Posted on 02 December 2004
Mangroves play a crucial role in fish nurseries and protecting tropical coastal ecosystems. Were it not for these salty forests, many of the world’s coral reefs and their communities of rainbow colored, protein-rich fish simply would not exist.
It is mid-day, humid, and stiflingly hot. The cicadas’ high-pitched song creates a sound curtain blocking out all other noise. Hundreds of spiky roots point up through the grey mud like a bed of nails. Roots weave in and out, creating a complicated maze at knee height. The mangrove forest is virtually impenetrable and hiking here is not exactly easy. But, it’s full of life.
Everywhere you step, fiddler crabs are waving their large blue and red claws before scurrying along to hide as one ventures too close. Later, as the rising tide laps in once again, the marine realm takes over. Sunbaked oysters open up to feed from the nutrient-rich water. Fish and prawns zigzag through the roots to feed and hide from predators roaming the adjacent coral reefs and open waters.
Mangroves play a crucial part in coastal tropical biodiversity. First and foremost, they act as a nursery for many species that live in and around coral reefs. The inter-weaving underwater roots in a mangrove forest create a multitude of niches where great numbers of fish, crustaceans, and turtles find shelter and breed out of reach of voracious predators found out on the reef.
A study on the Mesoamerican reef has revealed that there are as many as 25 times more fish of some species on reefs close to mangrove areas than in areas where mangroves have been cut down. This has an important effect for both the marine world and the people who rely on its resources.
”Mangroves play a vital role in coastal fisheries," says Dr Ghislaine Llewellyn, co-author of the study published in the February issue of Nature
magazine. "They are incredibly important for maintaining fish diversity and abundance in the tropics.”
Acting as a buffer zone between land and sea, mangroves also help create the right conditions for coral reef growth by filtering sediments and pollutants that would otherwise choke or poison the coral. Were it not for mangroves, coral reefs would be even more ’stressed’ than they already are from the variety of others pressures surrounding them. Mangroves perform a similar protective function for the coastline, saving millions of dollars each year in coastal erosion damage.
Despite these multiple values, mangroves are disappearing at an alarming rate. Once abundant along tropical tidal coasts and estuaries worldwide, many mangrove forests have disappeared or are degraded today.
”The current rate of mangrove deforestation has implications not only for coral reefs and mangrove forests, but also for biodiversity, fisheries, and livelihoods in the entire region,” says Dr Melanie McField, WWF’s coral reef scientist on the Mesoamerican reef.
”Coral reefs are incredibly valuable ecosystems and provide nearly US$30 billion in net benefits to the world economy yearly, in tourism, fisheries, and coastal protection. Mangroves are the guardians of this wealth.”
Mangroves were long considered wastelands and their destruction often encouraged by governments and planners. As coastal populations have soared across the tropics, the combination of deforestation for subsistence and commercial uses (e.g cooking fuel, construction, wood chips for paper production, coastal development for aquaculture, and/or toursim), together with land- and sea-based pressures (e.g. increasing sediments from inland agriculture, oil spills, industrial pollution, and sewage) now exert enormous pressure on many of these invaluable ecosystems.
The Philippines, for example, has lost 75 perc ent of its mangroves since the 1950s according to scientists. In parts of Central America, the rate of destruction is equally alarming. Here, mangroves are cut faster than the tropical rainforest, then filled and used as new land for construction. The rapidly expanding tourism industry is also driving much of the destruction, paving the way for resorts, marinas, and golf courses. In Belize City, a system of mangrove-lined ponds and mangrove-wetland drainage areas have served as a natural sewage treatment facility for much of the city’s waste water. Recently, dredging for a massive port expansion resulted in the destruction of more mangroves and the free ecosystem services they provided.
Even your choice of dinner may have an implication for mangroves. A luxury nibble like giant tiger prawns now feature prominently on menus in Europe and North America. But tiger prawns often come from intensive farming in clear-cut mangrove areas. Nearly 40 per cent of all mangrove loss world-wide is attributed to prawn farming. The single largest cause of mangrove loss in Vietnam is conversion to prawn farms.
But, prawn farming is not all bad news for mangroves. In Belize, prawn farms are generally constructed inland from the coastal mangroves, which are seen to be valuable as they provide some natural buffering and cleansing capacity for the farms’ effluent waters.
”WWF is working with prawn farmers in Belize to develop guidelines which would enable the production of eco-certified prawns, hopefully encouraging the adoption of more environmentally sensitive aquaculture practices in other locations too,” says Dr McField.
WWF has dozens of similar projects under way around the world to ensure that these cradles of marine life continue to rock.
As understanding of how tropical coastal ecosystems are functionally linked grows, conservation efforts will increasingly aim to protect corridors containing mangroves, seagrass beds, and coral reefs. Data shows time and again how networks of marine protected areas, including these three habitats, boost fish abundance on reefs, benefit local fishing communities, and contribute to sustainable livelihoods.
”Given the ever-increasing range and severity of disturbances to coral reefs, and their importance in tropical economies, every natural source of ecosystem production and resilience should be conserved. As such, mangroves also need urgent protection and wise management,” Dr McField adds.
As the tide rolls out once again through the maze of roots in the mangrove forest, juvenile fish make for shelter, oysters close their shells, and the fiddler crabs take up their positions on stems and roots. The cradle still rocks, for some time still. But it needs a helping hand. Notes:
• Mangrove forests exist in coastal areas throughout the tropics. The mangroves' stilt roots allow the trees to get a foothold as waves run high on the sea outside, and aeration roots enable the root system to breathe as the rest of their root system is stuck in mud devoid of oxygen, while their leaves can excrete excess salt.
• Mangroves are also an important economic benefit to coastal communities. In most areas with extensive mangrove growth, people have found many ways of using their timber, leaves, and flowers. Mangrove wood is traded and used in construction and boat making.
• Fish traps made from mangrove wood are more resistant to rot, and beekeepers use certain species of mangrove wood for their hives. Mangrove wood is also used for making charcoal and lime. Some mangrove species flower all year around and beekeepers therefore place their beehives in the mangroves, ensuring constant supply of honey.
• Certain mangrove species are used as contraceptives and aphrodisiacs, others for dye and insecticides. The young leaves are sometimes given to cattle as fodder.
For further information:
Global Marine Programme