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

Nassella neesiana
“Over the past few years is has become increasingly clear that the introduced grass Stipa neesiana is a serious threat to remnant
stands of native grassland ... and can almost completely displace perennial native grasses, including the dominant Themeda
triandra ... Prior disturbance does not appear to be necessary for invasion ... We request that urgent attention be given to control
of the plant ...”
M.J. Bartley, R.F. Parsons and N.H. Scarlett of the Botany Department, La Trobe University, in a letter to the Victorian
Department of Conservation and Environment, 7 May 1990
Nassella neesiana is a long-lived, perennial, cool season (winter-spring growing), C 3 , South American, monecious, tussockforming
grass, with flexible reproductive mechanisms and a high survival rate of all life stages (Cook 1999, Gardener et al.
1996a 1999, 2003a, Storrie and Lowien 2003, Benson and McDougall 2005). In Australia it is both a serious enivornmental
weed (Carr et al. 1992, McLaren et al. 1998) and a problem weed of agriculture (Grech 2007a).
N. neesiana is a widely recognised threat to Australian temperate native grasslands. One of the earliest reports was that of
McDougall (1987) who recorded it as a weed in native tussock grassland and Eucalyptus camaldulensis woodland in the western
region of Melbourne. Carr et al. (1992 pp. 41, 51) considered it to be a “very serious threat to one or more vegetation formations
in Victoria”. It has rapidly expanded its range (Lunt and Morgan 2000), is able to actively invade grasslands (Hocking 1998),
reportedly without prior disturbance (Bartley et al. 1990), is allegedly potentially able to outcompete C 4 (summer growing)
grasses such as T. triandra (Ens 2002a), and is rated, along with Nassella trichotoma, as the most significant weed threat to
grassland biodiversity (McLaren et al. 1998, Groves and Whalley 2002). Trengrove (1997) considered it to be “much more
invasive” in T. triandra remnants than N. trichotoma. Morgan and Rollason (1995) considered it to pose “by far the greatest
threat of any potential new invader” at one grassland, Kirkpatrick et al. (1995 p. 36) classed it as “a major threat to one or more
grassland communities”, Morgan (1998d) called it one of the “most potentially threatening species” to T. triandra grasslands and
Lunt and Morgan (2000 p. 98) rated it as “perhaps the most serious environmental weed in remnant native grasslands in southern
Victoria”. Humphries and Webster (1992 and 2003 p. 2) and Webster et al. (2003 p. 3) wrote that “the aggressive invasion” of N.
neesiana at Derrimut and Laverton North Grasslands in Victoria needed “immediate attention if the values of these grasslands
[were] to be preserved”. Ens (2005) stated that it “swamps all other ground flora and forms expansive monocultures”. According
to Beames et al. (2005 p. 2) it is “particularly well adapted to the intensively cultivated areas surrounding urban areas and poses
a signficant threat to mismanaged urban grassland remnants”.
N. neesiana is a successful invader because of it wide ecological flexibility, synanthropic dispersal mechanisms, the availability
of suitable habitats in a suitable climate, and limited biotic resistance in the invaded communities. It has many of the general
charactersitics possessed by successful invasive species (Cox 2004): a large native range, abundance and sometimes dominance
in its native range, high vagility (via seed), dispersal by abiotic processes, short generation time, high reproductive rate,
ecological flexibility, wide climatic and physical tolerances, reproduction involving a single parent individual and association
with humans. In addition many varieties and forms have been described, suggesting that the species has wide genetic variability,
at least in South America. Bourdôt and Hurrell (1989a p. 415) attributed its invasiveness in New Zealand sheep pastures not to
“high competitive ability” but rather to “adaptations that enable the plant to survive the hazards of semi-arid, low-fertility
environments”.
It is often generally contended that invasion only occurs as a result of disturbance and that Australia is more subject to invasion
than northern hemisphere biomes (e.g. New 1994 p. citing Di Castri 1990). Exotic stipoid grasses in Australia “generally invade
plant communities which are already highly degraded and have a history of disturbance” (Gardener and Sindel 1998 p. 76 citing
G. Carr pers. comm.), along with “lands with higher fertility soil often previously used for grazing or farming” (op. cit.). Since
temperate native grasslands often occupy highly fertile soils, and are communities dependent on disturbance for the maintenance
of their natural diversity, this appears to create a significant dilemma. The disturbance necessary to create gaps in the dominant
grass canopy, in which most of the native vascular plant diversity can flourish, simultaneously promotes exotic plants, which
comprise the bulk of the seed bank at least at some sites (Lunt 1990b, Morgan 1998c).
Gardener and Sindel (1998) advocated quantitative studies to evaluate the biodiversity changes associated with invasion by
exotic Nassella species, to determine if any of these changes result from general degradation and to evaluate the impact of
Nassella management techniques on the promotion or inhibition of biodiversity. To date, only a single study of N. neesiana, with
a narrow focus, has been undertaken, that of Ens (2002a). She found that diversity of insects declined in areas occupied by the
plant in Sydney woodlands, although some groups benefited. Monitoring of biodiversity is necessary to provide a sound basis for
evaluating the techniques and strategies used to manage N. neesiana as a weed (Grice 2004a). Mangement measures are
currently focused on minimising seed production using herbicides or grazing in agricultural (Grech 2007a) and natural areas, and
managing the rate of mineralisation of nitrogen to favour C 3 grasses in natural ecosystems (Craigie and Hocking 1998, Hocking
2002 2005b, Groves and Whalley 2002). However there are concerns that some current approaches may be counterproductive,
producing grasslands that are more impoverished, and more highly invasible.
The success of a plant invader depends on its biological attributes, the attributes of the community or ecosystem invaded and the
effects of human activities. These factors are examined in detail in the review below.
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