Ecology of Sulawesi. Tony Whitten
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on the long shorelines of low-lying parts of eastern Sumatra and southern Borneo, the most marked effect on Sulawesi would have been the separation of the blocks of land either side of the Tempe depression. Evidence for this has been found in the vegetation record (p. 29) and there are even stories among local people of a time when travellers did not have to sail around the southern tip of South Sulawesi but could instead sail from the Gulf of Bone, through Lake Tempe and emerge in the Makassar Straits (Sartono 1982). With the exception of the Tempe depression and a few other flat plains (such as Malengke), most of Sulawesi's coastline slopes quite sharply and minor rises in sea-level would not have had significant effects. During this period seasonality in rainfall would have been less, rainfall would have been similar to or even greater than now, and mountain zones of climate, vegetation and fauna would have been raised. However, as stated above, this period occupied only a small fraction of the Quaternary. The majority of the period was characterized by lower rainfall and humidity, greater diurnal and seasonal variations, and by more marked rain shadows. Thus, the seasonal areas of Sulawesi and elsewhere would have been more extensive and, conversely the areas subject to more stable, wetter climates would have been reduced in area.
Present Climate
The climate of Sulawesi is best described with reference to rainfall since temperature is relatively constant, and other climatic variables such as wind velocity, evaporation and humidity change within even small areas. Between September and March, cool northwesterly winds pick up moisture while crossing the South China Sea (between East and West Malaysia, Philippines and Vietnam) and arrive in North Sulawesi via the Sulawesi Sea in about November, and in the west coast of South Sulawesi via the Java Sea in late November or early December. The west coast of the central part of Sulawesi is sheltered from the effects of these winds by Borneo.
Figure 1.12. Changes in sea-level over the last 7,000 years determined from a study in the southwest peninsula.
After de Klerk 1933
After this period, variable, humid, southeasterly winds blow towards eastern Sulawesi and rainfall peaks on the southeast coast occur between April and June, and on the northeast coast somewhat later. The southeasterly winds from the dry and wintery Australian landmass become stronger and these dry winds have a significant influence on the southern tips of the southwest and southeast peninsulas. Manado experiences a short dry season from August to October, but Jeneponto in the south of the southwest peninsula is subject to a long dry season between April and November.
Areas on the west coast of Sulawesi therefore tend to have their highest rainfall in December whereas those on the east coast have their wettest month around May. One might expect to find intermediate areas with two dry seasons (a bimodal distribution) and this is indeed the case; Pendolo and Pinrang in the middle of the southwest peninsula are examples.
Where the orientation of a range of mountains is more or less at right angles to the prevailing winds, the rainfall is higher on the windward side because the water in the air rises and cools as it climbs over the mountains, and this moisture is released as rain. Thus Maros receives over 500 mm per month between December and February but towns on the leeward side of the peninsula receive little rain. Valleys orientated in a north-south direction are in a rain shadow for virtually the whole year and the sheltered nature of the central western coast results in the Palu valley being one of the driest areas of Indonesia with less than 100 mm of rain, on average, falling in each month and an annual total of less than 600 mm.
Various authors have mapped the climatic zones of Sulawesi. The map which corresponds closest to the distribution of vegetation is that which uses the ratio between dry and wet periods (fig. 1.13) (Schmidt and Ferguson 1951; Whitmore 1984a, b).
A second map based on suitability criteria for growing rice has also been devised (fig. 1.14) (Oldemann and Darmiyati 1977), in which five major zones are recognized.
| Zone A | - | an area with ten to twelve consecutive wet months and two or less consecutive dry months; |
| Zone B | - | an area with seven to nine consecutive wet months and three or less consecutive dry months; |
| Zone C | - | an area with five or six consecutive wet months and three or less consecutive dry months; |
| Zone D | - | an area with three or four consecutive wet months and two to six consecutive dry months; |
| Zone E | - | an area with zero to two consecutive wet months and up to six consecutive dry months. |
'Wet' and 'dry' are defined as more than 200 mm and less than 100 mm of rain per month respectively.
Sulawesi has a greater percentage of its area in agroclimatic Zone E than have the islands around it, but more in Zones B and C than Borneo (most of which is in Zone A), Nusa Tenggara and Bali, or the Moluccas (table 1.4).
The most recent and complex climatic map of Sulawesi concerns bioclimate (Fontanel and Chantefort 1978) and recognizes many zones determined by three criteria: mean temperature of the coldest month, mean annual rainfall (fig. 1.15), and number of dry months9 (fig. 1.16). One of the major differences between this map and that on agroclimatology is that it takes into account altitude effects, although not in an absolute sense since the degree of exposure to prevailing winds has a significant effect (p. 21).
The maps discussed above are based on long-term averages but uncommon climatic events, particularly periodic drought, can be extremely important in determining the distribution of certain animals and plants. Most crops, for example, experience stress after only about four days without rain. The variation in annual rainfall is quite considerable (fig. 1.17) with the total for one year sometimes being twice the total of another. Meteorological records normally extract maximum rainfall figures but these are not particularly meaningful ecologically because above a certain quantity, rain will simply run off saturated soils to rivers. Lack of water, with its associated cloudlessness, high temperatures and low humidity, is a much more potent factor and an examination of rainfall minima and their distributions reveal that these too are extremely variable (fig. 1.18). Dry seasons can in fact only be defined by probability since, for the stations examined, at least six different months were recorded as 'driest months' in at least one year. A single location (e.g., Mapanget or Watampone) can have minimum monthly rainfalls between years ranging from 0 to 100+ mm.
Figure 1.13. Rainfall types based on dry/wet period ratios.
After Schmidt and Ferguson 1951; Whitmore 1984a, b
Figure 1.14. Agroclimatic zones.
After Oldemann and Darmiyati 1977
* The figures in the text do not add up to 100%.
After Oldemann and Darmiyati 1977; Oldemann et al. 1980
Figure 1.15. Areas with different mean annual rainfall.
After Fontanel and Chantefort 1978
Figure 1.16. Areas with different numbers of dry months.
Alter Fontanel and Chantefort 1978