Chapter Three
INTRODUCTION
In addition to mangrove forests, five other types of ecosystems are found around the coasts of Sumatra. These are: beach vegetation on accreting coasts (pes-caprae formation), beach vegetation on abrading coasts (Barringtonia formation), brackishwater forests, rocky shores and coral reefs. The last of these is not strictly within the scope of this book as it is a marine ecosystem, but it is included as background material for the reader who may wish to pursue studies on coral reefs.
None of the coastal ecosystems is known particularly well, except for the economically important coral reefs, and so the following sections serve only as introductions upon which further investigations can be based.
BEACH VEGETATION
Pes-caprae Formation
Along accreting coasts, that is, where new sand is being deposited, the sandy beach is colonised by a form of vegetation known as the 'pes-caprae' formation. Such vegetation is found throughout the Indonesian Archipelago and the western Pacific (Richards 1952). The name refers to Ipomoea pes-caprae, a purple-flowered creeper related to convolvulus, which is just one of a number of low, sand-binding herbs, grasses and sedges which advance over the sand with long, deep-rooting stolons or stems. Other species expected in this formation would include the legume Canavalia, sedges Cyperus pedunculatus and C. stoloniferus, and the grasses Thuarea involuta (fig. 3.2) and the prickly Spinifex littoreus. Full lists are given by Whitmore (1984), Wong (1978) and Soegiarto and Polunin (1980).
These plants are well adapted, being tolerant of periodic drought, salt spray (although dependent on non-saline ground water), wind, low levels of soil nutrients and high soil temperatures. They also produce small seeds which are dispersed by floating on water; for example, the small seed of Thuarea is surrounded by an air-filled chamber which floats easily and is dispersed by tidal currents (Holttum 1977). The mat of low vegetation of the parent plants traps dead leaves and other organic material blown by the wind or tossed there by the tides, and provides a little shelter for animals. Thus nutrients increase and later stages of the plant succession (p. 80) can occur, and the landward fringe is colonised anew. She-oak Casuarina equi-setifolia is found at the inner edge of the pes-caprae formation where it represents a late stage in the plant succession. She-oak can form pure stands but is unable to regenerate in closed forest or even in open forest on the carpet of litter formed by its own dead twigs. Thus, unless the coastline advances and provides fresh habitat, the belt of she-oak forest will be supplanted by other species (Corner 1952). The 30 m wide mature she-oak forest examined by a CRES team near Singkil, South Aceh, had a basal area of 32.7 m2/ha for trees of 15 cm diameter and over.
Figure 3.1. Pes-caprae association on a western beach in southern Aceh.
A.J. Whitten
Figure 3.2. Thuarea involuta, a creeping species of grass found in the pes-caprae association.
From Holttum 1977
The environment of exposed sandy beaches is particularly hostile to animals because of the instability of the sand, and the wide variations in temperature, salinity and humidity. A great many small animals live permanently within the sand where they have some protection, but they have to cope with the difficulties of finding food, reproductive partners and the low oxygen concentrations in the spaces of air between the sand grains (Brafield 1978). Near the high water level, burrows about 18 cm across can be found with small piles of sand around them. Their occupants are adult, beige-coloured sand crabs Ocypode ceratophthalma, which are rarely seen during the day. It is difficult to dig one of these crabs out of its burrow or to catch it as it runs across the sand. Young Ocypode are very numerous and can be seen on the sand surface both by day and night. Ocypode feed mainly on organic material from the sand but are sometimes predatory (Tweedie and Harrison 1970).
On wider beaches the small ghost crabs Dotilla mictyroides may occur in thousands with densities of over 100/m2 (Hails and Yaziz 1982; Mclntyre 1968). Although some of the larger individuals are coloured light blue with pinkish legs, the majority are sand-coloured. As the tide rises and covers the beach, each ghost crab builds a shelter of wet sand pellets over its back. The crab traps air beneath itself and as it burrows down, so the air pocket is carried down with it. When the crabs emerge as the tide falls, they are followed by a stream of small bubbles (Tweedie and Harrison 1970). Isopod crustaceans can be found by careful examination of the sand and the organic material washed ashore onto the upper sandy reaches (Jones 1979), and wading birds can sometimes be seen feeding on these and other small animals.
Lower down the beach a variety of molluscs occur but are rarely seen because they burrow beneath the surface. Examples of the bivalves are the white and pinkish Tellina, the large Pinna, and the economically important edible cockle Area granosa.
The sandy beaches are used as nesting sites by sea turtles. Sumatra is visited by at least three species which nest on its sandy beaches:
• the most abundant is the green turtle which weighs up to 100 kg and has a carapace up to 1 m long. Its eggs are commonly seen on sale in western towns such as Padang;
• the hawksbill turtle, also common, is exploited for its shell, with the main trading centre at Sibolga. This species is slightly smaller than the green turtle, weighing up to 80 kg and with a carapace length of 90 cm;
• the rarer, massive leatherback turtle which can weigh up to 1 ton.
The distribution of a fourth species, the loggerhead turtle, may include Sumatra (fig. 3.3). There is hardly any information on nesting sites of turtles on Sumatra but the more important areas are almost certainly on the offshore islands and the west coast of the mainland (Soegiarto and Polunin 1980; van der Meer Mohr 1928; Rappart 1936). However, in an article on the shore life of Pantai Cermin in Deli Serdang, van der Meer Mohr (1941a) shows a picture of green turtle tracks on the beach, indicating that some turtles bred there in 1940. Turtles used to lay eggs on Berhala, a small island off the coast of North Sumatra in the Straits of Malacca (A. Jazanul, pers. comm.; van der Meer Mohr 1928), but there is no recent information. Turtles also nest at a small beach near Kuala Kambas, eastern Lampung, but since all the eggs are taken for local consumption, the turtles may not return for much longer (FAO/Wind et al. 1979) (p. 349). Any information on nesting turtles in Sumatra is valuable so that their distribution and conservation status can be more accurately determined.
Figure 3.3. Shells of four species of sea turtle found around Sumatra, a-green turtle, Chelonia mydas; b-hawksbill turtle, Eretochelys imbricata; c-leatherback turtle, Dermochelys coriacea; d-loggerhead turtle, Caretta caretta.
From Anon. 1979b
Barringtonia Formation
The Barringtonia formation is found behind the pes-caprae formation on sandy soils. It is also found on abrading coasts where sand is either being removed by unhindered ocean swells or where sand has at least ceased to accumulate; in such areas a beach wall about 0.5-1 m tall is formed. On this wall and inland, the second type of vegetation, the Barringtonia formation, is found. This formation is also tolerant of salt spray, nutrient-deficient soil and seasonal drought. This belt of vegetation is not very wide, usually between 25 and 50 m where the lie of the land allows it, but narrower where the coast is steep and rocky. This type of forest merges with lowland rainforest inland. Large trees sometimes sprawl across the upper parts of the beach, and as the beach wall is eroded away so these eventually fall over and die. The larger trees of the Barringtonia association are of two main species: Barringtonia asiatica (Wallwork 1982), which has huge 15 cm wide feathery flowers and unusual fruit (fig. 3.4), and Calophyllum inophyllum, which has transparent yellow sap and 3 cm round fruit, the seeds of which are dispersed by bats. It should be noted that Barringtonia itself is not invariably present in the formation which bears its name (van Steenis 1959; Steup 1941). The plants found in this (and also the pes-caprae) type of beach vegetation are found in similar locations throughout the Indo-Pacific region, and some are typical of sandy shores throughout the tropics. Many of the species are not found outside these formations.
Figure 3.4. The fruit of Barringtonia asiatica.
Figure 3.5. The fruit of Heritiera littoralis.
Figure 3.6. The fruit of Hibiscus tiliaceus (left), Thespesia populnea (right).
In addition to the trees mentioned above, other typical species include the large bush Ardisia elliptica with its pink young twigs and leaves, Heritiera littoralis with its peculiar boat-shaped floating fruit (fig. 3.5), and other trees such as Excoecaria agallocha, pandans (Stone 1983), particularly Pandanus tectorius, Scaevola taccada, Terminalia catappa, and two types of hibiscus Hibiscus tiliaceus and Thespesia populnea. Both hibiscus have large yellow flowers with purple bases but the former species has slighdy hairy lower leaf surfaces, heart-shaped leaves as long as they are broad, flowers which fall off as soon as they have dried and smaller fruit. The latter has smooth leaves longer than they are broad with a sharper tip, flowers which remain on the plant for some days after they have died and larger fruit (fig. 3.6). Hibiscus tiliaceus is commonly planted in towns and villages. The cycad Cyras rumphii is also sometimes found in the Barringtonia formation. Despite their appearance, cycads are not palms, neither are they ferns, but they are related to the now-extinct seed-ferns that flourished roughly between 280 and 180 million years ago (Corner 1964). Cycads are commonly planted in gardens. In addition to the species above, certain species from the pes-caprae association can also be found, particularly on the beach wall.
Barringtonia formation forests have been cleared in many areas to make way for coconut groves but some excellent examples still exist on the west coast of Siberut (Anon. 1980a; Whitten 1982b) and in remote parts of the western coast of northern Sumatra.
Almost nothing has been written about the animals of Barringtonia formation forests but Simakobu leaf monkeys Simias concolor have been observed in them on Siberut Island (Whitten 1982b), and the CRES team investigating the rocky shores near Painan, West Sumatra, observed silvered leaf monkeys Trachpithecus cristata in the Barringtonia trees. The fauna of the sand in front of the Barringtonia formation is more or less the same as that described for the sand in and below the pes-caprae formation.
Productivity
Little is known about the productivity of beach vegetation but there are obviously great differences between the low-lying pes-caprae and the trees and shrubs of the Barringtonia formations. On the beach below the vegetation zone, the majority of energy inputs originate from the sea, and flotsam such as wood, sea-grass leaves and algal fronds may be an important source of food at the upper limit of tides (Soegiarto and Polunin 1980). Details of plant succession on the shores of Krakatau are given on pages 343-347.
BRACKISHWATER FORESTS
Brackishwater forest can be found at the inner boundary of mangrove forest, and at the upper tidal limit of rivers. It may also occur where the seaward beach is merely a sand barrier thrown up by the waves and formed by the currents. Behind such barriers the land is usually flat and low and streams often flood, thus creating marshy lagoons suitable for brackishwater forest. Examples of these are shown in figure 3.7. Brackishwater forest is characterized by nipa Nypa fructicans which forms pure, often extensive stands, but most of the other elements of the vegetation are commonly found in either mangrove forests or the Barringtonia formation (Whitmore 1984; Wyatt-Smith 1963).
Only one plant, the mangrove date palm Phoenix paludosa, appears to be restricted to this type of forest. It is easily recognised because it is the only feather-palm in Sumatra whose leaflets, when viewed from above, form a trough rather than a ridge at the junction with the midrib (Jochems 1927). The mangrove date palm is the most easterly relative of the date palm of north Africa and Arabia. Although brackish water and deserts are very different habitats, neither has fresh water freely available to plant roots (Whitmore 1977).
Figure 3.7. Locations of three brackishwater forests behind sand bars: a) River Rangau, Bangka Island, b) River Perbaungan, Deli Serdang, c) River Lebekiu, Siberut Island. The arrows point north.
ROCKY SHORES
Rocky shores occur where hard and resistant rock faces the sea in such a manner that the results of weathering are swept out to sea rather than deposited to form a wide beach. Such shores are usually steep, with the scarp face continuing below the sea surface. There is commonly, however, a narrow shelf or beach with shingle rather than sand in the upper tidal regions. Such steep coasts and cliffs are usually formed of old limestone (e.g., west of Banda Aceh), volcanic rock (e.g., south of Padang), granites and Tertiary sandstone (e.g., Bangka, Belitung and Bintan Islands and the Riau/Lingga, Anambas, Natuna and Tambelan archipelagos) (Sopher 1977).
There is no one type of vegetation peculiar to rocky shores, but a few specimens of trees normally found in the Barringtonia formation, such as Barringtonia, Casuarina, and Calophyllum, and pandans may be seen clinging to the rocks. Where the slope lessens and the salt spray is less, some form of lowland forest would be expected to grow.
The fauna of rocky shores is rather poor but has representatives from a very wide range of phyla. The animals are adapted to withstand the force of the waves, periodic desiccation, high temperatures and variable salinity. They all have efficient means of retaining a grip on the rocks; some are permanently fixed but others can move around to forage or graze. Barnacles are common and, among the molluscs, small oysters, cap-shaped limpets and Nerita snails are usually found (Tweedie and Harrison 1970). Medium-sized, long-legged rocky crabs (probably Grapsus grapsus) will be seen near the water's edge, and at the rocky shore examined by a CRES team near Painan, small blenny fish jumped from rock to rock in the spray zone. The zonation of animals up a rocky shore has been studied in Singapore (Purchon and Enoch 1954), and the principles of zonation are discussed by Brehaut(1982).
Most of the limestone or granite cliffs that have clefts or small caves in them will support colonies of swiflets (p. 317). One species, Aerodramus fuciphaga, builds its cup-like nest purely from hardened saliva but others mix in bits of vegetable material (Medway 1968).
CORAL REEFS
The conditions necessary for coral to grow are:
• warm water (above 22°C)
• clear water
• water with near to normal seawater salinity, and
• light.
All the seas around Sumatra are warm enough for coral but the second and third conditions are not met where large rivers flow into the sea. These rivers reduce the salinity and their sediment loads cause increased turbidity which in turn reduces light penetration. Thus the coasts of eastern Sumatra, and the coasts along the plains in western Sumatra, have no coral (fig. 3.8).
Most of reefs around Sumatra are fringing reefs (i.e., growing out from land), but atolls (islands formed by waves throwing up piles of coral fragments) and barrier reefs (away from land where the seabed is near enough to the surface for coral to form) are found between the west coast of Sumatra and the islands of Simeulue, Nias and the Mentawai Islands. The fringing reefs found around the islands of the Riau/Lingga, Natuna, Anambas and Tambelan archipelagos are shelves exposed briefly at low tide. The surface is flat and slopes gently towards the open sea. Loosened coral blocks are tossed by the breakers and are piled up to form a low ridge at the edge of the coral shelf.
Figure 3.8. Distribution of coral reefs in Sumatra.
The animal life of the coral reef is stunningly beautiful in colour and shape, and astoundingly diverse. Corals themselves are examples of colonial animals. The outer layer of a block of coral is made up of numerous 'polyps'. Each polyp removes calcium carbonate from the seawater and secretes its own skeleton of limestone but is connected to its neighbours by strands that extend laterally through minute holes in their skeletons. As the colony develops so new polyps form, often on the connecting sections between the older polyps, and the new skeletons grow over and stifle the polyps below. Thus the reef-building corals grow. Each species of coral has its own characteristic pattern of budding and so constructs its own particular, and characteristic, shape. Many of the species around Sumatra's coasts can be readily identified (Henry 1980; Searle 1956).
Corals are unable to live deeper than light can penetrate, because they are dependent upon single-celled algae, called zooxanthellae, which grow within their bodies. The algae need light to photosynthesise and thus provide food for themselves and for the polyp. During photosynthesis they release oxygen which helps the coral to respire. During the day the corals may appear to be nothing but dead lumps of limestone, some of which are coloured, but most of which are rather dull. At night and under certain other conditions, however, the millions of polyps extend their brightly coloured, feathery tentacles into the water, blurring the shape of the coral block they form as they sift the water for food particles. Optimum growth conditions for reef-forming coral occur when vigorous wave action causes turbulence and a continuous supply of food is available to the polyps.
Polyps are not the only organisms involved in reef-building; a peculiar group of pink-coloured algae, known as coralline algae (Corallinaceae), also contribute. These plants are red algae which deposit calcium carbonate in their cell walls. Coralline algae exist as crusts, fronds, segmented branches, epiphytes, parasites on other coralline algae, or as gravelly nodules, and are the world's hardest plants; it is impossible even to scratch them with a fingernail. Nearly 22% of the surface of a coral reef off Hawaii was found to be covered by several species of coralline algae, each of which had its own distinct habitat preference (Johansen 1981).
There are innumerable types of organisms associated with a coral reef. Some of these are attached, such as sedentary worms and sea anemones, while some, including certain fish and molluscs, hide in crevices. Inshore fish and other wide-ranging organisms live close to the reef and exploit its high biological productivity (De Silva et al. 1980). Of 132 fish species listed as being of 'economic value' in Indonesia, 32 are associated with coral reefs (Soegiarto and Polunin 1980).
Green plants can also be found on and around coral reefs. Some are green seaweeds, such as Ulva, Halimeda and Padina, but more common are seed-plants known collectively as seagrasses. Most of the seagrasses to be found off the coast of Sumatra, particularly in the Riau Archipelago, belong to the family Hydrocharitaceae, which includes the cosmopolitan Canadian pond weed Elodea canadensis. Unlike Elodea, however, most seagrasses look remarkably grass-like, with long, narrow leaves (one important exception to this is Halophila, which has spoon-shaped leaves). Seagrasses form mixed-species meadows, particularly of Thalassia hemprichii and Enhalus acoroides (den Hartog 1958), or pure stands, depending on the substrate. Distinct zonation of species can also be observed (McComb et al. 1981).
Seagrass leaves may carry very heavy loads of epiphytes, so many in fact that half the above-ground biomass of a seagrass meadow can be accounted for by epiphytes such as bacteria, algae and various sedentary marine animals. These epiphytes tend to thrive in nutrient-enriched water caused by human activity, but the enhanced growth of epiphytes reduces the amount of light reaching the seagrass, somedmes to its ultimate demise.
The flowers of seagrasses open underwater and the pollen is dispersed either by water currents or, if floating, by being blown across the water surface. Some seagrasses do, however, exhibit various forms of self-pollination (den Hartog 1958).
Seagrass meadows have a high standing crop, high productivity (about one-third of a lowland forest), and they are able to concentrate available nutrients. They would therefore be expected to be important in food chains and this is indeed the case, but primarily through their detritus rather than through being grazed. The detritus is eaten by crabs and molluscs which are in turn eaten by fishes. Seagrass is grazed by green turtles and dugong Dugong dugon, but they take only a small proportion of the productivity (McComb et al. 1981). Dugong are now extremely rare and it is doubtful whether a viable population exists anywhere off Sumatra.
Productivity
The net primary productivity of coral reef ecosystems can be as much as 2,000g carbon per m2 per year. This is higher than almost any other terrestrial or aquatic ecosystem anywhere in the world. For example, the equivalent average figure for tropical forest and estuaries is l,800g, for open ocean 125g and for savanna 700g (Whittaker and Likens 1973). In reef lagoons (i.e., where there is little turbulence and thus little mixing of water), the net primary productivity is much lower. For example, Nontji and Setiapermana (1980) recorded values of 39-96g carbon per m2 per year for lagoons in the Seribu Archipelago off Jakarta.