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Managing the Murray’s water to combat invasive species

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Healthy wetland, Cobram, Victoria.Healthy wetland, Cobram, Victoria.

Dense weed infestations are a familiar sight along rivers the world over. Why are river banks, floodplains and floodplain wetlands so susceptible to alien species invasion? And what can we do about it?

While there are many competing ideas to explain riverside (riparian) weed invasion, it seems that restoring more natural flows is one sure way to reduce the number of alien plants along the River Murray.

A study published in the Journal of Applied Ecology earlier this year indicates that flow regulation has led to weed invasion and a reduction in native plant diversity along the River Murray.

By reducing the size of natural flood events, river regulation has provided conditions that favour alien plants at the expense of native ones.

The study, led by Dr Jane Catford of the School of Botany at the University of Melbourne, surveyed plant communities and modelled flood histories of 24 riparian wetlands along the River Murray between Albury and Echuca to determine the effects of river flow regulation on the flora.

Dr Catford said the study confirmed the notion that riparian weed invasion may be related to changes in river flows, and it allowed scientists to identify what kind of flow alterations have had most impact along the Murray.

“The control of river flows has altered flooding patterns and reduced the size of natural floods, which in turn has created a favourable environment for alien plant species at the expense of native ones,” she said.

“Floods that would have occurred every 20 years are now occurring every 32 years, and median flows to the sea are reduced by 71%.”

Wetlands that were impacted most by flow regulation were found to have the highest cover of alien plants and the lowest cover of native plants.

“Native species are adapted to natural environmental conditions. Changes to these conditions can reduce their abundance paving the way for alien invaders. This is exactly what has happened along the Murray”, Dr Catford said.
“The reduction of native species and increase in alien weeds marks a change in the structure and function of these wetlands.”

The observed decrease in biodiversity is particularly worrisome because it hampers the ability of flora and fauna to adapt to other stresses, like climate change.

But it is not aInvasive Sagittata platyphylla, Cobrawonga.Invasive Sagittata platyphylla, Cobrawonga.ll bad news.

Dr Catford and her coauthors note that environmental flows should help redress the balance between native and alien species.

This presents a novel way to control weeds in river ecosystems, which are less suited to chemical herbicides and mechanical weed removal.

Dr Catford recommends that environmental water be used to augment natural floods that typically occur in spring. Increasing the size of mid-range floods (i.e. 10-20 ARI under the natural sequence) in particular will kill most of the weeds and will encourage growth and reproduction of native plants.

“Some of these wetlands are literally wall-to-wall with a single invasive species. Restoring more natural flooding patterns will help control such species and increase native diversity”, Dr Catford said.

Wetland plants improve water quality and provide crucial habitat and food for birds, fish, turtles and other animals. Changes to wetland flora will impair these functions, which contribute to an estimated $2.1 billion dollars annually for local regions 
About 90% of wetlands in the Murray-Darling Basin have been lost since European settlement, so it is essential to look after the ones that remain.

Some of the study wetlands were in Barmah Forest, a Ramsar site recognized for its international importance. As a signatory of the Ramsar Convention, Australia is obliged to conserve these wetlands.
Jane Catford in wetland, CobramJane Catford in wetland, Cobram
 “I also examined the effect of human-mediated weed dispersal, grazing, soil and water characteristics etc, but flow regulation was the clearly the main factor associated with weed invasion”, Catford said.

That said, alien plants can easily escape from gardens, and people commonly have their compost heaps at the back of their block near the local creek.

“Choosing natives and disposing of alien plants in green waste bins – or regular bins for some species – would certainly help reduce the spread of invasive plants”, Catford advises.

By Dr Jane Catford, School of Botany, The University of Melbourne

Reference: Catford, J. A., B. J. Downes, C. J. Gippel, and P. A. Vesk (2011) Flow regulation reduces native plant cover and facilitates exotic invasion in riparian wetlands. Journal of Applied Ecology 48: 432-442.

For more information or a copy of the paper, please contact Dr Jane Catford, School of Botany, The University of Melbourne, E. [email protected]

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