| Summary: | Forest assemblage composition is determined by local ecological (e.g. patch area,
species interactions), landscape (e.g. patch connectivity) and regional (e.g. historical
change in forest distribution) processes. I investigated the relative effect of these
processes on bird and frog assemblage composition in two isolated archipelagos of
Afrotemperate forest in the Limpopo Province.
The linear relationship between local and regional species diversity suggests
that forest bird assemblages in the Limpopo Province are unsaturated. In addition,
66% of bird species and 42% of frog species in southern African forests are
generalist species (i.e., forest associated as opposed to forest dependent),
suggesting that matrix species have invaded forest assemblages. I thus argue that
forest bird and frog assemblage composition is primarily determined by regional
(historical) processes and that local ecological processes play a relatively minor role.
Forests in the Limpopo Province were eliminated by major climatic changes
during the Quaternary with major forest expansion only in the last 6000 years.
Limpopo Province forest assemblages have thus established fairly recently. No forest
dependent frogs and one forest dependent bird have established in the Limpopo
Province forests from the relatively proximate forests in eastern Zimbabwe. This
suggests that the Limpopo River catchment has acted as a significant barrier to the
dispersal of forest vertebrate faunas. Cluster analyses showed that the forest bird
and frog assemblages are essentially Afrotemperate and South African in origin with
all forest dependent frogs and 97% of forest dependent birds occurring in the
KwaZulu-Natal scarp forests. In addition the most important environmental gradient
of change in the southern African forest bird faunas was the geographical distance
from northern KwaZulu-Natal. This gradient is congruent with a major northward
radiation of faunas from the KwaZulu-Natal scarp into the Limpopo Province. As a
result the Limpopo Province forests have low biodiversity values compared to the
KwaZulu-Natal scarp because forest frog and bird faunas are largely derived from the
latter region. However, the importance of the Limpopo Province forests lies in their
protection of threatened vertebrates as well as in providing landscape heterogeneity
and ecological services to the surrounding matrix. Soutpansberg forest bird assemblages appear to be more robust and resilient
and comprise a significantly greater proportion of forest associated species than
those of the Limpopo Province Drakensberg. This is likely to be a consequence of
more severe climatic extinction filtering of these faunas caused primarily by the
proximity of the Soutpansberg forests to the arid Limpopo valley during the
development of these forests. Consequently, regional and historical processes have
played a relatively greater role in determining forest bird assemblages in the
Soutpansberg than in the Limpopo Province Drakensberg and species richness in
the former region was not significantly affected by local ecological processes
(including forest area, isolation and habitat heterogeneity). Forest area and habitat
heterogeneity did, however, affect forest bird species richness and abundance in the
Limpopo Province Drakensberg where the relatively lower importance of regional
processes (compared to the Soutpansberg) has combined with anthropogenic
disturbance of smaller forests to increase the influence of local ecological processes.
However, the role of local processes in determining local species richness is likely to
increase in both archipelagos if the current rates of anthropogenic change and disturbance to forests are sustained.
Forests greater than 138 ha (minimum critical patch size) are needed to avoid
an island effect on bird species richness in the Limpopo Province Drakensberg.
However, the long-term conservation of vertebrate assemblages in Limpopo province
forests depends upon the successful conservation of evolutionary and landscape
processes. This can best be achieved by maximising forest connectivity and
landscape heterogeneity through the protection of both riparian corridors and forests
of all sizes. The maintenance of historical dispersal routes, in particular connectivity
along the escarpment with the scarp forests of KwaZulu-Natal, is important. This
would require the protection of forests on the KwaZulu-Natal scarp and along the entire northern Drakensberg escarpment. === Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2003.
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