Weeds – the real alien invaders
Key text
This topic is sponsored by the Australian Government Department of the Environment and Water Resources.
They’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.
A 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
1. Cash for weeds
2. How to be a good weed
3. Prickly pear – a biocontrol success story
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Posted February 2006.