Man­groves – mas­ters of sur­vival on salty ground

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.

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.