Literature review: Impact of Chilean needle grass ... - Weeds Australia
Literature review: Impact of Chilean needle grass ... - Weeds Australia
Literature review: Impact of Chilean needle grass ... - Weeds Australia
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Opinions differ on the nature and intensity <strong>of</strong> disturbance required for weed invasion. Patton (1935 p. 175) considered intact<br />
<strong>grass</strong>lands on the Victorian basalt plains to be highly resistant: “So long as the natural vegetation covering, open though it be, is<br />
maintained, entrance to new-comers is denied.” Lunt (1990a) found that areas <strong>of</strong> Derrimut Grassland ploughed during the 19th<br />
century were amongst the most diverse, with an average <strong>of</strong> 17 native and 13 exotic vascular plant species per 15 m 2 , probably<br />
illustrating one <strong>of</strong> the dilemmas <strong>of</strong> native <strong>grass</strong>land management, that certain types <strong>of</strong> disturbance favour both weeds and the<br />
native flora (Trémont and McIntyre 1994). Some exotics allegedly invade without major prior disturbance, by virtue <strong>of</strong> their<br />
superior competive abilities (Carr 1993), but prior disturbance may <strong>of</strong>ten be difficult to detect, and effects <strong>of</strong> historical<br />
disturbance may resonate long into the future. Disturbance that destroys native species certainly facilitates invasion by weeds<br />
both in terms <strong>of</strong> species and population sizes, and their occupation reduces recolonisation by natives. Nutrient and water<br />
enrichment <strong>of</strong>ten favours the exotics (McIntyre 1993). Soil disturbance was found to be the most important factor determining<br />
variation in plant species composition in <strong>grass</strong>lands <strong>of</strong> the New England Tablelands, more important than altitude, topographic<br />
position, lithology or water enrichment, but the greatest floristic effects resulted from soil disturbance accompanied by grazing<br />
(McIntyre and Lavorel 1994a). Dorrough et al. (2004) found that the intensity <strong>of</strong> grazing explained significant increases in the<br />
exotic richness <strong>of</strong> Monaro Tablelands <strong>grass</strong>lands, but that exotic status itself did not significantly explain plant responses in<br />
matched areas with contrasting grazing history. Exotic plant diversity was high even at the lowest grazing frequency. In many<br />
areas the local species pool is more likely to contain exotics that are well adapted to wetter conditions than natives (Kirkpatrick<br />
et al. 1995). The annual <strong>grass</strong> Bromus hordeaceus can be abundant where fire has been absent for a long period but is quickly<br />
eliminated by burning (Kirkpatrick et al. 1995).<br />
The consensus position <strong>of</strong> most <strong>grass</strong>land specialists appears to be well represented by McIntyre and Lavorel (1994a p. 381):<br />
“With little exogenous disturbance, the native <strong>grass</strong>land consists <strong>of</strong> a matrix <strong>of</strong> dominant perennial tussock <strong>grass</strong>es (e.g. Poa,<br />
Themeda) as well as smaller statured interstitial species that form the bulk <strong>of</strong> <strong>of</strong> species richness... including some exotic<br />
‘tolerant’ species. With increasing disturbance, the tolerant species will persist and many ‘intolerant’ species will decline, to be<br />
partially replaced by disturbance specialists. At the highest levels <strong>of</strong> disturbance, the matrix <strong>of</strong> native perennial <strong>grass</strong>es is usually<br />
replaced by large statured exotic <strong>grass</strong>es or forbs ... species richness is very low and a only a few tolerant native species persist.”<br />
The presence <strong>of</strong> weeds in native <strong>grass</strong>lands may impact on native plants not only through direct competition, but also by<br />
ramifiying effects through the food chain. Foraging by cockatoos for the bulbs <strong>of</strong> Romulea has been recognised as a theat to the<br />
integrity <strong>of</strong> one northern Victorian <strong>grass</strong>land (Kirkpatrick et al. 1995).<br />
Prevention <strong>of</strong> exotic weed invasion in <strong>grass</strong>lands has two main components, minimisation <strong>of</strong> disturbance to maintain an intact<br />
ground vegetation stratum, and hygiene measures to prevent entry <strong>of</strong> propagules (Davies 1997). Edges <strong>of</strong> <strong>grass</strong>land remnants<br />
tend to be the most highly invaded e.g. at Evans St. the road edges have significantly greater exotic plant richness and<br />
significantly lower native plant cover and richness than in core areas (Morgan and Rollason 1995, Morgan 1998d). Morgan<br />
(1998d) found no correlation between exotic plant richness and the amount <strong>of</strong> ‘bare’ (cryptogam encrusted) ground, a weak<br />
negative correlation with T. triandra cover and a stronger negative correlation with native plant richness.<br />
Grasslands that are floristically rich with native species are considered <strong>of</strong> high quality and have high diversity <strong>of</strong> forbs, an open<br />
structure with a high proportion <strong>of</strong> intertussock space, few weed species and management regimes involving regular biomass<br />
reduction by grazing or burning (Henderson 1999). Floristically poor (low quality) <strong>grass</strong>lands have low forb diversity, a canopy<br />
<strong>of</strong> dominant <strong>grass</strong>es that is <strong>of</strong>ten closed, abundant weeds and <strong>of</strong>ten a management regime lacking regular biomass reduction<br />
(Henderson 1999).<br />
Rare and endangered plants<br />
“The most imminent threat to the biological diversity <strong>of</strong> <strong>grass</strong>land is the extinction <strong>of</strong> rare and threatened species and genotypes.<br />
These cannot be resurrected once lost, whereas the <strong>grass</strong>land communities could conceivably be re-established ...” (Kirkpatrick<br />
et al. 1995 p. 87).<br />
A large number <strong>of</strong> rare and threatened plant species occur in native temperate <strong>grass</strong>lands <strong>of</strong> south-eastern <strong>Australia</strong> (Tables 6 and<br />
7). Robinson (2005) stated that approximately 200 native forb species that occur in these <strong>grass</strong>lands are rare and endangered,<br />
many <strong>of</strong> which were formerly common and widespread. Scarlett et al. (1992) provided lists <strong>of</strong> depleted, rare, vulnerable,<br />
endangered and presumed extinct <strong>grass</strong>land plants in Victoria. Scarlett and Parsons (1993) compared the numbers <strong>of</strong> threatened<br />
vascular plants across major landscape types in Victoria, including a category consisting <strong>of</strong> <strong>grass</strong>lands, <strong>grass</strong>y woodlands and<br />
fertile lowland open forests (Table 6). The highest concentration <strong>of</strong> endangered species occurred in this ‘<strong>grass</strong>y’category,<br />
whether assessed in the Victorian context or in terms <strong>of</strong> species populations throughout <strong>Australia</strong>. There was no doubt that<br />
undescribed taxa were under threat including species in Senecio, Craspedia, Podolepis, Leptorhynchos and Microseris (all<br />
Asteraceae), although some have subsequently been described. Herbs and low semi-shrubs comprised the majority <strong>of</strong> rare and<br />
threatened taxa. Kirkpatrick et al. (1995) listed 24 lowland temperate <strong>grass</strong>land taxa considered to be nationally rare or<br />
threatened, and calculated that 11% (79 species) <strong>of</strong> the total lowland <strong>grass</strong>land flora was rare or threatened. These data can now<br />
be considered to be poor indicators <strong>of</strong> current circumstances: “Even the most common species <strong>of</strong> lowland <strong>grass</strong>land forbs are<br />
now comparatively rare ...” (Robinson 2005).<br />
Table 6. The number <strong>of</strong> extinct, endangered, vulnerable, rare and insufficiently known vascular plant taxa <strong>of</strong> Victorian<br />
<strong>grass</strong>lands, <strong>grass</strong>y woodlands and fertile lowland open forests in 1993. Source: Scarlett and Parsons (1993).<br />
ConservationStatus Extinct Endangered Vulnerable Rare Insufficiently<br />
known<br />
Total<br />
<strong>Australia</strong>-wide 3 16 19 5 5 48<br />
Victoria 9 33 54 43 3 142<br />
105