Geoscience Reference
In-Depth Information
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Wallace's Line
Alfred Russell Wallace was a preeminent bio-
geographer of his time. He is best known as
the British Victorian naturalist who happened
upon the theory of evolution at the same time
as Charles Darwin. While Darwin proposed that
species evolved because of direct competition
with each other, leaving only the fittest to sur-
vive, Wallace suggested that the mechanism
for evolution was the environment. Wallace
traveled extensively throughout the tropics,
spending a great deal of time in Southeast Asia
and the Pacific. During his travels, he noticed
that birds and mammals on the islands of New
Guinea and Lombok looked very different from
those on the nearby islands of Bali and Borneo
to the west. In fact, he noted that animals on
New Guinea and Lombok were similar to Aus-
tralian species, while those on Bali and Borneo
were more like Asian species.
Without knowing it, Wallace was seeing the
results of tectonic plate activity. New Guinea
and Lombok, Timor, Flores, and Sulawesi (Cele-
bes) were part of the Australian plate, whereas
the islands to the west were part of the Asian
plate. Wallace sketched out a line that sepa-
rated these regions, a line that is still called
Wallace's Line. Wallace's Line marks the separa-
tion of placental mammals and marsupials as
well as differences in birds and plants between
these areas. It also separates West Malesia from
East Malesia. The islands of West Malesia all lie
upon the Sunda Shelf, a shallow continental
shelf that was exposed during glacial maxi-
mums, so plants and animal could move freely
among the islands. The islands on the Austra-
lian plate lie upon the Sahul Shelf. The two
shelves are now only 15.5 mi (25 km) apart.
Between them is a deep oceanic trench that
forms a formidable barrier to dispersal.
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begin in the eastern Pacific, affect this region.
During an El Nino, the monsoon is weakened
and pushed equatorward, creating a period of pro-
longed drought. Detrimental to the rainforest, it is
devastating to the poor countries of the region
because it causes widespread crop failure and
food shortages. Some researchers argue that El
Ni
nos initiate the mass flowering events common
in the Asian-Pacific rainforest.
Tropical cyclones (typhoons, hurricanes) are
other important influences on the rainforest. The
areas most affected lie between 10-20
~
N and S
latitude and include Bangladesh, the Philippines,
and much of Melanesia into Australia to the
south. The cyclical incursions of cyclones into for-
ested areas have left stands of disturbed forest
where fast-growing, sun-loving pioneers tend to
dominate. Other catastrophic events affecting
these rainforests are earthquakes, landslides, vol-
canic activity, and tsunamis. Along with wide-
spread destructive effects on the forest, these
events have had disastrous consequences for the
human populations of the region.
Soils
Soils of the Asian-Pacific rainforest are similar to
those of other rainforests, but the percentage of
area covered by each soil type differs significantly.
While the deeply weathered and infertile oxisols
made up a large percentage of Neotropical and
African soils, in the Asian-Pacific region they
account for only 3 percent. Oxisols occur in
patches on the islands of Borneo, Sumatra, Java,
Sulawesi, and the Philippines, and on the main-
land in Thailand and Malaysia.
Ultisols are the most abundant soils in the
Asian-Pacific rainforest. They occupy a signifi-
cant portion of the forests in Malaysia, Sumatra,
Borneo, Sulawesi, and the eastern Philippines.
Like oxisols, ultisols are deeply weathered soils
