Overall, mangroves are mostly flowering plants less than
one metre tall, distinguishing them from tidal saltmarsh plants
in tidal wetlands. Most flourish in the intertidal zone, above
mean sea level around estuaries, and in sheltered embayments
of coastal margins and islands. They are most abundant in
soft sediments, such as around the mouths of tropical and
subtropical river estuaries. Most species extend into colder
regions only in places that have warm water temperatures.
Moreton Bay is such a place, its marine habitats combining both
tropical and temperate biota.
A ‘shore’ thing
In the tropics and subtropics, mangroves are the dominant
shoreline ecosystem. Well-known for their morphological and
physiological adaptations, these flowering trees and shrubs are
uniquely adapted to coastal and estuarine tidal conditions. They
easily cope with salt, saturated anoxic soils and regular tidal
inundation. Specialised attributes such as exposed, air-breathing
roots known as ‘pneumatophores’, which help with gaseous
exchange; extra, above-ground stem support structures; salt-
excreting leaves; and high intracellular salt concentrations help
them maintain favourable water levels even in brutally saline
environments. As mud makes it hard for plants to obtain oxygen,
pencil roots – such of those of the grey mangrove – protrude
from the mud and operate as snorkels. Mangroves also boast
unusual water-dispersed propagules that bud off a parent plant,
often seen as pointed suckers or small ‘capsules’ floating on the
waves, off to colonise new mudflats. These unique attributes
help mangroves play essential roles in coastal productivity and
connectivity, supporting a biodiversity and biomass that is often
lacking in other vegetation types, especially in arid regions.
Aboriginal people ate the fruit of
Avicennia marina and used its bark as a
medicine to treat stings from stingrays.FAST FACTSTo the nursery
Mangrove forests are recognised globally as breeding habitats
for fish, crabs and prawns. Of the fish and prawns caught in
Queensland annually, 75% have lived at least part of their lives
in mangroves. The use of mangrove forests as nurseries is based
on two ecosystem functions: (1) mangrove detritus provides
an important food source for marine life, and (2) the structural
complexity of pneumatophores, fallen branches, and shady,
overhead foliage reduces water temperature and the risk of
predation. A study I conducted with Alistar Robertson on the
mangrove habitats of Alligator Creek, in Tropical North Qld,
found that at least five fish species spent their entire life cycle in
or around mangroves and a further eight were regular ‘long-term’
residents. Researchers have a reasonable understanding of the
contributions various mangrove species make in terms of food
provision and protection, but the important message is about
mutual dependencies. As the animals depend on the trees for
habitat, the trees depend on the animals, too, enhancing the
growing conditions and survival rate of both through symbiosis.In northern Queensland and around the
Top End, mangroves provide shelter for
juvenile crocodiles, as well as for a variety
of marine life. Photo: Jürgen Freund.Mangroves are a favourite haunt of mistletoe birds (Dicaeum
hirundinaceum), and some forests are full of mistletoe plants.
The tiny, specialist birds depend on parasitic plants like Lysiana
species for nectar and fruit. Having eaten a nice ripe mistletoe
berry, these birds conveniently wipe their backsides on a
branch, ridding themselves of the sticky waste and ensuring a
well-placed planting that will bolster both populations.Mistletoe birds spread
the seeds of the
parasitic climber with
which they share a
name. Photo: Duncan
McCaskill [CC]30 | Wildlife Australia | SPRING 2017