Cattle and fire to limit wetland invader

A new collaborative research project conducted by CSIRO and Queensland Parks and Wildlife Service is trialling a combination of fire and grazing in the hope of controlling an aggressive wetland invader: para grass. Kate O’Donnell reports.

Treatment trials | A short history | 

Just on the edge of Townsville in north-east Queensland, is the city’s Town Common Conservation Park, which 30 years ago was one of the country’s most significant seasonal wetlands, home to crowds of water birds such as egrets, herons, ducks and geese. Go down to the Common today however, and you will see that large areas are covered in para grass—one of Australia’s most invasive wetlands weeds—which has displaced most of the native plant species so that far fewer birds now nest or feed there.

The use of cattle to rehabilitate weed-infested land is of course somewhat ironic—the weed in question was introduced as a ponded pasture in the late 19th century. When it escapes pasture lands, or is left to itself, it rapidly forms a dense monoculture that excludes other species. It is now a common weed in many tropical cane-growing areas, particularly in low-lying ungrazed areas.

Cattle were removed from Townsville’s Common in the 1970s when it was made into a conservation park. Cattle are not permitted in Queensland’s national parks, but because the Common is managed by QPWS for the Townsville City Council, it is legally possible for the research team to use cattle in this project. By 2005, dense para grass up to 1 metre high extends over most of the wetter areas.

Tony Grice from Townsville’s CSIRO Sustainable Ecosystems leads the weed management trial, and estimates there is about 20 tonnes of para grass per hectare. “Other methods of weed control—such as physically removing the grass, or poisoning it—are impractical with such a large infestation,” he says. “In fact, it is highly unlikely that the species can be eradicated, though some degree of control is possible.”

Information gathered from the project will be used to develop and promote methods to restore other northern coastal wetlands degraded by para grass—but it’s not a method that can be used to control all wetland weeds. However, the results of the work should be relevant to other palatable grasses in wetlands.

Brolgas and magpie geese are the two bird species most affected by para grass but any species that needs wetland habitat and prefers open vegetation will struggle in dense infestations. There are anecdotes that once hundreds of brolgas thrived at the Town Common, but today those numbers have been reduced to perhaps tens.

The project is a joint partnership between CSIRO, Queensland Parks and Wildlife Service and the Burdekin Dry Tropics Board. It is funded by the Natural Heritage Trust through the Burdekin Dry Tropics Board and will involve community groups including Conservation Volunteers, Bird Observers’ Club of Australia, the Townsville–Thuringowa Landcare Group, Wetland Care Australia and local Indigenous communities.

Study sites

The study is taking place over 72 hectares of the Common, with 12 experimental plots of 6 hectares each. Trials began in August and will continue for at least the next 18 months. Cattle are removed from the area at the beginning of the wet season, and will be re-introduced when the dry begins.

Four types of treatments are being applied across the sites (see pictures opposite page):

• No treatment (leave para grass as it is)
• Control burning, with no grazing
• Control burning and grazing
• Grazing with no burning

Every month since the study began the research team has conducted bird surveys. Mammal sampling has also taken place, and so far rodents, planigales, bandicoots and agile wallabies have all been observed, as well as at least 50 different bird species.

Rehabiliation

But to rehabilitate the Common the diversity of the area’s vegetation needs to improve. The initial survey undertaken during the dry season found fewer than 20 species of plants in the study areas. “We don’t know how diverse the wetland originally was, but it certainly would have been more diverse than this—but what can grow in all that para grass?” asks Tony. “What would once have been here were sedges, which is what you really want to get back for the brolgas who eat their bulbs.”

The team is hoping a combination of the grazing and the fire will hold the grass back. A lot of the grass has been taken away in the short term but, says Tony, “you need to inform any manage­ment decisions by documenting what happens after removing the cattle.”