Description of the key threatening process
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| 5.DESCRIPTION
Describe the threatening process in a way that distinguishes it from any other threatening process, by reference to:
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its biological and non-biological components;
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the processes by which those components interact (if known).
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Buffel grass (Cenchrus ciliaris and C. pennisetiformis) comprises a suite of species and ecotypes originating from Africa and the Middle East but now rapidly colonising arid ecosystems in Australia and North America. CLIMAX modelling has shown that 25% of Australia, primarily in central Australia, is highly suitable for buffel grass whilst a further 43% is suitable for C. ciliaris establishment (Lawson et al. 2004). Jessop et al (2004) report that “Bates considers that C. pennisetiformis may become a greater threat than C. ciliaris in southern Australia”. Therefore, both species are considered together in this nomination. Recent evidence suggests that climatic tolerances and hence potential distribution of buffel grass are increasing as a result of new cultivars (Hacker & Waite 2001) or hybridisation (Friedel et al. 2006). For example, BIOCLIM modelling predicts that 69.5% of South Australia is highly or very highly suitable, and no part of the State's land area is entirely unsuitable, for establishment of buffel grass (Marshall and Hobbs 2010).
Buffel grass is a long-lived, deep-rooted and high biomass tussock grass that out-competes native vegetation through a number of processes. It is characterised by prolonged flowering/fruiting periods, prolific seed production, high seed dispersal ability, relatively long seed dormancy and tolerance to drought, fire and grazing (Franks 2002; Franks et al. 2000). Unlike many other weeds and native grasses, new buffel grass germinants can grow and set seed in as little as 3 to 6 weeks with sufficient moisture, and re-shooting mature plants can flower within 10 days after a rainfall event (Puckey and Albrecht 2004), especially after wildfires, as the ashes are reported to make good seedbeds (Paul and Lee 1978). Although buffel grass favours creeks, alluvial plains, calcareous areas and rocky ranges (Albrecht and Pitts 1997), it easily naturalises on a range of soil types and quickly forms self-sustaining populations under a range of disturbance regimes (Franks 2002). Buffel grass predominates in areas where summer rainfall ranges from 150-550 mm, winter rainfall is less than 400 mm, mean minimum winter temperatures rarely fall below 5oC, and soil texture is loamy (Cox et al. 1988), however, it has been successful in a broad range of soil types and landscapes.
When buffel grass is dense it can dominate light and space, reducing opportunities for native vegetation establishment (Miller 2003). Even at lower densities, buffel grass reduces soil nitrogen (Humphreys 1967), exhausting the mineral pool (Cavaye 1991) and also inhibits plant regeneration and growth through allelopathic suppression (Cheam 1984). Buffel grass aggressively colonises riparian habitats where it forms dense monocultures, displacing native vegetation. These mesic areas are functionally critical in a landscape where water is limiting to growth. Such mesic areas are also nutrient sinks and tend to support higher flora and fauna productivity, including endemic or rare species (Humphries et al. 1993).
Although buffel grass monocultures pose a significant threat to native vegetation and adapted biodiversity within their own right, it is the subsequent risk that buffel grass poses through fire that is of most threat to biodiversity, indigenous culture, life and property. Buffel grass produces approximately 2-3 times the combustible material of displaced native grasses, which results in hotter, more intense wildfires (Humphries 1993). Buffel grass threatens plant and animal communities that are not adapted to fire, by increasing the intensity and frequency of natural fire regimes (Adair and Groves 1998; NBIIISSG 2005). Unlike most grassy weeds that primarily only affect groundcover vegetation, the invasion of the canopy shadow of trees by highly flammable buffel grass threatens keystone arid zone trees such as river red gums (Eucalyptus camaldulensis), corkwoods (Hakea species) and beefwoods (Grevillea striata, Friedel et al 2006) with flow-on effects to other plants and animals.
For example, the Rare and Threatened Flora Management Plan for the APY Lands of NW South Australia (Paltridge et al. 2009) identifies 12 flora species in that region alone under threat from buffel grass. Buffel grass threatens stands of long-unburnt vegetation, for example red gum and mulga woodlands, hummock and Triodia grasslands, and the fauna that rely on these. High species richness of vertebrates including a suite of hollow-dependent birds, mammals and reptiles is directly threatened by increased fire intensity and frequency in woodland habitats (Neave et al. 2004). Triodia specialist fauna, including grasswrens (Paltridge et al 2009), spinifex bird, and many reptile species also lose shelter/nesting sites and food resources when Triodia is displaced by buffel grass. Granivorous birds and rodents such as delicate mouse, finches and some parrots that do not include buffel grass seeds in their diets declined as the cover of buffel grass increased (Ludwig et al. 2000; Franks et al. 2000). Other localised populations of threatened species (e.g. warru (black-flanked rock-wallaby), malleefowl and great desert skink) are also threatened by buffel grass-driven transformation of their specialised habitats (Paltridge et al 2009). Buffel grass can also affect invertebrates through changes in habitat structure (Best 1998) and fire regimes (Butler and Fairfax 2003).
These biological processes that make buffel grass such a threat in arid and semiarid Australia closely mirror the effects of other high biomass and flammable introduced grasses (e.g. Gamba Grass, Mission Grass) that have already been recognised as a Key Threatening Process in tropical and subtropical regions of Australia. Through its dramatic and sustained ecological transformation, buffel grass invasion poses a far greater threat than all other invasive weeds of central Australia. Other arid zone weeds, including those recognised as Weeds Of National Significance, are either largely restricted to disturbed or fertile patches, are outcompeted by native species in dry seasons, or do not exhibit traits such as the aforementioned fire feedback loop that cause major ecosystem upheaval.
In summary, buffel grass is considered one of Australia’s worst weeds (Humphries et al. 1991) and a ‘transformer weed’ of the Australian rangelands (Grice 2006, Bastin et al. 2008) due to its ability to transform the basic attributes of habitats. It is widely considered to be the most debilitating weed of natural ecosystems in arid and semi-arid Australia and directly or indirectly displaces and threatens a large percentage of native and endemic plants and animals of arid and semi-arid Australia (Best 1998; McIvor 1998; Fairfax and Fensham 2000; Franks 2002; McIvor 2003; Griffen 1993; Latz 1997; Low 1997; Ludwig et al. 2000; Butler and Fairfax 2003; Miller 2003; Puckey and Albrecht 2004; Clarke, et al 2005; Paltridge et al. 2009; Smythe et al. 2009).
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