Eastern Arc Forests

Published: May 5, 2014, 7:50 pm

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Introduction

The Eastern Arc Mountains possess large numbers of endemic plants and animals at some of the highest densities in the world. The endemism comprises both newly evolved species, as well as those with an ancient history. Globally recognized plant groups such as African violet (Saintpaulia) and ‘Busy Lizzie’ (Impatiens) have their center’s of radiation in this ecoregion (Eastern Arc Forests) . The Eastern Arc forests are characterized by a high number of restricted-range species that occur in only a few localities, or are found throughout much of the Arc but not anywhere else. The unique biogeography, and the disjunct nature of this ecoregion in elevational patches makes species distributions and ranges more similar to true islands than to mainland regions. The main threats to the ecoregion are the expansion of agriculture into the remaining areas of forest, mostly in the village-owned lands, but also into the forest reserves, which protect the majority of the remaining forest areas.

Location and General Description

The Eastern Arc ecoregion extends in elevational patches along a chain of isolated mountain ranges, from the Taita Hills, close to the Kenyan border with Tanzania, down through eastern Tanzania to the gap between the Udzungwa Mountains and Mt. Rungwe (Makombako Gap). From north to south, the main blocks are the Taita Hills in Kenya, the North and South Pare, the East and West Usambara, the North and South Nguru, the Ukaguru, the Uluguru, the Rubeho, and the Udzungwas. There are also smaller isolated outliers such as Mahenge to the south of the Udzungwas, Malundwe Hill in Mikumi, and the Uvidundwa Mountains north of the Udzungwas.

Source: WWF

The long-term climatic stability of the mountains has been discussed in several publications. The forests of the ecoregion have likely persisted in this location up to 30 million years, and hence been formerly connected to a forest belt that stretched across the entire tropical region of Africa. These forests probably survived the driest and coldest periods of the Ice Ages, as the Indian Ocean did not cool appreciably and rainfall patterns may not have been greatly disrupted. This stability is viewed as one of the main reasons for the biological importance of the ecoregion. The current climate of these mountains is much wetter than the surrounding lands, with perhumid (rain every month) conditions and rainfall up to 3000 millimetres (mm) per year recorded in the eastern Uluguru Mountains. In most of the other mountains the wet and dry seasons are more clearly defined, with two seasons in the northern forests and one in the southern forests. There is some evidence that the climate has become drier and more seasonal in recent decades, with a lower likelihood of the forests being enveloped in mist. In the Usambara Mountains, for example, some native tree species are no longer regenerating, perhaps due to increased temperatures in recent decades.

Udzungwa Mountains, Tanzania. (Source: Photograph by WWF/ Hartmut Jungius)

The Eastern Arc is comprised of heavily metamorphosed Precambrian basement rocks, which have been periodically uplifted by faulting and weathering over millions of years. The soils derived from these sediments are typically less rich than those of the volcanic mountains of East Africa, but the consistent rainfall in the area increases the agricultural potential. These block-faulted mountains have been geographically isolated for millions of years. The main blocks are uplifted along ancient faults, with uplift events occurring periodically, probably at least since the Miocene (about 30 million years ago). However, the faults, and therefore perhaps also the mountains, appear to be considerably older than this.

The topography of the Eastern Arc is characterized by rugged mountain blocks rising from the more subdued topography of the coastal plain of eastern Africa. Some mountain blocks are so steep that there are substantial areas of exposed rocks, yet slopes of up to 50-70 degrees still support forest vegetation. These mountains rise to 2,635 meters (m) in elevation (Kimhandu peak in the Ulugurus), although maximum altitudes of 2,200-2,500 meters are more typical. The tops of the Uluguru and Udzungwa mountains are plateau-like, and support forest grassland mosaics, which in the Udzungwas have been heavily altered by humans.

The population densities are substantially higher than those of the surrounding lowlands where the rainfall is less predictable, with densities of 100-200 persons per square kilometer (km²) found in the East Usambaras. Populations are increasing not only due to birth rates (around 2.8% per year), but also from migration due to better agricultural potential. In other areas the reverse is occurring and people are leaving the mountains and moving to the cities. In such places the population density may be stable, or even falling marginally.

Udzungwa Mountains, Tanzania. (Source: Photograph by WWF/ John Newby)

In terms of the phytogeographical classification of White, the ecoregion forms part of the Afromontane archipelago-like regional center of endemism. The Eastern Arc subregion of this Afromontane center of endemism is unique in its long-term stability and isolation, and shares affinities with both the East African montane forests and South Malawi montane forests – and to a lesser degree with the Albertine Rift montane forests and Ethiopian montane forests.

The forest formations have been divided into upper montane (2,635-1,800 meters [m]), montane (1,250-1,800 m), and submontane (800-1,250 m) forest. At higher altitudes, the forests becomes dwarfed, and are covered with lichens. Additionally, open vegetation bog and grassland habitats can be found in the higher areas of the Ulugurus, whereas grasslands and stands of bamboo can be found in the Udzungwas. The species composition of the mountain forests grades at lower altitudes with that of the lowland coastal forests of the Northern Inhambane-Zanzibar coastal forest mosaics.

There are a large and as yet unknown number of forest tree community associations in these forests, with high regions being more temperate and lower regions more tropical. Upper montane forests are characterizes by high rainfall and large trees such as Ocotea usambarensis. The upper altitudinal limit of this strata is determined by regular occurrence of frost. Tree species here include Aphloia theiformis, Cassipourea malosana, Dombeya torrida, Olea capensis, and the endemic Allanblackia ulugurensis. The montane forests are much taller and contain bigger trees. The shrub layer is dominated by Rubiaceae and Acanthaceae. A number of tree species are endemic to this region, including Allanblackia stuhlmannii, Beilschmiedia kweo, Isoberlinia scheffleri, and Polyceratocarpus scheffleri. Submontane forests often overlap with montane forests, but are distinguishable by the presence of lowland species such as Afrosersalisia cerasifera, Milicia excelsa, and Parkia filicoidea. Endemics include Anisophylla obtusifolia, Cephalosphaera usambarensis, and Englerodendron usambarensis. Other common tree species found in the Eastern Arc Mountains include Cassipourea gummifera, Khaya anthotheca, Ochna holstii, Podocarpus latifolius, P. falcatus, Ilex mitis, Cola greenwayi, Cornus volkensii, Newtonia buchanii, Pachystela msolo, and Trichilia dregeana.

Biodiversity Features

The forests in East Africa, although small and insular, contain a large number of endemic species and are important centers of relictual species, including forms with affinities to Southeast Asia and Madagascar. They are also important in Tanzania for the water supply for many of the larger towns in the country, especially Dar es Salaam.

Tree frog (Leptopelis vermiculatus), Amani, Tanzania. (Source: Photograph by WWF/ Arne Schiotz & CalPhotos)

It has been estimated that there are over 2,000 plant species in 800 genera in these montane and surrounding forests. At least 800 of these species are believed endemic to this ecoregion. On a broader scale, the montane forests of Tanzania contain 7% of the endemic plant species of Africa on only 0.05% of the total area. The forests are the centers of global endemism for the African violet (Saintpaulia) and Busy Lizzies (Impatiens), which have been widely cultivated for house plants in Europe and America. When considered together with the Northern Zanzibar-Inhambane coastal forest mosaic ecoregion, the density of plant species endemism is among the highest in the world. There are also high rates of endemism in the non-vascular bryophytes, including 32 known strict endemics and a number of near-endemics, some shared with Madagascar. These endemic plants are not only found in the forests, but also in the montane grasslands, wetland areas, and on rocky outcrops.

Bird endemism in the Eastern Arc forests is high, with both strict and near-endemic species. Some of the near-endemic species exhibit disjunct distribution patterns indicating formerly widespread populations in the mountains of eastern Africa. Species with limited distributions include the Taita thrush (Turdus helleri, CR) and Taita apalis (Apalis fuscigularis, CR) which only occur in a few square kilometers of forest in the Taita hills. The Udzungwa partridge (Xenoperdix udzungwensis, VU) is known only from a single forested area on the Udzungwa Mountains, and the Uluguru bush shrike (Malaconotus alius, EN) is confined to one forest reserve on the Uluguru Mountains, of less than 100 km² forest area. Others occur on several mountains. These include the Usambara eagle-owl (Bubo vosseleri, VU), banded sunbird (Anthreptes rubritorques, VU), and Mrs Moreaus warbler (Bathmocercus winifredae, VU). Other more wide ranging species are generally shared with mountain forests further to the south in Malawi and Zimbabwe, or with lowland coastal forests of the Zanzibar-Inhambane coastal mosaic.

Mammalian endemism is also high, considering the relatively small area of these montane forest patches. There are no endemic large mammals, however, presumably because the areas of forest are insufficient to permit the persistence of unique large forest-dwelling mammal species. Six strictly endemic small mammals are known, including five species of shrews (Crocidura tansaniana (VU), Crocidura telfordi (CR), Crocidura usambarae (VU), Myosorex geata (EN) and Sylvisorex howelli (VU)), and one species of galago (Galagoides orinus). Other threatened mammals which occur in these forests include Abbot’s duiker (Cephalophus spadix, VU), eastern tree hyrax (Dendrohyrax validus, VU), and the black and rufous elephant-shrew (Rhynchocyon petersi, EN).

Amphibians and reptiles also exhibit high levels of species endemism. Notable among the 25 species of strictly endemic amphibians are species within the reed frogs (Hyperolius – five endemic species), forest treefrogs (Leptopelis - two endemic species), tree toads (Nectophrynoides – five endemic species), species in the Microhylidae family (four endemic species), and the Caeciliaidae family (five endemic species). New species continue to be discovered in this ecoregion; for example, the newly described Kihansi spary toad (Nectophrynoides asperginis), which is extremely threatened with extinction due to the diversion of water to a hydroelectric plant. Other new species await description from the Udzungwa, Ukaguru, Uluguru, and West Usambara mountains.

The high rates of endemism seen in other groups of vertebrates are also found in the reptiles. The strictly endemic reptiles include ten species of chameleons (seven Chamaeleo and three Rhampholeon), three species of worm snakes (Typhlops), and six species of colubrid snakes in four genera.

The invertebrates of the Eastern Arc also contain very high rates of endemism. Available compiled information illustrate that up to 80% of the invertebrate fauna of a single eastern Arc Mountain can be strictly endemic, with the next mountain along containing a similar high rate of strictly endemic species.

In addition to their high rates of endemic species the forests of this ecoregion are also notable from an evolutionary perspective. They contain both ancient relic species with various affinities to other geographical areas, and newly evolved species. In the bryophytes there are 45 species that are shared with Madagascar, indicating a very ancient link in the floras of these areas. These species are not found on Kilimanjaro or any of the other young volcanic mountains of northern Tanzania. In the plants, there are a number of affinities at the generic level with the forests of West Africa and also with the forests of the Northern Zanzibar-Inhambane coastal forest mosaic. Among the birds there are species with affinities to those of West Africa. Two of the birds, the Udzungwa forest-partridge (Xenoperdix udzungwensis) and the African tailor bird (Orthotomus metopias), have affinities with living species in Southeast Asia, suggesting an ancient forest connection to the East. This kind of evidence demonstrates that the Eastern Arc forests are very old and contain representatives of groups that are otherwise extinct regionally or within Africa.

Current Status

The remaining blocks of forest habitat have been recently measured from satellite-derived land-cover maps: Taita Hills (6 km²), Pare Mountains (484 km²), West Usambaras (328 km²), East Usambaras (413 km²), Nguru (647 km², incl. Nguu), Ukaguru (184 km²), Uluguru (527 km²), Rubeho (499 km²), Udzungwa (1960 km²), and Mahenge (291 km²). Some of these estimates are believed to be too high. For example, recent work in the Uluguru Mountains has shown that the forest area is closer to 350 km² rather than 527 km². There are also areas of remaining natural grassland and bamboo, although some of the grasslands are being converted to Pyrethrum and potato plantations.

Forest is protected in the 1,900-km² Udzungwa National Park (much of which is not montane forest vegetation), as well as in a larger number of forest reserves that are established for water catchment purposes. A nature reserve has also been recently established in the East Usambara forests. However, as most of the remaining forest area is found in forest reserves, these areas are extremely important for biodiversity conservation. Moreover, they supply water to major rivers. For example, the main water supply for Dar es Salaam comes from the forest reserves of the Uluguru Mountains.

Forest habitat has generally been lost on all lower slopes in the Eastern Arc, and has also been lost at higher elevations in some blocks (e.g., for tea plantations in the Usambaras and the Udzungwas). The German administration’s creation of forest reserves in the late 1800s was an important step in protecting the forests, which at that time were described as ‘largely vanished’ and ‘very sad’. These measures have helped prevent the total loss of forest in some areas, although losses of significant areas of forest have continued (for example, in the West Usambaras when forest was degazetted and converted to farmland). Evidence from the Udzungwas also indicates settlement on land that is now forested, and hence there may be more forest in this area currently, than 100 years ago.

The habitat is naturally fragmented in the main forested blocks. However, it has also become substantially more fragmented in recent centuries due to human activities. The favorable climates of the mountains, good supply of water and building materials, and reasonable soils have attracted humans to these mountains for the last 2,000 years. Reliable access to markets (to sell agricultural products) is an important driving force in these movements both to and from the mountains. The human population is also increasing in many areas at around 2.8% per year, which is placing increased pressure on existing farmland, and is spurring the conversion of unprotected forest and grassland areas to farmland.

Types and Severity of Threats

Within the Udzungwa National Park the level of exploitation is low and protection is high. Within catchment forest reserves, which contain the majority of remaining forest resources, the protection level is more variable and depends on access, presence of valuable timber, population pressure, and the interest of local officials protecting the forest. In some cases, there is significant commercial logging (mainly using pitsawing techniques), and in other areas there is encroachment for farm plots and for the collection of other wood products for firewood and poles. Outside of catchment forest reserves the forests have been largely cleared on agricultural lands, except for small locally protected forest patches that are used for burial grounds and for traditional ceremonial purposes. In some parts of the Eastern Arc these smaller forest patches are numerous. Although they have not been studied biologically, they may provide a refuge for some of the plants and smaller species of animals that used to live in the forests.

Justification of Ecoregion Delineation

The boundaries of the Eastern Arc Mountains ecoregion follows the ‘undifferentiated montane vegetation’ unit of White, modified around 1,000-meter contour of the Uluguru, Nguru, and Mahenge Highlands. Although this ecoregion shares affinities with both the East African montane forests and the Southern Rift montane forests, and to a lesser degree with the Albertine Rift montane forests and Ethiopian montane forests, the Eastern Arc Mountains are unique in their long-term climatic stability and isolation. This has created a number of endemics and relict species, many of which only occur on individual mountains. A high-elevation area in the Pare Mountains was also included, although there is some debate as to whether it is biogeographically more similar to the Kenyan Mountains.

Additional Information on this Ecoregion

For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.

Eastern Arc Forests

Contents

WWF Terrestrial Ecoregions Collection

Introduction

Location and General Description

Biodiversity Features

Current Status

Types and Severity of Threats

Justification of Ecoregion Delineation

Additional Information on this Ecoregion

Further Reading

Citation

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