Geology Reference
In-Depth Information
tree species such as
Antiaris toxicaia, Ce/tis
wightii, Filicium decipiens
and
Gy rocarpus amei-
ca nus
subsp.
ameicanus.
Amongst herbs, the bird
dispersed epiphic
Rhipsalis baccifera
is the only
naturally occurring member of the American
family Cactaceae on coninental Arica, and also
occurs in the Neoropics.
Lack of overseas input at species level reflects
the isolaion of coninental Africa's moist forest
flora. Many overseas family and generic links with
the Neotropics and Far East may date from the
breakup of Gondawanaland. At that ime Africa
was at the cenre of the superconinent, some
distance from oceanic influence, and so much of it
would have been relaively dry (Raven
&
Axelrod,
1974), though inland lakes could have provided
areas of high rainfall as they do today. As South
America and India drifted away, moist forest
plants would have dispersed onto an increasingly
wetter African continent. The relatively poor
palm flora of Africa compared with the
Neoropics and Far East may be explained by
difficulty in dispersing across the widening Atlan-
ic and Indian oceans. Present-day differences
between continental African and Madagascan
floras demonstrate the barrier to long-distance
dispersal presented by the Mozambique Channel,
though it does appear to have been crossed by
Chysalidocapus,
a Madagascan palm genus with a
Pemba Island endemic. An alternaive explanaion
for the poor continental palm flora is that
droughts in Africa long after the breakup of
Gondwanaland so restricted the area of rain forest
that many species were driven to extinction
(Raven
&
Axelrod, 1974; Dransfield, 1988).
Table 4.4.
Nu mbers of species occuring in dferent
Easten Arcorest pes endemic to the Easten Arc.
274 sp eies were included in the anaysis, of wh ich
67 or 25% are endemic
Number of species
a
Forest type
2
3
4
5
6
Dy lowland
5
1
0
0
0
0
Lowland
15
3
0
2
2
0
Submontane
19
3
1
II
1
2
Montane
12
1
1
6
2
2
Upper montane 2
1
1
1
1
0
Dry montane
0
0
0
1
0
0
Total
32
7
2
18
4
4
No te:
a
l. In both north and south Eastern Arc.
2. In both north and central Eastern Arc.
3. In both central and south Eastern Arc.
4.
In only northen Easten Arc.
5. In only central Easten Arc.
6. In only southen Easten Arc.
similar quaniies in the montane (36% of
endemics) and lowland forests (33% of
endemics). The northen part of the Arc is richest
in endemic trees, containing 50% of all endemics
of which 27% are restricted to it (Table 4.4).
Endemic richness of the northern sections is con-
centrated in the Usambara Mountains. There are
a number of possible reasons for this: the Usam-
bara are better known botanically than other
Easten Arc mountains; there may have been
exinctions on the other mountains; there are
more habitats on the Usambara; or the environ-
ment on he Usambara may give rise to a greater
number of endemics. The endemic richness of
the Usambara is probably a combinaion of these
factors. Recent exploraion of other Eastern Arc
mountains has produced a number of new taxa
and distribuions (Lovett
et al. ,
1988) and there
are undoubtably more to find. Extincions on
other mountains may have occurred during dry
periods as the southen Easten Arc has a much
more marked dry season than in the north and an
increase in the length of this dry season could
have resulted in the loss of moist forest taxa.
Exinctions could also have resulted rom
Easten Arc endemis
Over 27% of the 274 Eastern Arc tree species
analysed are widespread throughout continental
Africa, but 25% of the trees are endemic to the
Easten Arc. Of the endemics 45% occur in both
northern (Usambara, Pare, Taita) and southen
(Uzungwa, Mahenge) parts of the Arc. Endemic
tree species occur in all forest ypes, though there
are few in dry montane forest (only one, plus
some edge and thicket endemics) and most in
submontane forests (55% of endemics) with
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