4. Mar, 2016

Mangroves. The Forgotten Habitat

Mangrove habitats or mangals are woods along the shoreline that have adapted to the intertidal movement of saltwater.  In some mangrove swamps saltwater mixes with sweet water from rivers  diminishing the degree of salinity. Mangroves do not  enjoy a great popularity  among  tourists, for obvious reasons. The hot and humid air and the almost impenetrable swamps and numerous bugs are the major causes that they will not score high on a list of  pleasant holiday locations. Their low appreciation and the destroying of mangrove forests are expressed in the title of this article that I borrowed from Jeremy Stafford-Deitsch's wonderful book on Mangroves.* 

Mangroves are  also not the first choice of UW photographers, although the tidal zones offer interesting opportunities for close up or even macro shooting for the snorkelers. Small fish like gobies, blennies, cardinal fishes and  snappers are often hiding in the mangrove roots. Mangroves are also interesting for taking ‘over under shots’. For example a  school of snappers, perhaps even a caiman crocodillus  or a juvenile shark under the surface, and some lovely mangroves above the surface.  For people interested in topside pictures, wet suit booties, a pair of small binoculars, a telephoto lens for more distant objects like birds or mudskippers  are recommended.  And an insect repellent! Most easy entrance entrance is from the sea in a small boat. Which allows you to visit two worlds, the intertidal area and the forest itself, at least a part of it.

But from  the point of view of ecology and biodiversity mangrove forests are a true marvel. They are the habitats of numerous fish, rare birds, crabs, insects, plants and flowers. Birds find an enormous food reserve in the form of worms and small crabs in the mud floors. The mangroves form  a highly complex world,  and the exploration of  living species in the mangroves  is still in its infancy.  A mangrove forest is also a magic world with  their  human-like  trees, funny roots and the strange sounds echoing in the silence*. Perhaps  the mangroves whispering to each other, like Tolkiens trees in Fangorn forest? 

Richest mangrove forests are found on the eastern hemisphere along the coasts  of East Sumatra, North East Borneo, North East  Australia, Papua New Guinea, and in the Ganges Delta along the gulf of Bengal.  But there are also numerous mangroves on the Western hemisphere in the Caribbean, Bahamas (on Bimini mangroves are found in a fringed lagoon**) and the Americas.  The variety of mangroves is enormous: different  genera are  connected with different tidal zones (see picture above). Red mangroves are found mostly at  the seaward edge followed in succession by black, white and buttonwood  mangroves  more land inward. In each tidal zone  the mangroves have adapted to the environment dependent on  the amount of immersion during high and low tide.  Some mangroves are fully inundated by all high tides, others partly inundated  and still others only  by exceptional high tides.

Mangrove seeds (also called propagules)  look  like elongated  beans. They   are buoyant and therefore suited to water dispersal. They grow on the parent mangroves until they are about 3 cm long and then  drop  they into the water  when the tide is in.  The seeds are viviparous:   they germinate while they are still on the parent tree. that will often carry whole bunches of these seeds.

Mangroves can survive because they have learned to filter out much of the salt even when it is absorbed in their leaves or roots. Especially their roots are crucial in surviving in the salty and anaerobic mud floors of the mangrove swamps. They  allow mangroves to absorb gases directly from the atmosphere. For example,  mangroves of the genus Avicennia (mostly black mangroves) that are found on higher grounds have developed aerial roots looking like thin pencils, often surrounding the base of the tree  The same holds for the  Sonnerata genus  which  root resembles a peg or  snorkel sticking up. The Brugulera genus has strange roots  looking like knobbing knees.  Most conspicuous perhaps are the red mangroves (or Rhizophora) that have developed spreaded legs (also called  prop roots or stilts) that stabilizes the tree (see picture above taken on Bimini).  It is said that aboriginals of the Northern Australian coast believe that  the prop rooted mangroves once walked from the sea to the shore. Some  Rhizophora trees  have also developed cascades or ‘curtains’  of aerial roots hanging from the tree tops.

Mangrove forests are in danger. One factor is big tourism and clearance of mangrove forest for recreational parks en jetties for cruise ships. And tourism inevitably  brings in rubbish and plastic drifting everywhere. Oil  spill from off shore  companies,  industries and  tankers flushing out their tanks in sea are devastating for mangrove habitats.  And then there is the clearing of mangrove forests to make place for aqua cultural  ponds for  fish, shrimp and prawn, often subsidized by local governments.

Another area in danger is  the Sundarbans Reserve Forest located in the south-west of Bangladesh adjoining to the Bay of Bengal. This immense forest is threatened by climate change, in particular the rising sea levels and floods  entering the delta and its mangroves. The area contains not only crocodiles but also the rare Bengal tiger whose habitats are shrinking due to the floods.  Villagers now become aware that the mangroves are their protection and have stopped cutting down the trees for charcoal production, and  instead have started  new mangroves. 

*Jeremy Stafford-Deitsch. Mangrove. The forgotten  Habitat.1996 Immel Publishing London.

Tomlinson, P.B. (1986). The Botany of Mangroves. Cambridge  University Press, Cambridge.