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The vegetation of the islands fits the popular image of the tropical; it is mostly lush and green and there are small-holdings and plantations of various crops. The sand and mangrove flats of the coast are not noted specifically in the literature, and the general impression is that little of the original vegetation remains, that which does exist is of low value, and it is an environment that is ripe for development. These perceptions are largely accurate, but in any case it is evident from a survey of the literature and from discussions with local people, that the coastal environment is very much a part of Zanzibari culture and economy, and is significant in terms of resources and hazards. This is especially true given rapid population growth and the associated increased utilization of coast and marine resources. Hence it was decided that a study of the coastal vegetation would be of value, and that it could form an important part of the ecological research undertaken in Zanzibar in the future. A reconnaissance survey was carried out in July 1996, the objectives of which are detailed in the next section.
The term ‘coastal vegetation’ refers to all the plants which grow in the inter-tidal zone, and where excessive salt or periodic inundation prohibit the growth of most terrestrial plants. It is an important and interesting subject in its own right, and in addition is a crucial component of the marine ecosystem. The role of coastal plants as stabilisers of sand and sediment is a well-known one, but they are also the primary producers in this environment, and so form the basis of a food chain which includes marine animals and birds. It is also a habitat that is frequently disturbed by humans, who exploit it for resources such as timber and thatching grass, for grazing, and for cultivation. Coastal vegetation has often been marginalised in conservation and resource management, and is a habitat which is particularly vulnerable to damage and degradation.
The coastline and shallow waters around Zanzibar are dominated by thirteen types of vegetation. There are different kinds of seagrass, mangrove, and coastal forest or thicket. All of these types of vegetation are exploited in various ways by humans, yet play vital roles in the maintenance of the biophysical environment, particularly in relation to the stability of the coastline and the quality of the adjacent marine environment. It is widely recognized that coastal vegetation in general is important as a protective and stabilizing agent for the coastline. This is a complex issue which is based on the physical structure of the vegetation and its impact on sediment movement, but the final result is that areas without coastal vegetation, particularly on tropical coasts, generally suffer from significant coastal erosion. On the east coast of Unguja there is evidence of coastal erosion in many areas, often linked by local people to the degradation of coral reefs, however at areas like Chwaka and Uroa the receding coastline and the consequences for local people are directly attributed to clearing of coastal vegetation. In both areas the recent construction of sea-walls by foreign agencies has been a response to public demand, yet a proper understanding of the causes and effects of coastal erosion and the potential for management of the coast through rehabilitation of the natural environment would likely lead to very different interventions.
This need for an increased understanding of the coastal vegetation in Zanzibar was recognized in a project conducted by the Institute of Marine Sciences at the University of Dar es Salaam, with support from the Japan International Cooperation Agency. This project was the most recent and ambitious attempt to document the current status of coastal vegetation in Zanzibar. A field survey was conducted in 1999 in four locations: Zanzibar Ushelishelini Daraja Mbili near Fumba (Southwest Zanzibar), Uroa (Chwaka) in northeast Zanzibar and Paje and Bwejuu, also in the southeast. The main objectives of this project were to assess the current status of coastal vegetation in Zanzibar by recording the various types and their extent, compare the findings to the data and maps prepared in the early and mid 20th Century including an assess the changes that have taken place, provide guidelines for sustainable resource management and coastal land use planning in the context of holistic conservation and management of the coastal area, and finally produce detailed maps and reports as a management tool for government institutions and NGOs involved in marine and terrestrial coastal conservation and management. The results from this project are to be used to further environmental planning and management, and will form the most up to date and accurate information on Zanzibar’s coastal vegetation.
Zanzibar’s coastal vegetation is highly diverse and has been relatively well described and studied. Over the years, this island’s coastal landscape has experienced significant alteration due to increased human settlement and the associated removal and use of coastal resources. Unfortunately, most of the changes took place during a time when few studies were conducted, and no detailed maps or records exist of the location or extent of the coastal vegetation types present. Because of the large gaps in current knowledge and the importance of coastal vegetation to both the local communities and the environment, a comprehensive understanding of Zanzibar’s coastal vegetation is needed.
Pioneers on the beaches are able to survive in unstable shifting sands. Usually, the primary colonizers are Spinifex (Spinifex littoreus) or Beach Morning Glory (Ipomoea pes-caprae), depending on the level of sand movement. Spinifex, due to its vertical rolling action in front of sand, is able to build up a hummock and trap the sand, effectively stabilizing it. Spinifex has been widely used historically throughout the Pacific and Asia for this very reason. Other stable sand plants are Goatsfoot Convulvulus (Ipomoea pes-caprae) and Beach Spinifex (Spinifex hirsutus). Behind the pioneer zone, usually inhabit areas of sand with some soil content; these areas can be subjected to saltwater or become quite dry. Plants that inhabit the high strand line are usually very good at tolerating salt and have to be resistant to sandblast.
Mangrove forests are found on low wave energy shores, protected from the sea by coral rag or sand bars. This habitat has been reduced in Zanzibar, and around the world, by cutting of mangroves for timber and as charcoal, and by conversion of mangrove areas to cultivation.
Vegetated intertidal habitats are found primarily on low wave energy shores. They are important for several reasons. They may be sources of detritus to nearby areas and so help to maintain high productivity in adjacent areas. They also may serve as nursery areas, providing refuge from predators in the open sea for juvenile forms of fish and other animals. Finally, the vegetation binds the substrate and so prevents erosion. This is particularly important in Zanzibar where erosion from deforestation has led to greatly increased sedimentation around the coast and consequent damage to coral reefs and seagrass beds.
The coastline of Zanzibar is characterised by a great diversity of shores and associated vegetation. Intertidal rocks, boulders, cliffs, coral rag, sandy and muddy shores all have characteristic flora and fauna. There are also three main types of coast, based on wave exposure, which are more widespread than the various specific substrata mentioned above. These are exposed coasts with high wave energy, sheltered coasts with low wave energy, and estuarine coasts within river mouths.
Reflecting the important role of sea level and the formation of suitable substrates, Kainja and Mmanga (1993) have proposed that Zanzibar’s mangrove communities have evolved in two steps. A pioneer association of Avicennia marina first colonized emerged Pleistocene reef around the edge of the island, later giving way to mixed stands of Rhizophora species in more sheltered reef and coastline sites. The presence of relict mangrove on high island ground and these primitive communities on raised Pleistocene reef indicate that the most extensive phase of mangrove expansion also occurred during the mid-late Holocene.
Mangrove forests first became established on the coasts of East Africa during the second half of the Pleistocene (Forsyth, 1983). The main radiocarbon dating from the region has been done in Kenya. It is believed that during the height of the Holocene transgression between 7000-5000 years BP, large areas of forest became inundated with seawater, killing the trees and preserving the organic material in situ. At Ras Serani, dates range from 6300-2800 years BP. The younger dates come from organic material recovered a few meters below the present surface of the forest.
One of the principal causes of concern in the coastal environments of Zanzibar is the recent decline of seagrass meadows and the associated loss of resources and habitats for a wide range of organisms. Seagrasses are the dominant group of plants in many shallow subtidal communities. Five different species are found in Zanzibar and they form extensive meadows, particularly on the sheltered western coast of the main island of Unguja. Seagrasses provide a variety of resources for plants and animals, and their structure and complex physical matrix of the leaves and rhizomes provide refuge from predation for a number of small invertebrates and fish. The loss of such services is a matter of concern as coastal areas are often breeding and nursery grounds for many species of marine organisms. Seagrasses are particularly important as they are a primary producer and many detritivores feed on the epiphytic algae associated with seagrass leaves.
Forest is very rare around the coast of Zanzibar today, with its only stronghold remaining at Ras Nungwi and Kigomani. The once continuous coastal forest has been cleared for agriculture, human settlement, and charcoal production. The forest is dominated by Cocos nucifera, with scattered Tamarindus indica, Calophyllum inophyllum, and Terminalia catappa. In the understory, Artocarpus altilis used to be densely planted, but today little natural regeneration takes place. An epiphytic form of orchid, Ansellia africana, is present, but overcollection has caused it to become scarce. Today, the forest is the home of introduced species like Eucalyptus sp, Leucaena leucocephala, Azadirachta indica, making it difficult to recognize it as indigenous. Due to its inaccessibility, this type of coastal forest has been least affected by man and is the reason for Saoudan Peninsula and Ras Nungwi being the most botanically diverse coastal areas on Zanzibar today.
These coastal areas have had vegetation studies carried out on them: Saoudan Peninsula, Mwera, Chwaka, Uzi Island, Tumbatu, Fundo Island, the northeast coast, and Fungu ya Kofi. The vegetation was divided into forest, thicket, and grassland, and every community and species present were recorded. Due to the destruction of these areas over time, students may find it difficult to locate the different vegetation types discussed. An illustration of each is provided below.
This section deals with an assessment of the largest seaward zone found on the east of the island. This area is almost entirely covered by a species of palm known as Cocos nucifera, and it is these palms that provide Zanzibar with its famous exotic images. The Cocos palms act as stabilizers, enmeshing the substrate with their complex root system, and over time build up debris swept in by the sea to form land. They also provide a habitat for many plants growing epiphytically and hermit crabs living in the washed-up fruit. The fronds of the Cocos palms offer some protection from the sea’s energy for the remainder of the plant species, and as succession occurs, the proliferation of human activity is stopped with the destruction of habitat.
This essay will now focus on the coast of East Africa’s largest island, Unguja, Zanzibar. Coastal vegetation plays an essential part in Zanzibar’s ecosystem. Not only does it protect the land from being eroded by the sea by acting as a barrier, but it also provides an important habitat for countless animals, insects, and birds. The vegetation can be broken up into several zones, the most seaward being characterized by plant species that are subjected to regular inundation by seawater, and the most landward, which are rarely inundated and further subdivided into salt marsh and scrub zones.
Lantana camara is an invasive species that was introduced to Zanzibar from Central and South America. It was brought to Zanzibar in the 19th century because it was an ornamental plant and spread across most of East Africa, being a serious weed. The spread of Lantana camara was once thought to be through the use of firewood, but it is now suggested that the plant produces its own fruit to be dispersed by frugivorous bird species. In other countries where Lantana camara is also an invasive species, there have been some very negative effects on other flora and fauna populations. One example is in South Africa, where the spread of Lantana camara has caused a decline in certain animal populations, such as the Samango monkey. This is due to the toxicity of the plant affecting the animals. It is unknown what effect this will have on Zanzibar’s animals, but a close watch will need to be kept on changes in certain species’ numbers. The removal of Lantana camara is expensive and requires a lot of manual labor. This plant is now common around Zanzibar and is having a detrimental effect on native species. Lantana camara outcompetes other species for nutrients and space, preventing them from growing adequately. In some instances, the plant gives off a toxic acid through the soil, which kills nearby plant species. Another invasive plant species that is commonly found on Zanzibar’s coastal vegetation is the coconut palm (Cocos nucifera). Although this plant is native to Malaysia and the South Pacific, it is considered an invasive species due to its widespread location and the fact that it has become naturalized in many open coastal areas. The coconut palm is another plant that outcompetes native species, and in some places, it must be removed to enhance the growth of key species that bind sand dunes. The high economic value of coconuts means that they are an important crop in Zanzibar, and to some atoll areas, the coconut palm is seen as a crucial plant to these communities’ existence. This may cause conflict when attempting to remove this species from certain areas, especially in locations of mixed native and non-native vegetation.
The greatest number of animals are found in three major areas of the East Africa shoreline. These areas are the Somali-Masai, the Southern, and the Eastern. Each has its own endemic species as well as a number of species which are found in only one area. On the islands of Zanzibar and Pemba, some 35 km offshore from the Tanzanian mainland only 15 km from the east coast of Africa, there exist a few habitat islands of coastal forest which can be compared with the original climax associations and it is with coastal forests and thickets that this paper is mainly concerned. The number of mammals in the coastal forests and thickets of Zanzibar and Pemba is limited and does not exceed about 7% of the land mammal fauna. It is with the marine and littoral environments that most of the fauna occurs. These two are, however, very rich in species with over 120 species of birds and over 350 species of fish. The marine fauna of Zanzibar and Pemba has been given some attention by a number of authors but very little has been published on marine or littoral environments and it is hoped that this paper may stimulate more interest in this aspect of the islands. Two references should be made to the history of this fauna.
