Introduction to the biogeography and ecology of the rain forests of eastern Africa - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 3

1 Introduction to the biogeography
and ecology of the rain fores ts of
eastern Africa

SAMUEL K. WASSER and JON C. LOVETT

The tropical rain forests ofAfrica are divided by a
corridor of arid land that runs from the Horn of
Africa, through Kenya, Tanzania, Zambia, Zim-
babwe and Botswana to the Namib Desert of
Namibia (Werger, 1978). The arid corridor
reaches the sea on the east coast of Africa in
Mozambique, where Madagascar casts a rain
shadow, and in the north where the corridor
covers much of Somalia. A narrow strip of
relatively high rainfall lies between the deserts of
the Somalia coast in the north, the Madagascar
rain shadow in the south and the woodland of the
central African plateau to the west. It is this
humid area of eastern Tanzania, Kenya and
southern Somalia that we define here as easten
Africa (Figures 1.1 and 1.2).
The area oftropical rain forest in easten Africa
is not large; it is approimately 10 000 km2, a mere
0.1% of the estimated 10 million km2 of tropical
rain forest in the world (Mabberley, 1983). Unlike
the vast west and central African forests, the for-
ests of eastern Africa are highly fragmented -
discrete islands associated with localised areas of
high rainfall, surrounded by a sea of com-
paraively arid woodland. Contrasts between these
wet and dry areas are pronounced. For example,
on the eastern slopes of the Uluguru Mountains
of Tanzania there is a per-humid climate where
more than 100 mm of rain falls each month of the
year and the annual rainfall exceeds 3000 mm. A
high diversity of slender boled, narrow crowned
trees reach heights greater than 60 m in a closed,
epiphyte-rich canopy. Streams cascade down

steep slopes with banks of fe rns, begonias and
Impatiens. A few kilometres to the south is the
highly seasonal climate of the Selous Game
Reserve, where large populaions of game animals
roam wooded grasslands. In the mountain's rain
shadow to the west, there are sisal estates in semi-
arid land that receive less than 600 mm of rain per
year.
As described by Griffiths (Chapter 2) and
Lovett (Chapter 3), it is the geological and
climatological histories of easten Africa that
make its forests unique. Climaic vicissitudes
throughout the Pleistocene are thought to have
caused major extincions by substantially reducing
the total area ofAfrican rain forest. However, the
forest patches of easten Africa appear to have
escaped these changes owing to the remarkable
stability of the Indian Ocean currents that bring
moisture to the tropical East African coast (Figure
1.2; Hamilton, 1982). There probably has been
tropical rain forest in eastern Africa since approx-
imately 30 million years ago, when Africa drifted
to its present posiion at the end of the Oligocene
(Axelrod & Raven, 1978). The coninental divide
between East and West Africa was accentuated by
the Miocene uplift of the central African plateau
(Griffiths, Chapter 2). The great age, isolation
and fragmented nature ofthe easten African for-
ests have combined to produce remarkably high
levels of endemism and diversity. About one third
of the more than 2000 moist forest plant species
are endemic (found nowhere else; Lovett, 1988)
with over 20 endemic genera (Iversen, 1991). The

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