Literature review: Impact of Chilean needle grass ... - Weeds Australia

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