Geology Reference
In-Depth Information
clear that climaic changes during the last glacial
maimum have been very different in different
parts of Africa.
In contrast to glacial periods, the interglacials
were warm and wet. Lake levels rose between
12 500 and 10 000
BP
and pollen evidence shows
that forest expanded at around the same time
(Hamilton, 1982). The wettest post-glacial period
was 9000-8000
BP
when the north African deserts
were dotted with swamps and lakes (Kutzbach
&
Street-Perrott, 1985) with rainfall 125-135%
higher than today (Gasse et a., 1990). After 5000
BP
the climate became drier again. Assuming that
wetter conditions occurred throughout Africa,
moist forests would have been more widespread
during this period. Under present climatic condi-
tions the southern migration route along the
Rukwa Rift contains patches of montane forest on
mountain tops with lowland species at lower
alitudes along forested stream courses running
through fire climax Zambezian woodland.
Present-day ires reduce the distribuion of moist
forest on the Mbeya range, and fire has been used
in southwest Tanzania for at least 60 000 years
(Hamilton
&
Faden, 1974). Under wetter condi-
tions, and with less frequent fires, these forest
patches would be more frequent and so would
facilitate migraion of readily dispersed forest
organisms from west to east. Once through the
southern migration route lowland species would
still need to cross the Poroto and Kipengere
mountains to reach the Eastern Arc and coastal
forests.
The pattern of cold dry glacial periods with
warm wet interglacials was probably true of other
glacial periods, though two periods may have been
cold and wet at 176 000 and 220 000
BP
(Ros-
signol-Strick, 1983). This may be of importance
as a cooler high rainfall period would allow a
greater spread of forest vegetaion than a hot wet
period.
also been south of its present position with the
equator passing through the present-day Sahara
Desert. Fossil and oceanic current evidence sug-
gests that tropical moist forest covered much of
modern North Africa in the early Tertiary. By the
mid-Tertiary Africa had reached its present
latitudinal posiion. Towards the end of the Mio-
cene, uplift of the central plateau separated
eastern and westen Africa. At the same time
upwelling of the cold Benguela current brought
aridity to southwest Africa. Ancient western
Guineo-Congolian links in the Eastern Arc for-
ests predate Miocene uplift of the central African
plateau. Uplift of the Eastern Arc and Nyasa rift
mountains 7 Myr
BP
would have accentuated
rainfall gradients, with the easten side of the
mountains receiving a high rainfall and the
westen leeward side being in a rain shadow. This
would have increased the barrier presented by the
arid corridor to migration and dispersal of moist
forest organisms from west to east, though migra-
tion and dispersal should have been possible dur-
ing wet periods.
During the last 2.3 Myr Pleistocene climaic
fluctuations caused considerable changes in
central and north tropical African forest vegeta-
ion, but the Indian Ocean ameloriated the
climate in tropical easten Arica. Consequently
the forests did not markedly reduce in area or
undergo the altitudinal depression proposed for
other forests on the Westen Rift thereby allowing
the survival of western Guineo-Congolian relict
endemic species. More recent links could have
occurred during Pleistocene interglacial periods
by dispersal between forest patches either through
the Kenyan Rift, or more likely along the southen
migration route of the Rukwa Rift.
Reerences
ANDREWS, P. & VAN COUVERING, J. A. H.
(1975).
Palaeoenvironments in the East African Miocene.
App roaches to Primate Palaeobioloy 5,
62-103.
ANDREWS, P., LORD, J. M. & NESBIT EVANS, F. M.
(1979). Patterns ofecological diversity in fossil and
moden mammalian faunas.
Biological Jounal of the
Linnean Society
11, 177-205.
Summay
Before the breakup of Gondwanaland easten
Africa would have been relaively arid, being
inland from Madagascar and India. It would have
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