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

pasture. The least understood transitions are those that may be classed as restoration: from developed pastures through enriched
grassland back to native grassland (McIntyre and Lavorel 2007).
McIntyre and Lavorel (2007) tabulated known and hypothesised leaf, broad morphology and regeneration traits of the grasses
associated with each of the vegetation states. The traits considered were for leaves: specific leaf area, N content, dry matter
content, toughness and life-span; for morphology: size, form, annual/perennial; and for regneration: seed size (large, medium or
small), flowering period (early-late) and presence or absence of vegetative regeneration. European experience has demonstrated
the utility of such simple biological modelling in predicting and managing transitions between states. The traits are poorly
known for the Australian grasses, but if found to be universally applicable they should enable prediction of the effects of land
use change on ecosystem services.
ACT Government (2005) provided a useful tabulation of the degree of disturbance corresponding with various levels of botanical
significance in temperate grasslands of the ACT. Very low disturbance levels correspond with the highest botanical significance
level. The ground layer includes the most sensitive species including orchids (Diuris spp. Caladenia spp., Thelymitra spp.) and
lilies. Under low levels of disturbance the most sensitive species disappear but forbs such as Dichopogon spp., Bulbine bulbosa,
Pimelea spp. and Wurmbea diocea survive, along with T. triandra. At moderate disturbance levels, sensitive species are rarely
present, and the native herbs are generally disturbance tolerant, including Chrysocephalum apiculatum, Convolvulus erubescens,
Plantago varia and Asperula conferta. High disturbance levels may contain a range of native grasses but T. trianda and most
native forbs disappear. Very high disturbance sites are dominated by perennial exotic species or a low cover and diversity of
native species, mostly grasses. These categories have reasonable correspondence with the states in the McIntyre and Lavorel
model. Vey low and low disturbance = reference grassland. Moderate disturbance = native pasture. High disturbance = possibly
improved native pasture. Very high disturbance = enriched grassland.
Most of the native grasslands of south-eastern Australia have been irreparably altered by pastoralism and agricultural
intensification, but large areas have been destroyed by other developments including urbanisation (Sharp 1997). Much of the
remaining native grassland and native pasture is currently managed by grazing of livestock. The ecological effects of grazing
will next be examined in more detail.
Mammal grazing
“The countless herds of horses, cattle, and sheep, not only have altered the whole aspect of the vegetation, but they have almost
banished the gunaco, deer, and ostrich.” Charles Darwin, The Voyage of the ‘Beagle’, 19 September 1833, on the Argentine
pampas in the Buenos Aires region.
The grazing of domestic livestock has been more important than any other exogenous disturbance in altering the composition
and structure of Australian temperate grasslands (Trémont and McIntyre 1994). “Grazing has a detrimental impact on
communities with little history of grazing, but is necessary to maintain communities with a long history of grazing” (van Andel
and van den Bergh 1987 p. 11). Sheep and cattle grazing initially caused increases in plant diversity in south-eastern Australia,
resulting from the decline of the dominant native grasses, and invasion by exotics and native species from adjacent drier
communities (Moore 1993). But ultimately under the nearly geographically uniform regimes of continuous grazing at set
stocking rates, vascular plant diversity declined across large areas (Moore 1993). The irregular occurrence of severe drought,
combined with plagues of introduced grazers, particularly rabbits, led to to episodes of intense overgrazing which have severely
altered the natural grasslands. Overgrazed areas largely became “synthetic communities” (Trémont 1994 p. 511), lacking most of
the orginal native plants and with a high proportion of exotic species.
The long term detrimental effects of overgrazing by livestock on south-eastern Australian temperate grasslands need to be kept
in perspective. Grazing reduces the competitive effects of dominant species and creates open ground suitable for plant
colonisation, so in general results in increased diversity of plant species and functional groups, with annuals particularly
favoured (Trémont 1994, Trémont and McIntyre 1994, Lunt 1995c, Overbeck et al. 2007). Grazing at low intensities has
maintained vascular plant diversity over large areas of so-called native pasture. Trémont (1994) compared native pastures
intermittently grazed by sheep from the time of European tree clearing in the mid 1800s to 1976, and then either left ungrazed
for 16 years or grazed at a stocking rate of 6.7 sheep ha -1 over the same period. Both areas were dominated by native grasses and
most species present were native perennial forbs. The grazed treatment had a more open canopy, more bare ground, much greater
species richness including small forbs and grasses and more common exotic annuals, whereas the ungrazed treatment had dense
grass and litter cover, was species poor, but had a higher proportion of native perennials and more vegetatively reproducing forbs
species. Dorrough et al. (2004) compared areas that were frequently grazed (native pastures), infrequently grazed (travelling
stock reserves) and minimally grazed (roadsides) on the Monaro Plains and found that infrequently grazed areas had the highest
native and exotic richness
The maintenance of high plant diversity in many grasslands is dependent on continued grazing, but negative biodiversity
impacts, particularly on native species, may occur if grasslands have a limited evolutionary history of grazing (Hobbs and
Heunneke 1992). Any change in the grazing regime may constitute a disturbance, and the impact of the changes on diversity and
invasive species depends on their nature in relation to the historical regime (Hobbs and Heunneke 1992). As cogently observed
by Trémont and McIntyre (1994 p. 646), “the absence of grazing stock from grassy communities is as important, in terms of
community structure, diversity and composition, as its presence”.
Large grazing mammals coevolved with grasslands in the Americas (Webb 1977 1978), Africa and Eurasia and probably
Australia (Jones 1999b). According to Bouchenak-Khelladi et al. (2009 p. 2398) a “major and rapid radiation of vertebrate
herbivores ... occurred between 20 and 10 million years ago ... along with a near simultaneous rise to ecological dominance of
grasses ... suggesting that grasses coevolved tightly with vertebrate herbivores”. Occupation of C 4 -dominated grasslands by
ungulates is inferred to have commenced c. 26 mybp, with occupation by Bovideae and Cervideae occurring in the early to late
Miocene (23-5 mybp). Significant increases in silica density in C4 lineages, believed to be anti-herbivore defences that decrease
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