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

Bovids crop their food between the incisor-canine row and a hard pre-maxillary pad, with sheep cropping directly, close to the
ground through a cleft in the upper lip, or after the fodder has been pulled into the mouth by the tongue (Groves 1989). They
graze with a tearing motion, as opposed to the shearing, scissor-like motion of the incisors of marsupials and thus tend to pull
whole plants from the ground, so have a more detrimental impact on perennial grassland species like Psoralea parva than
macropods (Muir 2003). Sheep graze more closely to the ground than cattle and are more selective so can be more damaging to
natural grassland (Moore 1993). They can prevent regeneration of chenopod dominants in drier Atriplex-Maireana grasslands
(Moore 1993). Carr and Turner (1959) found that on the Bogong High Plains of Victoria Poa inflorescences were highly
palatable to cattle but the mature leaves were not. Cattle feeding on snowgrasses generally pulled parts of tussocks out of the
ground, causing damage to swards. Archer (1984 p. 85) found that cattle and horses not only ate the endangered Thesium
australe “but tended in the process to uproot them or break them off at ground level”.
The impact of feral sheep on vegetation and landscape in the absence of other livestock has been well documented for the
Mexican island of Socorro by Walter and Levin (2008). Sheep were introduced in 1869 and have been the key cause of
ecosystem degradation. Sheep trampled and pulverised the soil to dust, ate even tiny seedlings in overgrazed areas and
transformed the native forest and woodland into open habitats with a mix of native and exotic vegetation. The half of the island
without sheep was found to contain only one exotic vascular plant species, compared to 44 spp. in the sheep impacted half which
contained many hectares of denuded ground and was susceptible to severe erosion. Sheep grazing favoured some poisonous
unpalatable plants, enabled the rapid spread of some exotic grasses and was a causative factor in the decline and replacement of
endemic birds.
Slashing, with removal of slashed material, and fire can be interpreted as grazing surrogates in so far as they result in removal of
herbage. Slashing of T. triandra grassland once in April or twice in successive Aprils made no significant difference to
intertussock gap distance at Derrimut compared with similar burning treatments (Henderson 1999). Stuwe and Parsons (1977)
found that the only plant species more common in grazed sites than frequently burnt (railway) sites or roadsides were exotics:
two spp. of Briza and Centaurium spp.
Insect faunas are generally impoverished under increasing intensties of livestock grazing (Samways 2005). Grazing alters
invertebrate habitat by affecting plant components, vegetation structure, microclimates, litter and soil properties (Yen 1999). On
the New England tablelands fertilised, improved pastures grazed by sheep have been found to have a higher proportion of exotic
invertebrates than native pastures (Yen 1999 see his reference). Hadden and Westbrooke (1999) examined the effects of removal
of sheep and rabbit grazing on the Formicidae, Coleoptera and Araneae fauna of long-grazed native T. triandra pasture near
Ballarat, Victoria. 6 m x 6 m plots were fenced out of the pasture, grazed by sheep at 3 DSE/ha, for 2 years. Of the 7 most
abundant ant species, the abundance of 2 species significantly declined, as did the abundance of the hot-climate functional group,
a result attributed to increased plant cover. Of the most abundant Coleoptera, significant increases were found with 2 spp. and
significant decreases with 3 spp., due to a complex of effects. The nominal “decomposer” functional group (comprising only
Anthicidae) decreased. The two most abundant species of Araneae significantly declined in ungrazed plots. Changes in the
proportions of spiders were related to structural change in the vegetation. As part of the same study Hadden (1997) found no
significant differences in the richness or abundance of the same taxa individually or when grouped between ungrazed and grazed
plots on the Northern and Western Plains over the whole two year period. Removal of grazing appeared to offer no short term
improvements in the biodiversity of these groups. The fauna appeared to be well adapted to the grazing regime.
Rabbits and hares
The effects of overstocking with sheep and cattle are difficult to distinguish from those caused by large populations of European
Rabbits, Oryctolagus cuniculus Lilljeborg, in Australia (Rolls 1984) and it is impossible to retrospectively separate their effects
for the purposes of determining ecological history (Frith 1973). European Hares Lepus europaeus Pallas, have had a lesser
impact and their effects are also confounded with those of livestock. Although hares are approximately twice the weight of
rabbits, the two species are trophic competitors: both are herbage-feeding lagomorphs which include woody twigs in their diet
under extreme conditions during summer. Both have digestive systems with caecal fermentation of digesta, both re-ingest soft
faeces (caecotrophs) and daytime hard faeces and both are poor digesters of cellulose (Stott 2007).
Large populations of rabbits first became widely established in Australia on the Victorian basalt plains near Winchelsea in 1859
and by 1890 had occupied all suitable habitat in Victoria (Menkhorst 1995d). The first reports of pastures being “eaten out” were
made in 1868 (Rolls 1984 p. 30). The impact on native grasslands was ecologically disastrous e.g. factories at Colac and
Camperdown canned half a million rabbits from the stony rises in 1881 (Rolls 1984 p. 53) and a factory was opened in Hamilton
in 1892 which processed 720,000 animals in 1894 and a million in the first eight months of 1895 (Brown 1987). Rabbits had
spread widely on the Central Coast of NSW by 1883 and were present on the New England tablelands in 1862 after release in
1854, but did not become abundant there until the early 20th century (Rolls 1984), with significant impacts in the Armidale
region from c. 1909 (Johnson and Jarman 1975) and plague proportions in the Glen Innes district by 1920 (Cameron 1975).
Areas of the NSW western plains and Riverina were occupied in 1879 and suitable areas in the rest of the State were slowly
occupied by 1925, at a rate of about 15 km per year (Rolls 1984). Rabbits occupied the burrows of wombats in wetter areas,
bilbies in the inland (Rolls 1984) and burrowing bettongs Bettongia lesueur (Quoy and Gaimard) (Noble 1993, Noble et al.
2007).
Rabbits extensively graze grasslands, reducing total herbage yield markedly, altering floristic composition and facilitating weed
invasion (Long 2003, Bloomfield and McPhee 2006). Cameron (1975 p. 22) considered it “clear” that they “eradicated a number
of the species of better natural grasses” in the Glen Innes district of NSW. Depradation of rabbit populations following the first
myxomatosis epidemics resulted in the appearance of grassland on previously bare and stony lands (Frith 1973).
Rabbits are selective feeders, with a high rate of feed intake, rapid gut passage, selection excretion of fibre and selective
retention of non-fibre constituents for microbial fermentation in the hind gut (Stott 2007). Rabbits graze very close to the ground
(Long 2003) and preferentially eat new seedlings during autumn and winter, in spring increasingly consume grass flower heads
and leaves of broadleaf species and later the inflorescences of other dicots, and in summer the green feed of summer-growing
native grasses, which may be eaten ‘as fast as they grow’, along with inflorescences and roots of Trifolium spp. (Menkhorst
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