Mangrove trees gnarled and bent appearances reflect the dynamic conditions they live in. Not many plant species can survive such a harsh marine environment. These salty forests are constantly exposed to the elements during low tide events and inundated by salty water during high tide. But these extremes play to their favour, and for many animals, mangroves are nurseries, shelter and food.

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Nudgee Beach Mangrove Boardwalk, Brisbane, Queensland. Photo © Gabrielle Ahern

The exposed roots and tree trunks of mangroves provide habitat for juveline fish species like the mudskipper (Periophthalmus spp.,) mangrove jack (Lutjanus argentimaculatus), barramundi (Lates calcarifer), snapper (Pagrus auratus), blue threadfin (Eleutheronema tetradactylum), sea mullet (Mugil cephalus) and bream (Acanthopagrus australis).

Australia is home to a great diversity of mangrove species with 22 genera from 19 plant families containing 41 species. Some of the more common varieties that thrive along the foreshores of estuarine, riparian and wetland areas include Avicennia integra (endemic to Australia only), River Mangrove (Aegiceras corniculatum), Grey Mangrove (Avicennia marina), Orange Mangrove (Bruguiera gymnorhiza), Yellow Mangrove (Ceriops australis) and the Spotted Mangrove (Rhizophora stylosa).

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Nudgee Beach Mangrove Forest, Brisbane, Queensland. Photo © Gabrielle Ahern

Mangroves are similar to coral reefs, in that they are nutrient poor. So what is the secret to their success?

The secret is a combination of factors. One of these factors involves anaerobic bacteria in the sediment, which helps breakdown leaf litter produced by the mangrove trees, providing food to other animals. For example, mangrove or mud crab (Scylla serrate), mud lobster (Thalassina anomala), banana prawns (Penaeus merguiensis), king prawns (Penaeus plebejus), mudwhelks (Pyrazus ebininus), mangrove oysters (Saccostrea commercialis) and barnacles (attached to tree trunks and roots), plankton, pistol shrimps and polychaete worms.

Larger animal species that inhabit mangrove communities include estuarine crocodiles (Crocodylus porosus listed as vulnerable under the NC Act), white-bellied mangrove snake (Fordonia leucobalia), little file snake (Acrochordus granulatus), and lizards: mangrove monitor (Varanus indicus) and rusty monitor (Varanus semiremex).

Mangroves are linked to other marine and terrestrial communities (salt marshes, estuaries, freshwater wetlands and coral reefs). The moving tides transport food from mangrove sites to communities deficient in carbon.

Over thousands of years, deposits of iron sulphides have formed in the sediments around mangrove communities, as a result of an interaction with sulphides in seawater, which is rich in iron oxide and organic matter. The very conditions mangroves need to grow in. The fact this system successfully traps toxic acid sulfate soils and is trophically linked to other animals and plant systems, emphasises the importance of mangroves.

Unfortunately, mangrove communities have been managed badly. There are examples of mangroves being used as dumpsites for rubbish or removed to advance the development of urban, marine, mining, industrial and agricultural development.

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Nudgee Beach Mangrove Forest, Brisbane, Queensland. Photo © Gabrielle Ahern

If mangrove trees are removed, the sediments are exposed to the air, with sulphuric acid forming when oxygen reacts with iron sulphide. The reaction is catastrophic to the fine balance of marine ecosystems. Toxic chemicals leach into the water, causing widespread fish kills and long-term negative impacts to invertebrate communities. Their removal destroys breeding grounds, shelter, sources of food, disrupts the normal behaviour of animals that inhabit them and the environmental processes that protect the landscape.

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Nudgee Beach Mangrove Boardwalk, Brisbane, Queensland. Photo © Gabrielle Ahern

There are some beautiful walks that meander through these salty forests. Some of the inhabitants you might spot depending on the time of day / night or where you are located include the black flying-fox (Pteropus alecto), little red flying-fox (Pteropus scapulatus), and the grey-headed flying-fox (Pteropus poliocephalus listed as vulnerable by the EPBC Act); and the false water rat (Xeromys myoides).

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Nudgee Beach Mangrove Forest, Brisbane, Queensland. Photo © Gabrielle Ahern

Migratory birds use these areas as breeding grounds, and for some species, they are a permanent home. Some of the birds inhabiting mangroves are: egrets (Ardea species), Australian white ibises (Threskiornis molucca); royal spoonbills (Platalea regia); cormorants (Phalacrocorax species); mangrove robin (Eopsaltria pulverulenta); mangrove golden whistler (Pachycephala melanura); white-breasted whistler (P. lanioides); broad-billed flycatcher (Myiagra ruficollis); shining flycatcher (M. alecto); mangrove gerygone (Gerygone levigaster); red-headed honeyeater (Myzomela erythrocephala); varied honeyeater (Lichenostomus versicolour); black butcherbird (Cracticus quoyi); lorikeets (subfamily Loriinae) and Australian pied imperial pidgeons (Ducula bicolor).

Nudgee Beach Mangrove Forest, Brisbane, Queensland. Photo © Gabrielle Ahern

Mangroves are remarkable for their strange bent over shapes and the plethora of animals and plants that thrive in their forests despite the extreme conditions they are exposed to. For more information about mangroves and their interesting biodiversity, please check out the following links or references.

Written by Gabrielle Ahern

Salty Wave Blue – Into all things ecology.

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Online Resources

Mangrove Watch Australia                          

The Encyclopedia of Earth – Wetlands Mangrove Ecology

Mangroves of Australia by the Marine Education Society