Invasive weeds worse than climate change

By Troy Douglas 14 October 2010
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The spread of pesky weeds across the nation, from gardens to deserts, is having a profound effect on native flora.

THEY MAY MASQUERADE AS delicate flowers, but pervasive weeds are increasingly threatening already vulnerable biodiversity levels, according to a new report.

Conservation group, The Invasive Species Council (ISC), is calling for urgent action to tackle the spread of weeds throughout the New South Wales to prevent long term irreversible losses of native plants and wildlife.

The report, Stopping NSW’S Creeping Peril, released in August, recommends a raft of solutions to tackle the problem, including restriction of plants coming into the state and standardisation of laws across the country.

“People have this perception that weeds are fairly benign,” says Carol Booth, ISC policy officer and author of the report.

“They don’t really understand the nature of weed invasion and how long they will take to affect the landscape. [Weeds] can take decades to go wild. We need to change the overall approach so that these plants are considered unsafe unless they are assessed as low risk.”

Weeds taking hold

Exotic plants rank as the second biggest threat to the state’s native species after land clearing, according to the report. Most of the 340 weeds most harmful to the environment were deliberately introduced as garden plants and only 20 per cent of these are banned from sale across all or part of the state.

Nationally, weeds now represent about 15 per cent of all native flora, and of the more than 27,000 species introduced, about 3,000 have become established in the environment. This figure is increasing by more than 10 species per year, mostly due to seeds spread by garden plants.

There is now tighter control over imported plants but, with the exception of Western Australia, pre-existing species can still be moved throughout the country.

“The federal quarantine system is good at preventing entry, but it’s the ones introduced before that system began and introduced in different states…that are most problematic,” Carol says.


Invasive weeds can threaten existing biodiversity by dominating and changing the composition of the landscape, out-competing existing native species and making it harder for them to adapt or survive. Researchers have even cautioned that weed invasion is more of a threat to native plants than climate change.  

The lantana (lantana camara), for example, can fill gaps fast, nudging out other species, while the madeira vine (Anredera cordifolia) smothers existing plants.

Rapidly spreading weeds, such as the dense bitou bush (Chrysanthemoides monilifera), can have a severe “ripple effect” on entire ecosystems, speeding up the rate of nutrient absorption and soil erosion and impacting animal habitats, says Kris French, a conservation biologist at the University of Wollongong.

“It lowers plant species richness and changes bird communities, as there are fewer birds that want to live in them than in native vegetations.”
Management and eradication can cost billions and the lingering effect of weeds on an environment is yet to be fully understood, says Kris, meaning that “prevention is far better than cure.”

The Northern Territory Parks and Wildlife Service has been successfully working with traditional owners of the Rainbow Valley Conservation Reserve to eradicate buffel grass (Cenchrus ciliaris), the most widespread invasive plant in parks of central Australia.

It grows thicker and more dense than native grasses, says Parks and Wildlife ranger Pat Hodgens, and generates fuel for fires which can wipe out entire ecosystems. He says control efforts occur every few weeks and need to be long-term, but this type of early intervention program could work around the country.

“If people are motivated to do it and decide there are areas of conservation concern, it could definitely be expanded to other parks.”


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