alt=""/>
Figure 2.6. Physical changes across the black layer.
After Brafield 1978
Figure 2.7. Comparison between steep, coarse-grained beaches (a) and shallow, finegrained beaches (b).
After Brafield 1978
Figure 2.8. Typical limpets, a - Cellana testudinaria, b - Patella exusta, c - Patella pica, d - Cellana radiata.
The primitive gastropods known as top shells or limpets (fig. 2.8) cope with exposure in at least two ways. Some species return to a 'scar' in a rock when the tide falls and the shells grow to match this scar exactly. Others are not restricted to occupying a single home site and instead secrete a mucus sheet between the shell margin and the rock surface to reduce water loss. The effectiveness of their adhesion is soon realized when attempts are made to pry them off rocks. It is water loss, rather than temperature which is the main danger to limpets even though they are more tolerant of desiccation than most animals: they can lose about 80% of their water and still recover when water becomes available. These animals, as well as mussels, and Littomrid3 snails, can also lower their metabolic rate thereby allowing them to survive periods of exposure when the only oxygen available is in the water held within their shells (Brehaut 1982).
Animals and plants can survive short periods at high temperatures which would be lethal over a longer period. This may have a significant effect on the degree of exposure, or distance up a rocky shore, that an animal can endure (Brehaut 1982). In addition, most marine organisms are only able to tolerate very minor variations in salinity because they do not have mechanisms of regulating the salt and water balance of their body fluids except within narrow limits. The crabs and snails cope with this problem in different ways: crabs can regulate the concentration of salt in their body tissue, whereas snails are remarkably tolerant of a wide variation in the concentration of salts in their body fluids.
Figure 2.9. Percentages of total aquatic animal taxa recorded at three types of shore (omitting microscopic forms). Note that crustaceans and gastropod molluscs account for 74% of the total on the mangrove shore. Note that these data are not based on complete lists or on equally exhaustive surveys but they serve to illustrate the major differences.
Data from Berry 1972
The success of crabs and snails in the intertidal zone is illustrated by their predominance in mangrove, rocky and sand/mud environments (fig. 2.9).
MANGROVE FOREST VEGETATION
Mangrove forests would once have fringed much of the coast of Sulawesi (fig. 1.20) but major expanses of mangroves are now found in relatively few locations (fig. 2.10), the remainder having been largely felled and used for timber or fibre or converted into brackish fish and prawn ponds (p. 187). South Sulawesi has more mangrove forest than the other three provinces combined (Darsidi 1982) and small patches can be found along most shallow beaches and river mouths away from centres of human habitation.
Figure 2.10. Present distribution of mangrove forests around Sulawesi (indicated in black).
After Salm and Halim 1984
Composition
Mangrove forests are characteristic of tropical coastlines and have very similar compositions irrespective of climate. Only 19 tree species are commonly encountered in Sulawesi mangrove forest, although there are about 16 species of tree that may be found only occasionally or in the forest closest to dry land (table 2.3). In addition there may be 20 species of orchids and other epiphytes but these are generally rare. Some plants have been reported from only small areas, such as Camptostemon philip-pinense (Bomb.) (fig. 2.11) from Kwandang Bay in Bolaang Mongondow (Steup 1939), but this must in part be due to inadequate collecting. A detailed list of plants found in Philippine mangrove forests (Arroyo 1979) is useful to those working in North Sulawesi. A key to the trees most likely to be encountered in mangrove forest and other coastal vegetation is given in Appendix C.
In addition to higher plants, various algae and bryophytes (mosses and liverworts) are also found. Some of the algae appear to have adaptations for living in brackish conditions and these species can be quite abundant. The algae are greenish, brownish or reddish (Johnson 1979), but are unfortunately rather difficult to identify (Teo and Wee 1983). Bryophytes were found on most of the major species of mangrove trees in Thailand (all of which occur in Sulawesi), but not on all trees present. They comprised of five species of moss and 21 species of leafy liverwort. Rhizophora apiculata bore the most species (23) but R. mucronata only four (Thaithong 1984). This may have been due as much to microclimate differences as to differences between the substrates provided.
After Hickson 1889; Heringa 1920; Steup 1933, 1939; Anon. 1980a; Darneedi and Budiman 1984
Figure 2.11. Camptostemon philippinense an uncommon mangrove tree known from North Sulawesi. Scale bar indicates 1 cm.
After Anon. 1968
Many different plant communities of mangrove trees have been identified in Southeast Asia (Chapman 1977b), many dominated by a single species, but these are not discrete. It may be that given the wide range of micro-environmental conditions occurring, there may be a virtually infinite variety of mangrove forest types. Thus any effort to classify an area of mangrove forest that is being studied as a certain 'type' is probably misguided and ultimately not particularly useful.
Figure 2.12. Fruits of the three species of Sonneratia found in mangrove forests in Sulawesi. Scale bar indicates 1 cm.
After Backer and van Steenis 1951
Mangrove trees are tolerant of saline soils, that is they are halophytes (Walsh 1974), but they are facultative rather than obligate in that they can also grow successfully in freshwater. This is demonstrated by the growth, fruiting and germination of Bruguiera sexangula, B. gymnorrhiza, and Sonneratia caseolaris (fig. 2.12) in the Botanic Gardens in Bogor (Ding Hou 1958). This is typical of many plants that might be regarded as restricted to growing in certain soil conditions. In fact, there are many organisms which exist not where they fare best, their 'fundamental niche', but where they grow most successfully in competition with other species, their 'realized niche'. Mangrove trees can sometimes be seen growing at the sides of rivers in seemingly freshwater conditions but in these cases there is generally a wedge of (heavier) salt water permanently or periodically near the bed of the river which maintains saline conditions for the tree roots. The occurrence of these trees in such locations may also be due to the flooding regime of the river (J. Davie pers. comm.).
Mangrove forests, particularly those that are frequently flooded, differ markedly from dryland forests and from most inland swamp forest in the virtual absence of climbing and understorey plants (Ding Hou 1958). In essence, the only plants that grow in mature mangrove forests are trees whose crowns reach the canopy. The reason for this has yet to be confirmed, but it would seem
