Exotic Invaders Found in Australia's Northern Rainforest

ADELAIDE, Australia, November 14, 2005 (ENS) - Australian plants and animals are unique because the continental country developed in isolation for millions of years, but modern forms of transportation are penetrating that protective isolation, bringing non-native species into the mix. Now, Australian plant scientists say they have found invasive species taking over previously undisturbed sections of the northern rainforest.

Some of the invaders, such as coffee and guava bushes and mango trees, appear harmless, or even beneficial, but they are changing the rainforest ecosystem in a way that worries scientists at the Cooperative Research Centre for Australian Weed Management.

"Until now it was thought that weeds only established round the edges and in disturbed areas of rainforest," explain Dr. Helen Murphy and Dr. Dave Westcott of the Commonwealth research branch CSIRO, who are trying to establish the nature, extent and processes involved in the invasion.

"However, certain plants - such as pond apple, guava, coffee and mango trees - can establish in relatively undisturbed areas of forest. And some, like pond apple, are quite capable of dominating surrounding vegetation."

A miconia, Miconia clavescens, seedling shows its distinctive purple underside. (Photo by Melissa Setter courtesy Queensland Department of Natural Resources and Mines)

The finding adds to concerns over existing pressures on the rainforest caused by fragmentation and human impacts. The invasive shrub lantana has already penetrated many areas of rainforest throughout the wet tropics, and can persist by making its way into the canopy as a climber up other trees.

"The serious aspect of this is that an invading plant can cause changes to the structure of the rainforest - some of these changes may in turn make it easier for other invaders to penetrate," says Dr. Murphy.

"At present we have no idea how big the threat is, but in 200 research plots scattered across the wet tropics we have logged around 50 invasive plant species. Individual plots may have as many as 12 or 15 of these environmental weeds," Murphy says.

Controlling these invasive species is not an easy task. Dr. Westcott says the use of fire and bulldozers are out of the question in rugged terrain and conservation areas. As yet there is little in the way of biological controls for the invaders, while manual control is costly.

The cassowary, a flightless bird unique to Australia, spreads the seeds it eats throughout its habitat. (Photo courtesy Wet Tropics Management Authority)

Australia has around 64 birds and animals which disperse rainforest seeds - and around three quarters of the world's tropical forest plants have fleshy fruits capable of being spread in this way.

"Such diversity of species and interactions means that understanding and managing invasion in rainforest systems will not be easy. Indeed we are at a very early stage," says Dr. Murphy.

One answer may depend on the co-operation of landholders and gardeners who live adjacent to rainforest areas in not growing those plants or trees that have a potential to invade the native forest through seed carriers.

Dr. Murphy says her research aims to identify the characteristics of plants or trees which might make them successful as rainforest invaders, so they can be kept in check. She is also seeking to determine the first signs that an invasion may be about to occur, so it can be headed off.

Her research involves building an understanding of how an outbreak of invasive plants occurs and trying to predict how serious it will be - whether it will end up dominating the rainforest, or whether the native vegetation will eventually contain and suppress it.

Pond apple, Annona glabra infestation in the Queensland rainforest (Photo by Melissa Setter courtesy QDNRM)

"It seems awful to have to regard things like coffee and mangoes as weeds," says Dr. Westcott, "but in the wrong place they can do quite a bit of damage to our natural heritage."

Until very recently, almost no assessment was made of the risk invasive plants pose to primary production or natural ecosystems in Australia.

The consequence of this long period of thoughtless introductions has been "devastating," says Weed CRC, one of the cooperative research centers that were established in 1990 to bring together researchers and research users from universities, the public sector and business.

The cost of weeds to Australian agriculture now exceeds A$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 the country's most valued places. Native plants and animals often are unable to compete with these invasions.

Weed CRC predicts that within a generation or two, many places, including parts of national parks and World Heritage Areas, could be dominated by invasive plants from South America and Africa.