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Mangrove Habitats

Mangrove forests

Mangrove forests are closed forests, with a heavy continuous canopy, ranging from 30 m to 1 m in height. In Australia, mangrove forests cover 11,600 square kilometres, only a fraction of the area covered earlier when sea levels were lower and prior to 6,000 years ago. 22 per cent of the coast is fringed by mangroves.

Mangroves on the mudflat* form the environmental conditions for animal habitat - the plant food, shelter, a trap for drifting and swimming estuarine food and a suite of varied furniture from hollows, rotting logs and a dense canopy to an incredible tangle of aerial roots, and there are copious quantities of deep 'mud, glorious mud'.

The mud bed with its algae and tree leaf fall shelters and feeds many invertebrates like microorganisms, plankton, worms, molluscs and crustaceans. Around 60 species of crabs inhabit these mudflats. The diagram shows eight distinct microhabitats utilised by molluscs in Darwin Harbour. In the tropics, mudskippers are fish restricted to the mud habitat. Within this muddy province are all the elements of a complete living ecosystem of producers, herbivores, omnivores, carnivores and decomposers.

With the incoming tide a suite of visiting vertebrates in the form of fish arrive to browse algae and to prey upon the permanent inhabitants. Larger specimens prey upon other visitors. Barramundi, mangrove jacks, octopus, sea snakes and eels along with saltwater crocodiles head the food chains here. Meanwhile in the microworld, swarms and schools of larval fish prey upon the smaller drifting plankton and remain under the shelter of algae and sea grass along the mangrove edge when the tide drops, to return on the rise.

Where do mangrove habitats occur?

Mangrove habitats occur at the interface between marine and terrestrial environments so there is an exchange of animals between the upper storeys. Mangrove monitors and rusty monitors make the mangrove forest their primary habitat feeding on crabs, insects, fish and birds. The scent and colour of mangrove flowers attract pollinating animals, red and pink flowers for honeyeaters and white flower parts for moths. A pristine, mature, northern mangrove community flowers and fruits throughout the year. Over 200 species of birds have been recorded in mangroves but only 14 are confined to the habitat, though 72 species regularly frequent these areas. This is rich by world standards.

Particular characteristics of habitats are used by different animals, for example mobs of flying foxes regularly camp in mangrove forests throughout the day. Take out or disturb any part of this network and the future of the species may be threatened.

Mudflats
Like rock platforms, mudflats of the estuaries and sheltered coasts range from strips which are permanently submerged in saline or brackish waters, to areas which are rarely inundated but which have extremely high salinity. Plants belonging to 20 families have become adapted to living on various parts of the intertidal mudflat. Some can withstand almost continual submergence of their root systems and battering by wave action. Others need the shelter of a dense stand but can survive high salt content and a continually sodden soil. Still others adapt to annual flooding by fresh water as well as high salinity for the rest of the year.

By changes to the "normal" physiology of plants, some 40 species of 60 world wide, have formed complex mangrove communities able to utilise the high fertility of the coastal muds. Most diversity lies along the extensive, sheltered, tropical coastline which has extreme tidal ranges in excess of 10 metres. Around small Channel Island, Darwin, there are 26 species while a similar area in Botany Bay has two.