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

INTRODUCTION
Globally and locally, alien invasive plants are one of the most significant causes of degradation of natural ecosystems and
amongst the greatest threats to the conservation of biodiversity (Carr 1993, Adair 1995, Vitousek et al. 1997, Adair and Groves
1998, Williams and West 2000, Byers et al. 2002, Wang et al. 2009). Approximately 11% of the Australian vascular plant flora
consists of established exotic species (Vitousek et al. 1997), a 1990 total of c. 2000 species (Adair and Groves 1998).
The Poaceae (grasses) contains many of the most damaging invasive plants. 22.9% of all grass species are recognised as weeds,
the highest proportion of any of the major weedy plant families (Witt and McConnachie 2004). Globally, at least 668 genera and
c. 2176 species of grass weeds have been recognised (Randall 2002).
Communities subjected to anthropogenic disturbance and close to human development are more prone to invasion by exotic
plants (Fox and Fox 1986, Hobbs 1991, Adair 1995, Adair and Groves 1998). Temperate grasslands worldwide have been
conspicuously invaded (Aguiar 2005) and in Australia are one of the ecosystems most severely affected and heavily invaded by a
wide range of exotic weeds (McIntyre and Lavorel 1994a, Groves and Whalley 2002). Chilean needlegrass, Nassella neesiana
(Trinius and Ruprecht) Barkworth (Poaceae: Stipeae) is a relatively new threat.
N. neesiana has many of the characteristics of a successful invasive species. It is a perennial, long-lived (Cook 1999), cool
season (winter-spring growing), C 3 , South American, monecious, tussock grass, with a high survival rate of all life stages
(Gardener et al. 1996a 1999 2003a). It is self fertile (Connor et al. 1993) but can cross pollinate and has a flexible reproductive
strategy involving both chasmogamous and cleistogamous panicle seeds, along with concealed cleistogamous seeds on the stem
nodes (Connor et al. 1993, Slay 2002c). In Australia it has been identified as an “aggressive” (McDougall and Morgan 2005 p.
35), highly invasive (Morfe et al. 2003), high impact (Thorp and Lynch 2000) weed, which is rapidly expanding its range (Lunt
and Morgan 2000). It’s rapid inter-regional spread from initial successful population confirms its among a set of the “most
worrisome” invaders (Shea and Chesson 2002) and justifies it status as Weed of National Significance (Snell et al. 2007). It is
both an environmental weed (Carr 1993) and a weed of agriculture (Grech 2007a).
The invasiveness of N. neesiana in Australian native vegetation seems to have first come to be widely acknowledged as a result
of Carr et al. (1992 pp.41, 51) who considered it to be a “very serious threat to one or more vegetation formations in Victoria”.
In native ecosystems it is reportedly able to actively invade grasslands (Hocking 1998 2007) and is potentially able to
outcompete C 4 (summer growing) grasses such as Themeda triandra Forrsk. (Ens 2002a). Along with Nassella trichotoma
(Nees) Hack. ex Arechavelata, it is rated as the most significant weed threat to temperate grassland biodiversity in Australia
(McLaren et al. 1998, Groves and Whalley 2002) and “the worst environmental weed threatening native grasslands” (Snell et al.
2007).
According to Kirkpatrick (1995 p. 77) N. neesiana has “the potential to almost totally displace the native flora” in lowland
temperate grasslands. Kirkpatrick et al. (1995 p. 35) thought that it seemed to be “capable of dominating grasslands across cool
temperate southeastern Australia”. Puhar and Hocking (1996) considered it “a serious emerging weed threat” and Liebert (1996
p. 8) a “major threat” to native vegetation. Morgan (1998d) viewed it as one of the “most potentially threatening species” to T.
triandra grasslands. Lunt and Morgan (2000 p. 98) rated it as “perhaps the most serious environmental weed in remnant native
grasslands in southern Victoria” and Morgan and Rollason (1995) considered it to pose “by far the greatest threat of any
potential new invader” at one grassland. 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 significant threat to mismanaged urban grassland remnants”.
These opinions are based to various extents on supposition, personal observations and scientific study. Gardener and Sindel
(1998 pp. 76-77) stated that there is “anecdotal evidence” that N. neesiana causes loss of plant biodiversity in grasslands
“because litter from the tall tussocks accumulates in the inter-tussock spaces and excludes shade intolerant species”. However T.
triandra, the major dominant grass of temperate Australian grasslands, has a similar inhibitory effect as the time since fire or
thinning increases (Stuwe and Parsons 1977). Diversity of bryophytes (mosses, liverworts) and lichens reportedly shows similar
declines following N. neesiana invasion “because the mosaic of substrates such as rocks and bare soil becomes covered with
litter” (Gardener and Sindel 1998 p. 77, citing V. Stasjic pers. comm.), as also happens in dense T. triandra (Scarlett 1994). A
single study of N. neesiana impact on insects indicated that diversity declines, although some groups benefit (Ens 2002a 2002b).
The success of N. neesiana as a weed has been widely attributed to its ability to produce a large, long-lived soil seed bank (e.g.
Storrie and Lowien 2003, Gardener 1998) and to the widespread dispersal of seeds by human activities (Bourdôt 1988),
particularly by mowing and slashing, and on livestock (Gardener 1998, Grech 2007a). However recent evidence (Hocking
2005b) from southern Australia suggests that the seed bank in native grasslands is much lower and more transient than reported
in New Zealand (Bourdôt and Hurrell 1992) and agricultural grassland on the Northern Tablelands of New South Wales
(Gardener 1998).
The success of N. neesiana is also attributable to the widespread availability of suitable climate and habitat that apparently lacks
biotic resistance to it. N. neesiana has many of the charactersitics possessed by successful invasive species in general (New
1994, Williamson and Fitter 1996, Cox 2004): a large native range, abundance in its native range, high vagility (via seed),
dispersal by abiotic, synanthropic processes, short generation time (reputedly capable of seed production in its first year), high
reproductive rate, ecological flexibility, wide climatic and physical tolerances, reproduction via a single parent and a general
association with humans. In addition many varieties and forms have been described, suggesting the species has wide genetic
variability, at least in South America. Studies of invasive plants in native ecosystems has largely focused on”single-factor
explanations” for their success (Callaway and Maron 2006), but it is clear that N. neesiana invasion in Australia depends on a
complex combination of the plant’s characteristics and the environments invaded.
Williamson and Fitter (1996) stressed the importance of distinguishing between species specific to abundant habitats, and species
that are habitat generalists, and note that a unique combination of factors account for each successful invasive species. N.
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