The components of the physical environment are constantly changing as a result of both natural processes and human action. The Aborigines produced marked effects on the vegetation by their deliberate or accidental burning over thousands of years; however, the advent of European settlement suddenly accelerated the pace of change. Hardly any element of the environment - vegetation, soils, wildlife, surface and underground waters or the coastline - has escaped the purposeful or unintended effects of human action. Even The Great Victoria and Simpson deserts and the remote Musgrave Ranges now support introduced populations of rabbits, foxes, cats and camels. Much of the plant cover in the arid regions has not withstood persistent grazing by domestic and feral livestock, while in the agricultural regions the modification of indigenous vegetation and faunal populations has been profound.
Many hundreds of species of plants have been introduced to South Australia, both intentionally and accidentally, since the arrival of Europeans. There have been intentional introductions of ornamental and culinary use, for use in erosion control, and as animal fodder. Accidental introductions have come as contaminants of agricultural and horticultural seeds; in earth attached to footwear, implements and livestock; or as seeds adhering to animal coats, sacking, clothing and other materials. The rate of accession averages about six new species per year and unfortunately seems to be increasing.
Of the 900 alien species of plants that have become established, most are from Europe, and in particular from the Mediterranean Basin. South Africa is also another important source. The niches created by the destruction of South Australian native plant communities by the advent of agriculture, pastoralism and forestry, have provided ample opportunity for the spread of those alien plants suited to the South Australian environment.
'Weeds' are those plants that people regard unfavourably. The successful establishment and spread of weeds depends upon environmental factors such as rainfall and soil type, and land-use and management practices. Some plants have very specific requirements, while others will thrive under a wide range of conditions. Particular weeds are of such concern that they are proclaimed as 'pest plants' and there is a legal obligation on landholders to control them. The legal framework for their control is provided by the Pest Plants Act of 1975, the latest in a series of Acts beginning with theThistle Act of 1851, which was the first noxious weed legislation in Australia
Salvation Jane, also known as Paterson's curse (Echium plantagineum L.), is a herbaceous annual found over most of the settled areas except for Kangaroos Island and much of Eyre Peninsula. It extends into the pastoral country, but where the average annual rainfall is less than about 200 mm, it is confined to areas receiving extra water from runoff. This pant is a garden escapee. Originating in the Mediterranean, and southern and central Europe, it was grown for centuries in northern European gardens and from there it was introduced into South Australian gardens by 1858. Noted as a serious weed by early this century, Salvation Jane is a valuable honey plant and provides some feed for stock in low rainfall country. However, it is also a poisonous plant causing liver damage which may be fatal in grazing animals.
Montpellier broom (Genista monspessulana [L.] Johnson) is a woody shrub which is confined to high rainfall areas, where it is particularly common in the Adelaide Hills. It is an aggressive weed as it is able to invade the remnants of native vegetation. Following fires, this prolific seeder colonises bare ground where it forms dense stands. This shrub was introduced as an ornamental and hedge plant by 1858, but had escaped near Adelaide by 1881. It is native to the Mediterranean Basin and was formerly confused with Canary (Islands) broom.
Pheasant's eye (Adonis microcarpa DC.) is a herbaceous annual found scattered throughout the agricultural areas but occurs in quantity only on highly calcareous soils. It is a major weed of first year pastures in some cereal areas. The potential distribution of pheasant's eye in South Australia is considerably more extensive than its present extent. Also an escaped ornamental, pheasant's eye was introduced to South Australia by 1876. By 1915, it was spreading in the Lower North but appeared to accelerate during the 1950s and 1960s. Pheasant's eye is apparently favoured by rotations with a short pasture phase.
Sea spurge (Euphorbia paralias L.) is a herbaceous perennial now found along most of the South Australian coast, including the offshore islands, but particularly the western coast of Yorke Peninsula. It is largely restricted to the first dune above the high-water mark where it grows with little competition, as very few other species are able to colonise such saline and infertile sands and withstand the salt-laden winds. Sea spurge is not considered a weed in economic terms and could even be seen as beneficial to some extent as it helps to stabilise the sand-dunes; however, it is more aggressive than the native species that would otherwise inhabit the dunes.
Native to the south and west coasts of Europe, sea spurge was first found in 1934 near Port Victoria, Yorke Peninsula. It is thought to have been introduced perhaps in ballast dumped from grain ketches. Within South Australia it has spread remarkably quickly for a plant with no obvious means of dispersal.
The importance of the River Murray to South Australia is profound. Regulation and diversion of its flow have provided a reliable year-round-water-supply, enabling development that might otherwise have been impossible.
Active modification of the river flow followed the 1914 River Murray Waters Agreement between the Commonwealth Government and the States of New South Wales, Victoria and South Australia. The River Murray Commission was established in 1971. Its functions were to regulate the sharing of the water resources of the River Murray between the States in prescribed proportions and to construct a series of locks, weirs and barrages. By 1940, six weirs and locks had been completed between Blanchetown and the South Australian border.
A series of barrages was also constructed on the lower lakes near the Murray Mouth to prevent the intrusion of seawater during periods of low river flow. These barrages also stabilise the terminal lakes at a level which enables gravitational irrigation on to the flats of the lower reached of the river. The cumulative effect of these works is an overall rise in the water level of about 1 metre. This allows safer navigation but also has the effect of drowning thousands of river red gums and flooding large areas of low-lying land upstream of each lock.
There are three principal sources of salt inflow to the River Murray: natural inflows, irrigation-induced flows, and lock-induced flows from groundwater upstream of the locks. Much of the salt derives from places in New South Wales and Victoria where the River Murray flows across land characterised by very saline groundwater, presumed to be the legacy of a period several million years ago when the valley and plains were covered by the sea.
Wasteful irrigation practices have been a major cause of the rising salinity content of the river. Over-irrigation and seepage from the open channels and evaporation basins have resulted in a large increase in the salt content of the water finding its way back to the river. The continued application of increasingly saline water has resulted in decreasing crop and pasture yields.
Numerous measures have been proposed to combat the salinity problems. Since the drought of 1967-68, no further expansion of irrigation allotments has been permitted. Other recent measures have included the construction of the large Noora North evaporation basin, 20 Kilometres east of Loxton. Unlike the existing basins near the river, this scheme disposes of irrigation drainage water well outside the River Murray trench. The existing evaporation basins have also been upgraded and the open channels of conveying irrigation water are being replaced by concrete underground pipelines. Farmers are also being encouraged to replace their furrows or overhead sprinklers with the more efficient micro-jet or drip systems. If managed correctly, these systems involve minimal wastage: they not only reduce the seepage of saline water into the river but also optimise the use of water consigned to growers.
For the first 125 years of European settlement the clearance of native vegetation was usually equated with 'progress'. For example, in the 1840s woodcutters' camps were scattered throughout the stringybark eucalypt forests of the Mount Lofty Ranges. Selective cutting of the largest trees in this area led to the coppice form of regrowth which today dominates the surviving forests. The more open woodland were the first to be cleared for farming; however, some large trees in three areas survive to adorn the landscape.
The first significant clearance of the mallee scrub was undertaken in order to supply timber for the copper smelters at Wallaroo, but with the development of the mullenising technique in the late 1860s the once 'impenetrable' scrub north-west of Gawler and on Yorke Peninsula was rapidly cleared. By the First World War the expansion of farms had made considerable inroads into the mallee scrub on Eyre Peninsula and in the Murray Mallee. A succession of technical improvements - the invention of the stump-jump plough, the use of superphosphate in the 1890s, the correction of trace element deficiencies after the Second World War - maintained the incentive for farmers to replace native vegetation by cropland and pasture. For many years after the Second World War land clearance was a deductible expenditure under Commonwealth tax law.
The example of native vegetation in the southern Mount Lofty Ranges in 1945 and in 1980 gives a clear illustration of the changes that occurred following the development of fertilisers capable of maintaining pastures and livestock on extremely impoverished soils. Of about 240,000 hectares of native vegetation in 1945, less than 90,000 hectares remained by 1980, a reduction of over 60%. Only three significant stands of native vegetation remained in 1980.
Commonwealth taxation incentives for vegetation clearance on farms have not only been minimised but also the State Government's Vegetation Retention Scheme now provides incentives to encourage property owners to retain native vegetation. In 1983, the State Government amended regulations made under the Planning Act, and in 1985 introduced the Native Vegetation Management Act. The latter Act required landowners in the agricultural regions of the State to obtain permission before clearing native vegetation. This action was in response to mounting concern over the environmental consequences of vegetation clearance. Over 80% of the native vegetation once covering the settled districts has now been cleared. In the absence of controls over clearance of native vegetation on private land, the only substantial remnants in the agricultural regions could, within a few years, be limited to isolated reserves on public land and in roadside strips.
The Adelaide Coastline
The population of Adelaide is fortunate in enjoying a 25 kilometre long sandy beachfront from Seacliff in the south to Outer Harbour in the north. Unfortunately, the beaches are not able to sustain themselves naturally because the first European settlers did not understand the coastal processes involved and allowed urban development to occur too close to the seafront.
The quantity of beach sand and dune sand along the Adelaide coastline is finite. This sand is thought to have come from the gulf floor as the sea-level rose between 12 500 and 6500 years ago, and there is evidence that sand continued to accumulate until 3000 years ago. The sand has been driven northwards ever since by waves generated during the south-westerly winds. The net drift of sand is northward at an estimated 30,000 cubic metres each year, which produces the sand accumulations on the southern sides of the Glenelg and Outer Harbour breakwaters. Another longer-term natural process which affects Adelaide beaches is a rise in sea-level of approximately 2 mm per year. It is estimated that this accounts for a loss of 17,000 cubic metres of sand each year.
The most important short-term process on any beach is sand movement in response to different wave conditions. Storm waves take sand from beaches and dunes and deposit it on offshore sand-bars. As the sand-bar builds up it has the effect of reducing the wave energy-reaching the shore. In calm conditions the sand returns to rebuild the beach and dune.
Since European settlement, buildings and other structures have been erected on the dunes. Damage suffered in periodic storms has led to the erection of protective seawalls. In 1948 and again in 1953 major storms demolished many of the timber and concrete walls, the cost of the damage in present-day values was about $40 million. Storms in 1981 were slightly less severe, although record storm tides were experienced. The relatively small amount of damage (less than $0.5 million) indicates the effectiveness of coast protection measures undertaken since the passing of the South Australian Coast Protection Act in 1972.
The 1972 Act, which established the Coast Protection Board, arose out of a major study into Adelaide beaches undertaken at the University of Adelaide in 1970. The findings of that study have been confirmed and updated in the Adelaide Coast Protection Strategy Review (1984). This review argued that protection of Adelaide beaches should continue, or else some beaches would be lost and severe property damage would occur during storms. The recreational value of the beaches that would be lost if no action was taken was estimated at $3.8 million per year, while property losses of up to $28 million could be expected over the next 50 years.
To combat the net northward drift on Adelaide beaches the Coast Protection Board regularly takes sand by truck from northern areas of accumulation and replenishes the eroding southern sections. The cost of this programme was approximately $400,000 in the financial year 1984-85, when 110,000 cubic metres of sand was trucked. New walls have been constructed, and others are projected, in order to establish wider beaches with a strip of sand-dunes in front of the existing seawalls. The advantages of this method of restoring the natural environment far outweigh those afforded by other protection measures. The beach and dunes south of the Brighton jetty have been replenished in this way, and the Coast Protection Board is now concentrating on other eroded beaches.