expressed his view that the Indonesian Archipelago was inhabited by two distinct faunas, one found in the east, one in the west. The following year he defined these two regions, based on the distribution of birds, by placing the boundary between Lombok and Bali and between Borneo and Sulawesi. He was struck that Borneo and Sulawesi should have such different birds and yet be separated by no major physical or climatic barrier. He believed that Borneo, along with Java and Sumatra, had once been part of Asia, and that Timor, the Moluccas, New Guinea and perhaps Sulawesi had once been part of a Pacific-Australian continent. The fauna of Sulawesi seemed so peculiar that he suspected it might have been connected with both the Asian and the Pacific-Australian continents (Wallace 1859). He insisted that an explanation of the origin of the fauna of Sulawesi would have to accept that there had been vast changes in the surface of the earth, a concept which challenged the established view but which we now know to be true (p. 2). The line that Wallace drew east of the Philippines, through the Makassar Straits and between Bali and Lombok (Wallace 1863) came to be known as Wallace's Line. In 1910, three years before he died, Wallace decided that the predominance of Asian forms on Sulawesi should be reflected in the Line being moved east of Sulawesi (Wallace 1910). Many other analyses have been performed on the distribution of animal species resulting in several different Lines (fig. 1.29) (Simpson 1977). Weber's Line attempts to delimit the boundary of faunal balance, that is, where the ratio between Asian and Australian animals is 50:50 (Weber 1904). Weber used molluscs and mammals in his analysis but the exact position of the Line differs from one group of animals to another. For example, Asian reptiles and butterflies penetrate further east then do its birds and snails. Lydekker's Line delimits the western boundary of the strictly Australian fauna in much the same way as Wallace's Line delimits the eastern boundary of the Asian fauna; both these Lines effectively trace the 180-200 m depth contours around the Sahul and Sunda continental shelves respectively. The area between these two Lines has been nominated as a separate region, subregion or transition area called Wallacea (Dickerson 1928). This concept was first suggested by Wallace in 1863, but has been strongly criticized as the area does not comprise a homogenous fauna, and there is no gradual change in species composition across it; instead there are large number of endemic species (Stresemann 1939; Simpson 1977). The name Wallacea should be retained, but to describe the area between the Oriental and Australian regions rather than as the name for a strict biogeographical entity.
Figure 1.29. Biogeographical Lines through insular Southeast Asia.
From Simpson 1977
The concept of Wallace's Line has fascinated biogeographers and it has been found that its validity differs between groups of animals and plants. For example, an early analysis of Sulawesi's flora showed similarities with Borneo, Sumatra and Java, rather than with the Moluccas and New Guinea (Lam 1945). This analysis was based, however, on a limited number (about 700) of species. An analysis at the generic level of the whole Malesian flora25 demonstrated the existence of three provinces in Malesia, of which East Malesia comprises New Guinea, the Moluccas, and Sulawesi (van Steenis 1950). A recent analysis of 4,222 species in 540 genera that have been subject to recent taxonomic revisions revealed that the Sulawesi flora was most closely related to the floras of other relatively dry areas in the Philippines, Moluccas, Lesser Sunda Islands and Java (van Balgooy in press). There is no clear affinity between Sulawesi and the islands east or west of it, but the flora of the lowlands and of the ultrabasic soils show a stronger similarity with that of New Guinea, whereas the montane flora (1,000 m and above) shows a stronger similarity with that of Borneo26 (fig. 1.30). The higher the altitude, the greater the distance between areas of similar altitude, and the less chance there is of receiving plants from similar habitats. This would explain the greater proportion of Bornean plants on Sulawesi mountains, but the greater affinity with New Guinea flora among the lowland plants may be a result of New Guinea having more relatively dry areas than Borneo and therefore being a more suitable source of plant species. Some of these may have been brought to Sulawesi via the 'Sula Spur' (p. 6), whereas others could have island-hopped. An examination of the percentage of taxa that do not cross imaginary lines between or within landmasses in a given direction revealed that the strongest such 'demarcation Line' was for plants of western origin between Borneo and Sulawesi. About 50% of the non-endemic plant species of Borneo do not occur in Sulawesi. This suggests that the Makassar Straits have been a barrier to dispersal for a very long time. Interestingly, however, this Line is very weak when considering non-endemic plants of eastern origin crossing from Sulawesi to Borneo. The easiest routes by which plant species appear to have entered Sulawesi are those via Java and the Lesser Sunda Islands and via the Philippines and Sangihe. The former route argues for the existence of an island chain between Java, Lesser Sundas and Sulawesi in the not too distant past.
When specific groups of animals or plants are examined, many interesting distribution patterns are found. The percentage of a total species list shared between neighbouring islands (fig. 1.31) indicates a generally closer affinity between Sulawesi and islands to the east but this is at least in part an artefact of the relatively impoverished floras and faunas to the east.
The mountain flora of Sulawesi is derived from two sources: those which originated locally (autochthonous) and those for which the centre of origin is outside the area concerned (allochthonous) (van Steenis 1972). The allochthonous flora, although a minority of the total mountain flora, allow hypotheses to be made regarding its origin. This part of the flora belongs to genera whose species are found only in cold climates (i.e., microtherm species), and in the tropics they are generally found only in the subalpine forests on mountains 2,000-2,500 m high. These genera, such as Rhododendron (Eric.) and Gentiana (Gent.), are found in many tropical and subtropical countries yet none can tolerate a hot climate. Soils seem to have little or no influence on the distributions since a single species will be found on soils originating from igneous, sedimentary or recent volcanic parent material. The age of the rocks does seem to be relevant, however, with some species being absent from recent volcanic soils (van Steenis 1972).
Figure 1.30. Floristic affinity of Sulawesi to Borneo (solid line) and New Guinea (dashed line) for each 500 m increase in altitude.
From data in van Balgooy in press
From an analysis of the distribution of about 900 of these cold-adapted mountain species, it has been concluded that there are three tracks by which plants arrived in Sulawesi during some period or periods in the geological past (fig. 1.32) (van Steenis 1972). Continuous ranges of high mountains do not, of course, exist along the entire lengths of these tracks. During the coldest times of the Pleistocene the mean temperature dropped only about 2°C (p. 18) which, with rates of temperature change being about 0.6°C/100 m (not necessarily applicable at that period) is equivalent to a drop in the levels of the forest zones of 350-400 m. The number of suitable mountain tops would obviously have been greater during the cooler periods thereby forming more 'stepping stones' across which plants could disperse.
Palms are a useful group for biogeographical study because their genera, at least, are well known and they represent an ancient group of plants in which several genera had evolved by the Oligocene (30 Ma ago) (Dransfield in press). Only two genera, Gronophyllum and the monotypic Pigafetta, are found in Sulawesi but nowhere further west. This is surprising in the case of Pigafetta because it is a tall fecund palm of secondary growth with small seeds (p. 396). There are, however, 13 genera of palms that are found no further east than Borneo illustrating again that Wallace's Line is most noticeable travelling west to east. Two genera, the spiny Oncosperma and the fan palm Pholidocarpus, cross Wallace's Line from Borneo to Sulawesi but are found no further east, and another sixteen are found in Sulawesi and to the east and west. Of these, the rattans belonging to Calamus, the undergrowth palm Licuala, and the tree palms Cyrtostachys, Areca, and Livistona exhibit a peculiar distribution having species in Sunda-land and New Guinea, but few or none in Sulawesi (Dransfield 1981 in press). Since it is known that Sulawesi was perhaps drier in the Pleistocene than were the continental landmasses
