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The term mangrove refers to a diverse group of salt-tolerant trees
and other plant species that are found along sheltered tropical
and subtropical shores and estuaries. Mangrove wetlands are
home to many rare animals and plants, but also have wider ecological
and economic importance, and provide numerous services to
Different mangrove tree species have evolved different adaptation to exclude excess salt from the water.
Osmosis – Why is salt harmful?
Much of the salt may be excluded by the roots prior to water uptake but some salt often enters the plants and could potentially cause damage if not taken care of. Rhizophora sp. can transport salt ions to old leaves. The old leaves, that look yellow in colour, thus serve as salt storage compartments and are eventually shed by the tree.
So do mangroves only live in salt water?
Mangroves actually do not need the salt water at all to survive but are relatively poor competitors against other plants that occupy the zones farther up the coastline. Their special adaptations to survive in salt water allow these plants to live in a habitat only a few species of flowering plants can. Tests have been done with mangrove seeds and it has been found that they grow even faster and taller in fresh water compared to salt water if competition with other species is reduced. Around the world, mangrove communities are often found in river deltas where salty oceanic water meets freshwater from river output. Great tidal differences create strong mixing of water and often result in very turbid water with low visibility.
Where are mangroves found?
Mangrove communities are only found in tropical or sub-tropical areas of the world. To survive they need water temperatures above 24ºC in the warmest month. Mangrove communities also require annual rainfall that exceeds 1250mm, which is often the case in the tropics. These requirements are very similar to the needs of tropical coral reefs.
The ability of these trees to live in these rather exposed environments is aided by a number of mechanisms. Highly elaborate root systems give the trees support and prevent them from falling over as waves and tides exert their forces on the coast-line.Rhizophora mangle, a mangrove tree species generally found farthest out in the water, have aerial roots that look like a network of stilts standing out of the water. Rhizophora sp. is often what people first think of when talking about mangrove trees. The stilt roots provide extremely good support against physical environmental impact. Aerial roots are found amongst most mangrove tree species although they may be of different appearance. In Mexico the species Avicennia germinans is predominantly found inside the outer zone of Rhizophora. Avicennia has another distinct rootsystem that looks like pencils sticking out of the sediment. These may get exposed during low tide.
A very important function of these aerial roots besides giving the plant support is oxygen uptake for root respiration since the roots cannot receive much oxygen in the sediment.
Mangrove species reproduce by producing flowers and rely on pollination by bees and insects. After pollination, the seed remains on the parent tree. It germinates and grows roots before dislodging. It then has a better chance of establishing itself in the sediment before getting swept away by outgoing tides. Many mangrove species release their seed inside a capsule. The capsule floats and is deposited in a location where it germinates and sends out its shoots and then becomes a new mangrove tree.
An additional positive but indirect effect of this dense network of roots is that they also bind sediment. Tropical areas of the world often have seasonally heavy rainfall. The rain flushes large amounts of sediment out to sea. Sedimentation is one of the major threats to the coral reefs of the world. The symbiotic zooxanthellae algae, providing sometimes almost all the energy needed by the coral polyp, is dependent on sunlight for photosynthesis. Increased turbidity decreases the depth that sunlight can penetrate and can thus lead to coral death. Sediment can also cover the corals directly and kill it. Since both mangroves and coral reefs are generally found in the same areas o the world, the mangroves serve as a sieve that retains much of the sediment and thus prevents it to get flushed out to the coral reefs.
The sediment around mangrove areas is generally composed of very fine silt or mud particles. These densely packed particles make the sediment very oxygen poor. The only organisms that can survive and thrive in this oxygen free environment are bacteria. The bacteria can reduce sulphur instead of oxygen for respiration giving these environments a very distinct and strong smell of sulphur (rotten eggs). The bacteria on the other hand provide an important first step in the mangrove community food chain. Bacteria are decomposers that break down and convert all dead organic material into useful nutrients that are released into the water. These can ,in turn, be utilised by other organisms in the surrounding water. So even though the sediment in mangrove communities itself does not provide a good habitat for much marine life, the surrounding water can sustain an enormous diversity of organisms.
Benifits of Mangroves
Mangroves prevent coastal erosion, and act as a barrier against
typhoons, cyclones, hurricanes, and tsunamis, helping to minimize damage
done to property and lif. Mangrove tree species that inhabit
lower tidal zones can block or buffer wave action with their stems, which
can measure 30m high and several metres in circumference. The trees
both shield the land from wind and trap sediment in their roots, maintaining
a shallow slope on the seabed that absorbs the energy of tidal
Mangrove forests store and process huge amounts of organic matter,
dissolved nutrients, pesticides and other pollutants that are dumped into
them by human activities, and by absorbing excess nitrates and phosphates
prevent the contamination of coastal waters. In so doing, they
play a vital role in protecting coral reefs and seagrasses from siltation and
eutrophication. Mangroves also function as a sink for atmospheric carbon
dioxide, a major contributor to global warming.
Although they are not particularly species rich, mangrove ecosystems
are important nursery areas and habitats for commercially valuable
shrimp, shellfish, and fish species. Globally, nearly two thirds of all
fish harvested depend on the health of wetlands, such as mangroves,
seagrasses and coral reefs for various stages in their life cycle. An
authoritative study carried out by an international group of scientists in
the Caribbean, found that mangroves play a vital role in nurturing and
protecting juvenile coral reef fish. Coral reefs were found to have more
than twice as many snappers (Lutjanus apodus) and grunts (Haemulon
sciurus) where healthy mangrove forests were found nearby. However,
the destruction of mangroves may have caused local extinction of one
of the largest herbivorous fish in the Atlantic – the rainbow parrotfish
(Scarus guacamaia). The researchers concluded that if the current rate of
mangrove deforestation continues there are likely to be serious impacts
on ecosystems and the productivity of fisheries.
People derive many harvestable benefits from mangrove forests:
wood for fuel, furniture and construction, a source for charcoal, tannin,
paper, dyes and chemicals, thatch, honey and incense. The foliage
of mangrove species is used to feed livestock, and several mangrove
plants are used for traditional medicine. The annual economic value of mangrove habitats is estimated at £40,000 - £140,000 per acre.
Threats from Global Warming
Rising sea levels and changing salinity pose the most serious threats to these ecosystems. Where mangroves are sheltered by coral reefs killed by global warming, damage to mangroves from increased wave action is expected to rise.
Loss of mangroves will have a serious economic impact on both fisheries and coastal communities. In developing countries, mangroves have proven critical for saving human lives by their dampening of the wave heights and wind speeds during coastal storms.
3.6 million hectares destroyed
The world has lost around 3.6 million hectares of mangroves since 1980, equivalent to a 20 percent loss of total mangrove area, according to FAO's recent mangrove assessment study, entitled, ‘The world's mangroves 1980-2005.’
The total mangrove area has declined from 18.8 million hectares in 1980 to 15.2 million hectares in 2005, according to the report. Since 2000, there has been a slowdown in the rate of mangrove loss, the report shows, reflecting an increased awareness of the value of mangrove ecosystems.
Areas of major loss of mangroves
- Asia has suffered the largest net loss of mangroves since 1980, with more than 1.9 million hectares destroyed, mainly due to changes in land use.
- North and Central America lost about 690,000 hectares and Africa lost 510,000 hectares over the last 25 years.
- At the country level, Indonesia, Mexico, Pakistan, Papua New Guinea and Panama recorded the largest losses of mangroves during the 1980s. A total of some one million hectares were lost in these five countries - a land area comparable to the island nation of Jamaica.
- In the 1990s, Pakistan and Panama succeeded in reducing their rate of mangrove loss.
- But Vietnam, Malaysia and Madagascar increased their mangrove clearing and became three of the five countries that lost the most mangroves since 1990.
On a positive note, a number of countries have had an increase in mangrove area over time, including Bangladesh. Part of the largest mangrove area in the world, the Sundarbans Reserved Forest in Bangladesh, is well protected and no major changes in the extent of the area have occurred during the last few decades, although some damage to the mangroves was reported after the recent cyclone in 2007.
In Ecuador, the abandoning of ponds and structures for shrimp and salt production led to a rebuilding of various mangrove sites.
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