The arrival of Europeans in Australia has been followed in the last 200 years by over 28 000 foreign plants. Some introductions were accidental, but most were imported for pasture, horticulture or for ornamental reasons. Of the nearly 300 plants known to have established themselves as weeds in the wild between 1971 and 1995, for example, two-thirds were introduced as ornamentals. Of the 460 pasture and legume species trialled in northern Australia 1947-85, 60 became weeds, and 13 of these are now serious crop weeds. Only four proved useful without also causing weed problems. One plant introduced for pasture became a major weed within a decade.

Until very recent times, almost no assessment was made of the risk these plants posed to primary production or natural ecosystems. The consequence of this long period of thoughtless introductions has been devastating. The cost of weeds to Australian agriculture now exceeds $4 billion per year, and almost all the plants involved are foreign. Over 2 500 species of introduced plants are now established in the wild in Australia, and many threaten the integrity of some of our most valued places. Half a million dollars a year, for example, is spent trying to keep just one species (Mimosa pigra) out of Kakadu.

In recent years salinity, which has been estimated to cost the nation at least $200 million per year, has finally received the serious public attention it deserves. The recent National Action Plan for salinity was worth $1.4 billion. Clearly, the $4 billion per year cost of weeds means the issue is at least in the same league as salinity. Indeed, it appears to be a substantially larger problem.

An independent study in 2000 by the Centre for International Economics in Canberra showed that available control measures for many invasives are highly cost-effective. However, they require education, long-term strategy and investment. Investing in control now will repay us well in future, but we urgently need to begin. Indeed, as all farmers with weed problems know, we cannot afford not to act.

Examples of invasive plants having a high impact on the natural environment are all around us, but often go unrecognised. Slow but wholesale landscape change is happening on a large scale in many regions. In many respects it is the rabbit problem repeated hundreds of times over, quietly and slowly degrading whole landscapes, largely out of the public eye. Within a generation or two many once-loved places, including parts of national parks and World Heritage Areas, could be dominated by invasive plants from South America, South Africa and elsewhere. Already it is clear that many of our native plants and animals are unable to compete with these invasions. They decline in numbers, and even disappear from affected areas altogether.

Many invasive plants are also decidedly unhealthy. Apart from being toxic, causing rashes, stings or injury by spines, many common weeds cause severe respiratory problems, especially in children. Rye grass, parthenium, ragweed, plaintains and privet are all well known culprits. Some water weeds can entangle swimmers. Many invasive plants are also toxic to native animals and livestock, and others of no food value can simply displace nutritious plants. Both cause major losses to farmers.