Mangroves are unusual trees that can live in salt water from the oceans. Most land plants are killed by salt, but mangroves are able to get rid of the salt. Most plants die if their roots are drowned in water and have no oxygen, and in the mud of mangrove swamps, the rotting leaves usually use up all the oxygen. However mangrove trees have developed special kinds of roots that stick up out of the mud into the air to get oxygen. As a result, mangrove forests (or at least a fringe of mangroves) are common along the coasts of many tropical countries and islands wherever the shore is soft and muddy enough for them to take root.

In the Pacific, the number of kinds of mangrove trees is greatest in the west, and gets smaller going to the east across the Pacific until the mangroves disappear. Mangrove forests in the western Pacific and Indian Ocean are bigger and richer, with different kinds of mangroves growing under different conditions. Mangroves are also very important in the Caribbean. Some mangroves prefer water that is not as salty as seawater, and they are thus more common at the mouths of rivers where fresh-water and salt-water mix.

Usefulness of mangroves

Most people 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 in their special conditions, and, like the tropical forest, they produce a lot of leaves and other organic matter. Instead of accumulating in the soil, the leaves fall in the water, where they rot and provide food for microbes and many tiny animals. This rich food is not only eaten in the mangrove swamp, but much of it may be carried out into the lagoon or to coral reefs and other coastal fisheries areas, where it helps to feed the fish. The areas near mangroves are thus often very important for fishing.

Because there is lots of food in mangrove areas, and good shelter among the mangrove roots, some kinds of fish come there to reproduce, and many baby fish grow up there before going to live in the lagoon or on the reef. The mangroves are a critical habitat upon which these species depend for survival, and if something happens to the mangroves, the future of these kinds of fish will be affected.

Mangroves also build land or keep it from being washed away, which can be very important on islands and coasts where land is limited. Mud and sediment are often washed down rivers and streams. When there is a mangrove swamp at the river 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 land plants. In this way the mangrove forest advances slowly outward, leaving land behind. Even in areas where there is not enough sediment 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 washed away when the mangroves were cut and removed.

Mangroves are 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. Mangrove wood is often collected as firewood, and it can also be used for building. The bark has tannin which has craft and medicinal uses.

Even in the city, mangroves can be important. The city wastes run off and pollute the nearby coastal waters. When the wastes from all the people run into a mangrove swamp, they can be taken up and used by the plants and animals in the swamp. In a way the swamp filters the water, leaving clean water to go out the other side. As long as there is not too much waste for the mangroves, and no poisonous wastes from industries, the mangroves are an excellent waste treatment system, and much cheaper than a sewage treatment plant.

Threats to mangroves

Unfortunately in spite of their usefulness, mangroves are being destroyed in many places. Sometimes they are drained as a sanitary measure, although mosquitoes, for instance, do not like the salty water of most mangrove swamps. Often mangrove areas are used to dump rubbish or garbage. In places where unoccupied land is in short supply, mangroves are often cleared to make agricultural land, or filled in for construction. Large areas have been lost to development in this way. However, such low-lying land may be vulnerable to flooding in storms, so the development is not always so successful.

In some places mangroves are cleared to make aquaculture ponds for raising fish or shrimp. Ponds may also be built to treat the wastes from cities, towns or factories. Other kinds of construction can also damage mangroves. Anything that changes the way water circulates or its saltiness can kill the affected mangroves. Taking water from rivers for irrigation can reduce the amount of fresh-water available to mix with the salt-water. Frequently a causeway for a road, or some other construction project, may keep the sea-water from coming into the mangroves. As the sea-water is replaced by fresh-water, the mangroves will die.

Mangroves are also sensitive to pollution, particularly oil pollution. If an oil spill goes into a mangrove area, the oil covers the aerial roots, and the tree roots can no longer get the air they need to live. The roots will die, and with them the whole forest. Mangroves are also very sensitive to herbicides.

With all these different threats, and the fact that few people appreciate how important the mangroves really are, it is no wonder that the area of mangroves is getting smaller. The steady reduction in mangroves means the loss of an important resource. As each little bit is taken, the remaining natural area becomes that much more important for such things as fish breeding and nursery areas. On some islands, only tiny areas of mangroves remain. Their loss could be a tragedy for coastal fisheries.

Mangrove management

In coastal areas and islands where many different needs must be fit together, the careful management of mangroves is important. If there are large areas of mangrove forest, then some parts can probably be developed, allowing for a balance of uses. Special attention needs to be paid to the percentage of the total area developed, and to avoiding critical breeding habitats and other areas of particular interest. Where only small areas of mangrove remain, they probably should be protected.

Every effort should be made to avoid changes in salinity or water circulation in mangrove swamps. If a road needs to be built through a swamp, enough bridges or culverts should be provided to allow water movement into and out of the swamp. In areas where sedimentation is important, the mangroves should be allowed to go ahead with their stabilizing and protective role. Similarly mangroves should be strictly protected wherever they are important in controlling coastal erosion.

Because of their vulnerability to oil pollution, mangroves should receive special attention for protective measures in oil spill contingency plans. Oil loading and storage facilities should not be located near mangrove areas.

Not all uses of mangroves will go together. A forest that is heavily cut for firewood will not produce as much food for the lagoon and reef. The mangroves areas in a city that are used for waste treatment should probably be closed to fishing to avoid the danger that shellfish and other seafood from the area might pick up and spread diseases.

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 good. Where temporary damage at a construction site cannot be avoided, at least the trees can be replaced afterwards. It is even 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 is already there than to try to replace it once it has been lost.

QUESTIONS

Are there mangroves in your area? What are they like?

What is unusual about a mangrove forest?

How can mangrove roots live in the swamp mud where there is no oxygen?

Why do many people think mangroves are bad? Is this true?

Why are mangroves important for baby fish?

How do mangroves build or protect the land?

What are other ways that mangroves are important?

What are the most important uses of mangroves in your area?

What are the threats to mangroves?

Have mangrove areas been lost where you live? Why?

How much of the original area of mangrove still remains on your coast?

Is anything now being done to manage your mangroves?

What do you think needs to be done so that the mangroves will meet all of your requirements?

Do you know of places where it might be good to replant mangroves?

Instructions for trainers in the use of this unit