Written by Philip Briggs & Chris McIntyre
Following a fault line that runs east of the more geologically recent Rift Valley, these are the oldest mountains in East Africa, having formed at least 100 million years ago…
The phrase ‘Eastern Arc‘ was coined by Dr Jon Lovett in the mid 1980s to describe a string of 13 physically isolated East African mountain ranges that share a very similar geomorphology and ecology. All but one of these crystalline ranges lies within Tanzania, forming a rough crescent that runs from Pare and Usambara in the north to Udzungwa and Mahenge in the south.
Following a fault line that runs east of the more geologically recent Rift Valley, these are the oldest mountains in East Africa, having formed at least 100 million years ago, making them 50 times older than Kilimanjaro.
For the past 30 million years, the Eastern Arc has supported a cover of montane forest, one that flourished even during the drier and colder climatic conditions that have periodically affected the globe, thanks to a continuous westerly wind that blew in moisture from the Indian Ocean.
It was during one such dry phase, ten million years ago, that these became isolated from the lowland rainforest of western and central Africa. More recently, each of the individual forested ranges became a discrete geographical entity, transforming the Eastern Arc into an archipelago of forested islands jutting out from an ocean of low-lying savanna. And as with true islands, these isolated ancient forests became veritable evolutionary hotspots.
The Eastern Arc Mountains host an assemblage of endemic races, species and genera with few peers anywhere in the world. In the two Usambara ranges alone, more than 2,850 plant species have been identified, a list that includes 680 types of tree, a greater tally than that of North America and Europe combined. At least 16 plant genera and 75 vertebrate species are endemic to the Eastern Arc forests.
Their invertebrate wealth can be gauged by the fact that 265 invertebrate species are thus far known from just one of the 13 different ranges – an average of 20 endemics per range. Little wonder that the Eastern Arc is classified among the world’s 20 top biodiversity hotspots, and is frequently referred to as the Galápagos of Africa.
Eastern Arc endemics fall into two broad categories: old endemics are modern relics of an ancient evolutionary lineage, while new endemics represent very recently evolved lineages. A clear example of a ‘living fossil’ falling into the former category are the giant elephant shrews of the suborder Rhynchocyonidae, whose four extant species are almost identical in structure to more widespread 20-million-year-old ancestral fossils.
In many cases, these older, more stable endemics are affiliated to extant West African species from which they have become isolated: Abbott’s duiker and the endemic monkey species of Udzungwa are cases in point.
The origins of new endemics are more variable. Some, such as the African violets, probably evolved from an ancestral stock blown across the ocean from Madagascar in a freak cyclone. Others, including many birds and flying insects, are local variants on similar species found in neighbouring savannas or in other forests in East Africa.
The origin of several other Eastern Arc endemics is open to conjecture: four of the endemic birds show sufficient affiliations to Asian species to suggest they may have arrived there at a time when moister coastal vegetation formed a passage around the Arabian peninsula.
The forests of the Eastern Arc vary greatly in extent, biodiversity and the degree to which they have been studied and accorded official protection. A 1998 assessment by Newmark indicates that the Udzungwa range retains almost 2,000km² of natural forest, of which 20% has a closed canopy, while the forest cover on Kenya’s Taita Hills is reduced to a mere 6km². The most significant forests in terms of biodiversity are probably Udzungwa, East Usambara and Uluguru.
However, ranges such as Nguru and Rubeho remain little studied compared with the Usambara and Udzungwa, so they may host more endemics than is widely recognised.
The Eastern Arc forests are of great interest to birdwatchers as the core of the so-called Tanzania–Malawi Mountains Endemic Bird Area (EBA). This EBA includes roughly 30 forest pockets scattered across Malawi, Mozambique and Kenya, but these outlying forests cover a combined 500km² as compared with 7,200km² of qualifying forest in Tanzania. Of the 37 range-restricted bird species endemic to this EBA, all but five occur in Tanzania, and roughly half are confined to the country.
In terms of avian diversity, the Udzungwa Mountains lead the pack with 23 regional endemics present, including several species found nowhere else or shared only with the inaccessible Rubeho Mountains. For first-time visitors, however, Amani Nature Reserve has the edge over Udzungwa in terms of ease of access to prime birding areas.
Distribution patterns of several range-restricted bird species within the EBA illuminate the mountains’ pseudo-island ecology, with several species widespread on one particular range being absent from other apparently suitable ones. The Usambara akalat, for instance, is confined to the Western Usambara, while Loveridge’s sunbird and the Uluguru bush-shrike are unique to the Uluguru.
The most remarkable distribution pattern belongs to the long-billed tailorbird, a forest-fringe species confined to two ranges set an incredible 2,000km apart – the Eastern Usambara in northern Tanzania and Mount Namuli in central Mozambique. Stranger still is the case of the Udzungwa partridge: this evolutionary relic, discovered in 1991 and known only from Udzungwa and Rubeho, has stronger genetic affiliations to Asian hill partridges than to any other African bird.
The Eastern Arc has suffered extensive forest loss and fragmentation in the past century, primarily due to unprecedented land use pressure – the population of the Western Usambara, for instance, increased 20-fold in the 20th century.
Of the 12 Eastern Arc ranges within Tanzania, only one – the inaccessible Rubeho massif – has retained more than half of its original forest cover, while five have lost between 75% and 90% of their forest in the last two centuries. Fortunately, none of Tanzania’s Eastern Arc forests have yet approached the crisis point reached in Kenya’s Taita Hills, where a mere 2% of the original forest remains.
Given that many Eastern Arc species are highly localised and that animal movement between forest patches is inhibited by fragmentation, it seems likely that 30% of Eastern Arc endemics have become extinct in the last century, or might well do so in the immediate future. True, the salvation of a few rare earthworm taxa might be dismissed as bunny-hugging esoterica, but the preservation of the Eastern Arc forests as water catchment areas is an issue of clear humanistic concern. Most of the extant Eastern Arc forests are now protected as forest reserves.
The proclamation of a large part of the Udzungwa Mountains as a national park in 1992 is a further step in the right direction. Even more encouraging is the more recent creation of Amani Nature Reserve as part of a broader effort to introduce sustainable conservation and ecotourism with the involvement of local communities in the Eastern Usambara.
Anybody wishing to come to grips with the fascinating phenomenon of ‘island’ ecology in the Eastern Arc Mountains (and elsewhere on the African mainland) is pointed to Jonathon Kingdon’s superb book Island Africa.