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

tend to benefit more from relaxed competition under grazing pressure (McIntyre et al. 1995). In pampas grasslands, livestock
grazing markedly reduces the average distance between plants by dividing large tussocks into multiple smaller plants (Soriano et
al. 1992). Grazing may affect larger-scale patterns in vegetation including the ‘dual-phase mosaic’ or ‘banded vegetation’ of
semi-arid rangelands, which consist of patches or bands of high cover vegetation in a matrix of low cover with different
dominant plants; and since the matrix controls the hydrology and water availability in the patches, any disruption to it can impact
the whole system (Aguiar 2005). At the small scale, grazing tends to reduce the biomass of dominant grasses, enabling higher
vascular plant diversity. It commonly enables continued existence of the range of native grassland species, but allows the entry
of a pool of exotics (e.g. Soriano et al. 1992 p. 392). Grazing in flooding pampas grasslands has increased plant diversity at the
stand scale in this way, but has decreased it at the landscape scale because the exotics are mainly generalists with wide
environmental tolerances (Perelman et al. 2001).
Grazing can exert strong selective pressure by altering the population structure of grazed and ungrazed species. Populations that
are reduced in size can lose significant genetic diversity through genetic drift and inbreeding depression, while palatable species
may evolve traits that provide grazing resistance or enable grazing avoidance (Aguiar 2005). Development of pastoral agriculture
in the South American pampas has led to an increase of species that produce toxic secondary metabolites such as alkaloids and
terpenoids (Aguiar 2005), and similar concerns have long been a focus of pastoral weed research in Australia.
Grazing directly affects litter degradation rates, and, by altering the floristic and growth form composition of the vegetation by
removal of palatable and less grazing-tolerant species, alters litter makeup, decay rates and nutrient turnover (Villarreal et al.
2008). Reduction of litter by grazing reduces fuel loads and the probability of wild fire (Aguiar 2005).
Trampling can create openings for seedling establishment, reduce the dominance of tall growing species or directly reduce
fragile species (Hobbs and Heunneke 1992). Direct evidence of trampling damage has been noted for example by Archer (1984)
who recorded cattle and feral horse damage to colonies of Thesium australe (although not in temperate grassland).
Grazing animals also disperse seeds of native plants and exotic weeds, in their dung and externally (Hobbs and Heunneke 1992),
discussed in more detail below. Other less direct effects include soil disturbance and compaction, destruction of the cryptogam
crust, redistribution of nutrients via urine and dung, and the creation of bare ground (Mack 1989, Kirkpatrick et al. 1995. Sharp
1997). All of these changes can facilitate exotic grass invasion. The effects can be highly concentrated by congregation of
herding livestock, resulting in wide variability in the spatial arrangement of damaged patches (Hobbs and Heunneke 1992).
Studies in arid Australia have shown that sometimes >50% of the grazing in a paddock occurs in 30% of annual production is
removed by grazing animals (Mott and Groves 1994). Drought periods are common in many Australia grassland areas and
throughout the historical record, stocking rates have commonly not been altered to correspond with the accompanying reduction
in forage production (Mott and Groves 1994). Continued grazing at accustomed levels during periods of climatic stress and
alterations in other disturbance regimes can result in transformative shifts in grassland state (Aguiar 2005).
Grazing reduces the insect biodiversity of grasslands by simplifying the structural complexity of the plant components, with
different major taxonomic groups affected in different ways (Tscharntke and Greiler 1995). Invertebrates can suffer particularly
negative impacts from trampling (Hobbs and Heunneke 1992). Greenslade (1994) found indications that grazing has a strong
impact on the composition of the Collembola fauna of ACT grasslands, one of the important detritivore groups, reflecting a
general trophic cascade through the system.
Sheep and cattle
Audas (1950 p. 472) considered many Australian native grasses to be “excellent pasture or fodder plants ... equal, and in some
cases superior to, the cultivated exotic kinds”, including T. triandra, Austrodanthonia penicillata, Microlaena stipoides, eight
species of Adropogon, and fifteen species of Panicum (Mitchell Grass and Umbrella Grass) (“splendid fodder” op. cit. p. 472).
He noted the “rich, succulent and varied character” of indigenous pasture during spring and summer but the paucity of green
foliage in winter. However quality is mainly determined by environmental conditions, and all species, native or exotic, have
periods in which their forage quality is low (Johnston et al. 1999). The lack of cool season feed provided by native grasses
contributed to a strong focus on the introduction, breedings and widespread planting of exotic C 3 grasses for pastoral use in south
eastern Australia. A widespread perception developed from the late 1950s, based on unreplicated and otherwise biased studies,
that native species were of little pastoral value (Johnston et al. 1999).
Long term continuous grazing by introduced bovid livestock, primarily by sheep Ovis aries and cattle Bos taurus, has resulted in
major changes in the vascular plant species composition of grasslands throughout Australia (Moore 1993, Trémont 1994,
Kirkpatrick et al. 1995, Groves and Whalley 2002). The most immediate effect of continual grazing, particularly by sheep, was
to suppress or eliminate the most palatable and most easily damaged plants, which were eaten, trampled or failed to regenerate.
Intertussock herbs were the first to disappear (Stuwe and Parsons 1977, Groves and Whalley 2002, Groves et al. 2003a).
Murnong, Microseris spp., a staple food of aborigines, and once very abundant on the open plains, was greatly diminished or
eliminated in some areas by 1846 due to depradation by sheep, which learnt to root up the whole plant, or continually defoliated
it (Gott 1983). This species was also highly palatable to rabbits (Lunt 1996). Of 59 Victorian grassland sites sampled by Stuwe
and Parsons (1977) Microseris was present at only one, an ungrazed railway grassland. Other Asteraceae including Senecio
macrocarpus Belcher and Rutidosis leptorrhynchoides were unable to tolerate heavy grazing and are now severely depleted
(Morgan 1995a, Humphries and Webster 2003, Hills and Boekel 1996). The grossly contracted range of R. leptorrhynchoides
known in the mid 1990s consisted entirely of areas protected from domestic livestock (Morgan 1995a). Native legumes were
probably also heavily affected: e.g. grazing and trampling by cattle is considered an important current threat to Psoralea parva
(Muir 2003). By the 1990s, all remnants of Victorian basalt plains grassland with high vascular plant richness had been protected
from livestock grazing for decades (Morgan 1998c). The fragile cryptogam crust also sufferred major early impact.
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