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Mangroves – masters of survival on salty ground

Nowhere else do sea and land organisms share the same habitat to such a great extent as in the mangrove swamps of the tropical and subtropical coasts. The biotic communities of the mangroves are unique. Here, true terrestrial organisms colonise the upper storeys of the tree and shrub layer, while true marine organisms live underneath them.

 

Mangroves have extended buttress roots which slow the tidal flow and promote the deposition of mud and silt.

Mangroves are trees and shrubs from different plant families, up to 30 metres in height. There are almost 70 species which are superbly adapted for life in the conditions found on salty shores and in brackish river estuaries. It is species diversity which differentiates the mangrove communities of the eastern hemisphere (including the Indian Ocean and the western Pacific) from the western hemisphere (including the Caribbean and the west coasts of America and Africa). The Indo-Pacific group, which generally forms denser and taller stands, is more species-rich overall.

 

Filters to combat sea salt

In order to be able to absorb any water at all from the salty brine, their plant cells maintain a very high osmotic pressure. In other words, the salt concentration inside the cell is higher than that of seawater. A complicated ultrafiltration mechanism in the mangrove roots allows the diffusion of water towards the higher salt concentration within the cells, but prevents any intake of salt.

 

Like cacti, some mangrove plants can also store water (salt succulence) in order to dilute high salt concentrations. In addition, they can shed leaves impregnated with salt, and some have salt glands and salt hairs for the purposes of excreting excess salt. Since the different species have varying degrees of success in coping with the excessive salt levels, moving inland the progressively higher concentrations of salt influence the distribution of species in the mangrove forest.

 

Variation of salt concentration influences the distribution of mangrove species in the mangrove forest (above: Africa, below: South-Australia).

Mangroves need calm tidal coasts, and like coral reefs they are important 'land builders', directly involved in creating land, protecting it from erosion and extending the coasts. Their distribution is mainly restricted to the tropics between the 30th parallels north and south of the equator; but mangroves also develop in suitable locations in Japan, the Bermudas, in southern Australia and New Zealand.

 

The ideal conditions are found in sites sheltered from wave action with a regular supply of water. In addition to the salty sea tides, particularly in periods of high rainfall, the evergreen forest can be flooded with rainwater which carries additional nutrients and sediments.

 

The fruits of the mangroves germinate on the tree before they fall into the water.
Atlantic horseshoe crabs (Limulus polyphemus) congregate in the mangroves in their thousands for the mating and spawning season every spring.
The pink flamingo (Phoenicopterus ruber ruber) feeds on micro-organisms which are filtered through a complex mechanism in its long beak.

Specialised root system

All mangrove fruits and offshoots can float, and have developed different strategies to ensure successful reproduction. The fruit germinates while still attached to the tree. There it grows into a cigar-shaped seedling of considerable size, with the first roots and leaves already developed before it falls into the water. It drifts until it lands in a suitable spot, where it takes root.

 

A characteristic feature of mangrove swamps, however, is the dense mat of buttress, stilt and knee roots. While providing a habitat for numerous marine life forms, these also anchor the plants in fine, loose soils and on hard substrates alike. In among them, countless spiky suckers project upwards from the silt. Because there is no oxygen in the sludgy soil beyond the top few millimetres, these shoots contain vital respiratory organs called pneumatophores. Without them the roots would die off.

 

Habitat diversity in a confined space

Compared to the unvegetated tidal flats, the dense mangrove root systems multiply the available space for other organisms, offering them a large number of microhabitats in a confined space. Countless fish, crustaceans and bivalves populate the water. The roots of the trees are colonised by algae, barnacles, oysters, sponges and molluscs. In the free-flowing channels, pistol shrimps and fish abound. Large numbers of fiddler crabs are found on the silt surfaces.

 

The upper storeys of the mangrove forest overhead are home to reptiles, birds and mammals. Sea cows head for the sheltered mangroves to calve, and monkeys venture onto the shore to catch crabs. Numerous water birds including cormorants, kingfishers, ibises, herons and frigate birds take advantage of the rich pickings, and nest in the treetops.

 

Mangroves, together with coral reefs and tropical rainforests, are the most productive ecosystems on earth. Their falling leaves, flowers and fruits supply more than three kilograms of organic matter per square metre per year, to be decomposed by bacteria and fungi and returned to the food chain. Small fish, shrimps and invertebrates feed on this detritus, which is enriched with micro-organisms, and they in turn become prey for other organisms.

 

Suspended matter from the mangroves is washed out to sea with the tides, providing nearby coral reefs and sea grass meadows with organic material. Meanwhile the sheltered waters between the roots provide ideal conditions for the larvae and young of numerous fish species.

 

Highly endangered

An estimated 50 percent of the mangrove forests that once existed worldwide have been destroyed in recent decades. Traditionally, mangrove wood has been cut for use as fuel, charcoal or tanning materials. But the removal of comparatively small quantities of wood by coastal populations has never threatened the integrity of the mangroves.

 

Only large-scale destruction, resulting from conversion into rice and coconut palm plantations or even construction land, following drainage, has made the situation critically acute. Above all, the establishment of breeding units by shrimp farmers has contributed substantially to the declining area of mangrove in all parts of the world. For instance, in Ecuador and the Philippines, the Shrimp Aquaculture Industryand its unrestrained expansion to date has been responsible for deforesting 70% of mangrove forests in those regions. The use of an area for shrimp breeding is problematic because after three, to a maximum of ten, years' use, shrimp ponds have to be abandoned due to contamination of the pond bottoms with chemicals. Reforestation is usually impossible for decades afterwards.

 

 

Land Period Former Area [ha] Current Area [ha] Lost Area[%]
Cuba 1969 - 1989 476,000 448,000 6
Bangladesh 1963 - 1990 685,000 587,000 14
Thailand 1961 - 1993 300,000 219,200 27
Vietnam 1969 - 1990 425,000 286,400 33
USA 1958 - 1983 260,000 175,000 33
Indonesia 1969 - 1986 4,220,000 2,176,000 48
Philippines 1968 - 1995 44,000 140,000 69
Puerto Rico 1930 - 1985 26,300 3,000 89
Kerala (India) 1911 - 1989 70,000 250 96

 

 

Brown herons make the most of the abundance of fish in the waters.

Although the importance of mangrove swamps for the marine as well as the terrestrial ecosystem is well known, its destruction is continuing almost without relent, and mangrove forests are in danger of disappearing altogether from the coasts in the next twenty years. However, the close interdependency of the mangrove ecosystem and the marine environment will only be realised when these networks are disrupted and the impacts of destruction become tangible.

 

Wherever mangrove forests have been cleared on a large scale, the yields of coastal fisheries have drastically fallen. The reason is that many economically important fish species reproduce in the mangroves, and the loss of these refuges removes a life-supporting resource, not just for these fish populations but also for the coastal population. When the forest is felled, a natural protective belt is lost, exposing the adjacent coastal region to storms, tidal waves, flooding and erosion.

 

Against a backdrop of evident degradation, varying degrees of effort are being made in many regions to re-establish the mangrove forests. But it is difficult and time-consuming to restore them to a near-natural condition. The older the trees, the more they are covered in orchids, ferns, tendrils and mosses. Only when the intricate root network is fully formed can it actually fulfil its filtering and protective functions properly.

 

 

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