely cleared for coconut plantations.

This beach vegetation was almost identical to that of the Cayman Islands. Before striking Belize, Hurricane Hattie passed close enough to Grand Cayman to cause washing out of the outer few meters of vegetation on some beaches, but the recovery was completed in about 5 years.

On the Belize cays, Hurricane Hattie caused catastrophic damage to beach vegetation over a swath about 65 km wide. Destruction was not as total as in the mangroves. Outpost beach vegetation was damaged mainly by storm waves reworking the substrate. In the most extreme cases, vegetated cays as long as 120 m were completely washed away and have not since reappeared above the sea. More commonly, the outpost vegetation merely lost some ground to beach erosion and washovers or was buried under fresh coral rubble. Parts of the cays were stripped of all loose sand and shingle down to beachrock. In places, losses were selective: waves swept away Ipomoea and Sesuvium but left Sporobolus , and Hymenocallis reemerged where neighboring species were killed by burial.

The beach ridge thickets suffered less from direct wave action than from defoliation and breakage. Here, also, losses were partly selective, Cordia being uprooted more often than neighboring species. The native Thrinax palm stood up to the wind better than the planted Cocos .

Within 6 months, outpost species were recolonizing some of the bare sand, either vegetatively or by seedlings; Sporobolus, Sesuvium , and Ipomoea were among the first. Seedlings of a thicket shrub, Tournefortia , were common. The former distinction between zones was blurred, with outpost species appearing inland of their former position. Some weedy annuals that were not noted on beaches before the hurricane, for example, Portulaca oleracea and Cenchrus incertus , were prominent as early colonizers of fresh sand.

By 4 years after the storm, there was vigorous but patchy colonization of loose sand by both outpost and thicket species. All the outpost species that were common before the storm were well represented and were displacing Portulaca . Among the shrubs that had been prehurricane dominants, Tournefortia was best represented by seedlings; there were also some seedlings of Suriana, Sophora, Thrinax, Conocarpus , and Borrichia . The other thicket species were present as survivors but not as recolonists. Areas of stripped beach rock, coarse shingle, and mats of dead coconut roots remained mostly bare.

By 11 years after the storm, effects of the hurricane were no longer generally obvious except for relic dead and broken trees. Resurveys commonly found more species than the prehurricane roster, perhaps because of more thorough search. However, only a few seedlings of some formerly important thicket species had appeared: Coccoloba, Cordia, Bursera , and the naturalized Terminalia .

Comparative mapping over the same 11-year time span extended to cays south of the swath of Hurricane Hattie. These showed less change, of course, but they still showed quite dynamic cay morphology and vegetation patterns. The maps usually showed only single dominants for each area, so changes may be quantitative rather than advances and retreats of species borders. However, they leave no doubt that micromigrations have gone on, partly as recovery from a storm 15 years before Hattie and partly as a result of chronic instability of sand cays. During the decade, a few species became extinct on some of the smaller cays; some of these had been present originally as individual bushes or trees.

Coral Sand Beaches, Mauritius

(McIntire and Walker 1964; Sauer 1962)

Mauritius is a large, rugged volcanic island, but along with basalt sea cliffs and boulder shores, it has extensive coral sand beaches behind fringing reefs.

Early in 1960, after 15 years without any significant storms, Mauritius was raked by two strong cyclones; the second, Carol, was probably the most

intense in the recorded history of the southwestern Indian Ocean. Instrumentally recorded wind velocities at several stations on Mauritius reached 240 km/hr. Waves superimposed on the storm surge left piles of drift 3 to 4 m above normal wave reach on exposed coasts.

A few months before the storms, the coastal vegetation had been surveyed with belt transects and maps of small sample areas; the vegetation was then in apparent equilibrium and neatly zoned. Outpost vegetation on coral sand beaches was dominated by the same pantropical herbs and vines as on Belize cays. Beach ridge thickets also had the same pantropical shrubs, but two of the dominants were Indo-Pacific species, Tournefortia argentea and Scaevola taccada . Inner beach ridges behind the thickets were generally planted with an introduced timber tree, Casuarina equisetifolia . There were only a few coconut plantations.

Here, as in Belize, destruction by the hurricane of beach outpost vegetation was mostly due to reworking of the sand substrate by storm waves rather than to direct damage to the plants. This was dramatically evident where the same species survived pounding by storm waves where rooted in crevices on basalt but were completely swept away where rooted on sand. Waves also bit into the outer margins of thickets on loose sand and shingle ridges, but survivors of all the species were left in each site.

By contrast, destruction of Casuarina trees extended far beyond the reach of the waves, and mortality in large areas was nearly total. The coconut plantations had mortality of only about 25%.

Three months after Cyclone Carol, a resurvey showed complex changes in beach profiles but little change in shoreline positions because sand had been redeposited as the storm abated. Most of the area reworked by waves was still completely bare, but recolonization was beginning. The commonest new colonists of the outpost zone were Canavalia maritima seedlings and vegetatively propagating Ipomoea pes-caprae and Cyperus stoloniferus , the latter more abundant than before the storm. Some beaches were invaded by seedlings of inland weeds, such as the Mexican prickly poppy, Argemone mexicana , perhaps aided by low salinity after the heavy rains. Except for these weeds, recolonization of the outpost zone was invariably by species that had been present in the same sites before the storm. This outcome was not inevitable, since Ipomoea pes-caprae was the only species that had been present in over 50% of the sites. The storm was evidently ineffective in coastwise dispersal within the outpost zone, probably because seed afloat during the storm was stranded mainly in the thicket zone or farther inland.

In the thicket zone, a few survivors of all the previously present species were found in almost every site; the dominant Tournefortia and Scaevola were resprouting vigorously where damaged. Shrub and tree seedlings were generally few.

Where the storm waves had left bare sand back in Casuarina and Cocos

plantations, there were many new seedlings of species not present before the hurricane. These included all the common outpost and thicket species and also some inland weeds, for example, Portulaca oleracea and Ricinus communis , and crops, for example, Nicotiana tabacum and Ipomoea batatas . Drift stranded during the storm evidently contained a mishmash of propagules derived partly from excavation of beaches and partly from flooding rivers.

During 1961 and 1962 several other cyclones passed near enough to Mauritius to produce heavy surf and beach erosion, mainly on the west coast. The strongest, in 1962, had winds gusting to 200 km/hr. When revisited in 1963, west coast beaches showed additional losses of shrubs that had survived Carol in 1960, but new colonies of outpost herbs, especially Sporobolus and Cyperus , had appeared. On the east coast, recolonization by these and other outpost herbs and vines was further advanced; there were also scattered seedlings of thicket shrubs, mainly Scaevola . The wreckage of the Casuarina plantations had mostly been converted to charcoal, and some replanting had begun.

Pyroclastic Beaches, Krakatau

(Docters van Leeuwen 1936)

After the cataclysmic eruption of Krakatau in 1883, the first expedition to the new island found no sign of plant life. After 15 years, the beaches of volcanic ash and sand had acquired a rich flora including outpost vines, such as Canavalia maritima and Ipomoea pes-caprae ; thicket shrubs, such as Scaevola taccada and Morinda citrifolia ; and beach trees, such as Cocos nucifera, Casuarina equisetifolia, Terminalia catappa, Calophyllum inophyllum , and Pongamia pinnata . Within another 10 years, the island had nearly all the species to be found on beaches of any tropical Indo-Pacific island. The most notable missing species were Tournefortia argentea and Suriana maritima. Tournefortia seedlings have been found occasionally on Krakatau beaches but have not survived. Suriana has not been reported, although there can be little doubt that its sea-dispersed seeds can reach the island. These two species are probably barred from the island by unsuitability of the volcanic substrate; they generally grow on coral and limestone.

The rapid colonization by sea-dispersed beach species is explainable partly by the location in the Sunda Strait between Sumatra and Java. Ocean currents there vary greatly with tides and monsoonal wind shifts, so the island receives drift from both Sumatra and Java, as well as from more distant sources. Success of some species was probably partly due to symbiosis with nitrogen-fixing bacteria; these species include not only the legumes Canavalia and Pongamia but also Casuarina .

Introduced Indo-Pacific Species, Mauritius

(Sauer 1961, 1967b )

Several Indo-Pacific shrubs and trees that are now naturalized along the Mauritius coast would probably be mistaken for natives without the historical record of their introduction.

Mauritius was uninhabited and visited only occasionally by Portuguese and Dutch ships before being temporarily colonized by the Dutch in the seventeenth century. Permanent settlement began with arrival of the French in 1715. Soon after, Mauritius began playing a leading role in the great exchanges of tropical plants between botanical gardens of European colonial powers. Its great botanical gardens at Pamplemousses and Reduit were world famous. Beach species were a very minor part of their collections.

In the mid-eighteenth century, Mauritius acquired Casuarina equisetifolia and Calophyllum inophyllum from Madagascar and Terminalia catappa from Bengal. In the early nineteenth century, Morinda citrifolia was imported from India and Desmodium umbellatum from an unspecified source. In the late nineteenth century, Pongamia pinnata was imported from the Seychelles. All of these have become naturalized as volunteers in natural coastal habitats on Mauritius, probably spreading around the island by floating seeds. Terminalia is also dispersed by the native fruit bats or flying foxes.

The story of Cocos nucifera is less clear. Coconut palms were reported on the Mauritius coast in 1598 by the first Dutch expedition. Whether they arrived on their own or were introduced by the Portuguese is unknown. In any case, coconuts are rarely found today along the Mauritius coast except where planted.

It is not immediately obvious why any of these species should have had to await human transport in order to reach Mauritius. All except Casuarina are known to be capable of long-range dispersal. Casuarina seeds survive immersion in salt water indefinitely but are not buoyant. They are believed to cross ocean barriers by rafting, particularly by floating pumice on which they have been seen germinating. Moreover, all these species were native on various other Indian Ocean islands, including the Seychelles, which lie in the same longitude as Mauritius (fig. 2).

The Seychelles were uninhabited when discovered by a British ship in 1609; wild coconut palms lined the beaches then. The archipelago remained uninhabited until colonized by French from Mauritius in the mid-eighteenth century, initially to exploit the wild coconuts. The first colonists also encountered coastal Casuarina, Calophyllum , and Terminalia groves. They were amazed at the size of the Terminalia trees, which they had known in Mauritius as saplings. Morinda and Pongamia were probably also naturally established

Figure 2. Prevailing Indian Ocean Currents Upstream from Seychelles and Mauritius. Although the currents
are not constant, drifting seeds of tropical beach plants are more likely to reach the Seychelles than Mauritius.
Some such species that arrived prehistorically in the Seychelles have since been artificially introduced to Mauritius.

in the Seychelles, although the first clear records are from the nineteenth century.

The rich native beach flora of the Seychelles, as compared to Mauritius, may result from the pattern of ocean currents. The Seychelles lie in the Equatorial Current downstream from Java, the rest of the East Indies, and many Pacific islands. Mauritius is far enough south to lie within the great counterclockwise circulation around the southern Indian Ocean and is thus downstream from the southern and western coasts of Australia. Such currents are variable, of course, but evidently not enough to erase different effects on dispersal.

Transfers Between Old and New Worlds

(Sauer 1982)

Some of the Indo-Pacific beach species introduced to Mauritius were also introduced through botanical gardens to the West Indies. Casuarina equisetifolia and Terminalia catappa from the East Indies were being grown in Jamaica by 1790, and Morinda citrifolia probably arrived soon after. All three were widely planted in the New World tropics during the nineteenth century, Casuarina for timber and fuel, Terminalia as a shade tree, and Morinda as a medicinal and dye plant. Herbarium specimens show that by mid-nineteenth century, all three had escaped from cultivation and were spreading on their own along seashores. Once in the region, Terminalia and Morinda were quite capable of island hopping by sea dispersal; both are recorded as having arrived in Grand Cayman, for example, as drift seeds. Casuarina and Morinda mainly colonize open seashore habitats, but Terminalia invades dense coastal thickets and overtops native trees.

Not all introduced Indo-Pacific seashore species have been so successful. Cocos nucifera, Barringtonia asiatica , and Scaevola taccada are commonly planted in the Caribbean region, often around beach houses, but are not spreading on their own.

Various Caribbean beach species have been widely planted in the Indo-Pacific region, including Mauritius, and have escaped to a few beaches, for example, spiderlily, Hymenocallis caribaea ; yellow bells, Tecoma stans ; whitewood, Tabebuia pallida ; frangipani, Plumeria spp.; and cocoplum, Chrysobalanus icaco . The cocoplum has become particularly successful on beaches of Viti Livu, Fiji. It had escaped to several beaches by 1962, and by 1984 it was extensively codominant with the native shrubs in beach ridge thickets on Viti Livu.

Anacardium occidentale , the cashew, has naturalized more extensively. Native to coastal dunes of northeastern Brazil, the cashew was introduced

by the Portuguese early in the colonial period to West and East Africa, India, and the East Indies; other people soon joined in planting the trees. The fruits are attractive to many animals, including monkeys, and the seeds are capable of at least local sea dispersal. Herbarium specimens show that by the mid-nineteenth century, the cashew was naturalized on many seashores of East and West Africa, tropical Asia, and the East Indies.

Comment

Like mangrove swamps, tropical sea beaches have a rich and very discrete flora. Few species transgress between these beaches and other kinds of habitats. There are minor exceptions, including the cases noted above when inland weeds temporarily invaded beaches after hurricanes and torrential rains. Conversely, beach species can invade adjacent inland sites when their normal competitors are removed. For example, in the Caribbean and Indian Ocean after World War II, abandoned military installations were commonly overrun by Canavalia maritima and Ipomoea pes-caprae vines.

For the outpost species, normal survival in a habitat within reach of storm waves involves perpetual micromigration, even on a stable coastline. Less extreme situations than the hurricanes discussed above include winter northerly storms and summer calms along the Mexican Gulf coast and monsoonal wind shifts on the East African coast. Beach ridge thicket species live in habitat largely controlled by a salt spray gradient and only marginally affected by storm waves. Where a coastline is progressively eroding or advancing, both outpost and thicket species may survive by local migration without any species extinctions within sites or recruitment of species from other sites.

Sea dispersal of tropical beach species has allowed them to colonize habitats that are spatially extremely discontinuous, not only on islands but on mainland coasts. Both outpost and thicket species are excluded from topographically sheltered shorelines by mangroves and inland species. Some beach species are excluded from bedrock or shingle coasts or from black sand or quartz sand beaches. Moreover, unlike mangroves, beach species need fresh water. They can survive a dry season by drawing on the fresh water lens floating in the sand above the denser salt water, but they are excluded from coasts where there is too little rain to maintain this lens.

Unlike many mangroves, the beach species produce seeds that remain dormant while afloat, with the embryos protected from seawater. As discussed in the Introduction, in two beach ridge thicket species, Tournefortia argentea and Scaevola taccada , breaking of dormancy requires a sequence of flotation in the sea, brief or long, followed by stranding and wetting by rain. Other beach species have seeds with extremely thick, hard integuments that remain impermeable during flotation for at least several months; they also

resist scarification by surf and sand. Presumably they germinate when the integument decays after long burial in moist sand.

As in the mangroves, adaptation for sea dispersal of beach species is probably not the result of selection for long-range dispersal per se, but for persistent buoyancy, since any seeds that sink at sea are eliminated as contributors to the gene pool of their species. The effectiveness of such selection is shown most dramatically by the outpost species, a dozen of which have naturally pantropical ranges. A few thicket species also have long-buoyant seeds and have achieved naturally pantropical ranges. Most thicket species have seeds that float for more moderate time spans or have fruits attractive to birds and bats, or both. It is within this latter group that recent human dispersal has permitted certain species to naturalize on beaches they had not been able to reach before.