The Linyphiid spider fauna (Araneae: Linyphiidae) of mountain forests in the Eastern Arc mo11ntains - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 118

2. The total lack of representaives of genera
typical for the mountains in central East
Africa and the Alberine Rift (Trachyneta,
Aroneta and Microyba).

both in the rain forest on the mountain and in
gallery forest along the rivers are widely dis-
tributed over the region, and probably occur upon
several of the mountains' (Holm, 1962: p. 195).
Thus, the fauna in the montane forest zone was
not considered sufficiently isolated, or its ability
for long-distance dispersal was considered so
great, that contact between the different forest
areas was envisaged, with a shared fauna on the
different mountains as a result.
The last observaion was in good accordance
with the knowledge at that ime about the avifauna
of the areas invesigated by Holm (1962). These
areas have almost no endemic species and most of
the birds are shared with other montane forest
areas in Kenya, Uganda and eastern Zaire
(Moreau, 1966).
As regards the higher alitudes, above the forest
zone, Holm pointed out that the degree of
endemism was much higher. The presence of
many endemic linyphiids at high alitudes (Holm,
1962) was quite surprising, as these spiders are
small, agile and supposed to be good aeronauts
(ballooners). Thus Holm's endemic species could
easily be rejected with the argument that the pro-
posed endemic species would be found on other
East African mountains which had not yet been
invesigated in 1962.
Many new areas were invesigated for
linyphiids during the following decades (Figure
7. 4) and these collecions, together with other
inormaion on the environmental histoy of East
Africa, have made it possible to test some of
Holm's hypotheses.
Even though the Eastern Arc mountains are well
known for their many endemic species among
birds, butterflies and repiles (Loveridge, 1933;
Carcasson, 1964; Moreau, 1966), entomological
eploraion started very late. Apart from isolated
descripions of single species collected by ravel-
ling naturalists, no major effort had been made to
collect and describe the arthropod fauna of the
Eastern Arc mountains until 195 7. The first

Each mountain is thus well defined by a num-
ber of endemic species, but the group itself (the
Easten Arc) is arificial (paraphyleic as regards
spiders).

istoical backround
Even though the invesigaion of the African
linyphiid fauna started in the last century, only 56
species had been discovered and described up to
1960 (Russell-Smith, 198 la). Nearly 30 years
later, a total of 358 linyphiid species had been
recorded from Africa south of the Sahara
(Scharff, 1990a) and many more will no doubt be
found in the future. A more detailed historical
account for the period before 1960 s given by
Holm (1962) and van Helsdingen (1969). Infor-
maion covering the period after 1960, together
with more detailed inormaion on the total East
African linyphiid fauna, is given in Scharff (1992).
Our present knowledge and the increased
interest in East African linyphiids are based
primarily on two papers (Denis, 1962; Holm,
1962), both of which deal with the subfamily Eri-
goninae (then considered to be a family). Denis
(1962) reviewed the erigonines then known from
Africa, and Holm (1962) presented a preliminary
discussion of the zoogeography of the East Afri-
can erigonines. Based on a large amount of new
material, Holm could show that the 'individual
mountains carry numerous endemic taxa, and that
these taxa often consist of closely related species
belonging to the same genus, and replacing each
other on the different mountains' (Holm, 1962: p.
194). He also pointed out that 'endemism
increases with height, depending obviously upon
the degree of isolaion which is greater in the
higher, and smaller in the lower zones' (Holm,
1962: p. 195).
With regard to the lower zones of montane for-
ests, Holm found that the few species then known
tended to occupy several mountains: 'on the
whole we can reckon that the species which occur

Figure 7.4. Map of easten Africa. The stars represent areas
that have been invesigated for spiders of the family
Linyphiidae. Stars within the encircled area represent forest
areas invesigated by the author.