The montane butterflies of the eastern Afrotropics - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 133

8 The montane butterflies of the
easten Afrotropics

R. DE JONG AND T. C. E. CONGDON

With real islands the ecological difference
between the island and its surroundings (the sea)
is so extreme that an origin of the great majority of
the terrestrial island organisms from the sea is
most unlikely if not out of quesion. This may be
different for ecological islands like forests sur-
rounded by open formaions. Here the ecological
difference is, in principle, bridgeable. In a genus
like Sp ialia (Hesperiidae) with 26 mainly African
species only one of which (S. ploetzi (Aurivillius) )
is a forest species, a change in ecological require-
ments must have taken place, in this case an adap-
taion to forest environment by an open formaion
species (de Jong, 1978). Similarly, and maybe
even more instrucive, is the genus Colotis which
has 42 species in Africa, all living in woodland and
open, often arid, habitats except . elgonensis
which is restricted to highland forest. Indeed, in
any monophyleic group of species, some of which
live in open formaions and some other in forests
such a change must be postulated. Thus with eco-
logical islands we must also deal with the
quesion:

Inroducion
In the easten half ofAfrica the forests are mainly
restricted to mountains and surrounded by
savanna or even semi-desert. They have very
appropriately been compared with an archipelago
by White (1981). Their isolation and the high
degree ofendemism offlora and fauna raise ques-
ions about the evoluionary and geographic
history ofthe forests as a whole and ofthe species
living there. As with real islands the cenral ques-
ions are:

1. From the geographic point of view, have
these islands always been isolated or is
there a history of interconnecions?
2. From the biological point of view, how are
the species distributed and related, and
where do their sister species live?

Staring from an alloparic speciaion model
there is a causaive correlaion between the two
quesions in such a way that the geographic
history of the islands must have influenced the
evoluion of the species living in the islands. Bio-
geography is concened with this correlaion. To
put it in a simple way, the biogeographic quesion
is:

4. What is the ecological origin of the
consituent species?

In this chapter we shall address these quesions
by way ofthe butterflies ofthe mountain forests of
the easten part of Africa, roughly east of Long.
30° E. Forests are not the only ype of natural
habitat at higher elevaions. There may be marshy
areas and large sretches of grassland within the
forest belt, and where the mountains are high
enough there is a reeless ericaceous belt or even
an alpine belt (see Hedberg, 1986). The butter-

3. Have the species of a paricular island
originated on the spot (and if so, how
about the ancestors) or are they colonists
from elsewhere (either by jump dispersal
or following a range expansion of the
habitat), or a mixture of both (and if so,
what are the proporions)?

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