Geology Reference
In-Depth Information
A comparison ofthe total number ofspecies on
well invesigated mountains in the Easten Arc
and other parts of easten Africa reveals very dif-
ferent numbers. This is obviously related to col-
lecing effort, as some areas are better
invesigated than others, but is also a result ofthe
size and diversiy ofhabitats, ecological condiions
and environmental history. Thus, Mount Kenya is
a much higher mountain than the Uluguru
Mountains and has more different habitats
(among others, a well-developed
Eica
and
Afroalpine zone). It is therefore not srange that
41 linyphiid species have been recorded from
Mount Kenya, while only 17 have been recorded
from the Uluguru Mountains. Interesingly, the
number of 'true forest species' is nearly the same
on the two mountains, with 15 species on Mount
Kenya and 14 species on Uluguru Mountains.
1976; Schi0tz, 1981). These forests have thus
acted as refuge areas for forest species during dry
climatic periods in the past. Repeated extensions
and contracions of the forest cover, caused by
climaic changes, enabled forest faunas of dif-
ferent mountains to make contact in the forest
extension phase and caused isolaion in the orest
contracion phase. If isolaion was long enough,
speciaion would be expected. The genus
Op hynia
could be taken as an example of this
hypothesis (Figure 7.3). Such a distribuion could
be the result of an ancestral, once widespread
populaion of a
Op hynia
living in forest which had
a larger distribuion in East Africa than that at
present, bearing in mind the extensive forest
destrucion during the last 100-200 years. When
climaic condiions changed, the forest cover
shrank to the small disjunct forest 'refuge' areas
we observe today. The ancestral populaion of
spiders was thus fragmented, and because ofpro-
longed reproductive isolaion, new species
evolved in each refuge area. In this way, a number
of closely related species would tend to replace
each other geographically.
This patten could have changed during later
orest extensions/contracions where species
could perhaps spread to new areas, or to areas
which were already occupied by other species.
This kind of range extension, where species
spread throughout suitable habitats that acted as
corridors or bridges, should be disinguished
from dispersal, where a species acively moves
across a barrier of unsuitable habitat(s). Dispersal
cannot be ruled out as a factor responsible for
some patterns observed, and could blur the pic-
ture of distribuions caused by geographical
segregaion, but both factors should be used with
care in the interpretaion of the distribuions
observed.
How the distribuion and diversiy of the East
African linyphiid fauna fit into the theory of'refu-
ges' cannot be determined by just looking at the
data from the Easten Arc mountains and the
Central East Africa Forest Group. It must await a
total phylogeneic analysis of monophyleic
groups with representaives in these areas and
anywhere else (also if they occur outside post-
ulated 'refugia'). These results should then be
Endemsm
Perhaps the most remarkable thing about the
linyphiid fauna of the Eastern Arc is the very high
number of endemic species. This high degree of
endemism could be refuted by the argument that
the assumed endemic species would be found on
other mountains not yet invesigated, or that the
number of endemic species would be reduced by
taxonomic synonymy. However, many mountains
have been invesigated since the work of Holm
(1962), including the Easten Arc mountains and
mountains in Malawi, and this has not changed
the fact that some mountains, most notably those
of the Easten Arc, harbour considerable num-
bers of endemic species. The reason for the high
degree of endemism on the Easten Arc moun-
tains, compared with that of other mountains in
Kenya and Tanzania, could be a result of dif-
ferent environmental history, i.e. a difference in
the geological and climatological history of these
areas which is reflected in the present-day dis-
tribuion of flora and auna.
A number of authors have suggested that the
high degree of endemism in the Eastern Arc
mountains is a result of a long-established forest
cover, which has probably changed in extent but
always been present and of a rather moist type
(Carcasson, 1964; Moreau, 1966; Hamilton,
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