Mangroves are a collection of salt-tolerant evergreen trees that live in tropical and sub-tropical coastal environments and line approximately 8% of the world’s coastlines. They are unique because they occupy both land and water and are sometimes referred to as ‘floating forests’. This unique aspect of mangroves that enable them to ‘float’ is due to their aerial roots that develop in fine muds or sandy sediments. These roots form a dense tangled network below the water surface providing a home and shelter for a diverse number of species. They also prop the tree up, hence the term ‘prop roots’ and take in oxygen at low tide.
There are over 50 species of trees and shrubs that are classified as ‘true’ mangroves. Each of these species have adapted to the salty water conditions they grow in. Most mangrove shorelines are made up of two or three zones, each dominated by different mangrove species. The largest mangrove coverage is in the America’s, where just four species exist. These consist of the red, black, white and button mangroves. Red mangroves grow nearest the shoreline as they have a particularly high salt-tolerance with black mangroves growing further landward in more swamp-like conditions. Some species of black mangroves develop pencil-like tubes called pneumatophores to aid breathing in the rich oxygen-free sediment they form in. White and button mangroves grow further inland in drier conditions and are only in contact with seawater at high tide.
Over half the worlds mangrove forests have been destroyed over the last 30- 40 years to make way for commercial enterprises such as aquaculture (mainly shrimp farming), agriculture and coastal development. Intensive shrimp farming has devastating environmental effects. Not only does the practice clear large areas of coastal habitat including mangrove forests but it also pollutes nearby coastal waters and marine habitats such as coral reefs with waste matter from the shrimp ponds.
Increased human settlement along our coastlines also leads to agricultural expansion. This is believed to be the most destructive human impact on mangrove forests due to the scale of the problem. Unregulated urban development increases pollution and alters the distribution and use of water and with increased tourism into tropical regions over recent decades; this is only compounding the problem.
Furthermore, overfishing is another (indirect) factor affecting mangrove communities. Many commercial fisheries are overfishing fish stocks and removing juvenile as well as adult fish from our seas. A large majority of juvenile fish would go onto spawn and use the mangrove habitat as a breeding ground and as a fish nursery. Mangroves support a complex community of species and fish play a vital part in this community as they consume large deposits of decomposed leaf, bark and twig litter produced by the mangrove trees. Small fish in turn attract and feed larger fish and this forms a healthy cycle that supports the community. The removal of fish in this cycle can lead to a major imbalance and could compromise the health of the forests.
The importance of mangroves and why we should care
Mangrove forests are important habitats for a number of reasons. Firstly, they are vital ecosystems that deliver a full range of what are referred to as ‘ecosystem services’. These services include provisional services, such as food, wood and fibre; regulating services such as flood regulation and water purification; supporting services such as nutrient cycling and finally cultural services, including recreational tourism and education.
Mangroves are also known as rich centres of biodiversity as they provide a home and shelter for many species including fish, birds, frogs, snakes, insects and several endangered crocodiles. Mammals also occupy these forests ranging from small animals like swamp rats and monkeys to large carnivores like tigers, that use the dense foliage as cover. Mangroves are also important nursery areas for many species of fish. Overfishing is a global problem and we are fishing at an accelerated rate without allowing fish stocks to recover; mangroves are therefore vital in providing breeding grounds for fish.
Along with protecting coastlines from erosion by acting as a natural barrier and flood defence, mangroves also filter pollutants from river run-offs and prevent a harmful build up of sedimentation from reaching the oceans and nearby marine habitats such as coral reefs. Mangroves and coral reefs have a symbiotic (mutual beneficial) relationship – the reef protects the coast where the mangroves grow from being eroded by the sea, and the forest traps sediment washed from the land preventing it reaching the reef. Both mangrove forests and coral reefs found in coastal areas provide protection and breeding grounds for fish – a key source of income and nutrition for people in these regions.
The solution: what we can do
Efforts to save mangroves from commercial development are becoming more popular as the benefits of mangroves become more widely known. In Thailand, community management volunteer programs have been effective in restoring damaged mangrove forests and in some areas across Western and North East Africa mangrove reforestation are also underway. However, establishing new mangrove plantations on coastal mudflats has not always been easy due to the lack of nutrients (nitrogen, phosphorus and iron) from inland water flow in some coastal regions where reforestation projects have been carried out. Ideally, the best solution is to protect existing mangrove forests and to restore damaged forests in regions where they naturally occur. Establishing marine reserves or Marine Protected Areas (MPA’s) in mangrove-lined coastlines are one of the most effective ways to prevent deforestation and urban development occurring. You can help by signing a petition to establish more marine reserves.
It must also be noted that mangroves grow in some of the poorest regions of the world where commercial enterprises such as shrimp farming and agriculture are in the short term more economically viable than protecting mangroves. Therefore, it’s vital that Conservation organizations and Governments work with local communities and provide incentives (possible employment) education and training through community-led projects that aim to highlight the benefits of mangroves in order for people to want to conserve them. Unfortunately, it’s often lack of funds that prevent projects such as these from succeeding; but there are many great projects urging people to get involved by actively volunteering or by asking for a small donation to support field projects. More information on a successful (and now completed) community-led reforestation project run by the United Nations Development Programme (UNDP) can be found here.
Mangroves are wonderfully diverse habitats that carry out their processes and functioning modestly ‘behind the scenes’ without a big fuss or drama. Many people do not really know or fully understand how important these forests are and are not even aware of the many natural benefits they provide; benefits we all take for granted. We often try and replicate these benefits, but as with all things in nature, no man-made system can ever be more effective at doing its job or replace the thing that naturally evolved to do it. Mangroves ‘do their job’ extremely well providing they are left alone to get on with it (through protection and necessary conservation). We must act now to conserve these incredible habitats by voting for more marine reserves and by educating people about the multi-benefits these floating forests provide us with. For more information about ways to make a difference and for ways to get involved, please click here.