Geology Reference
In-Depth Information
tested against informaion on the environmental
history (geology, climatic and vegetaion history)
of the areas. Only then will it be possible to con-
struct zoogeographical hypotheses that are not
just speculaion.
It should be noted that the theory of the
Easten Arc forest reuges is based on quaternary
climaic invesigaions (including palynological
surveys) of the high mountains of East Africa
(Mount Kilimanjaro, Mount Kenya, Mount
Elgon, and the Rwenzori Mountains) and dis-
tribuional data offauna and flora. No study ofthe
vegetaional history ofthe Easten Arc mountains,
to document past composiion of vegetaion on
the individual mountains, has been carried out,
and this should have high prioriy in the future.
Valuable bogs eist on both the Ulugurus and on
Mount Rungwe.
Linyphiids are able to disperse passively by the
wind and it is temping to use this ability to
eplain disribuion pattens. It is nevertheless
difficult to explain the distribuion of the Easten
Arc linyphiids as a result of ballooning. The
eistence of many endemic species within certain
genera does not support a theory that postulates
frequent and easy contact between closely set
mountains. Thus, although the Uluguru Moun-
tains are situated only 250 m south of the Usam-
bara Mountains, they do not share a single true
forest species. It seems that even though the dis-
tance between the forests is not too great to
prevent acive dispersal, the habitats lying
between the forests act as srong barriers.
According to previous studies (Holm, 1962;
Bosmans, 1977, 1979), the linyphiids in the mon-
tane forest belt and the gallery forests ofthe East
African mountains should be widely disributed.
The disribution oflinyphiids in the montane or-
ests of the much lower mountains of the Eastern
Arc is, however, very different. There are very few
widely distributed linyphiid species and most of
these are well-known African ransgressor species
occurring in a wide range of habitats.
Historical factors can be used to eplain why
the Eastern Arc mountains do not have any
representaives of the genera Aroneta, Trachyneta
and Miroyba, even though these mountains
should be high enough to harbour these species.
Representaives of these genera do occur on
nearby mountains in Zai're, Kenya and Tanzania.
First of all, the fauna of the Easten Arc could
have another origin which did not include these
genera or their ancestors, and representatives of
these genera have not been able to colonise the
Easten Arc since, even though condiions for
colonisaion may have been favourable during
periods in the past.
Several studies in both ropical and temperate
regions have shown that the lower alitudinal limit
of montane species varies with the size of the
mountain. Thus species which occur on high
mountains often descend to alitudes that are
lower than the summit of surrounding mountains,
from which they are absent (White, 1978). This
phenomenon has been eplained by various
authors as a result of wind exposure, different
temperatures at the same alitude on big and small
mountains, or exposure to fog. Historical reasons
have also been suggested. According to the latter,
the montane organisms on the smaller mountains
became exinct during a warmer period in the
past, whereas they could find refuge in higher
alitudes of the taller mountains (White, 1978).
Conclusion
We do not yet know the total Easten Arc
linyphiid fauna. There are sill mountains that
have never been invesigated. Of these, those of
importance for the delimitaion and disincion of
the Easten Arc Mountain Forest linyphiid fauna
should be especially targeted (Pare, Rubeho,
Nguru, Kanga and Ukaguru mountains in
Tanzania and Taita Hills in Kenya).
As for the linyphiids, and maybe many other
groups, delimitaion of faunas based on auteco-
logical studies of single species does not work.
As an alternaive we should ry to work out the
phylogeneic relaionships among the genera with
endemic representaives on individual Easten
Arc mountains. Through this approach, faunas
will be delimited by their patten of sister-group
disjuncions (Griswold, 1991: p. 75).
Based on our present knowledge, he individual
mountains in the Easten Arc appear to harbour a
disinct linyphiid fauna ith high numbers of
Search WWH ::




Custom Search