Ecology of Sulawesi. Tony Whitten
Читать онлайн книгу.
bacteria and algae growing on and within it and is therefore an important food source for many 'detritivore' animals such as zooplankton, other small invertebrates, prawns, crabs, and fish. These detritivores are eaten in turn by carnivores which are dependent to varying degrees on these organisms. It is probable that most of the micro- and macro-fauna in the mangroves and surrounding coastal areas are dependent on the productivity of litter from mangrove forests (Ong et al. 1980a, b). A major initiative to study the important issue of transport of material in mangrove estuaries in the Indo-Australian region is currently underway (Ong et al. 1985).
Mangrove forests are highly productive ecosystems but only about 7% of their living leaves are eaten by herbivores (Johnstone 1981), and most of the mangrove forest production enters the energy system as detritus or dead organic matter (fig. 2.22). This detritus plays an extremely important role in the productivity of the mangrove ecosystems as a whole and of other coastal ecosystems (Lugo and Snedaker 1974: Ong et al. 1980a, b; Saenger et al. 1981; Mann 1982). Its importance to offshore ecosystems is not clear (Nixon et al. 1980).
The high productivity of mangroves and the physical structure and shading they provide forms a valuable habitat for many organisms, some of which are of commercial importance. At present the most valuable mangrove-related species in Indonesia are the penaeid prawns. The juvenile stages of several of these prawn species live in mangrove and adjacent vegetation, while the adults offshore (Soegiarto and Polunin 1980).
The influence of mangroves extends far beyond the prawn fisheries (p. 187). For example, carbon from mangrove trees has been found in the tissues of commercially important bivalves such as the cockle Anadara granosa, oyster Crassostrea, shrimps such as Acetes, used in the making of belacan paste, crabs such as Scylla serrata, and many fish (Rodelli et al. 1984) such as mullet Mugil, milk fish Chanos and barramundi or giant perch Lates (MacNae 1968; Moore 1982; Polunin 1983).
Figure 2.22. Major pathways of energy flow in a mangrove-fringed estuary.
After Saenger et al. 1981
OTHER COASTAL VEGETATION
There are three main types of beach vegetation: the pes-caprae formation, the Barringtonia formation and the vegetation of rocky shores. The removal through development of the vegetation on a sandy beach may not be regarded as a particularly serious loss in itself, but its ability to hold together a loose sandy substrate means that in its absence more or less continuous coastal erosion occurs. This, and its resultant impact on human settlements is most damaging during severe storms, because the power of the wind and waves is no longer countered by deep-rooted vegetation.
Pes-caprae Formation
The pes-caprae formation is found along sandy beaches which are actively accreting; that is, where sand is being deposed, or on already-developed beaches that are now being eroded. Its name is derived from the conspicuous, purple-flowered creeper with two-lobed leaves, Ipomoea pes-caprae (Conv.).6 The other plants found in this formation also tend to be low, sand-binding herbs, grasses and sedges whose long, deep-rooting stems or stolons spread across or just under the surface of the sand.7 The herbs include the small legume Canavalia, the herb Euphorbia atoto (Euph.), the sedge Cyperus pendunculatum (Cype.) and various grasses. Full lists are given elsewhere (van Steenis 1957; Wong 1978; Soegiarto and Polunin 1980; Whitmore 1984). Most of the species are confined to this habitat type and many are pantropical in their distribution.
The actual composition of the vegetation depends to some extent on the type of sand. There are two main forms on Sulawesi beaches: black, andesitic (volcanic) particles found in the north, and white, calcareous sand from the erosion of coral reefs as found in the southern part of Sulawesi and most of the offshore islands. The black beaches of Minahasa have a poor beach flora probably because the black surfaces absorb more heat and become extremely hot.
Plants are dependent on non-saline soil water but are tolerant of the periodic droughts, salt spray, almost constant winds, low levels of soil nutrients, and high temperatures found in the habitat. The plants also typically have small seeds which are dispersed by water, some even having air sacks around the seeds to assist floating.
The green mat formed by the pes-caprae formation traps leaves and other organic material blown by the wind or tossed up by waves at high tides. Small animals can also take refuge there. As a result, soil conditions improve, nutrients increase and plant succession proceeds (table 2.4).
One of the first large plants to be seen at the landward edge of the pes-caprae formation is the she-oak Casuarina equisitifolia (Casu.), which frequently forms pure stands at the top of the beach. She-oak seedlings are intolerant of shade, but even in open conditions, if there is a carpet of she-oak twigs and litter, the seedlings will not grow. This may indicate the presence of some chemical or allelopathic prevention of regrowth, but this has yet to be proven. Thus, unless the shoreline advances, the belt of she-oak will be replaced by other species.
Barringtotiia Formation
The Barringtonia formation is found behind the pes-caprae formation on sandy soils. It is also found behind on abrading coasts, where sand is either being removed by unhindered ocean waves or where sand has at least ceased to accumulate; in such areas a beach wall about 0.5-1 m tall can be found and the formation is found inland of this. The plants are generally tolerant of salt spray, nutrient-deficient soil and seasonal drought and grow in a belt along the coast, usually between 25 m and 50 m wide, where the lie of the land allows it. The belt will be much narrower where the coast is steep and rocky. Large trees sometimes sprawl across the upper parts of the beach, and as the beach wall is eroded away these eventually fall over, die and become shelters for many small seashore animals.
The larger trees of the Barringtonia association are of three species: Barringtonia asiatica (Lecy.) which has huge 15 cm wide feathery flowers and unusual-shaped fruit (fig. 2.23), Calophyllum inophyllum (Gutt.) which has transparent yellow sap and round fruit of 3 cm diameter, and Terminalia catappa (Comb.) whose large leaves turn red before falling and whose boughs stand out at right-angles to the trunk in a manner similar to kapok trees Ceiba pentandra (Bomb.). Barringtonia itself is not invariably present in the formation which bears its name (van Steenis 1957) and it is sometimes found on sandy ground away from the coast. As with the pes-caprae formation, the plants found in this 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. In addition to the trees mentioned above, other typical species include the coconut palm Cocos nucifera, the large bush Ardisia elliptica (Myrs.) with its pink young twigs and leaves, Heritiera littoralis with its peculiar boat-shaped floating fruit (fig. 2.23), and other trees such as Excoecaria agallocha (Euph.) with sticky, white sap which may cause temporary blindness (Burkill 1966), pandans Pandanus, the white-flowered and large-leafed Scaevola taccada (Good.) the fruits of which are dispersed by birds (Leenhouts 1957), and two types of hibiscus Hibiscus tiliaceus (Malv.) and Thespesia populnea (Malv.) (van Steenis 1957). Both hibiscus have large, yellow flowers with purple bases, but H. tiliaceus has slightly hairy lower-leaf surfaces, heart-shaped leaves which are as long as they are broad, black-coloured longitudinal glands on the leaf undersur-face near the base, flowers which fall off as soon as they have dried, and smaller fruit. T. populnea has smooth leaves, longer than they are broad with a sharper tip, no black glands on the base of the leaf undersurface, flowers which remain on the plant for some days after they have died and larger fruit (fig. 2.24). Hibiscus tiliaceus is commonly planted in towns and villages.
After L. Clayton pers. comm.
Figure 2.23. Fruits of Barringtonia asiatica (left) and Heritiera littoralis (right). Scale bars indicate 1 cm.
The Calophyllum trees near the mouth of the Lariang River in northern South Sulawesi bear the epiphyte8 Myrmecodia (Witkamp 1940). Myrmecodia and certain other epiphytes are able to grow where there is insufficient