Weeds – the real alien invaderThey’re green, they’re mean, and they cost Australia billions of dollars each year. Invasive weeds threaten the environment, agriculture, and people’s health – and the threat is growing.
Key text
Key textA green menace Invasive weeds are a serious problem in Australia. Weeds endanger the biodiversity of Australia’s unique waterways, National Parks, and National Heritage listed areas. By out-competing native plant species, weeds threaten their survival and reduce the plant diversity needed to support indigenous insects, birds and animals. Changes to the natural flora cover can lead to other negative environmental impacts, including increased soil erosion and greater bushfire intensities that further damage the land and the native organisms that rely on it. Agriculturally, weed species can reduce or contaminate crop yields and poison or injure livestock, decreasing productivity. Farmers are often forced to invest significant amounts in controlling invasive species on their properties, adding to the costs of production. Less commonly recognised is the impact weeds have on people’s health. Weeds cause significant human health problems, with introduced species making up 20 out of the 25 major seasonal allergens in Australia. Asthmatics and hay fever sufferers pay the price when introduced allergenic plants like ryegrass and ragweed release their pollen each year. Most weeds in Australia were, and still are, intentionally introduced for agricultural or ornamental purposes. While these plants have formed the basis of important industries and beautiful gardens, many have also gone on to become serious weeds – 70 per cent of Australia’s weeds are ornamental garden plants that have ‘jumped the garden fence’. Some important commercial species, such as the radiata pine and the European olive, are also serious environmental weeds (Box 1: Cash for weeds). A growing problem More than 27,000 species of alien plants have been introduced to Australia since European colonisation, with new species being introduced every year. About 10 per cent of introduced species become naturalised in their new habitat, meaning they can maintain their populations in the wild without human cultivation. One in ten naturalised plants become weeds, and have a negative impact on native biodiversity, agriculture, or both. The price paid for species which become weeds is tremendous, both in economic and environmental terms. Economically, weeds have been estimated to cost Australian agriculture a total of $4 billion every year: One in every seven dollars of farm income is spent on the control of weeds. The health costs of weeds are estimated at over a billion dollars annually, when medical treatment, drugs and time off work are added up – and money can’t measure the human suffering caused by persistent health problems. Environmentally, weeds have caused the extinction of four known species of Australian native plants, and 57 more are, or will soon become, threatened because of invasive introduced plants. Species like rubber vine in North Queensland have the potential to destroy entire native ecosystems by supplanting indigenous vine thickets, threatening native animals as well as plants. Vast areas of Australia and nearly every kind of native ecosystem are affected by introduced plants, from oceans and waterways to rainforests, grasslands and deserts. And the weed problem is growing. Not only is the rate at which new plant species are introduced to Australia increasing, but changing conditions are giving weeds more opportunities to thrive. Global warming and environmental degradation can favour weed species – warmer weather and increased carbon dioxide levels provide perfect conditions for fast-growing weeds to spread (Box 2: How to be a good weed). Areas where natural vegetation has been disturbed by human or animal activities offer new opportunities for weeds to establish themselves, compounding the initial damage. What can be done to stop weeds? Preventing new weeds from becoming established in the first place is Australia’s best defence against invasive species. To restrict the introduction of problem species, the Australian Quarantine and Inspection Service (AQIS) imposes conditions on the importation of plant material into Australia, and inspects goods arriving at international mail centres, airports and seaports. It is impossible to stop every potential weed from reaching Australia’s shores. Once a weed is already here, several methods of control can be implemented to rein in weed numbers, including herbicides, physical controls such as burning or slashing, biological control and cultural control. Weed control often requires the integration of several or all of these methods to be effective. Biological control – or biocontrol – reduces weed populations by introducing the plant’s natural enemies to weed-infested areas. One of the reasons introduced plants become weeds is because the organisms that normally attack them are not present and they grow out of control as a result. By introducing the right species of insect or pathogen to areas with large weed populations, weed numbers can be controlled. Biocontrol has had many successes in Australia, from the famous prickly pear to more recent successes against water weeds salvinia and water hyacinth (Box 3: Prickly pear – a biocontrol success story). Developing a biocontrol is a long and expensive process, because researchers have to carry out testing to make sure the control agent will attack the weed and not native species. Only when the monetary and environmental costs of controlling weeds through more conventional means are unacceptably high does biocontrol become a viable option. Cultural control is a weed management technique that involves educating people about habits that will reduce weed spread, while promoting the growth of desirable plant species. In an agricultural setting for example, farmers can reduce weed infestations on their properties by rotating crops to make weed adaptation difficult, keeping equipment and clothing free of seeds and planting crops that are able to out-compete weeds. For the home gardener, prevention is everything. By not planting potential weeds, and taking steps to ensure garden plants don’t jump the fence and establish themselves in the wild, everyone can do their bit to fight the green menace. Boxes 3. Prickly pear – a biocontrol success story Related Nova topics: Integrated pest management – the good, the bad and the genetically modified A plague on the pest – rabbit calicivirus disease and biological control
Box 1: Cash for weedsAlthough the majority of weeds in Australia started as garden plants, a number of important species were introduced for commercial or agricultural purposes. Radiata pine, a commercial timber species, is a weed in parts of Australia where plantations exist. It is a fast-growing, water-hungry species that depletes the soil of nutrients and excludes natives from their natural habitat. It spreads easily because of its winged seeds and because some native cockatoos feed on its cones, carrying them into bushland. The European olive is another commercial species that has become a weed. Olives are an environmental weed and a health concern in southern and south-eastern Australia. They invade open forests and woodlands, crowding-out native species, and their extremely allergenic pollen triggers hay fever and asthma in those susceptible. Introduced in 1800, olives have until recently been a marginally profitable crop in Australia due to high picking costs and low domestic demand. Many unprofitable groves were abandoned and these unmanaged groves are responsible for the olive spreading and naturalising in Australia. For potential weed species currently under commercial cultivation, proper management with a view to weed containment is necessary to reduce the risk of escape into the environment. This includes monitoring nearby bush and road sides for seedlings, and managing birds and other wildlife that might spread seeds. Selectively bred cultivars of crop plants with reduced fertility and superior fruit are also under development. These new varieties may eventually play an important role in reducing escapes of commercial weeds into the environment. Related sites
Box 2: How to be a good weedSuccessful weeds have certain traits that allow them to become a threat in their new environment. A common trait is the ability to produce large numbers of seeds, allowing the weed to spread quickly to new areas. Seeds can be carried by birds, animals, humans, wind and water, depending on the species of weed. Weedy species are usually quick-growing, and rapidly colonise areas disturbed by fire, human activity, or over grazing by livestock or animal pests. Their quick growth rate allows them to out-compete native species for resources such as sunlight, nutrients and living space. The weed’s new home is usually free of the pests and diseases that kept their population in check within their native habitat, allowing numbers to expand until the plant becomes a problem. Some introduced species that are established but have not yet spread widely are known as ‘sleeper’ weeds – they may have the potential to become a problem if conditions change. For example, the introduction of an appropriate pollinator or a change in climate could cause a relatively localised weed to become a widespread problem. Many Australian species have the right characteristics to become weeds if introduced to a new part of the country, or if exported to foreign soil. For example, bluebell creeper, a climbing vine native to Western Australia, has become a weed in parts of southern Australia. It has infested hundreds of hectares, and in some places now makes up 60 per cent of the plant cover. Even that iconic Australian tree the eucalypt is a weed – introduced to California 150 years ago for their wood and medicinal properties, they now form monoculture forests that exclude most native flora and fauna. Related sites
Box 3: Prickly pear – a biocontrol success storyA classic example of successful biological control in Australia is the control of the noxious prickly pear cactus through the use of the Cactoblastis caterpillar. One species of prickly pear was first brought to Australia with the First Fleet to help establish an industry in cochineal – the red dye used to colour the coats of the British soldiers. The dye was extracted from the cochineal beetle, which lives and feeds on prickly pear. By the late 19th century a different species of prickly pear had spread out of control in New South Wales and Queensland, and by the 1920s covered more than 25 million hectares. The spiky cactus formed dense stands that made the areas it infested nearly impossible to raise animals on, driving many farmers from their land. Various methods of chemical and physical control of the cactus were tried without much success. In 1926, the caterpillars of the Argentinian Cactoblastis moth were first released into affected areas. The moth larvae burrow into and eat the cactus, eventually killing it. Cactoblastis was a great success, and in less than 10 years after its introduction the prickly pear population had been brought largely under control. Biocontrol agents do not completely eradicate the organism they target. Instead, they reduce and control its population enough that it no longer causes serious harm, bringing it into a sustainable balance with its new environment. Related sites
Activities
Further readingATSE Focus May-June 2002 The problem with weeds (by John Kerin) Summarises the weed problem in Australia and research efforts to control it.
Australasian Science March 2007, pages 31-32 Taming a wild radish (by Ed Newbigin) Describes efforts to develop a contraceptive to stem the proliferation of one of Australia’s most important weeds.
January-February 2007, page 11 Set a pest to catch a weed Reports on the feral camels taste for an invasive weed.
October 2006, page 12 Weeds winning the war Suggests that weeds are rapidly developing herbicide resistance and looks at options for weed control.
July 2005, page 6 Weed 'em out New legislation removes a loophole in the law which allowed the import of some weeds.
July 2005, page 7 Our least loved exports Looks at a number of Australian plants that have become weeds overseas.
March 2005, page 9 Door shutting on weeds New legislation removes a loophole in the law which allowed the import of some weeds.
Ecos No. 135, 2007, page 6 Loophole closed to keep out plant aliens Reports that a loophole in national quarantine law has been closed with a recent review of plant species that can be legally brought into the country.
No. 134, 2007, pages 28-30 Foreign problems in the desert landscape (by Denise Lawungkurr Goodfellow) Reports on the problem of introduced plant species becoming weeds that is being made worse by a tropical climate and the planting of exotic ornamentals.
No. 126, 2005, page 4 Sale of weeds online highlights the need for tighter controls Looks at the online sale of weeds in Australia and the need for more controls.
No. 120, 2004, pages 26-29 Fighting the triffid take-over (by Louise Lawrence) Looks at the role of the community and the biological control of a number of temperate and tropical weeds.
No. 118, 2004, page 5 Rust wins against rampant rubber vine Describes the successful release of a rust fungus against the rubber vine weed.
No. 114, 2003, pages 24-27 Managing mimosa (by Kate Smith) Covers the variety of methods being used to control mimosa in northern Australia.
New Scientist 28 April 2007, page 16 Weed-friendly GM crops can help the environment Looks at a novel way to control weeds.
27 January 2007, page 50 Himalayan balsam Reports on efforts to use biological control on an aggressive weed in Europe, US, Canada and New Zealand.
9 September 2006, page 21 Weeds shall inherit the Earth (by Stephen Meyer) Comments on the role of human selection on extinction of plant species.
10 September 2005, pages 41-43 We shall eat them on the beaches (by Joe Roman) Suggests the possibility of eating invasive species of plants and animals as a means of control.
11 December 2004, page 48 Farmer Buckley's exploding trousers (by Stephanie Pain) In 1931, farmers' trousers all across New Zealand started to explode, thanks to the pesticide sodium chlorate.
12 October 2003 Global warming to put gardens in bloom (by Fred Pearce) Suggests that the growth of plants generally, and weeds in particular, will accelerate due to global warming.
25 June 2003 Exotic fungus may save India's tea (by James Randerson) Covers the release of a rust fungus to control the spread of a weed that is threatening India's tea crop.
4 June 2003 Weedkilling robots slash herbicide use (by Duncan Graham-Rowe) A robot has been developed to identify and remove weeds from crops to decrease the use of herbicides.
12 May 2003 Weevils save lakes from water hyacinth pest (by James Randerson) Describes the impact of two weevils on the water hyacinth weed in West Africa.
Useful sitesCommonwealth Scientific Industrial Research Organisation (Australia)
World Wildlife Fund
Weeds Cooperative Research Centre (Australia)
Environmental weeds in Australia (Australian National Botanical Gardens)
Lists some of the significant weeds in Australia. Provides information on habitat, the nature of the threat and a photo for identification.
Environmental weeds in Australia (Australian Government Department of the Environment and Water Resources)
Summarises the threat of environmental weeds in Australia and provides information on the Government's National weed strategy: A strategic approach to weed problems of national significance.
Weed identification tool (Weeds Australia)
Provides a map to access weed identification information for all states and territories of Australia.
Australian Broadcasting Corporation
Glossaryallergens. An allergen is any substance that triggers an allergic reaction. Common respiratory allergens are grass pollen, mould spores or house dust mite faeces (present in dust); other allergens may affect the skin or the digestive system. allergic reaction. Allergies are inappropriate reactions of the immune response to substances (allergens) that normally wouldn't cause any noticeable effects. Most allergic reactions involve the allergen binding on to special immune system cells and causing these cells to release compounds that affect the surrounding tissue. One such compound is histamine. It causes itching and inflammation. Chemicals that block the effect of histamine are called antihistamines, and they are standard allergy medication. biodiversity. A measure of the variety of life. It is usually calculated from the number of species of organisms - although genera, families, classes and phyla can all be counted too. biological control. A strategy for the control of pests or disease-causing organisms that relies on the use of other living organisms rather than chemical pesticides. ecosystem. A term used to encompass all the organisms in a community together with the associated physical environmental factors with which they interact (eg, a rockpool ecosystem, a forest ecosystem). extinct. Occurring no more. The word is usually used for species but can apply to any level of classification. Recent extinctions are hard to prove, and an 'official' limit of 50 years with no recorded sightings of the species is now used. global warming. An increase in the average temperature of the Earth's surface. Global warming is one of the consequences of the enhanced greenhouse effect and will cause worldwide changes to climate patterns. habitat. 1. The place normally occupied by a particular organism or population. 2. The sum of all the factors that determine the existence of a community (eg, the freshwater habitat). herbicide. A chemical used to kill plants or inhibit their growth. Herbicides can be selective (kills some plants but not others) or non-selective. They can affect the whole plant, or only part of a plant, at particular stages of development. pathogen. An organism capable of causing a disease. pollen. The male sex cells of seed plants that carries the gametes to fertilise female eggs.
External sites are not endorsed by the Australian Academy of Science. Posted February 2006. The Australian Foundation for Science is also a supporter of Nova. This topic is sponsored by the Australian Government Department of the Environment and Water Resources.
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