Mangroves

Article | Updated 7 years ago

Mangroves are a specialised group of salt-tolerant, terrestrial plants that inhabit shorelines between low and high tide levels. Mangroves perform several important roles in the environments where they occur. Their root systems reduce the erosion of the soft sediment of the shoreline by holding the sediment together and also restricting the flow of water in the area. These plants also contribute to the recycling of nutrients.

As mangroves grow in sand or mud that is saturated with water, their ability to exchange gases through their root system is reduced. To overcome this problem, some mangrove species possess special structures known as pneumatophores. These structures extend above the mud or sand allowing oxygen to enter the plant’s roots when exposed to the air at low tide.

To overcome the problem of a high salinity environment, mangroves possess adaptations that prevent the uptake of salt by their root systems and allow the excretion of excess salt from their leaves.

Along the mainland and islands of the Dampier Archipelago there are six species of mangrove. The most abundant species growing in the region is the Grey Mangrove (Avicennia marina).

Plants and Animals of Mangrove Habitats

The mangrove habitat can be divided into different zones, and different organisms can be found in each area. Behind the mangroves, away from the water’s edge, is the driest area in this habitat, the salt flat. This area is only inundated during high spring tides i.e. when tides levels are highest. Thus, at low spring and high neap tides the salt flats are left exposed. Consequently, animals that live on the salt flat are at risk of drying out and, therefore, have special adaptations that allow them to survive. Some animals, including several species of crab, avoid this by living in burrows sealed by ‘plugs’.

Mudflats are the areas in front of the mangroves. As the incoming tides flood this area, animals, such as fishes, move in to feed. These animals retreat with the falling tide. Other animals, including snails and sea cucumbers, retreat into burrows as the water level drops, while other such as fiddler crabs, emerge to forage. Some animals such as barnacles, oysters and snails live on the trunks and leaves of the trees. Few species feed directly on the mangrove trees, instead they feed on decomposing leaves and wood.

Mangroves provide important nursery grounds for many species of fishes and crustaceans (e.g. prawns). The mass of roots and the muddy waters allow the juvenile animals to evade their predators.

Methods for Sampling Mangrove Habitats

Surveying of the organisms that live amongst the mangroves is generally carried out by hand collecting during periods of low tide. Researchers must contend with the network of the mangrove roots, the thick soft mud, and mosquitoes and sand flies, making it a difficult task.

The surveying of fishes in the mangrove habitat is also difficult as the muddy water reduces visibility. To overcome this problem authorised researchers may release rotenone, a substance made from extracts of mangrove roots, into the water. When released into the water fish become temporarily paralysed, allowing them to be caught and identified.