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The East Usambara Mountains are situated in northeastern Tanzania
within 40 km of the coastal town of Tanga between 4o48' -
5o13'S and 38o32' - 38o48'E. These
mountains form part of a chain known as the Eastern Arc that
stretches down the coast of East Africa from southern Kenya to
southern Tanzania. This is a chain of isolated mountains composed of
Precambrian rock exposed by block faulting and slow uprising
[Griffiths, 1993]. Being adjacent to the Indian Ocean, considerable
orographic rainfall occurs in this area. The rainfall distribution
is bi-modal, peaking between March and May and between September and
December. The dry seasons are from June to August and January to
March. However, precipitation occurs in all months. Rainfall is
greatest at higher altitudes and in the southeast of the mountains,
increasing from 1,200 mm annually in the foothills to over 2,200 mm
at higher altitudes. Due to topographic and climatic interactions,
the west-facing slopes of the mountains are drier compared to the
east-facing slopes. Due to their age, isolation and their role as
condensers of the moisture from the Indian Ocean, the East Usambara
Mountains support ancient and unique forests, rich in endemic
species [Hamilton, 1989].
Research in the East Usambara Mountains began in the late 1890s
with substantial botanical collections being undertaken. Later, in
1928 surveys were undertaken on amphibians and by the 1930s detailed
ornithological work had begun. Since these early studies biological
research in the mountains has steadily increased. Recently, work in
the area has also included an attempt to understand the drainage and
catchment value of the mountains' forests [Bruen, 1989; Litterick,
1989].
The East Usambara forests have been likened to the African
equivalent of the Galapagos
Islands in terms of their endemism and biodiversity [Rodgers and
Homewood, 1982; Howell, 1989]. They are considered to be one of the
most important forest blocks in Africa [Tye, 1994]. Currently, at
least 3,450 species of vascular plants have been recorded in the
Usambara Mountains in which it is suggested that over one quarter
are endemic or near endemic [Iverse, 1991a]. Many are threatened
[Rodgers, 1996].
The forests of the East Usambara Mountains are not only important
for their biodiversity, they also play an important role in
maintaining the hydrological cycle that fees the Sigi River. The
river is the water source for the local communities as well as
supplying water for the large coastal town of Tanga. Deforestation
in the area will lead to increased soil erosion, particularly from
the steeper slopes. Soil erosion is liable to result in a more
irregular run off and deterioration in water quality due to
siltation.
The latest survey of the East Usambara Mountains, conducted by
Johansson and Sandy [1996] shows that approximately 45,137 ha of the
East Usambara Mountains remain as natural forest. This can be
divided into two types: submontane forest and lowland forest.
Altitude is the factor differentiating these two forest types
[Hamilton, 1989], with submontane forest generally occurring above
850 m.
The mammals of the East Usambara Mountains show limited endemism
[Kingdon and Howell, 1993]. However, there are several species of
special interest. These include: the restricted Zanj elephant shrew Rhynchocyon
petersi which is common in the Usambara Mountains [Collar and
Stuart, 1987] yet listed as globally "Endangered" by IUCN
due to a decline in habitat extent and quality; Eastern tree hyrax Dendrohyrax
validus, listed as "Vulnerable" by IUCN
[Hilton-Taylor, 2000], and the Lesser pouched rat Beamys hindei
which is also considered "Vulnerable" by IUCN
[Hilton-Taylor, 2000].
There are at least 11 species of reptiles and amphibians endemic
to the East and West Usambara Mountains [Howell, 1993]. The East
Usambara Biodiversity Surveys provide further information on new
species and species' range extensions. A new species of snake,
Prosymna semifasciata, was recently found in Kwamgumi and
Segoma Forest Reserves [Broadley, 1995] and a recently described
amphibian species, Stephopaedes usambarae [Poynton, 1999] has
been recorded by the surveys in Mtai and Kwamgumi Forest Reserves.
The forest avifauna of the East Usambara Mountains has a high
diversity with at least 110 species [Stuart, 1989]. Six species
occurring in the lowland forests are considered
"Vulnerable" to global extinction: Sokoke scops owl Otus
ireneae; the endemic Usambara eagle owl Bubo vosseleri;
Swynnerton's robin Swynnertonia swynnertoni; East coast
akalat Sheppardia gunningi; Amani sunbird Anthreptes
pallidigaster and the Banded green sunbird Anthreptes
rubritorques [IUCN, 1996],
The East Usambara Mountains are essentially forest 'islands'
[Lovett, 1989]. There has been natural forest in the area for
several million years. The Usambara Mountains harbor many species
that have been geographically separated from their closest relatives
for long periods. They also serve as a refuge for formerly
widespread flora and fauna that have become extinct over much of
their former area [Iversen, 1991].
These forests have been under continuous exploitative human
pressure for at least 2,000 years [Schmidt, 1989]. Until recently,
especially before the past 50 years, [Kikula, 1989], this pressure
was sustainable. However, the growing human population in the area
is leading to increased pressure on the remaining natural forests,
and represents the main threat to their survival.
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