Malay Archipelago

A schematic eastern delimitation of the Indomalayan Region follows the border of the Pleistocene Sunda land from the straits between Bali and Lombok in the south, between Borneo and Celebes (Sulawesi) and between Palawan and the Philippines in the north. This is "Wallace's line" of most zoogeographers (Fig. 4.15). A corresponding western limit of the Australo-Papuan Region follows the western border of the Sahul Shelf which includes the islands of Aru, Misol and Waigeu ("Lydekker's line"). There is a third line where western and eastern faunal elements of mammals and birds reach similar proportions of faunal balance (50:50%), "Weber's line" west of Tenimber, Buru and the northern Moluccas. Weber's line separates the islands with predominantly Indomalayan fauna in the west from islands with a predominantly Papuan fauna in the east. Mayr (1944d) documented that the shelf margins of both the Sunda land and Sahul land represent conspicuous breaks in a general faunal transition zone between the Indomalayan and Australo-

Fig. 4.15. Malay Archipelago, the contact zone between the Indo-Malayan and the Australo-Papuan faunas. The shaded area in the west is the Asian (Sunda) shelf, and in the east lies the Australian (Sahul) shelf. The area between the two shelves, never connected by a land bridge, is referred to as Wallacea. The real border (line of balance) between the Asian and the Australian faunas is Weber's Line (from Mayr 1944d)

Fig. 4.15. Malay Archipelago, the contact zone between the Indo-Malayan and the Australo-Papuan faunas. The shaded area in the west is the Asian (Sunda) shelf, and in the east lies the Australian (Sahul) shelf. The area between the two shelves, never connected by a land bridge, is referred to as Wallacea. The real border (line of balance) between the Asian and the Australian faunas is Weber's Line (from Mayr 1944d)

Papuan Regions (see also Keast 1983; Clode and O'Brien 2001). Generally speaking, this island region is inhabited by such a faunal mixture of western and eastern provenance and so heterogeneous that according to Mayr (l.c.), it should not be considered as a separate zoogeographical unit "Wallacea." All four areas that are supposed to make up "Wallacea," the Lesser Sunda Islands, the Celebes region, the Philippines, and the Moluccas, are more different from each other and each of them is more similar to an area outside Wallacea than to make sense whatsoever of the recognition of "Wallacea." The western elements of reptiles, butterflies and plants penetrated farther to the east than those of birds and mammals. Plant geographers even consider New Guinea part of the Malayan (Malesian) Region.

The islands of Timor and Sumba are inhabited by a predominantly Indomalayan bird fauna containing numerous eastern elements from the Papuan Region (Mayr 1944e) all of which reached these islands by dispersal across ocean barriers. Like Wallace (1869) Mayr found no support for any of the numerous land bridges in this region that other zoogeographers, e.g., Rensch (1936), had proposed to explain the immigration of birds and other animals. Four phenomena confirm the interpretation of dispersal across ocean barriers in the Malay Archipelago and contradict the assumption of land bridges (Mayr, l.c.):

(1) Faunal relationships are independent of submarine contours (below the 200-m line) but are closely correlated with the distances of the islands from each other (providing size and ecology are comparable);

(2) Faunal affinities exist even between geologically unrelated islands;

(3) The small percentage of endemic species on the islands of the Sunda arc is explained by the superior dispersal faculties of the birds of the Lesser Sunda Islands which crossed easily the water gaps to the north and south;

(4) The spread of birds inhabiting high mountains in the Malay Archipelago can be understood only by assuming great dispersal powers, because their montane habitats have never been directly connected, not even during the coldest periods of the Pleistocene.

This important zoogeographical analysis (which Mayr considers one of his best papers in this field) established the "oceanic" nature of the eastern islands of the Malay Archipelago strongly supporting Stresemann's (1939) conclusions. When the latter congratulated Mayr, in early 1946, on his zoogeographical studies of the bird faunas of the Malay Archipelago ("You succeeded in making zoogeography a fascinating subject again"), Mayr refused "credit for having started anything new. It was your zoogeographical analysis in the Birds of Celebes (1939) which made me think and got me interested in analyzing the birds of Timor (1944e) in a somewhat similar fashion" (21 March 1946). This is another instance of the important influence of Stresemann's methods and thinking on Mayr's systematic and zoogeographic work (see also Glaubrecht 2002; Vuilleumier 2005b:66-67).

About Rensch's book, History of the Sunda Island Arc (1936) Mayr had written to Stresemann: "I thoroughly disagree with most of Rensch's conclusions. He compares the faunas oftwo islands and then draws conclusions as to their possible connection on the basis of the percentages of the animals common to both. In this, he pays no attention whatsoever (except in the general introduction) to the fact, that these islands might be totally different in their ecology: If you compare a savanna-covered island with a completely wooded island, you may get only a slight agreement of the faunas, although the geological history of the two might easily be the same. On the other hand, Rensch makes very little allowance for the distribution across the open sea. My work on Polynesian islands and even such islands, like Rennell and Biak , has convinced me that the open sea is not always as much of a barrier as commonly supposed. As soon as two islands are fully settled such as Batanta and Salawati, there is only very little exchange of individuals, but as long as one island is full of open ecological niches, it will permit the settling down of immigrants. This point has been largely disregarded by Rensch. I believe you cannot make such definite statements as he does in many cases, neither do I believe that all of the island connections have existed as postulated by him" (25 April 1939).

The distribution of birds in the Philippines is influenced by three main factors (Delacour andMayr 1946k): (1) Climate and vegetation, (2) former land connection between islands, (3) proximity to neighboring areas. These factors jointly have contributed to the special bird faunas of the various islands and island groups.

Was this article helpful?

0 0

Post a comment