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.
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).
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Mangrove forest at Nudgee Beach. Photo © Gabrielle Ahern |
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).
Mangroves
are similar to coral reefs, in that they are nutrient poor. So what is the
secret to their success?
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Mangrove trees growing at Nudgee Beach. Photo © Gabrielle Ahern |
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).
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High tide inundates the mangrove forest creating protected nurseries for fish, crustaceans and sharks.
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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.
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Mangrove trees are opportunists and take advantage of space along the coastline to establish forests.
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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.
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.
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 boardwalk meanders through the mangrove forest. There are places along the way to enjoy the views, listen to the sounds and spot a few crabs in the mud. 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).
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Shorebirds are a familiar presence. The mangroves provide valuable shelter and food.
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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.
If
you would like to see images of mangrove forests, please take a look
at my Pinterest site: https://www.pinterest.com/saltywave
Online
Resources
Mangrove
Watch Australia
Accessed:
18 February 2016
The
Encyclopedia of Earth – Wetlands Mangrove Ecology
Accessed:
18 February 2016
Mangroves
of Australia by the Marine Education Society http://www.mesa.edu.au/mangroves/mangroves04.asp
Accessed:
18 February 2016
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