Wetlands Conservation 2: Management Issues
WETLANDS CONSERVATION 2: Management Issues
Visitors using mangrove boardwalk, Channel Is (Michael Michie)
The Environmental Research Institute of the Supervising Scientist (eriss) carries out research in the Alligator Rivers Region, particularly relating to management issues. According to their website, wetlands in northern Australia face a number of increasingly serious management threats including:
- invasion by exotic plant species, such as Salvinia (Salvinia molesta), Mimosa (Mimosa pigra) and Para Grass (Brachiara mutica)
- damage from feral animals, such as buffalo, pigs and horses
- drainage, vegetation clearance and development
- saline intrusion and rising sea levels associated with climate change
- inappropriate or altered fire regimes
- inappropriate pastoral practices
- potential impacts from mining, eg gold, bauxite, uranium
- decreased water quality and interruption of natural flow regimes, eg ponded pastures, mining, irrigation.
Some of these issues are explored further in this chapter and you can also check out the eriss website, http://www.ea.gov.au/ssd/wetlands/northern/management.html.
The definition of a weed is 'a plant growing out of place'. Plants that invade our natural ecosystems show the following features:
most are introduced and therefore have no natural enemies (weevils, caterpillars, bugs)
to keep them in check they compete with and displace native species through vigorous growth habits
by displacing native plant species they disrupt the normal food pyramids of ecosystems
they reduce the biodiversity of plant and animal communities
they are prolific seed producers and may also reproduce through stem, leaf or root fragments
they are tough and tolerate extreme conditions (salinity, dehydration)
they affect natural processes (fire intensity, stream flow)
they appear unnatural.
Some introduced pasture species have taken over areas of native grasses by choking them out, including para, mission, aleman, olive hymenachne and gamba grasses. Mission grass has been declared a noxious weed that landholders are under a legal obligation to eradicate it.
Of the remainder, the most widespread is para grass that is a problem because of its debatable status. It is quite palatable to stock and is therefore tolerated by the pastoralists. It grows rapidly, is spread by seed or stem fragments (carried by birds, vehicles or water) and is resistant to brackish water and shade. Clearing an area of para grass entails reduced grazing and expensive spraying. Pastoralists see no convincing reason to control a plant that is of some benefit as fodder and is not classed as a noxious weed.
The government, on the other hand, is concerned at the loss of native grass because of the part grasses play in wetlands ecosystems by supplying seeds to a variety of waterbirds. Each species of waterbirds has its preference for a few particular species of native grasses, and their lifestyle is linked to the seeding and growth periods of these grasses.
At present the management strategy is for for both NT and Commonwealth governments to control para grass on their parks and reserves and to maintain an education program for pastoralists and landcare groups aimed at controlling the grass outside reserves.
There is another group of plants about which there is little argument. Because of the environmental damage they can cause, they have been declared noxious weeds and there are thirteen such plants in the wetlands region of the Top End. These plants are not only a threat to natural ecosystems but as intruders diminish the heritage value of the wetlands.
Noxious weeds of the Top End wetlands include:
mimosa, Mimosa pigra
spinyhead sida, Sida acuta
flannel weed, Sida cordifolia
sicklepod, Senna obtusifolia
coffee senna, Senna occidentalis
mission grass, Pennisetum polystachion
water lettuce, Pistia straniotes
salvinia, Salvinia molesta
water hyacinth,Eichhornia crassipes
snake weeds, Stachytarpheta spp.
candle bush, Senna alata
grader grass, Themeda quadrivalvis
|Mimosa thicket (Mimosa pigra) (C M Finlayson)
The plant that causes the most concern is Mimosa pigra. It is a prickly shrub that can grow 6 m high, spreading quickly and forming dense impenetrable thickets. It completely displaces native vegetation destroying nesting sites for waterbirds and transforming sedgeland and grassland on floodplains into shrublands virtually devoid of wildlife.
Mechanical control measures (grubbing, chaining, burning) and chemical control measures (spraying) are effective but costly. Biological control, which involves importing natural enemies of the invader from its native home, tropical America, may offer a more economic solution in the long term. By May 1996 there were 8 insects and 2 pathogens (rust-type fungi) being trialled in the NT as biological controls for mimosa. The aim of biological control is not to eradicate a weed completely, but to establish a long-term, self-sustaining balance between the plant and its enemies.
You can access a slide presentation on Integrated control of Mimosa pigra by clicking here (courtesy of NT Department of Primary Industries and Fisheries; you will need Microsoft PowerPoint to see it).
Alien animals - the ferals
|Water Buffalo (Bubalus bubalis)
|Horses (Equus caballus)
|Pigs (Sus scrofa)
|Cats (Felis catus)
Cane Toad (Rhinella marina)
(formerly Bufo marinus)
|Feral Animals invading the Top End wetlands
Until the 1980s large herds of feral Water Buffalo (Bubalus bubalis) roamed the Top End wetlands. They were the descendants of animals imported from Asia for the early settlement on Coburg Peninsula and had adapted well to the tropical Australian environment. The buffalo caused heavy damage to the wetlands by churning up waterholes and reducing them to muddy wastelands unsuitable for aquatic life or waterbirds. The worst damage done by buffalo was to break down natural levee banks that held back salt water in the tidal creeks. At high tides the level of water in many places in the coastal creeks is above that of the surrounding freshwater swamps of the floodplains and a break in the creek bank allows salt water to spill out onto the floodplains.
With saltwater intrusion, freshwater habitats become saline ponds and there is a complete loss of the plant and animal life that once lived in the affected area. Melaleuca forests that lined the edges of some parts of the lower Mary River floodplain became ghost forests of dead white trunks. At the time of writing 17,000 hectares of grasslands and forest have been lost to saltwater intrusion, and a further 50,000 hectares are under threat.
Nearly all of the feral buffalo have been eradicated from the wetlands between Darwin and the East Alligator River, but although the animals themselves are no longer an ecological threat, the damage that they initiated will continue. Some buffalo remain in Arnhemland, particularly on the Blythe-Cadell floodplain and in the Arafura swamp. The problems associated with their presence is the concern of local land management groups assisted by the Northern Land Council's Caring for Country Unit and Parks Australia North.
Feral pigs (Sus scrofa) are the major alien species problem in the wetlands because in feeding they turn over the soil with their snouts, destroying vegetation and devastating the habitats of ground-nesting birds and native mammals. They breed rapidly, gather in large numbers, are cunning and difficult to eradicate. Feral cats (Felis catus), wild horses, or brumbys, (Equus caballus) and European honey bees are other alien species that have an effect on the wildlife of the region. Some concern is also expressed that feral domestic dogs (Canis familiaris) breeding with wild Dingoes (Canis lupus dingo) are weakening the Dingo’s genetic pool. The Cane Toad (Bufo marinus) has had a massive impact on the ecology of Top End wetlands and the implications of this are still emerging (2015).
A major step in the conservation of the wetlands environment has been the growing realisation that. the issue of good land management is of concern to all stakeholders in each particular catchment - government bodies, traditional owners, pastoralists, tour operators, miners, fishing interests, landcare groups and environmentalists. By gathering together and discussing management options, it is possible that integrated management plans for each catchment can be established.
The Mary River Landcare group is already playing an important part in the stewardship of the area by encouraging the concerned parties to identify common management objectives.
Salt water intrusion
|Effect of saltwater intrusion on the Mary River (C M Finlayson )
We have already noted that the cause of salt water intrusion was probably the impact of buffalo on natural levee banks. The invasion of the Mary River floodplains was first noticed back in the 1940s and at the same time, two creeks that drain the coastal swamps grew in width and depth and started to create a network of new channels. Because the tidal creeks could hold more water, incoming high tides had more power to erode the creek banks even further and the creeks continued to grow. This natural cycle has become progressively more threatening, making salt water intrusion the biggest immediate environmental problem of the region.
The two main methods of control are to build barrages – long walls of earth – to hold back the salt water (either at the mouth of the creeks or at the furthest point of intrusion) or to build a tidal choke. This is an engineered structure that holds back most of the incoming tide at the creek's mouth but allows some salt water to pass through with reduced volume, energy and erosion potential. The tidal choke would also allow Barramundi (Lates calcarifer) to move in and out of the river. Building barrages and levees directly onto the floodplains is difficult because of the swampy terrain and because the land is so flat that the water goes around any obstacle.
In 1995 the NT Parliament appointed a number of members to a bipartisan Sessional Committee on the Environment. The committee recommended the construction of a tidal choke. It also made the comment that if nothing was done to prevent saltwater intrusion, the remainder of all the wetlands of the Mary River would be seriously threatened and would eventually cease to exist as a freshwater system.
Important factors in the control of visitor impact on wetlands are access and visitor infrastructure. If an area is remote, a long way off sealed roads and without any facilities (parking areas, water, toilets, fireplaces, or shelters) it will not attract large numbers of visitors, especially the large tourist coaches. On the other hand, poor access makes it difficult to fight fires and monitor the condition of wetland habitats in the area. Having no good roads also encourages those owning four wheel drive vehicles to drive through the bush, a practice that has the potential to cause considerable damage to the natural environment.
Sealed roads make it possible for increased numbers of people to visit an area of scenic interest, and this has the potential of increasing public awareness of the natural environment by having a range of information services available – interpretive signs, map displays, rangers, and brochures. It is argued that by visiting sites of interest and beauty, members of the public will be more aware of the importance of wilderness values and be sympathetic to the conservation of wetlands for the national estate.
However, by improving access to sites exposes those sites to increased visitor numbers. Large numbers of people using the same patch of bushland day after day place enormous pressure on the ecology of the area. Vegetation may be damaged, seedlings can be trampled underfoot, soil becomes compacted and eroded, litter may accumulate and the native animals move away because of the noise. The growth of weeds is encouraged on the disturbed roadside and each new track and road increases the number of road kills of native animals. Sealed roads affect the acidity of surrounding soils. Rainwater running off the road surface is alkaline (it has a high pH value) and this upset in the chemical balance of acidity of the water can affect vegetation, sometimes causing dieback to sensitive plants.
The issue of access roads – where and how many – is one that depends upon the priorities of the authorities in charge of a park or reserve. Ideally, the situation at environmentally sensitive localities are constantly being monitored and sites can be closed to visitors if they are being overused. Management strategies for controlling visitors include limiting numbers on certain tracks and dividing a reserve or park into zones so that some areas are subjected to intensive use, whilst others have restricted access. Kakadu National Park has five zones: intensive management, intermediate management, minimum management, wilderness and scientific research zone. The use of four wheel drive vehicles for off-road driving is prohibited.
Besides the problem of land degradation by sheer weight of numbers, visitors may bring a number of other problems with them. Rangers and others involved in managing wetland reserves have to exercise control over the use of firearms, lighting fires in unsuitable places or in times of bushfire danger, bringing in dogs and other pets, and the use of noisy generators.
The management of fire in the Top End is one of the most thoroughly researched elements of the region's ecology. Because of the large amounts of grass (sometimes 3 m tall) produced each wet season there is abundant fuel for wildfires that may occur during the dry season. The later into the dry season the more the grass has cured (dried out) and the fiercer the fires are likely to be. If some of the grass can be burnt off in the early or middle parts of the dry season the less likelihood there will be of destructive late dry season fires.
The Aboriginal people traditionally used fire as a management tool. Not only was it used as described above, to prevent big fires in the latter part of the dry, but to provide fresh new growth (green pick) for kangaroos and wallabies, to clear undergrowth for easier travelling, to signal each other and for cultural and spiritual reasons. Fire and smoke were seen to have purification qualities and there were special areas of land that were required to be burnt before being entered.
Modern fire management practices in Kakadu National Park are described in this extract:
By attempting to re-establish (as far as is practicable) traditional Aboriginal burning patterns, contemporary Park practice aims to reduce the frequency, extent and intensity of wildfires within the Park and to protect species and habitats particularly sensitive to fire. A practical implication of adopting the traditional fire management model is that considerable effort must be given to breaking up grassy fuels in the early-mid dry season, in effect create a mosaic of burnt and unburnt patches across the landscape. (Press and others, 1995)
Present data indicates that the average annual burning over a 15 year period covers 40-45% of the country in the Top End.
In those parts of the Top End where there is multiple land use, it has been found that different properties have different objectives in fire control. However, moves are being made to develop fire management under the umbrella of a coordinated catchment plan.