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It is difficult to conceive of a single indicator that could measure the impact of invasive species on Australia, because of the difficulty in measuring their environmental and financial cost. Few national data are available on the impact that many of the thousands of invasive species have had. Although it is difficult to assess change in this area, invasive species have had an important impact on aspects of Australian progress. This commentary discusses some of those species, together with the ways in which they have become established and what is being done to control them.
The Australian continent's long isolation from the rest of the world has endowed us with a unique set of plants and animals. Like other islands, our isolation has also made our flora and fauna susceptible to the impact of invasive species: native species have not before had exposure to organisms like many of those that have arrived from overseas. Some invasive species thrive in Australia because the predators and parasites that controlled them at home do not exist here, while some species grow more quickly, breed more prolifically or have more varied diets than their Australian counterparts.
Environmental disturbance, particularly clearing and modification of native vegetation and habitat fragmentation, is widely thought to help many invasive species to establish and spread. (SEE FOOTNOTE 1)
In 2001, some 25 mammals, 20 birds, four reptiles, one amphibian and at least 23 freshwater fish species introduced from overseas were established in Australia, (SEE FOOTNOTE 1) along with about 2,000 plants. (SEE FOOTNOTE 3) Because of human activity, the abundance and range of a number of native animals and plants have also changed. Not all of these species are invasive or widespread at the moment, but many are a cause for concern wherever they are established.
Species threatened by invasive animals(a)
SOME INVASIVE SPECIES FROM OVERSEAS
Introduced predators like the fox and cat have spread over much of Australia and have contributed to the decline or extinction of some native species, through predation or the spread of disease. Cane Toads have advanced through Queensland to Cape York, south to Port Macquarie and into the Northern Territory, and have just reached Kakadu. They eat mainly insects, but also frogs, small mammals and snakes. And because they are poisonous, they kill many animals that prey on them such as goannas, quolls (tiger cats); (SEE FOOTNOTE 1) and some birds (although certain birds are learning to kill the toads and eat their organs while avoiding the poisonous glands). (SEE FOOTNOTE 4)
Rabbits have at times reached plague proportions over much of Australia, competing with native animals for scarce resources, overgrazing vegetation and digging holes which damage soil structure. Goats strip vegetation, erode slopes and compete with rock wallabies for food and shelter. Donkeys and pigs cause erosion and spread weeds (pigs also eat rare plant species).
Commercial honeybees are an invasive insect, found in nearly every habitat. (SEE FOOTNOTE 5) They compete for nectar with native insects as well as birds and mammals from which they also take nestholes.
All States and Territories have populations of fish introduced from overseas, which are thought to have played a role in the decline of 17 threatened and five other native fish species. Trout alone are assumed to be wholly or partly responsible for declines in five native fish species. (SEE FOOTNOTE 6) And exotic marine animals (often introduced into coastal waters from ships' ballast or riding on hulls), have entered and disrupted native food chains, and can dominate local communities.
Other introduced organisms, such as dieback fungus (Phytophthera cinnamomi), invade plant communities, killing selected species, and disrupting ecological processes. Dieback is the most important threat to the biodiversity of the Stirling Range National Park in Western Australia. Some plants (such as banksias and grevilleas) are highly susceptible, and 80 to 100% of infected individuals may die. The exposed ground is often invaded by weeds. (SEE FOOTNOTE 6)
NATIVE SPECIES WHICH ARE INVASIVE
Outside their natural range or in increased numbers, native species may be as serious a threat to biodiversity as foreign ones. Many are spreading and increasing in abundance because of recent human activity. Plant species native to one part of Australia have been introduced to other parts where they have become invasive. The Sweet Pittosporum, for example, is a rainforest tree from south-east Australia that now grows wild in Western Australia, South Australia and western Victoria. It grows now where it has never grown before, invading open woodlands and shading out rare plants. (SEE FOOTNOTE 5)
Large areas of grass and crops, together with more watering points, have encouraged Galahs, for example, to expand their range and colonise much of Australia. When Galahs arrive they compete for nest sites with birds native to the area, like Carnaby's Cockatoo, an endangered black cockatoo from south-west Western Australia. (SEE FOOTNOTE 6)
WEEDS - INVASIVE PLANTS
The National Weeds Strategy states that weeds are among the most serious threats to Australia's primary production and natural environment, and are increasingly moving into or towards almost all ecosystems of immediate economic, social or conservation value. (SEE FOOTNOTE 2) They displace native species, and the effects flow on to animals, such as insects and birds, that rely on native plants for food and shelter. Many weeds also interfere with agricultural production.
About 370 weed species in Australia have been declared noxious. (SEE FOOTNOTE 9) To help focus national efforts addressing the weed problem, a 'Top 20' list of 'Weeds of National Significance' has been compiled (see table).
Weeds also cause environmental damage that is difficult to quantify. Some species cover very large areas. Blackberry ranges over 9% of Australia and could potentially occupy twice this area. Weeds also affect important conservation regions. Mimosa, which threatens the Kakadu World Heritage Area, can grow to a height of six metres, and produces so many seeds that it can double in area every year, turning species-rich tropical wetlands of northern Australia into a Mimosa monoculture. (SEE FOOTNOTE 10) These weeds, and many more, pose a serious threat to biodiversity.
So-called sleeper weeds (weeds that are established or newly arrived but are not as yet a widespread problem) are recognised as a particular source of concern. For years Athel Pine did not pose a problem until the wet year of 1974, when thousands of seedlings, washed from homestead gardens, sprouted along inland waterways. (SEE FOOTNOTE 5) It now grows along water courses in central Australia, changing the river flow, displacing red gums and raising water tables to contribute to salinity.(SEE FOOTNOTE 6)
Weeds also cause flow-on effects. Some weeds are either more flammable or more fire retardant than the species they displace, and can alter the fire patterns of the communities they invade (which may have effects on native animals living in those communities). Other weeds provide food and shelter for invasive animals.
Weeds of national significance, distribution - 1999
A HISTORY OF INTRODUCTIONS
Despite Australia's isolation, over millions of years species have arrived naturally from elsewhere in the world. Birds have flown here, and seeds have been carried by ocean currents or blown by the wind. But since European colonisation, the rate of invasion has changed: thousands of foreign animals, plants, insects and fungi have arrived and become established since 1788, compared to an estimated rate before that of one or two species per millennium. (SEE FOOTNOTE 5)
Exotic mammals have existed in Australia for a long time. Dingos, which were bred from wolves in Asia, first arrived in Australia some 4,000 years ago, probably brought here by people from Indonesia.(SEE FOOTNOTE 5) Dingos kill native wildlife and they have been implicated in the extinction of the Thylacine from mainland Australia (Dingos did not colonise Tasmania). Experts are still debating whether cats arrived in Australia before Europeans.
However, the vast majority of foreign species have arrived since European colonisation. Many were introduced deliberately. Early settlers brought species like pigs and blackberries with them. They released the animals into the wild and sowed seeds as they travelled to provide a source of food for those who followed them. Rabbits and foxes were introduced to be hunted for sport. And the acclimatisation societies of the nineteenth century introduced animals which became pests, like sparrows, starlings and carp, to enrich Australia's native fauna.
Trout were introduced as game fish, and an American minnow, commonly known as the Mosquito Fish, was introduced in the hope it would eat mosquito larvae and rid our cities of mosquitoes.
The Cane Toad was introduced in the 1930s to help sugar cane farmers to control a native beetle that was eating their crop. The toad had little effect on the beetle, but it has had a very significant impact on many native species. And research agencies have introduced many foreign grasses, some of which have become major weeds, in trying to improve pasture.
Many plants and animals could potentially become invasive species in Australia, if introduced. There are many ways in which they might arrive.
Research agencies and pastoralists continue to introduce foreign grasses and legumes in an attempt to make rangelands more profitable. Between 1947 and 1996, for example, over 460 exotic plant species were introduced as pasture. Only 5% of these have proved useful as fodder, yet 13% have become major weeds, including Para Grass, which has spread into Kakadu National Park, reducing habitats for water birds. (SEE FOOTNOTE 6)
Agriculture, Fisheries and Forestry~Australia has set up a risk assessment process for invasive species. This assesses the potential invasiveness of species that people want to bring into Australia, to try to prevent the importation of further invasive species. (SEE FOOTNOTE 11)
However, nurseries and garden centres still sell many species of recognised weeds, and garden plants comprise many of the top 20 worst weeds and are the main management problem in some national parks. (SEE FOOTNOTE 5) For example, Rubber Vine from Madagascar now smothers large areas of woodland and forest (its current distribution is some 600,000 km2 , but it could potentially spread over five times that area). (SEE FOOTNOTE 8)
The pet trade imports millions of live fish each year, some of which carry diseases that can infect native species. Exotic aquarium fish, plants and snails have entered our waterways, sometimes after owners have dumped them, or when ponds overflowed. Two of the top 20 worst weeds, Cabomba and Salvinia, are aquarium plants.
Australia's growing trade links with the rest of the world provide a threat. Because Australia exports so many bulk commodities, we are a net importer of water carried as ballast by ships, water which has originated in other parts of the world and carries foreign plants and animals. Some, like the Northern Pacific Starfish, which eats oysters, mussels and other sedentary species, are having a major impact on our waters. (SEE FOOTNOTE 6) Ships also carry barnacles from around the world, while insects, spiders and reptiles arrive in cargo crates.
International travellers can carry foreign seeds on their clothing, and those travelling within Australia move native and non-native species around on their cars, while diseases such as Dieback Fungus have invaded reserves on road building machines. (SEE FOOTNOTE 5) And exotic diseases, such as the virus which killed very large numbers of pilchards in our southern waters during the mid-1990s, are difficult to detect and can enter the country in a variety of ways. (SEE FOOTNOTE 5)
LINKS TO OTHER DIMENSIONS OF PROGRESS
Invasive species have had significant impacts on Australian biodiversity. Weeds have affected agricultural productivity, have contributed to salinity and have affected the quality of our freshwater ecosystems. As the health of those ecosystems has declined, some foreign fish have been able to out-compete native species. Animals such as rabbits, pigs and goats have caused erosion and grazed heavily on native vegetation.
Many invasive species appear to do best in a disturbed environment, and land clearance is recognised as helping many invaders to spread. Gardeners and agriculture have also been responsible for the introduction of many invasive species from overseas.
See also the commentaries Biodiversity, Land clearance, Land degradation, Inland waters, Marine ecosystems, National income, and Openness.
1 Bomford, M. and Hart, Q. 2002, "Non-indigenous verterbrates in Australia", in Environmental and Economic Costs of Alien Plant, Animal and Microbe Invasion, CRC Press, New York (in press).
2 Weeds Australia 1999, The National Weeds Strategy: A Strategic Approach to Weed Problems of National Significance, URL: http://www.weeds.org.au last viewed 22 February 2002.
3 Australian National Botanic Gardens 1998, Australian Flora & Vegetation Statistics URL: http://www.anbg.gov.au/anbg/australian-flora-statistics.html last viewed 14 February 2002.
4 Frog Decline Reversal Project 2002, The Unwanted Amphibian URL: www.fdrproject.org.au/pages/TDprogress.htm last viewed 22 February 2002.
5 Low, T. 2000, Feral Future, Viking Books, Ringwood.
6 State of the Environment Advisory Council (SoE) 1996, Australia - State of the Environment Report 1996, State of the Environment Advisory Council, CSIRO Publishing, Melbourne.
7 Environment Australia 2001, Threat Abatement Plan for Dieback caused by the root-rot fungus, Environment Australia URL: www.ea.gov.au/biodiversity/ threatened.tap.phytopthora.index.html last viewed 22 February, 2002.
8 Thorp, J. Lynch. R. 2000, The Determination of Weeds of National Significance, Natural Heritage Trust/National Weeds Strategy Executive Committee, Launceston.
9 Weeds Australia 2000, Noxious Weed List URL: www.weeds.orh.au/noxious.htm last viewed 22 February 2002.
10 Lonsdale, M. 1999, "Weeding out the enemy", Ecos July.September1999, pp. 32-37.
11 AFFA 2002, The Weed Risk Assessment System Agriculture Fisheries and Forestry~Australia URL: http://www.affa.gov.au last viewed 22 February 2002.
12 Tasmanian Parks and Wildlife, Fox Sightings, Tasmanian Department of Primary Industries, Water and Environment, Hobart. URL: www.dpiwe.tas.gov.au/inter.nsf/WebPages/SJON-56P2RG?open last viewed 22 February 2002.
13 Queensland Department of Primary Industries 2001, Fire ants - Where are they in Australia, Queensland DPI URL: www.dpi.qld.gov.au/fireants/8061 last viewed 22 February 2002.