Buffelgrass, used widely by the pasture industry, is also an ecosystem transformer, carrying hot fires that kill woody plants. photo by DD
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Weed risk set to rise

Thursday, 1 January 1970  | 

Australia already pays a high price for introducing hundreds of exotic plant species for livestock pasture production. Many of these species have become weeds with major environmental, social and economic impacts. Some introduced grasses increase fire intensity, transforming natural savanna woodlands to exotic-dominated grasslands. Gamba grass, introduced to northern Australia, has increased flammable biomass from six to ten tonnes per hectare, with the cost of fire management now nine times higher.

Many new pasture plants have a high risk of becoming invasive weeds. One of the most reliable predictors of a species’ invasiveness is whether or not it is invasive elsewhere. Globally, over 90% of plants developed for pasture are regarded as weeds, and one third are classed as weeds in the country in which they are sold.

If new varieties of existing weeds are introduced, the weed problem could escalate. New varieties are bred to increase pasture production, but also possess characteristics that increase invasion risk. Some are inoculated with bacteria or fungi that increase reproduction and growth, or can interact with the soil to increase nutrient availability, which could exacerbate weed invasion. Pasture varieties that resist disease, tolerate drought or grow in poor soils can become more successful than their less tolerant relatives, increasing the threat to ecosystems with high conservation value. Introducing additional genetic variation into existing weed populations can enable weeds to invade new habitats, and to take advantage of changing climates.

Although proposals to introduce new species are scrutinized for potential environmental and economic impacts, the likely impacts of new varieties of permitted exotic species are not considered. Without regulations that take into account the environmental impacts of new pasture varieties, existing weeds will likely become more invasive.

Supporting Research

Driscoll D. A., Catford J. A., Barney J. N., Hulme P. E., Inderjit, Martin T. G., Pauchard A., Pyšek P., Richardson D. M., Riley S. & Visser V. (2014) New pasture plants intensify invasive species risk. Proceedings of the National Acadamy of Science USA in press.
Takes a global perspective to consider whether new pasture taxa are likely to become environmental weeds, and whether there are mechanisms in place to limit potential risks.
Lonsdale W. M. (1994) Inviting trouble - introduced pasture species in northern Australia. Aust. J. Ecol. 19, 345-54.
Explore the history of pasture introductions in northern Australia, to examine the trade-off between usefulness and weediness, and, if possible, find predictors of weediness that will allow us to select species that will minimize tntersectoral conflict.
Fensham R. J., Donald S. & Dwyer J. M. (2013) Propagule pressure, not fire or cattle grazing, promotes invasion of buffel grass Cenchrus ciliaris. J. Appl. Ecol. 50, 138-46.
Address the following hypotheses: (1) buffel grass invasion will be more advanced as propagule pressure increases; (2) fire promotes the invasion of buffel grass; (3) cattle grazing promotes the invasion of buffel grass and (4) intercanopy areas will be more vulnerable to invasion than under tree canopies.
Uchitel A., Omacini M. & Chaneton E. J. (2011) Inherited fungal symbionts enhance establishment of an invasive annual grass across successional habitats. Oecologia 165, 465-75.
Examine how seed-borne endophytic fungi influenced the establishment of the invasive annual grass Lolium multiXorum Lam. (Italian ryegrass) in three communities—fallow Weld, grassland and forest—representing distinct stages of an old-Weld succession. We hypothesised that endophyte infection increases host invasion ability by enhancing its performance at various life stages
Godfree R. C., Woods M. J. & Young A. G. (2009) Do virus-resistant plants pose a threat to non-target ecosystems? II. Risk assessment of an Australian pathosystem using multi-scale field experiments. Austral Ecol. 34, 525-44.
Report the results of a series of field experiments designed to determine the impact of Clover yellow vein virus (ClYVV: Potyviridae) on wild populations of the perennial pasture legume Trifolium repens L. (white clover). This pathosystem is important in Australia, where it has been targeted by both GM and conventionally bred virus resistant host genotypes.
Rout M. E., Chrzanowski T. H., Westlie T. K., DeLuca T. H., Callaway R. M. & Holben W. E. (2013) Bacterial endophytes enhance competition by invasive plants. Am. J. Bot. 100, 1726-37.
Test effects of bacterial endophytes on soil nutrients, and growth in the invasive grass Sorghum halpense. Also examine ability of endophytes for transmission in space and across generations
Ellstrand N. C., Prentice H. C. & Hancock J. F. (1999) Gene flow and introgression from domesticated plants into their wild relatives. Annu. Rev. Ecol. Syst. 30, 539-63.
Review extent of hybridisation of crops with wild relatives, and the consequences
Lu B. R. & Snow A. A. (2005) Gene flow from genetically modified rice and its environmental consequences. Bioscience 55, 669-78.
We focus on possible environmental risks that are related to gene flow from transgenic rice
Lavergne S. & Molofsky J. (2007) Increased genetic variation and evolutionary potential drive the success of an invasive grass. Proc. Natl. Acad. Sci. USA. 104, 3883-8.
We focus on the invasive wetland grass Phalaris arundinacea L. and document the evolutionary consequences that resulted from multiple and uncontrolled introductions into North America of genetic material native to different European regions.
Grossman J. D. & Rice K. J. (2014) Contemporary evolution of an invasive grass in response to elevated atmospheric CO2 at a Mojave Desert FACE site. Ecol. Lett. 17, 710–6.
To test if invasive plants can evolve very rapidly (i.e. within 7 years) in response to elevated atmospheric CO2 under natural field conditions, we examined the evolutionary response of an introduced grass species Bromus madritensis ssp. rubens (L.) to elevated CO2 treatments at the Nevada Desert.
Prober S. M. & Wiehl G. (2012) Relationships among soil fertility, native plant diversity and exotic plant abundance inform restoration of forb-rich eucalypt woodlands. Divers. Distrib. 18, 795-807.
We asked how floristic composition changes with increasing nutrients and whether any native species are limited by low soil nutrient concentrations.
Grice A., Friedel M., Marshall N. & Van Klinken R. (2012) Tackling Contentious Invasive Plant Species: A Case Study of Buffel Grass in Australia. Environmental Management 49, 285-94.
In this paper we consider biophysical and social factors in examining prospects and approaches for developing broad-scale strategic solutions for one contentious naturalised pasture species in Australia, buffel grass Cenchrus ciliaris L. (syn. Pennisetum ciliare (L.) Link).
Cook G. D. & Grice A. C. (2013) Historical perspectives on invasive grasses and their impact on wildlife in Australia. Wildlife Society Bulletin 37, 469-77.
In this paper, we place the current concern about the declines in northern Australian fauna into the context of historic policy developments that have influenced scientific directions and led to a legacy of invasive plants across the region. The evidence for direct and indirect effects of invasive grasses on fauna is briefly reviewed and the implications for management considered.
Clarke A., Lake P. S. & Dowd D. J. (2004) Ecological impacts on aquatic macroinvertebrates following upland stream invasion by a ponded pasture grass (Glyceria maxima) in southern Australia. Marine and Freshwater Research 55, 709-13.
In this study, we compared paired upland stream sections that were either invaded or not invaded by Glyceria maxima in Gippsland, Victoria to determine some of its impacts on the abundance, morphospecies density and functional feeding group (FFG) composition of aquatic macroinvertebrates. We also briefly considered the implications of G. maxima invasion for channel morphology, stream hydrology and sediment loading.
Bunn S. E., Davies P. M., Kellaway D. M. & Prosser I. P. (1998) Influence of invasive macrophytes on channel morphology and hydrology in an open tropical lowland stream, and potential control by riparian shading. Freshwater Biology 39, 171-8.
The aims of this study were (i) to describe the influence of para grass and accumulated sediment on channel morphology and hydrology in a tropical lowland stream and (ii) to determine the effectiveness of shading as a means of controlling invasive riparian weeds such as para grass.
Weber J., Dane Panetta F., Virtue J. & Pheloung P. (2009) An analysis of assessment outcomes from eight years’ operation of the Australian border weed risk assessment system. Journal of Environmental Management 90, 798-807.
Assessments in Australia undertaken by Biosecurity Australia using the WRA system have been electronically recorded since 1998 (Walton, 2001); these address species proposed for importation as well as those held in genetic resource centres within Australia but not yet released. The resulting eight-year database provides a long-term dataset to evaluate the working performance of the WRA system. The aims of this study were to investigate how the WRA system has behaved in its official use and, in particular, to determine what questions are frequently associated with system outcomes.
van Kleunen M., Weber E. & Fischer M. (2010) A meta-analysis of trait differences between invasive and non-invasive plant species. Ecology Letters 13, 235-45.
To calculate effect sizes (Hedges’d) and their variances for differences in trait values between invasive alien plant species and non-invasive, either native or alien, plant species from 117 published studies. They used meta-analysis to assess whether invasive species and non-invasive species differ in traits related to physiology, leaf-area allocation, shoot allocation, growth rate, size and fitness (Table 1). This meta-analysis allowed them to test for general patterns across multiple studies and a wide range of plant species. Moreover, it allowed them to identify gaps in general knowledge on determinants of plant invasiveness, and consequently to provide recommendations for future research.
Setterfield S. A., Rossiter-Rachor N. A., Douglas M. M., Wainger L., Petty A. M., Barrow P., Shepherd I. J. & Ferdinands K. B. (2013) Adding Fuel to the Fire: The Impacts of Non-Native Grass Invasion on Fire Management at a Regional Scale. Plos One 8, e59144.
To assess the effect of A. gayanus invasion on regional fuel load and the consequences of this invasion on grassland fire danger index, fire management practices and their associated costs.
Pauchard A., Kueffer C., Dietz H., Daehler C. C., Alexander J., Edwards P. J., Arevalo J. R., Cavieres L. A., Guisan A., Haider S., Jakobs G., McDougall K., Millar C. I., Naylor B. J., Parks C. G., Rew L. J. & Seipel T. (2009) Ain't no mountain high enough: plant invasions reaching new elevations. Front. Ecol. Environ. 7, 479-86.
This paper reviews evidence that plant invasions are increasing in mountain systems and discusses the relevant drivers that have led to the current situation, and those that may increase invasion risk in the near future