The Wallace Line or Wallace's Line is a faunal boundary line drawn in 1859 by
the British naturalist Alfred Russel Wallace that separates the ecozones of
Asia and Wallacea, a transitional zone between Asia and Australia. West of the
line are found organisms related to Asiatic species; to the east, a mixture
of species of Asian and Australian origin is present. Wallace noticed this
clear division during his travels through the East Indies in the 19th
century. The line runs through Indonesia, between Borneo and Sulawesi,
and through the Lombok Strait between Bali and Lombok. The distance between
Bali and Lombok is small, about 35 kilometres. The distributions of many
bird species observe the line, since many birds do not cross even the
smallest stretches of open ocean water. Some bats have distributions that cross
the line, but other mammals are generally limited to one side or the
other; an exception is the crab-eating macaque. Other groups of plants and
animals show differing patterns, but the overall pattern is striking and
reasonably consistent. Flora do not follow the Wallace Line to
the same extent as fauna. Background
Antonio Pigafetta had recorded the biological contrasts between the
Philippines and the Maluku Islands in 1521 during the continuation of the
voyage of Ferdinand Magellan, after Magellan had been killed on Mactan.
Moreover, as noted by Wallace himself, the observations in faunal differences
between the two regions had already been made earlier by George Windsor Earl. In
Earl's pamphlet On the Physical Geography of South-Eastern Asia and
Australia, published in 1845, he described how shallow seas connected
islands on the west with the Asian continent and with similar wildlife, and
islands on the east such as New Guinea were connected to Australia and were
characterised by the presence of marsupials. Wallace used his extensive
travel in the region to propose a line to the east of Bali since "the whole of
the islands eastwards from Java and Borneo do essentially form a part of a
former Australian or Pacific Continent, from which they were separate." The name
'Wallace's Line' was first used by Thomas Huxley in an 1868 paper to the
Zoological Society of London, but showed the line to the west of the Philippines.
Biogeography Understanding of the biogeography of the
region centers on the relationship of ancient sea levels to the continental
shelves. Wallace's Line is visible geographically when the continental
shelf contours are examined; it can be seen as a deep-water channel that marks
the southeastern edge of the Sunda Shelf which links Borneo, Bali, Java, and
Sumatra underwater to the mainland of southeastern Asia. Australia is likewise
connected by the Sahul Shelf to New Guinea. The biogeographic boundary known
as Lydekker's Line, which separates the eastern edge of Wallacea from the
Australian region, has a similar origin to the Wallace line.
During ice age glacial advances, when the ocean levels were up to 120 metres
lower, both Asia and Australia were united with what are now islands on
their respective continental shelves as continuous land masses, but the deep
water between those two large continental shelf areas was, for over 50
million years, a barrier that kept the flora and fauna of Australia separated
from those of Asia. Wallacea consists of islands that were not recently connected
by dry land to either of the continental land masses, and thus were populated by
organisms capable of crossing the straits between islands. "Weber's Line"
runs through this transitional area, at the tipping point between dominance by
species of Asian against those of Australian origin.
See also Australia
References Further reading
van Oosterzee, Penny. Where Worlds Collide: the Wallace Line.
Pleistocene Sea Level Maps Wallacea - a transition zone from Asia
to Australia, specially rich in marine life and on land.
Dawkins, Richard. The Ancestor's Tale. Weidenfeld & Nicolson. ISBN
0-7538-1996-1. Chapter 14 - Marsupials. Abdullah, M. T.. Biogeography and
variation of Cynopterus brachyotis in Southeast Asia. PhD thesis. The
University of Queensland, St Lucia, Australia.
Hall, L. S., Gordon G. Grigg, Craig Moritz, Besar Ketol, Isa Sait, Wahab
Marni and M. T. Abdullah. "Biogeography of fruit bats in Southeast Asia".
Sarawak Museum Journal LX(81):191-284. Wallace, Alfred Russel. "On the Physical
Geography of the Malay Archipelago". Royal Geographical Society 7: 205–212.
with the original paper Wilson D. E., D. M. Reeder. Mammal
species of the world. Washington DC: Smithsonian Institution Press.
External links Too Many Lines; The Limits of the
Oriental and Australian Zoogeographic Regions George Gaylord Simpson,
Proceedings of the American Philosophical Society, Vol. 121, No. 2,
pp. 107–120 Wallacea Research Group
Map of Wallace's, Weber's and Lydekker's lines