Geology Reference
In-Depth Information
gascar and easten and southen African coastal
vegetaion led Leroy
(1978)
to propose that the
coastal srip should be included in a broadly
defined Madagascan region. Examples of links
between the Easten Arc forests and Madagascar
include: Aphloia with one polymorphic species in
easten and southen Africa and Madagascar;
Ludia with one easten African species and about
20
species in Madagascar and the Mascarenes;
Dais with one easten and southen African spe-
cies and one Madagascan species; and Lpidio-
trichilia with one species in East Africa and three
in Madagascar. Species with Madagascan affi-
niies also occur in the westen Guineo-Con-
golian region or have links there. For example,
Agauria salicifolia which, like Aphloia, is a variable
monoypic genus occurring on Madagascar, is
vey widespread; Necpsia castaneolia has two
subspecies in easten Africa, two subspecies in
Madagascar and the other two species in the
genus are West African (Bouchat
&
Leonard,
1986);
and Ocotea which was once thought to be
exclusively easten and southern African, Mada-
gascan and Neoropical (Leroy,
1978;
Chapman
&
White,
1970),
now has a West African species
.
gabonensis.
Easten Arc endemics with Madagascan affi-
niies are Saintpaulia and Zimmennannia. The
Easten Arc and easten coastal moist orest
lithophic and epiphyic herbaceous endemic
genus
Saintpaulia
appears to be derived from
Saintpauia-like Strptocapus
from Madagascar
(Hilliard
&
Burtt,
1971).
Similarly, the shrubby
moist forest and forest edge
Zimmermannia
has six
species in the Easten Arc, and a seventh
.
ecayi
in Madagascar which appears to be inter-
mediate between the Easten Arc Zimmennannia
and the more widespread Meineckia (Poole,
1981;
Webster,
1965).
Another link is in the small
saprophyic herb Sychelaria which has three spe-
cies: S. aficana from the southen Easten Arc, S.
thomassettii rom the Seychelles, and S. maa-
gascaiensis rom Madagascar (V ollesen,
1982).
Easten Arc-Madagascar links can be ex-
plained either by long-distance dispersal, or
direct overland contact prior to the breakup of
Gondwanaland. The latter eplanaion is used by
Vollesen
(1982)
to eplain the disribuion of
Sychelaia, and by Pocs
(1975, 1985;
Bizot, Pocs
&
Sharp,
1985)
to eplain the disribuion of
bryophytes such as the Rutenbergiaceae. The
Africa-Madagascar separaion began
165
Myr
BP
and ended
121
Myr
BP
(Griffiths, Chapter
2).
It is
possible that such ancient links eist in primiive
plants like bryophytes. Sychelaia also belongs to
a potenially primiive family, the Triuridaceae,
from the evidence of its free carpels (Mabberley,
1987).
The altenaive to relictual Gondwanic
distribuion is long-distance dispersal. None of
the plants linking Madagascar with the Easten
Arc has paricularly large seeds or fruits and could
have arrived by bird or wind dispersal. Genera
such as
Saintpaulia
and
Zimmemannia
almost
certainly evolved after the breakup of Gondwana -
land. The age and climaic stability of the Easten
Arc has resulted in a buildup of Madagascan con-
necions through long-distance dispersal over
ime. More may be revealed as knowledge of
Madagascan taxonomy improves; for example, the
species Aenoplusia ulugurensis was once thought
to be endemic to upper montane forest edges in
the Uluguru but is now known to be the Mada-
gascan Budlja axilaris (Leeuwenberg,
1979).
Links with the Far East and Neotropics are also
a mixture of long-distance dispersal and ancient
Gondwanic disribuion pattens (Ehrendorfer,
1988;
Raven
&
Axelrod,
1974),
though again it is
difficult to disinguish between an ancient long-
distance dispersal event and a ruly relictual dis-
ribuion. It has been suggested that long-distance
dispersal is an adequate explanaion of all the
florisic links between ropical America and Africa
(Ayensu,
1973;
Brenan
1978;
Thome,
1973).
In
the hemiparasiic Loranthaceae all African genera
derive from Asia, and the three primiive genera
Helixanthera, Dendrophthoe and T. xi/us extend
from Asia to Africa. Both Helixanthera and Den-
drophthoe occur in the Easten Arc (Polhill,
1989).
The large Easten Arc montane forest ree Bal-
thasaria schliebenii is closely related to the Asiaic
genus Adinandra (hite,
1981);
and also in the
Theaceae the upper montane orest ree
Tenstromia popetala is one of two African species
in a genus of
80
or so Asian and Neoropical
species. More recent long-distance dispersal from
overseas appears to be represented by widespread
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