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