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

asexual seed production, so local fungal epidemics can damage whole populations, increasing the likelihood of further ecological
damage (Puckey and Albrecht 2004).
Temperate Australia appears to be less susceptible to an invasive species grass-fire cycle, in part because climatic conditions
mitigate against very high biomass production. Milberg and Lamont (1995) inferred increased fire susceptibility due to invasion
by Ehrharta calycina and Eragrostis curvula on roadsides in Western Australia. E. calycina was also implicated as a cause of
more frequent fire by Virtue and Melland (2003), as were Cortaderia spp. in Tasmania by Harradine (1991). McArdle et al.
(2004) suggested that Hyparrhenia hirta has the potential to induce a positive feedback fire cycle because of its dense tussock
form that may protect the growing points from fire damage. Stoner et al. (2004) demonstrated that invasive Phalaris aquatica
produced approximately three times the fine fuel biomass of T. triandra, the grass it replaced in their study area of southern
Victoria, and argued that the increased fire intensity and flame residency and burnout times would be more likely to irreversibly
damage native plant communities.
Invasive plants may also decrease the intensity or frequency of fire. Succulent plants or mesic species can have this effect (Carr
1993). However, as with Pittosporum undulatum in south-eastern Australian it may be difficult to tell whether the plant is
reducing the fire-proneness of the vegetation or invading as a result of a pre-existing reduction of burning (Carr 1993). N.
neesiana might reduce the incidence or severity of fire in spring in Themeda triandra grassland by increasing the ratio of green
to dry vegetation in the standing crop (N. neesiana being a spring grower and T. triandra a summer grower), or it might possibly
reduce fire in general by producing a smaller amount of flammable material than the plants it displaces.
Impacts on nutrient cycling
Several African grasses are known to fix significant levels of N in their native habitats (Rossiter et al. 2003). Invasive grasses
can also alter N fixation rates by displacing legumes or by reducing the litter of other plants that support non-symbiotic N fixers
(Rossiter et al. 2003). Invasive grasses may produce litter with different physical and chemical properties which accumulates and
decays at altered rates and seasons (Grice 2004a). Higher C:N and lignin:N ratios in the foliage and litter may reduce nitrogen
mineralisation rates (Levine et al. 2003). The decomposition rates of invasive grass litter was lower than that of native grasses in
three of six cases reviewed by Rossiter et al. (2003).
Where invasive grasses displace summer growing species there is reduced uptake N mineralised in summer, so more is lost by
leaching after autumn and winter rains (Sinclair 2002).
Other effects
Another class of feedback effects include erosion and soil stabilisation. According to Heyligers (1986) the introduced coastal
dune grasses Ammophila arenaria and Thinopyrum junceiforme are more efficient at trapping sand and better colonisers of the
backshore zone than native dune grasses. The dunes they build are larger and have a different shape. They also build foredunes
in areas where the native grasses would be ineffective sand stabilisers. Changes in erosion patterns resulting from substrate
stabilisation are also caused by Spartina (Gray et al. 1997) and Cynodon dactylon (Mack and D’Antonio 1998).
More complex alterations to disturbance regimes occur with grazing. Caldwell et al.(1981) found that invasive Agropyron repens
(L.) Beauv. had greater photosynthetic capacity in its new growth and recovered more quickly after grazing than a dominant
native species Agropyron spicatum (Pursh) Scribn. and J.G. Sm., and that these factors were driving species replacement over
large areas. Thus invasive grasses have the potential to alter successional dynamics (Grice 2004).
Impacts of N. neesiana
Hocking (1998 p. 86) argued that the biodiversity impact of N. neesiana in Australia was “likely to be major” in part because it
was known to be “actively invading high quality grassland remnants at much higher rates than serrated tussock and to have a
greater potential for invasion of grassy woodlands, over a wide range of climatic conditions” (Hocking 1998 p. 89). Earlier,
Morgan (1994 p. 88) considered it to be“one of the most troublesome grassy weeds of grasslands”. However major biodiversity
impacts are more likely to arise from weeds with “growth forms that are novel to the invaded ecosystem [rather] than growth
forms for which there is a native ecological analogue” (Grice 2004a p. 55). Such weeds are more likely to be ‘transformer
species’. N. neesiana has a growth form similar to a number of native species that are commonly dominant or subdominant in
temperate native grasslands in south eastern Australia. Various Austrostipa and Austrodanthonia species have similar tussock
forming habits and stature, have similar cool-season growth periods and probably a markedly similar phenology. Hocking (1998
p. 86) also observed that “some well-managed” native grassland remnants have shown resistance to invasion “but further
documentation is needed”.
Exotic stipoid grasses including N. neesiana have been identified as “one of the most significant issues ... threatening nationally
important remnant grasslands in Australia” (McLaren, Stajsic and Iaconis. 2004). N. neesiana has been identified as a particular
threat to numerous grasslands, e.g. notably by Craigie (1993) as a “very serious threat to the integrity” of the Laverton North
Grassland, because few native plant species survive beneath dense infestations. According to Craigie (1993) “prior disturbance”
did “not seem to be necessary” for N. neesiana invasion and it was “invading the margins of swamp depressions and spreading
out from those” with most infestations in areas where Themeda cover was sparse. She observed that “It grows back more quickly
than other perennials after burning and [cleistogenes] ... may partially escape burning”, that it “aggressively colonise[d] the intertussock
spaces” and initially “grows faster than native species”. Liebert (1996 p. 8) noted that it “quickly invades disturbed
soils” resulting from revegetaion programs, while a report by Bob Bates (Jessop et al. 2006 p. 108) observed that it wa: “able to
become established on even the hardest bare sites on disturbed ground”.
Despite inclusion amongst the few exotic perennial grasses listed as a key threatening process in NSW, N.neesiana is not listed
by Coutts-Smith and Downey (2006) as posing a threat to threatened biodiversity in NSW. However this indicates a failure both
of their literature review technique (for Ens (2005) had previously stated that N. neesiana “threatens the ecological integrity of
affected natural ecosystems”) and the administrative process of threat identification in NSW, and also reflects the general lack of
integration of weed impact literature. A poor historical linkage between biodiversity conservation and invasive species
management is also to blame (Downey and Cherry 2005).
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