Mangroves in the Casamance (southern Senegal)© FAO/afrol News
Environment - Nature | Science - Education
an undervalued ecosystemHistorically classified "unhealthy wastelands" or "useless swamps" by development-eager authorities and businesses, the mangrove forests actually are one of the most fascinating resources in tropical Africa. The trees manage to live on the edge between flooding rivers, tidal waves intruding with salt water and the drylands, where they create new land and environs rich in fish, birds, wood and other resources. Finally, their value is being discovered. The mangroves are a characteristic forest biotope in tropical river estuaries and tidal zones. They constitute an incredible adaptation to the environmental conditions of entering salt, sea water and escaping sweet, riverine water. The forests are highly productive areas and in many places an underdeveloped resource. They still are widespread along the West African coast from Senegal to Congo and locally in East Africa.
Mangrove forests are found at the edge of tropical oceans where regular flooding occurs. Mangrove forests range in stature from mere shrubs to up to 40 meter tall trees. The forests are however characterised by a very low floristic diversity compared with most inland forests in the tropics. The same few species totally dominate wide tracts. This is because only very few plants can tolerate the harsh environment and actually flourish there - mud saturated with salt that frequently is inundated by ocean and river water.
Most land plants are killed by salt, but mangroves are able to filter it out. Most plants die if their roots are drowned in water thus leaving them without oxygen, and in the mud of mangrove swamps, the rotting leaves usually consume all available oxygen. However mangrove trees have developed special kinds of roots that stick up - out of the mud and into the air - to get oxygen.
Mangrove trees have pitchfork-like roots which grow out from the lower part of their trunks. The trees use their roots for additional support. These stilt or prop roots also trap debris, which provides the trees with nutrients, and are also important for stabilising the shoreline. Living on the edge - between river, land and ocean - the mangroves therefore also actively create their own environment, making it liveable for them and other species by stabilising the silt-rich soil and creating a new land environ.
WildlifeMangroves are highly productive biotopes and as such have a vibrant, rich and endemic wildlife. Mangrove forests and the salt marshes connected to them provide food and a home for fish, shellfish, molluscs, wildfowl and threatened marine mammals. Most of these species are endemic to the mangroves, meaning they cannot live in any other place. Most of the endemic species are an enormous variety of crabs.
But also many other species need the mangroves in periods of their life. Ducks, geese and other wild birds stop over at coastal wetlands - mostly the mangroves - during migration. Flounder and bluefish use the marshes as nurseries, winter quarters and occasional feeding grounds. The mangroves further offer nursery and breeding grounds for freshwater and marine life - especially shrimp.
The manatees may serve as our example of the many endangered species of the African mangroves. The West African manatees (also called sea cows and sirenias, Trichechus senegalensis) live in rivers, bays, estuaries, and coastal areas.
Although they are marine animals, the manatees are related to neither whales, walrus nor dolphins. Surprisingly, their closest relatives are the elephants. They move freely between freshwater and saltwater habitats. Manatees are slow-moving creatures that feed on aquatic vegetation. Adults range in length from 2.5 to 4.5 meters and may reach nearly 700 kg in weight.
Adult manatees have no natural enemies but in some areas, they are heavily hunted for meat, hides, and oil. Where boat traffic is heavy, the slow manatees are often injured or killed by boat propellers. They are mostly protected by law because of their usefulness in keeping waterways clear of aquatic vegetation. The West African manatee totally depends on the mangrove habitat, but very little is known about the rare species, as they have not been widely studied.
Apart from the manatees, the most characteristic species of the mangroves are the seabirds. Mangroves are an ideal sanctuary for birds, many of which are migratory. Closed mangrove forests provide a secure nesting and feeding habitat for a variety of waders, terns and flycatchers.
According to FAO, the total list of mangrove bird species included between 150 and 250 species. Worldwide, 65 of these are listed as endangered or vulnerable, including for instance the milky stork (Mycteria cinerea), which lives in the rivers of mangroves.
Thus, reserves in the mangroves have turned bird watcher's paradises, where tourists can observe pelicans, pink flamingos, storks and other birds. Indeed, in the mangroves of southern Senegal's Casamance province, bird watching expeditions are a major income source for locals, and authorities in Guinea-Bissau, Sierra Leone and Gabon hope to repeat the Senegalese success story.
The mangrove ecosystemPrecise data on global mangrove resources are scarce as this unique ecosystem has been studied surprisingly little. However, a major part of Africa's subtropical and tropical coastline is dominated by mangroves. Estimates are that there are some 16 million hectares of mangrove forests worldwide.
The general distribution of mangroves mostly corresponds to that of tropical forests, but extends somewhat further north and south of the equator, sometimes beyond the tropics. However, over the past several decades, the area covered with mangroves globally has increasingly been reduced as a result of a variety of human activities, such as over-harvesting, freshwater diversion and conversion to other uses.
There are two distinct bio-geographic zones of mangroves in the world: those of West Africa, the Caribbean and America; and those on the eastern coast of Africa, Madagascar and the Indo-Pacific region. While the first - the Atlantic region - contain only ten tree species, mangroves of the Indo-Pacific are richer, containing some 40 tree species (excluding palms).
Mangrove forests are considered being vital for healthy coastal ecosystems. The forest detritus - consisting mainly of fallen leaves and branches from the mangroves - provides nutrients for the marine environment and supports immense varieties of sea life in intricate food webs associated directly through detritus or indirectly through the planktonic and algal food chains.
A primary factor of the natural environment that affects mangroves over the long run is the global sea level and its fluctuations. Other shorter-term factors are air temperature, salinity, ocean currents, storms, shore slope, and soil substrate. Most mangroves live on muddy soils, but they may also grow on sand, peat, and coral rock.
If tidal conditions are optimal, mangroves can flourish far inland, along the upper reaches of coastal estuaries. Especially in Western Africa, mangrove forests reach far into the hinterland. River Gambia, the Sine-Saloum river system of Senegal, the Casamance, rivers of Guinea-Bissau, River Niger and Cameroonian rivers are edged by a wide zone of mangroves, sometimes more than 100 kilometres from the outer coast.
In other cases - where vast amounts of sweet water pour into the ocean, entire islands tens of kilometres off the coast may be entirely covered by mangroves, as is the case of most of Guinea-Bissau's Bijagos archipelago. Several of the outer Bijagos islands, such as Ilha de Orango - lie around 100 kilometres from the coast and as such fresh water supply, but in the shallow waters off Guinea-Bissau, salinity is still sufficiently low for trees to filter out salt from the sea water that is mixed up with fresh, river water.
Evolutionary adjustments to varying coastal marine environments have produced some astounding biological characteristics within mangrove plant communities. Certain species of mangroves exclude salt from their systems; others actually excrete the salt they take in via their leaves, roots, or branches. In salt excluding mangrove species, the mangrove root system is so effective filtering out salt that a thirsty traveller could drink fresh water from a cut root, though the tree itself stands in saline soil.
The mangroves as a resourcePeople mostly have thought of mangroves as noxious impenetrable swamps full of diseases, and they used to be destroyed as a public health measure. But now we know better. Mangroves are very productive coastal resources that are useful in many ways.
Mangrove trees grow well within their special conditions - where no other trees could be planted - and, like the tropical forest, they produce a lot of leaves and other organic matter, detritus. Instead of accumulating in the soil, the leaves fall in the water, where they rot and provide food for microbes and planktons. This again is excellent fish fodder and the areas near mangroves thus often are very important for the fisheries.
Mangroves have proven to be an important source of food and materials for many coastal people. Crabs, clams, oysters, fish and other food are often collected there. Even the mangrove fruits are sometimes eaten.
Also the trees in themselves are useful. Mangrove wood is often collected as firewood, and it can also be used for constructing. The bark has tannin, which has craft and medicinal uses. If properly managed, mangroves can provide timber for construction, charcoal for energy, food for livestock, shellfish for local consumption, and so on. In fact, the natural resource base is that rich, that a Fijian cost benefit analysis analysing a possible conversion of mangroves into agricultural land concluded that this would not increase revenues produced per hectare per annum. Most conversions actually would reduce revenues drastically.
One very important environmental service produces by the mangroves is that they also build land or keep it from being washed away. Mud and sediment are generally washed down rivers and streams. When there is a mangrove swamp at the river's mouth, the water spreads out into the mangroves, and the sediment settles to the bottom where it is trapped by the mangrove roots. As the bottom gets shallower, the mangroves can grow further out, while those on the inside eventually find themselves on dry land, where they are replaced by ordinary land plants.
In this way the mangrove forest advances slowly outward, leaving dry land behind. Even in areas where there are not arriving enough sediments from the rivers to build new land, the mangroves protect the shoreline from being washed away in storms. The roots and trunks break the force of the waves, and the leaves and branches reduce the effects of the wind and rain. There are examples of islands which were built by mangroves, and then were washed away when the mangroves were cut.
Even in the city, mangroves can be important when the city wastes run off and pollute the nearby coastal waters. When these wastes run into a mangrove swamp, they normally are absorbed and used by the plants and animals in the swamp. The swamp filters the water, making use of the nutrients and also absorbing toxics and leaving clean water. As long as cities do not produce too much waste for the mangroves, and the waste does not contain too much toxic from industries, the mangroves are an excellent waste treatment system, and much cheaper than any sewage treatment plant. Mangroves, however, are sensitive to pollution, particularly oil pollution. Too much toxic waste will kill the forest.
Lately, also the value of the mangroves for tourism has been discovered. Senegal in particular has known to appreciate this resource. Two important national parks are based on the mangrove resource - Parc National du Delta du Saloum and Parc National de la Basse Casamance. The Casamance park is located close to the tourist centre Cap Skirring and with its over 200 species of sea birds, it is a popular resort for photo safaris and bird watching excursions.
For Western African countries, poor of lions and elephants, this is a major resource when it comes to eco-tourism. Even in classic "safari countries" such as Kenya and Tanzania, coastal tourism is becoming of great importance, and the coastal national parks are based upon the mangrove and/or coral reef resources.
Several other African nations are in the beginning of a tourism exploitation of their mangrove forests. Guinea-Bissau - the nation geographically most dominated by mangroves - hopes to include its remarkable coastline on UNESCO's World Heritage list and thus market itself as a tourist destination. Sierra Leone and Liberia hope to revive their tourism industry, mainly focusing on their coastline of beaches and mangroves. Gabon during the last years has protected much of its unique mangrove coast and aims at becoming Africa's main ecotourism nation.
Threats to the mangroves and conservation effortsToday, mangrove forests are among the most threatened habitats in the world - disappearing at an accelerating rate, yet with little public notice. Many threats occur. Lenticels in the exposed portions of mangrove roots are highly susceptible to clogging by crude oil and other pollutants, attacks by parasites, and prolonged flooding from artificial dikes or causeways. Over time, environmental stress can kill large numbers of mangrove trees. In addition, the charcoal and timber industries have also severely impacted mangrove forests, as well as tourism and other coastal developments.
But the rapidly expanding shrimp aquaculture industry poses the gravest threat to the world's remaining mangroves. Literally thousands of hectares of lush mangrove forests have been cleared to make room for the artificial shrimp ponds of this boom and bust industry. This highly volatile enterprise has grown exponentially over the last 20 years, leaving devastating forest ruins in its wake.
Mangroves in most countries do not enjoy any special protection. On the contrary: Until recently, mangrove forests have been classified by many governments and industries alike as "wastelands", or useless swamps.
This erroneous designation has made it easier to exploit mangrove forests as cheap and unprotected sources of land and water for shrimp farming. Environmentalists say the amount of mangrove forest destruction is "alarming". Globally, as much as 50 percent of mangrove destruction in recent years has been due to clear cutting for shrimp farms. Destructions have been greatest in Asia and Latin America, but coastal developments have taken their share of the mangroves in Central and Eastern Africa as well, and to a lesser degree in Western Africa.
But also the fragility of the mangrove ecosystem leads to its reduction. In the Gulf of Guinea, oil spills and occasional catastrophic oil blowouts have released thousands of barrels of oil into the sea. Though the exact total discharge is unknown, the quantities spilled so far have created problems ranging from the contamination of beaches and physical infrastructure of ports, destruction of sea birds, to the killing and contamination of marine life resources such as mangroves.
A new threat to the mangroves has risen from global warming and a rising sea level. Global warming in itself might influence the temperature, precipitation - and thus river flow - and carbon dioxide level - and thus the photosynthesis. In the tropics, it is assessed that the mangrove biotope will be the hardest affected by global warming. This is especially due to less rainfall and sweet water supply from rivers.
On a longer term, however, the largest problem is a rising sea level - also due to global warming. This may have the most disastrous effect on the mangroves, as they are a coastal biotope. Their survival will depend on their ability to move inland as sea level rises and if there is land available for the mangroves further inland. Mostly, the belt directly behind the mangrove is one of the most intensively used land recourses in the humid tropics.
Fortunately a mangrove forest can often be replanted if it is damaged, just like a forest on land, assuming that the environmental conditions are still satisfactory. Where temporary damage at a construction site cannot be avoided, at least the trees can be replaced afterwards. Some environmentalists say it should be possible to require a developer who destroys part of a mangrove swamp to replace it with an equal area somewhere else, so that the total area of mangroves does not change. However, it is much easier to keep the mangrove that already exists than to try to replace it once it has been lost.
Conservation efforts are now finally targeting the mangrove environ as its true value is getting more known. Several African governments and environmentalist NGOs are by now implementing conservation and management efforts in limited areas. Efforts to protect mangrove and other wetland areas have also recently been introduced by the UN through its environmental agencies UNEP and FAO.
Indeed, the need to protect the world's mangroves was discovered much earlier. In 1971, a convention to protect "Wetlands of International Importance" was adopted in the Iranian city of Ramsar. This was unusual because it focused on specific wetland sites that were considered to be of importance especially as waterfowl habitat. To become a signatory to the Ramsar Convention, a country had to designate at least one such site and guarantee its protection.
By now, around 100 countries have become signatories to the treaty, which is proving to become a major salvation to the world's mangroves. Some 850 "Ramsar sites" have been designated by these countries covering over 53 million hectares. About a third of these contain mangroves, so Ramsar and its partners have embarked on the protection and wise use of over 15 million hectares of mangrove wetland. More mangroves are sure to be registered Ramsar sites in the near future.
Meanwhile, the battle over the mangroves is mostly fought locally. In some cases, the tourism potential of these coastal forests is understood and destruction is halted. In other cases, local populations are making use of the mangroves' rich fisheries resources for nutrition, their firewood for cooking, their wood for constructions and the lands stabilised by them as home. Their fight to protect their livelihood against authorities or companies - eager to use the mangroves for other developments - is going on daily, but seldom reported about and mostly lost.
Sources to this feature article include UNEP, Ramsar, FAO, afrol's archives and more
By staff writers
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