The tropical savannas region is rich with wetland
areas that, compared to other wetlands in southern Australia, are
in good ecological condition. However, they are increasingly facing
a number of serious threats, most of which fall under the following
categories. For more information on some of these threats, click on
the Continuing Pages above.
Changes to natural water flows |Pollution
|Vegetation loss |Saltwater intrusion
|Introduced plants and animals | Natural processes
| References |
This can be caused by activities such as artificial drainage,
the pumping of groundwater for irrigation, the construction of dams
and weirs and the loss of vegetation. Plants and animals that
inhabit wetlands may exist in a fine balance with a particular
water regime. When this water regime is changed it can have
disastrous consequences for the plants and animals that depend on
it.
Water pollution can be caused by many activities and can impact
on wetlands in a variety of ways. Using chemicals, such as
fertilisers and pesticides, in the catchment of wetlands can lead
to a build of up too many nutrients in the water. The result can be
an algal bloom. This alga can be toxic to some animals, and can
deplete the water of oxygen; suffocating fish.
Pollutants and toxic materials may find their way into wetlands
from mines or processing plants, industrial areas and through
accidental chemical/oil spills. Potential pollution of wetlands
from the mining of bauxite, gold and uranium mining in the region
is a constant possibility. Pollutants from the hulls of boast
entering estuaries, creeks and rivers can also create a
problem.
More commonly, visitors to creeks, rivers and rock holes pollute
wetlands by leaving fishing gear behind, throwing rubbish overboard
or swimming in small rock holes with lashings of sunscreen or
insect repellent on! These chemicals will wash off the skin and
affect the finely balanced ecosystems that exist in these pools.
These contaminating substances can be harmful to wildlife and can
greatly reduce the water quality of the wetland.
Surrounding vegetation can play an important role in the
functioning of wetlands. The vegetation assists in maintaining
regular wetland water regimes, provides habitat and food for fauna
and protects against salinity and erosion; which together create a
healthy wetland. The loss of vegetation through clearing will
result in the loss of these values.
Saltwater intrusion into freshwater wetlands is a problem in the
tropical savannas region. It is set to become the number one threat
to wetlands in the region with predicted climate change and
associated sea level rise.
The issue is that low-lying coastal plains that support
freshwater wetlands are often close to or even below the level
reached by the highest tides. Saltwater intrusion can occur if
tidal banks are eroded, for example when buffalo or cattle trample
them. Saltwater from the tidal waters can then flow into
surrounding low-lying swamps and other fresh water wetlands making
them salty.
It is predicted that future climate change will cause some rise
in sea level. Even a small rise in sea level combined with the
large tidal range of northern Australia will create a rapid
extension of tidal-creek systems. This is seen on the coastal
plains of the Mary River in northern Australia. Tidal-creek systems
have extended more than 30 km inland in 50 years, invading
freshwater wetlands and destroying associated vegetation over an
area of at least 17000 ha (Knighton et al 1991). In the case of the
Mary River, the large tidal range, the very flat floodplains and
uncontrolled feral buffalo have all contributed to the rapid rate
of expansion. Saltwater intrusion changes the vegetation
communities and aquatic fauna species present. Entire paperbark (
Melaleuca ) forests once living in shallow wetland areas
have died because the level of salt in intruding water has become
intolerable.
Invasion of wetlands by exotic plant species such as salvinia (
Salvinia molesta ), mimosa ( Mimosa pigra ) and para
grass ( Brachiara mutica ) have created large-scale
ecosystem change in regions of the tropical savannas. In places
these introduced species can create monocultures, outcompeting
native plants and reducing habitat availability and diversity.
Weeds also choke wetlands, prevent fauna from accessing the water
and reduce good nesting sites. Research into biological control
agents to control one weed, Mimosa pigra, has had some
success in thick stands of the plant, see link at right.
Wetlands of the tropical savannas also suffer from the effects
of feral animals. These include cattle, buffalo, pigs, wild horses,
cane toads and fish (e.g. carp and mosquito fish). Introduced fish
can out-compete native species while cane toads poison most native
species that eat them. Cattle, buffalo and horses trample banks,
muddying and fouling wetlands and destroying habitat whilst pigs
root around disturbing plants and wetland fauna whilst spreading
the seeds of weeds that stick in their course muddy hair.
Fire, floods, cyclones and drought are all naturally occurring
processes that have the potential to damage wetland environments.
Fire can cause a massive change to the habitat available for
wildlife around wetlands. Floods, cyclones and drought can impact
the natural water regime of the water body and the vegetation
surrounding the wetlands. The tropical savannas region experiences
the impacts of several cyclones each season. Large areas of wetland
habitat, including Melaleuca forests were toppled during the
2005-2006 cyclone season. It will take decades for some of these
habitats to return to pre-cyclone conditions.
Other threats to wetlands come from fire, poor pastoral
practices including overstocking and pumping of water from wetlands
for irrigation and the creation of ponded pastures.
An example of one tropical savannas wetland region under threat
is that found in the Gulf Country. The wetlands here are
biologically rich and important for local and international fauna.
However they face threats typical to many other wetland areas in
the tropical savannas.
The Gulf has one of the largest intact wetland systems left in
Australia: the Karumba–Burketown wetlands, which encompass an
area of 2.2 million hectares. Extensive marine, estuarine and
freshwater wetlands are the distinctive features of the land. These
wetlands are an extremely important site for more than 22 species
of migratory wading shorebirds and waterbirds, which visit the Gulf
each year. The sheer number and variety of migratory birds makes
the Gulf a place of international conservation significance.
Twenty-three percent of Australia's surface water flows through
this landscape. The health of the Gulf's rivers supports the
existence of many of the communities and industries including
fishing and tourism that exist in the region. Threats to these
wetlands potentially include the development of irrigated
agriculture in northern Queensland including cotton. Broad scale
irrigation will draw water from the wetlands and could alter the
natural wet/dry seasonal cycle, increasing the potential for
salinity. There are also concerns that proposals to expand existing
mining operations in the region could also pose a threat to the
Gulf's spectacular rivers and wetlands.
Knighton, A, D., Mills, K. & Woodroffe,
C.D. 1991, 'Tidal-creek extension and saltwater intrusion in
northern Australia', Geology: Vol. 19, No. 8, pp.
831–834.
