What’s wrong with introducing a plant or animal?

By: Mark Kesby, Projects Officer
April 30, 2010

On my recent visit to South Africa, I was constantly surprised how many eucalypt and wattle trees I saw. Often, the locals were complaining that they are a major weed. This got me thinking – what turns a plant into a weed and why?

The Macquarie dictionary defines a weed as a plant growing wild, especially in cultivated ground to the exclusion or injury of the desired crop. So in our ordered agricultural or garden sense, a weed is any plant that is growing where we don’t want it! This means that a plant can be highly desirable in one place and a weed next door. For example, clover is a weed in our lawn yet is a highly desirable pasture species. Wheat is an important crop in this district but is a weed when growing in chick peas.

Some plants are always a weed. Bathurst burrs for example. It has been brought into Australia and has no use and few (if any) natural agents of control. Nothing eats it, is easily spread and is a great opportunist on disturbed or bare ground. So in paddocks that are poorly managed it is a rampant weed.

Most of our worst weeds (and pests) have been introduced from elsewhere – either a different part of Australia or overseas. Classic examples are Prickly and Tiger Pear, rabbits and cane toads. Rabbits, for example, were released into the wild by Thomas Austin (Winchelsea Vic) in 1859 – along with hares, partridge and sparrows! At the time Austin stated “the introduction of a few rabbits could do little harm and might provide a touch of home in addition to a spot of hunting”.  When introduced, all these pests found conditions to their liking and importantly, nothing in their new environment to control them. After much research and expense, science has sometimes been able to bring some of these pests back into a new balance by introducing a biological control but never eradication.

Often no practical control agent can be found. 

Around 1000 cane toads were introduced into Cairns (Qld) in 1935 in a vain attempt to control cane beetles. Today they are continuing their march west and south. They have now invaded Western Australia and are being found further south in NSW. They are not just an annoying pest to us, they are doing major damage to local wildlife. They eat native animals especially insects and other small invertebrates, outcompete local skinks and frogs for food and any animal that eats a toad is destined to die (they are highly poisonous).

As I discovered in South Africa, we have exported plants and animals to other parts of the world that have then become major pests. A stark example of this is the humble brushtail possum. Sure it can be a bit annoying in suburbia but we all think that they are great. New Zealanders on the other hand, consider them one of their major pest species and run regular control programs (1080 poisoning and trapping). Since being introduced in 1837 their numbers have risen to at least 30 million – that’s 21,000 tonnes of leaves, berries and fruit every night! One estimate puts the number at 70 million! Biological control methods are being researched. Possums are doing great damage in the NZ forests and are driving out native animal species.

Many of Australia’s major weeds are garden escapees! Often as the result of plant material thrown over the back fence or dumped down at the river. Prickly Pear and Lippia are good examples. With our recent move to hardier, less water demanding garden plants, the risks are increasing. Some of the decorative tall grasses are a particular concern.

Sometimes our thoughtless actions can have major consequences for agriculture and the environment. So I’d like to leave you with two important messages. Be really careful with what you plant in the garden and how you dispose of the residues. Either compost them or put it in the green waste bin for council to compost. Never throw them over the back fence or dump them along the road. You just might be starting a major weed issue.

The other important reminder is never bring plant or animal material back into Australia without declaring it. Let the quarantine inspectors decide what’s a threat – that’s their job. We might think that there’s no danger but so did the bloke who brought rabbits into Australia.

The Rabbit Plague

LPLM 6741 8375

www.lplmc.com.au

A healthy catchment for a productive future