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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INTRODUCTION<br />
Globally and locally, alien invasive plants are one <strong>of</strong> the most significant causes <strong>of</strong> degradation <strong>of</strong> natural ecosystems and<br />
amongst the greatest threats to the conservation <strong>of</strong> biodiversity (Carr 1993, Adair 1995, Vitousek et al. 1997, Adair and Groves<br />
1998, Williams and West 2000, Byers et al. 2002, Wang et al. 2009). Approximately 11% <strong>of</strong> the <strong>Australia</strong>n vascular plant flora<br />
consists <strong>of</strong> established exotic species (Vitousek et al. 1997), a 1990 total <strong>of</strong> c. 2000 species (Adair and Groves 1998).<br />
The Poaceae (<strong>grass</strong>es) contains many <strong>of</strong> the most damaging invasive plants. 22.9% <strong>of</strong> all <strong>grass</strong> species are recognised as weeds,<br />
the highest proportion <strong>of</strong> any <strong>of</strong> the major weedy plant families (Witt and McConnachie 2004). Globally, at least 668 genera and<br />
c. 2176 species <strong>of</strong> <strong>grass</strong> weeds have been recognised (Randall 2002).<br />
Communities subjected to anthropogenic disturbance and close to human development are more prone to invasion by exotic<br />
plants (Fox and Fox 1986, Hobbs 1991, Adair 1995, Adair and Groves 1998). Temperate <strong>grass</strong>lands worldwide have been<br />
conspicuously invaded (Aguiar 2005) and in <strong>Australia</strong> are one <strong>of</strong> the ecosystems most severely affected and heavily invaded by a<br />
wide range <strong>of</strong> exotic weeds (McIntyre and Lavorel 1994a, Groves and Whalley 2002). <strong>Chilean</strong> <strong>needle</strong><strong>grass</strong>, Nassella neesiana<br />
(Trinius and Ruprecht) Barkworth (Poaceae: Stipeae) is a relatively new threat.<br />
N. neesiana has many <strong>of</strong> the characteristics <strong>of</strong> a successful invasive species. It is a perennial, long-lived (Cook 1999), cool<br />
season (winter-spring growing), C 3 , South American, monecious, tussock <strong>grass</strong>, with a high survival rate <strong>of</strong> all life stages<br />
(Gardener et al. 1996a 1999 2003a). It is self fertile (Connor et al. 1993) but can cross pollinate and has a flexible reproductive<br />
strategy involving both chasmogamous and cleistogamous panicle seeds, along with concealed cleistogamous seeds on the stem<br />
nodes (Connor et al. 1993, Slay 2002c). In <strong>Australia</strong> it has been identified as an “aggressive” (McDougall and Morgan 2005 p.<br />
35), highly invasive (Morfe et al. 2003), high impact (Thorp and Lynch 2000) weed, which is rapidly expanding its range (Lunt<br />
and Morgan 2000). It’s rapid inter-regional spread from initial successful population confirms its among a set <strong>of</strong> the “most<br />
worrisome” invaders (Shea and Chesson 2002) and justifies it status as Weed <strong>of</strong> National Significance (Snell et al. 2007). It is<br />
both an environmental weed (Carr 1993) and a weed <strong>of</strong> agriculture (Grech 2007a).<br />
The invasiveness <strong>of</strong> N. neesiana in <strong>Australia</strong>n native vegetation seems to have first come to be widely acknowledged as a result<br />
<strong>of</strong> Carr et al. (1992 pp.41, 51) who considered it to be a “very serious threat to one or more vegetation formations in Victoria”.<br />
In native ecosystems it is reportedly able to actively invade <strong>grass</strong>lands (Hocking 1998 2007) and is potentially able to<br />
outcompete C 4 (summer growing) <strong>grass</strong>es such as Themeda triandra Forrsk. (Ens 2002a). Along with Nassella trichotoma<br />
(Nees) Hack. ex Arechavelata, it is rated as the most significant weed threat to temperate <strong>grass</strong>land biodiversity in <strong>Australia</strong><br />
(McLaren et al. 1998, Groves and Whalley 2002) and “the worst environmental weed threatening native <strong>grass</strong>lands” (Snell et al.<br />
2007).<br />
According to Kirkpatrick (1995 p. 77) N. neesiana has “the potential to almost totally displace the native flora” in lowland<br />
temperate <strong>grass</strong>lands. Kirkpatrick et al. (1995 p. 35) thought that it seemed to be “capable <strong>of</strong> dominating <strong>grass</strong>lands across cool<br />
temperate southeastern <strong>Australia</strong>”. Puhar and Hocking (1996) considered it “a serious emerging weed threat” and Liebert (1996<br />
p. 8) a “major threat” to native vegetation. Morgan (1998d) viewed it as one <strong>of</strong> the “most potentially threatening species” to T.<br />
triandra <strong>grass</strong>lands. Lunt and Morgan (2000 p. 98) rated it as “perhaps the most serious environmental weed in remnant native<br />
<strong>grass</strong>lands in southern Victoria” and Morgan and Rollason (1995) considered it to pose “by far the greatest threat <strong>of</strong> any<br />
potential new invader” at one <strong>grass</strong>land. Ens (2005) stated that it “swamps all other ground flora and forms expansive<br />
monocultures”. According to Beames et al. (2005 p. 2) it is “particularly well adapted to the intensively cultivated areas<br />
surrounding urban areas and poses a significant threat to mismanaged urban <strong>grass</strong>land remnants”.<br />
These opinions are based to various extents on supposition, personal observations and scientific study. Gardener and Sindel<br />
(1998 pp. 76-77) stated that there is “anecdotal evidence” that N. neesiana causes loss <strong>of</strong> plant biodiversity in <strong>grass</strong>lands<br />
“because litter from the tall tussocks accumulates in the inter-tussock spaces and excludes shade intolerant species”. However T.<br />
triandra, the major dominant <strong>grass</strong> <strong>of</strong> temperate <strong>Australia</strong>n <strong>grass</strong>lands, has a similar inhibitory effect as the time since fire or<br />
thinning increases (Stuwe and Parsons 1977). Diversity <strong>of</strong> bryophytes (mosses, liverworts) and lichens reportedly shows similar<br />
declines following N. neesiana invasion “because the mosaic <strong>of</strong> substrates such as rocks and bare soil becomes covered with<br />
litter” (Gardener and Sindel 1998 p. 77, citing V. Stasjic pers. comm.), as also happens in dense T. triandra (Scarlett 1994). A<br />
single study <strong>of</strong> N. neesiana impact on insects indicated that diversity declines, although some groups benefit (Ens 2002a 2002b).<br />
The success <strong>of</strong> N. neesiana as a weed has been widely attributed to its ability to produce a large, long-lived soil seed bank (e.g.<br />
Storrie and Lowien 2003, Gardener 1998) and to the widespread dispersal <strong>of</strong> seeds by human activities (Bourdôt 1988),<br />
particularly by mowing and slashing, and on livestock (Gardener 1998, Grech 2007a). However recent evidence (Hocking<br />
2005b) from southern <strong>Australia</strong> suggests that the seed bank in native <strong>grass</strong>lands is much lower and more transient than reported<br />
in New Zealand (Bourdôt and Hurrell 1992) and agricultural <strong>grass</strong>land on the Northern Tablelands <strong>of</strong> New South Wales<br />
(Gardener 1998).<br />
The success <strong>of</strong> N. neesiana is also attributable to the widespread availability <strong>of</strong> suitable climate and habitat that apparently lacks<br />
biotic resistance to it. N. neesiana has many <strong>of</strong> the charactersitics possessed by successful invasive species in general (New<br />
1994, Williamson and Fitter 1996, Cox 2004): a large native range, abundance in its native range, high vagility (via seed),<br />
dispersal by abiotic, synanthropic processes, short generation time (reputedly capable <strong>of</strong> seed production in its first year), high<br />
reproductive rate, ecological flexibility, wide climatic and physical tolerances, reproduction via a single parent and a general<br />
association with humans. In addition many varieties and forms have been described, suggesting the species has wide genetic<br />
variability, at least in South America. Studies <strong>of</strong> invasive plants in native ecosystems has largely focused on”single-factor<br />
explanations” for their success (Callaway and Maron 2006), but it is clear that N. neesiana invasion in <strong>Australia</strong> depends on a<br />
complex combination <strong>of</strong> the plant’s characteristics and the environments invaded.<br />
Williamson and Fitter (1996) stressed the importance <strong>of</strong> distinguishing between species specific to abundant habitats, and species<br />
that are habitat generalists, and note that a unique combination <strong>of</strong> factors account for each successful invasive species. N.<br />
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