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

are therefore less likely to disturb the soil and damage the cryptogam crust. The digestive system and digestive physiology of the
large macropods have many similarities to those of ruminants (Frith 1973), but they are much better able to process high fibre
grass than ruminants, whose intake increasingly declines as the proportion of cell wall constituents in the feed increases (Hume
et al. 1989). Large grazing Macropus spp. “appear to be ideally adapted to maintaining their feed intake as grasses mature during
dry periods and increase in fibre content” (Hume et al. 1989, p. 684) and some species can efficiently use low-quality forage
(Frith 1973, Ellis 1975). Macropod diets consist mainly of leaves of monocots and dicots, grasses being the most important food
of the larger species (Ellis 1975, Hume et al. 1989, Morgan 1994, Bennett 1995), with readily digestible green leaves being
prefered. The diet of M. fuliginosus (Desmarest) includes “many other plants, particularly forbs” and dietary shifts during
periods when grasses decline commonly occur (Bennett 1995 p. 138).
Kangaroos eat many plants which livestock usually avoid. They are able to be more selective feeders than the blunt-muzzled
bovids because they possess narrow muzzles, bearing narrow incisor arrays (Groves 1989). Under good conditions macropods
can coexist with livestock, particularly cattle and horses, because they select a very different set of food items (Frith 1973, Hume
et al. 1989). However where weedy dicots are a problem, their preference for grasses may exacerbate invasion (Morgan 1994).
Other macropods, including the Spectacled Hare-wallaby Lagorchestes conspicillatus and the Bridle Nail-tailed Wallaby
Onychogalea fraenata have decreased greatly as a result of overgrazing of livestock on Astreleba grassland and chenopod
shrubland (Ellis 1975).
Many macropod species, including Macropus spp., cause minor soil disturbance at resting sites, often in the shade, or at basking
sites (Hume et al. 1989).
Overgrazing by kangaroos is also a threat to grasslands. The population of Senecio macrocarpus at Yan Yean, Victoria, is
thought to have dramatically declined rapidly between 1987 and 1992 due to high kangaroo density (Hills and Boekel 1996
2003).
No information appears to be available on interactions between N. neesiana and macropods. Existing knowledge of their ecology
suggests they may be better able to suppress it than domestic livestock, and theoretical considerations based on recent analyses
of biotic resistance (Parker and Hay 2005) suggest that they might prefer it to native grasses. Macropus spp. are probably poor
vectors of seed in comparison to sheep, since they have short pelage and more flexible mode of foraging, making them less likely
to contact seeding culms, and have coexisted with the similarly hazardous seeds of Austrostipa spp. in evolutionary time.
The role and effects of fire
“In grassy plains unoccupied by the larger ruminating quadrupeds, it seems necessary to remove the superfluous vegetation by
fire, so as to render the new years growth serviceable”. Charles Darwin, The Voyage of the ‘Beagle’, 15 September 1833, on the
Argentine pampas between Bahía Blanca and Buenos Aires.
Fire is a regular feature of temperate grasslands throughout the world. In Australia fire became an important environmental
factor in the late Miocene (c. 10 mybp), coincident with the decline of rainforest (Martin 1994). It has been a constant and
generally increasing feature through the Quaternary period (c. 1 mybp +) but “it has not proved possible as yet to quantify
vegetation/fire relationships” (Kershaw et al. 2000 p. 482).
Fire is a critical process in maintaining open patches and the maintenance of high productivity in fire-dependent grasslands
(Hobbs and Heunneke 1992). Fire reduces the cover of all herbaceous grassland plants to close to zero, with generally only ‘fireresistor’
perennial caespitose grasses retaining some standing live biomass, slightly above the soil surface, after burning
(Overbeck and Pfadenhauer 2007). Post fire, the contribution to total plant cover from caespitose grasses gradually increases
(Overbeck and Pfadenhauer 2007) and after several years can approach 100%, under suitable climatic conditions, leading to the
loss of a high proportion of other vascular plants. Thus plant diversity is “directly linked to the fire cycle” (Overbeck and
Pfadenhauer 2007 p. 36). Biomass accumulation has been well documented in Australian T. triandra grasslands, where Lunt and
Morgan (1998a) recorded doubling of T. triandra density in the first year post-fire, doubling again in the second year to c. 5
tonnes ha -1 with c. 50% the biomass dead, and doubling again >6 years post-fire, by which time biomass levels of 8 t ha -1 had
been reached of which over 5 t ha -1 consisted of dead material.
The mechanisms by which fire alters the composition and functioning of communites include the removal of the litter layer,
creation of bare ground for seedling establishment, removal of shade and transpiration and thus alteration of microclimate,
addition, depletion and creation of nutrients and the formation of ash beds. Fires in grasslands usually result in brief increases in
soil fertility (Hobbs and Heunneke 1992), with temporary increases in N, K, Ca, Mg and pH in the uppermost layer, but can
reduce fertility in the long term, depending on frequency, severity and season of burning (Overbeck et al. 2007). Nutrient
addition from fire could result in increased invasion (Hobbs 1989). Fire results in liberation of plant-stored N to the atmosphere
and alterations to the fire cycle resulting from grass invasion may thus impact on N fluxes in a grassland (Rossiter et al. 2003).
Particular plant species may be promoted or disadvantaged depending upon the frequency, seasonal timing and intensity of fires,
the condition of the vegetation, and the biology of the native and exotic plants in the community (Hobbs and Heunneke 1992,
Adair 1995, MacDougall and Turkington 2007). Fire probably commonly has differential effects on different life stages of
particular species (Overbeck and Pfadenhauer 2007) so deliberate grassland management using fire must consider not just the
seasonal phenologies of the native species (MacDougall andTurkington 2007) but the demographic structure of the species
populations (Hobbs and Heunneke 1992).
The effects of fire on the flora of native grasslands is dependent on its thoroughness (patchiness), which is related to fire
intensity but dependent on landscape features and local site characteristics such as presence of rocks and steep slopes, and on the
season of burning (Stuwe 1994, Overbeck and Pfadenhauer 2007). Fires during different seasons favour different sets of plants.
For instance annual species can be lost if fire occurs after full germination of their seed bank but before flowering (Kirkpatrick et
al. 1995). The patchiness of fires determines the amount of diversity at the community level in grasslands (Lunt and Morgan
2002).
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