Thursday, May 17, 2018

Hanneke Meijer - Wallace’s enigma: how the island of Sulawesi continues to captivate biologists

“We now come to the Island of Celebes, in many respects the most remarkable and interesting in the whole region, or perhaps on the globe, since no other island seems to present so many curious problems for solution.” (Wallace 1876)

Wedged in between the continental landmasses of south-east Asia and Australia lies the vast island realm of Wallacea. Named after Alfred Russel Wallace, the 19th-century explorer and naturalist who traversed this area, it hosts floras and faunas that are incredibly rich and often include species found nowhere else on Earth. The natural history of Wallacea is complicated, and heavily dictated by geological forces such as plate tectonics and volcanism.

As the oldest and largest island within Wallacea, Sulawesi (formerly known as Celebes) hosts a rich fauna with a large number of species that are unique to the island. Although its fauna is pre-dominantly Asian in origin, it is the only island in south-east Asia with marsupials (the bear cuscus and dwarf cuscus), a typical Australian element. In addition, it hosts the smallest primate in the world, the tarsus tarsier, which fits in the palm of your hand. You can find miniature buffaloes, or anoas, whose lovable appearance is said to hide an aggressive demeanour. And there are wild pigs, babirusas, with wrinkled skin and impressive upper tusks that instead of growing down, grow up and backwards toward the skull. (According to Wallace, some old writers supposed that the tusks served as hooks, so that the animal could rest its head on a branch.)

Wallace was puzzled by the origin of Sulawesi’s unusual fauna, and the stark difference between Sulawesi and its closest neighbouring island, Borneo. In his seminal work The Malay Archipelago from 1869, Wallace noted that there seemed very little affinity between many of Sulawesi’s birds and mammals and those in other parts of the world. This led him to propose that Sulawesi perhaps represented “a rather ancient land”, and that its unique fauna had its origin “in a remote antiquity”

Shaped like a giant “K” with four arms pointing in different directions, the geological history of Sulawesi is complicated. The island is a composite of several fragments of land that, over time, were driven into each other as they sat atop continental plates headed for collision. Some of these fragments are thought to be rather old, possibly dating as far back as the Cretaceous. Such an old origin for parts of Sulawesi would explain its unique biota and high species richness; species would have rafted along on these fragments from worlds since long lost (a process known as vicariance).
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