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New Tactics in the War on Weeds

The native herb Lomandra stands in front of the invasive weed African lovegrass

The native herb Lomandra stands alone in front of the invasive weed African lovegrass, which was introduced into Australia for pasture improvement but was found to be unpalatable to grazing livestock and native animals.

By Jennifer Firn

Sometimes fertilisers can be more effective than herbicides when it comes to controlling weeds.

Invasive weeds are one of the most serious threats to the unique and rich biodiversity of Australian rangelands. Many Australians may not know much about rangelands, as only two million people live in this region, but they cover more than 70% of the land mass – more than 6 million km2 – and are packed full of unique and highly valued plants and animals, including 53 of Australia’s 85 biogeographical regions and five of 15 identified biodiversity hotspots.

Although these rangelands are far away and out of sight from most of the population, they support the most lucrative Australian industries, including pastoral, mining and tourism. Because of high economic, social and environmental values, sustainable management of these vast landscapes is critical for Australia’s future.

Many of the invasive weeds within rangelands were intentionally introduced for pasture improvement, soil stabilisation and ornamental purposes, but haven’t work out as planned. Instead they have become serious management issues that threaten biodiversity, key ecosystem functions such as nutrient and water cycling, and production.

In the Australian National Weeds Strategy, 20 weeds are listed as having impacts of national significance, with eight of these occupying rangelands. The list of weeds in Australian rangelands is much longer, with 622 exotic plant species being present of which160 are identified as a serious threat to biodiversity.

Australia spends more than $1.4 billion per year to control weeds, yet most continue to spread. For decades the methods used to control weeds have centred on killing the invaders with herbicides, slashing and bulldozers. But these measures create harsh disturbances, the very conditions that favour the growth and survival of many invasive species. Consequently, one weed may be removed from an area only to have the same or another one take its place.

When the dominance of a weed species is facilitated by other changes in the ecosystem – such as land clearing, changed fire regimes or disturbances such as cattle grazing – generic control strategies may not work, and simply targeting the weed for local removal could lead to the re-establishment of the same weed or another. This is because the weed may not be the cause of its own establishment.

One such invasive weed is African lovegrass, a perennial grass native to southern Africa, which was intentionally introduced into Australia for pasture improvement and soil conservation. Although it is easy to establish, particularly in regions with low rainfall and low nutrient soils, it has been found to be unpalatable to grazing livestock and native animals, low in nutritional value and difficult to contain, and has spread into many sites and regions where it was never intentionally introduced.

Despite large populations of lovegrass across Australia, it has not been declared a weed in all regions. It is classified as a locally controlled weed (category C4) in NSW, and is declared in 33 council areas in the State; a Category 2 weed in South Australia; a declared pest plant in the ACT (category C3) and Tasmania (category D); while in Victoria it is a regionally prohibited weed (category PS/C2) in five of 11 regions. Victorian authorities have also placed it in the very serious threat category. In Western Australia, Lovegrass is “unassessed”, meaning it is not officially permitted or prohibited but, until a decision is made, it remains prohibited.

In Queensland, lovegrass has yet to be declared a weed, although several scientific articles have specifically named it as a weed of concern in Queensland because of its ability to invade and dominate disturbed and/or degraded pastures and grasslands. It has also been estimated that the dominance of lovegrass on properties in southern Queensland and northern NSW reduces production by more than 40%.

In 2006 I set up a large research trial on a private rangeland property to examine and compare the efficacy of 24 different combinations of control measures for reducing the abundance of lovegrass and returning a more desirable native pasture community. This experiment is ongoing, and over time I have found that continued cattle grazing and the addition of fertiliser were the most effective combination for reducing the abundance of African lovegrass.

This combination of control strategies appears to be at odds with conservation because ungulate grazing and nutrient supplemention are generally associated with the degradation of grassland communities – particularly in Australia because ungulates such as cattle and sheep were only introduced to the continent a little more than 200 years ago and therefore Australian plant species are not well-adapted to the feeding and trampling impacts of ungulates, with many native species lacking the growing strategies needed to continue to survive and persist.

I found that a key reason why lovegrass dominated was that it is unpalatable to grazing animals such as cattle and kangaroos. This meant the animals were choosing to feed on the native plant species first and only choosing to eat lovegrass when no other species was available.

So the most effective control measure was to keep grazing but make lovegrass tastier using a low application rate of fertiliser. This method decreased lovegrass abundance without using herbicides and labour-intensive slashing.

In addition, the native grasses became more abundant because they were grazed less and had access to more nutrients in the soil. This meant that the abundance of another weed, a low-lying perennial forb called Mayne’s pest, was kept at low levels because of increased competition from the natives. The abundance of Mayne’s pest increased substantially when lovegrass was killed with herbicides, probably because a large opening was made in the community where light and moisture conditions were suitable for another weedy species.

Overall, the recommendation from this study is not to use fertiliser and grazing for all invasive species. Instead the findings point to a need for a different approach to invasive species management – one where we understand how the invasives grow, what the natives need and then change the conditions to return our native species.

Managing the impact and abundance of invasive plant species requires a broader ecological focus then the invasive species itself. Understanding the historical context of how and why an invasive plant species has become established is the first piece of the puzzle. Three other pieces are needed, including an understanding of how the new invader-dominated ecosystem functions, a clear definition of how these ecosystems should function in the future, and the characteristics needed to attract and sustain populations of more desirable native species. Without this understanding the application of the stock standard control strategies may in some situations lead to even further degradation and little or no change.

Making the effort to understand the dynamics of the new invader-dominated ecosystem and the needs of the native species first will be more difficult because it requires deeper knowledge of the impacts of invaders and may also require more effort, money and resources in the short-term. In the long-term, however, it has the potential to lead to more effective management strategies for the rehabilitation and possible restoration of the environmental and economic functions of rangelands.

Dr Jennifer Firn is OCE Postdoctoral Fellow at CSIRO Ecosystem Sciences.