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

palatability, digestibility and nutritional value, coincided with increases in ungulates with hypsodont characters, i.e. highcrowned,
longer lasting teeth (Bouchenak-Khelladi et al. 2009).
During the late Tertiary and the Pleistocene epoch of the Quaternary in Australia a wide range of herbivorous birds and
mammals developed in association with the wide occurrence of grassland. These included the 2-3 tonne Diprotodon optatum,
giant macropods Sthenurus, Procoptodon and Protemnodon, the small grazing diprotodon Palorchestes azael and open country
flightless birds (Frith 1973, Hope 1994, Johnson 2009). But in all continents except Africa the open country megafauna largely
became extinct during the late Pleistocene, and the grazing regimes were therefore completely changed, with major ecological
repercussions (Johnson 2009). The Australian herbivorous mammalian fauna was more severely affected than the Americas and
Africa, losing almost all species over 100 kg in weight (Johnson 2009). Some 20 of the 50 Australian species that disappeared
were grazers, with Macropus spp. being the largest survivors (Jones 1999b). Loss of the megafauna due to aboriginal overkill, as
argued by Flannery (1994) remains contentious, but the major role of hunting in the extinctions throughout the world now
appears to be widely accepted (Johnson 2009). The extinctions appear to have occurred over a long period from 50-30 kybp for
the larger species, through to c. 10 kybp for the smaller (Hope 1994). Kershaw et al. (2000) argued that people coexisted with
the megafauna for many thousands of years, but conceded (pp. 502-503) that recent redating of significant fossil megafauna sites
largely reduced the overlap, making it unlikely that climate or habitat change were to blame. Loss of the megafaunal grazers
would have resulted in major change in the vegetation (Flannery 1994), particularly by reducing tree and shrub herbivory, as in
African savannas (Hope 1994). Knowledge of megaherbivores indicate that they effectively acted as ecological engineers. Their
extinction has likely led to the replacement of large areas of open vegetation with closed, woody formations, a decline in the
heterogeneity of vegetation across a range of scales, increased fire, primarily due to increased fuel loads and possibly resulting in
new areas of uniform grassland, and the decline of coevolved plants, including the species they dispersed or which had evolved
defences against them (Johnson 2009).
Grazing of sheep and cattle is now the major form of management in a large range of native grasslands, notably in the Australian
Capital Territory (ACT Government 2005) and in many privately owned native pastures (particularly in southern NSW), and is a
substitute for burning to reduce the biomass of dominant grasses (Stuwe 1994). Moderate or more intense grazing pressure can
be more effective than fire in maintaining bare ground and the low biomass of dominant grasses required to maintain high
diversity of indigenous annuals and prostrate species, but disadvantages the taller, upright herbs (Kirkpatrick et al. 1995). Native
pastures near Armidale NSW ungrazed for 16 years had a species richness of only 20 species per 30 m 2 because of large grass
tussocks, dense grass litter and little bare ground, while areas grazed continuously by sheep at moderate to low stocking rates
were much shorter, with small tussocks, little litter and abundant small bare areas, and had a diversity of 36 species (Trémont
and McIntyre 1994). Based on experimental results from disturbance treatments Sharp (1997) recommended retention of
livestock grazing as a management regime in the ACT, particularly at sites with high diversity of small intertussock forbs. Thus
it appears that sites that have been managed by livestock grazing for long periods will degrade if grazing is removed, but can be
managed without significant biodiversity loss by continuing the established grazing regime.
The effects of introduced livestock grazing on native grasslands are complex, being dependent, inter alia, on the grazer species,
the intensity and duration of the grazing pressure, and interactions with other environmental and management factors
(Kirkpatrick et al. 1995, Lunt 1995c, McIntyre et al. 1995, Aguiar 2005). Some of the most basic components of grazing impact
in Australia have been poorly understood and await adequate investigation (Johnston et al. 1999). For instance conceptions of
what plants are important fodder have significantly changed historically. White (in the foreword in Leigh and Mulham 1965, p.
v), commenting on the findings of pastoral research in the NSW Riverina, wrote that the pastoral research group had investigated
“the question of which species are important at various seasons of the year. Often thirty or more species are available for
selection and the extent to which the sheep exercises this opportunity is quite surprising. In many instances, in spite of
appearances, inconspicuous plants or rather unattractive shrubs are more important to sheep than was formerly believed”. Lack
of understanding of appropriate grazing regimes for natural grassland management continued into the period when grassland
preservation became a cause célèbre: “Currently it is not known if it is preferable to maintain a continuous, light grazing regime
or to graze intermittently, either lightly or heavily ... avoiding peak flowering and seeding times for native plants ... It may be
that particular sites require different regimes, on the basis of the different species mix present, type of exotic infestation, drainage
or other factors” (Sharp and Shorthouse 1996).
One effect that was believed to result in the deterioration of native grassland as a productive pasture was depletion of soil P
through the harvest of livestock and from continual removal of wool (Wadham and Wood 1950). However Groves et al. (2003a)
considered nutrient addition from continuous grazing to be a major factor in historical changes in grassland species composition,
initially due to increased nutrient mobilisation through soil disturbance and death of native plants, and increased returns from
faeces and urine, later through the addition of fertiliser and the sowing of legumes. Grazing adds highly labile mineral N direct to
the soil, leads to higher average plant tissue N concentrations, decreased root: shoot ratios and sometimes less efficient N-uptake
(Wedin 1999). Deposition of urine and faeces also alters the cycling rates of other nutrients. Nutrient enrichment and soil
compaction associated with livestock camps in native grasslands favours weeds (Kirkpatrick et al. 1995).
Mammalian herbivore grazing impacts on patterns of recruitment, production and mortality of grazed and ungrazed species, on
plant resource use and on resource availability. The most direct effect is removal of and damage to the above-ground parts that
are consumed or trampled (Kirkpatrick et al. 1995), but the effects on plants that are not eaten may be among the largest impacts.
Interactions between defoliation and release from competition can result in complex changes in structure and composition
(McIntyre et al. 1995). Grazing alters the composition of the community by differentially altering the population density, genetic
composition and structure of the component plants (Aguiar 2005) and by creating bare patches suitable for colonisation
(McIntyre et al. 1995). It may also affect the structural diversity at the community level, although this possibility has been
widely ignored (Aguiar 2005). Status as a grassland may be dependent on grazers that differentially destroy juvenile trees and
shrubs, preventing reversion to woodland or shrubland (Hobbs and Heunneke 1992). The accessibility of perennating buds to the
grazing animals is an important determinant of survival, so suvival of hemicryptophytes (with buds close to ground level) tend to
decline only when grazing is intense (McIntyre et al. 1995). One immediate structural effect is that green leaf material becomes
more concentrated close to the soil surface (Soriano et al. 1992). Plants that avoid grazing because of their small size or height
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