All you need to know about Mangrove forests- one of nature’s frontline sea defences

Mangrove forests are one of nature's frontline sea defences; self-building, self-repairing and solar-powered, they do the job for nothing and provide a home for a myriad of creatures- from sponges to sharks, fish to frigatebirds.

All you need to know about Mangrove forests- one of nature's frontline sea defences
All you need to know about Mangrove forests- one of nature's frontline sea defences

Mangrove forests are one of nature’s frontline sea defences; self-building, self-repairing and solar-powered, they do the job for nothing and provide a home for a myriad of creatures- from sponges to sharks, fish to frigatebirds.

Yet worldwide, the world already lost 20 percent of the mangrove ecosystems due to coastal development. According to UNESCO, these frontier forests are disappearing three to five times faster than overall global forest losses, with serious ecological and socio-economic impacts.

As well as protecting low-lying islands and vulnerable coastlines against storm surges, tsunamis, rising sea levels and erosion, their soils are highly effective carbon sinks, sequestering up to five times more carbon from the atmosphere than forests on land.

Mangroves also provide valuable nursery grounds for fish, conchs, lobsters and other crustaceans which, in turn, support coastal fishing communities and contribute to food security.

UNEP research has calculated that by underpinning fisheries and buffering coastlines, every hectare of mangrove forest represents an estimated US$33-57,000 per year. Not only that, but protecting a mangrove forest is 1000 times less expensive, per kilometre, than building a seawall.

Mangroves have evolved a range of adaptations for withstanding the salt in seawater. Some species have special membranes covering their roots which prevent the salt from entering, but most varieties draw in seawater and then find a way of ridding themselves of the salt at a later stage.

A common method involves secreting it from pores or special glands, giving the leaves of many mangroves a shiny, sweaty appearance. Found
throughout the Caribbean, the red mangrove is a pioneer.

Supported by arching prop roots it establishes a tenuous existence on a beach or a shallow reef-top where the tidal range is small. The unusual roots have several important functions other than providing stability.

Underwater they create a complicated woody meshwork which strains the sand and debris from tidal currents. This sediment accumulates as a submerged bank and becomes enriched with the decaying remains of plants and animals swept in from the reef.

Dead mangrove leaves are also decomposed by the thriving hordes of marine bacteria which feast on the rotting compost. This concentrated source of nutrients provides the foundation for a successful mangrove swamp.

Finely branching systems of nutritive roots spread through the mire, absorbing the organic compounds. A major drawback of this muddy substrate is its lack of oxygen, but this fundamental deficiency is overcome by the mangrove‘s aerial roots.

By emerging from the trunk or stems, a section of each root will always be exposed to air, allowing vital streams of oxygen to diffuse into the plant tissue.

Viewed underwater, the roots of a mangrove form an eerie world with shafts of sunlight filtering through from the canopy above. Huge shoals of juvenile fish swerve in and out of the woody pillars, glistening with silver as they pass through the rays of light.

Like seagrass beds, mangroves are important nurseries for reef fish. Young lobsters also find shelter amongst the dense root thickets. They’re largely temporary visitors, migrating towards the bustling waters of the coral reef as adults.

Other forms of marine life are permanent inhabitants of the mangrove forest. Covering the surface of every submerged root is a profusion of plants and animals.

Orange and purple encrusting sponges grow in lumpy nodules or coat the roots in a smooth skin; anemones and sea squirts form dense clusters, while periwinkles. oysters, mussels and barnacles fill in the gaps between.

If travellers venture deeper into the forest, beyond the arching roots of the red mangrove zone, they’ll often encounter a glistening mudflat, riddled with the protruding spikes of breathing roots and pock-marked with pools of shallow water.

Sheltered from the sea breeze, the heat and humidity can be almost overwhelming, but as uncomfortable as it might feel to us, this part of the mangrove swamp is teeming with life and the first animals you’re likely to see are fiddler crabs.

They appear from their mud burrows at low tide to feed on detritus. Timid creatures, they constantly survey their surroundings from eyes set on long stalks. At the slightest sign of danger an entire colony will vanish into their individual burrows.

Male fiddler crabs are slower feeders than the females since they only have one pincer that is small enough to convey food to the mouth. The other is greatly enlarged and used for displaying to females.

Different species attract mates in a variety of ways. Some simply twitch their huge pincer from side to side, but others have a more elaborate courtship display, standing on tiptoe and weaving it in circles.

In many mangroves, a curious type of semi-aquatic fish may be encountered. Using their strong pectoral fins and long supple tails to walk and wriggle across the mud at low tide, mudskippers have solved the problem of how to breathe out of water.

Their bulbous heads accommodate a set of large gill chambers in which they carry around their own supply of water. When the oxygen dissolved in each watery mouthful is depleted, the fish return to their shallow pools to collect a fresh supply.

While partly submerged in the water, the mudskippers continue to monitor events above the surface using their bulging eyes, which act as periscopes. On the flat stretches of mud, these are also useful for detecting prey, such as juvenile fiddler crabs and worms.

Several species of bird feed or nest in the mangroves of the Caribbean, from scarlet ibis and brown pelican to the lesser Antillean bullfinch, crested hummingbird and yellow warbler.

None, however, are as conspicuous as the magnificent frigatebird. Although the canopy of a red mangrove forest may only reach 4-5m in height, it can support dense nesting colonies of these seabirds.

Males build a rudimentary nest of twigs before turning their attention skywards to where potential mates are gliding back and forth. They then begin an extraordinary courtship display which involves inflating a small wrinkled pouch of bare skin beneath their throats into a huge red balloon.

When a female flies past, the males lift their heads and send quivering vibrations through the rosy air-filled sacs. Any show of interest from a female frigatebird sends the male into a head weaving, bill snapping frenzy.