Geology Reference
In-Depth Information
endemic forest species, much higher than have
been found on any other East African mountains
invesigated. No species are resricted to a range
covering only the Eastern Arc.
The Usambara, Uluguru, and Uzungwa moun-
tains appear to harbour very closely related
linyphiid species that replace one another on the
individual mountains. Each mountain thus has its
own species composiion at both low and high
alitudes, but it is not possible to draw any criical
alitudinal line between a lowland and highland
linyphiid fauna or to define a particular inter-
mediate forest linyphiid fauna. There is a gradual
change in the species composiion rom lowland
to highland and it is therefore important to
preserve the whole coninuum of habitats, when
and if conservation of these unique forests should
be considered.
This zoogeographical patten indicates that the
linyphiid faunas of the Usambara, Uluguru, and
Uzungwa mountains have been effecively
isolated from each other and from other forest
areas for a long period of ime. The high degree
of taxonomical isolaion and geographical
replacement of closely related species within
certain genera of linyphiids point towards a zoo-
geographical indicator group of great value.
the different mountains, the geologically old
mountains do not show paricularly high species
richness.
In the northen hemisphere, linyphiids are
known for their ability to disperse by ballooning.
For unknown reasons, linyphiids move upwards
from the lower vegetation to the ips of grasses
and branches where they assume a iptoe posiion
in which the abdomen ofthe spider is raised while
the spider simultaneously pulls out silk from its
spinnerets. The silk thread is caught by the wind
and the spider is carried away with it, using it as a
kind of glider. Air samples taken up to several
thousand metres above the ground have shown
that spiders make up an important part of the
airborne animals (aerial plankton) carried long
distances by the wind (Greenstone, 1990). Bal-
looning has been used to eplain the wide dis-
ribuion of spider species and the occurrence of
mainland spider species on isolated oceanic
islands (Bristowe, 1939, 1941; Gertsch, 1949;
Foelix, 1982).
Linyphiid spiders are found in both lowland
and highland forests. Preliminary results from
collecting along alitudinal gradients in the
Uzungwa Mountains show differences between
lowland and highland forest faunas, but the
changes are gradual and no criical borderline can
be identified (Scharff, 1992).
Invesigations carried out on the East African
mountains have also revealed a number of genera
each with closely related species replacing one
another on certain mountains. Each of these
mountains has its own representaive(s) of par-
icular genera and the distribuional ranges of
species belonging to these genera are limited to
certain geographical areas (Holm, 1962; Jocque
&
Scharff, 1986; Scharff, 1990b, 1992). Although
only scattered inormaion is available on the East
African lowlands (all kinds of habitats), and there
are still many uneplored areas in the highlands,
general pattens can now be presented and
discussed.
Sumay
Based on present knowledge, and data from the
best invesigated mountains in Africa, the geologi-
cally old East African mountains (mountains in
the Albertine Rift and in the Eastern Arc) harbour
a more disinct linyphiid fauna, with higher num-
bers of endemic species, than can be found on the
geologically younger East African mountains such
as Mount Kilimanjaro, Mount Kenya and Mount
Meru. The number of endemic species is high on
each of the geographically close, old coastal base-
ment mountains in Tanzania (the Easten Arc
mountains), despite the fact that some linyphiids
are known for their strong ability to disperse pass-
ively with the help of wind. Other East African
mountain areas, which are also geographically
close, hold a much larger proporion of wide-
spread species. However, when one compares the
total species richness of true forest linyphiids on
Acknowledgements
I would like to extend my sincere thanks to
Henrik Enghoff, Konrad Thaler, Bent Muus,
Rudy Jocque, Jon Lovett, Sam Wasser, Kim
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