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

2004, Richardson and van Wilgen 2004). Complex, simultaneous negative and positive effects are probably usual. For example
Lenz et al. (2003) found that the presence of annual exotic grasses on a hillside in one South Australian grassland facilitated
native perennial grass growth on upper slopes but impeded it at the lowest elevations.
Feedback processes, in which the invasive plant modifies the invaded environment or habitat for other organisms are doubtless
frequently important. The invader may increase temporal or spatial resource fluctuations and may increase the heterogeneity or
homogeneity of the area invaded in a wide variety of ways (Melbourne et al. 2007).
Impact on the invaded systems may include changes to:
1. Competitive interactions with other plants for light, nutrients, water, pollinators and other resources - resulting in changes in
species composition, niche displacement, or replacement of another species (Weiss and Noble 1984, Adair 1995, FFG SAC
1996, Woods 1997, Prieur-Richard and Lavorel 2000, Williams and West 2000, Levine et al. 2003, Vidler 2004).
2. Species richness or dominance patterns (Adair 1995, FFG SAC 1996,Woods 1997).
3. Physical structure and chemistry of the habitat (Adair 1995, FFG SAC 1996, Woods 1997, Williams and West 2000).
4. Alterations to animal health, habitat, food chains and trophic structure of communities (Williams and West 2000, Groves
2002, Low 2002, Levine et al. 2003).
5. Phenology of native species (Woods 1997).
6. Facilitating or allowing invasion of other species, including other plant or animal pests, or pathogens (Groves 2002).
7. Genetic changes, including rates and details of evolutionary interactions, introduction of foreign genes, hybridisation and
gene swamping (Carr 1993, FFG SAC 1996, Williams and West 2000, Cox 2004).
8. Disturbance regimes and successional pathways (Woods 1997, Vitousek et al. 1997, Mack and D’Antonio 1998, D’Antonio
et al. 1999, Prieur-Richard and Lavorel 2000).
9. Ecosystem function and ecosytem services (Versfeld and Van Wilgen 1986, Adair 1995, FFG SAC 1996, Prieur-Richard
and Lavorel 2000, Levine et al. 2003, Richardson and van Wilgen 2004) including nutrient cycling (Vitousek et al. 1997,
Rossiter et al. 2006), hydrological processes (Vitousek et al. 1997, Versfeld et al. 1998, Williams and West 2000),
geomorphological processes including soil erosion and landform (Adair and Groves 1998, Williams and West 2000), fire
cycles (D’Antonio and Vitousek 1992) and C storage (Seabloom et al. 2003).
10. Management regimes, resulting from altered management directed against the weed (Groves 2002).
More detailed explanations and examples of each of these rather arbitrary categories are provided below.
Competitive interactions
Competition by invasive plants is by far the most frequently invoked cause of invasive plant impact on biodiversity (Levine et al.
2003). ‘Weed competition’ with native plants had been identified as by far the most important cause in NSW, however in
approximately half of the instances the competing weeds were not identified, and only rarely have competitive mechanisms been
investigated (Downey and Coutts-Smith 2006). Mechanisms of competition between plants include sequestering of resources
(space, water, nutrients, light), and alterations to the pathways and rates of cycling of energy, water and nutrients (Levine et al.
2003, Grice 2004a, Vidler 2004). In general, the more resources an invasive plant obtains, the fewer are available for the invaded
community, and invaders that dominate resources can reasonably be expected to have high biodiversity impacts (Grice 2006).
The foliar cover achieved by successful invasive plants is a useful general indicator of their potential impact. Canopy dominance
reduces light availability to subsidiary species and clearly reflects biomass dominance, which ultimately indicates the extent to
which the plant monopolises available resources. A ‘canopy dominant’ environmental weed can totally or largely alter the nature
and functioning of an ecosystem by dominating, overtopping or replacing the natural canopy, while a subcanopy dominant can
have similar effects in a lower stratum (Swarbrick 1991). Woods (1997) argued that few experimental studies had demonstrated
that competition for light by invasive plants is a causal factor of community change and there was similarly little support for the
contention that competition for other limiting resources is important. This may largely be due to the lack and difficulty of
adequate study, rather than the unimportance of such effects. Multi-factor competition may well be usual (Levine et al. 2003).
Hautier et al. (2009) clearly demonstrated that competition for light causes losses of understorey species in experimental
grassland communities.
Asparagus asparagoides L. Druce (Liliaceae) has a strong negative impact on the endangered Pterostylis arenicola M. Clements
and J. Stewart (Orchidaceae) probably because both grow from tuberous roots in autumn and winter and senesce in spring and
summer (Groves 2002 2004). Root competition of this species has also been demonstrated to significantly reduce germination of
the endangered Pimelea spicata R.Br. (Thymelaeaceae) (Groves 2002 2004).
Competitive superiority of the invader to an ecologically similar native has been partially demonstrated for Chrysanthemoides
monilifera subsp. rotundata (DC.) Norl. which directly displaces Acacia sophorae (Labill.) R.Br. on coastal dunes in New South
Wales (Weiss and Noble 1984). Similarly a species may replace an ecological guild - Woods (1997, citing Chilvers and Burdon
1983, etc.) cites the example of Eucalyptus spp. replacement by Pinus radiata D. Don. in Australia.
Successful invasion resulting from superior competitive abilities may not result in any functional changes in ecosystem
properties, the invasive species essentially functioning like the displaced native (Adair and Groves 1998), but the examples cited
have, or will likely lead to more profound shifts in dominance patterns and the conversion of ecosystems to new types.
Species richness and dominance patterns
A consistent negative relationship has been found between the abundance or presence of invasive plants in Australian and that of
native species (Grice 2006). Invasive plants are recognised threats to whole communities, e.g. the aforementioned
Chrysanthemoides monilifera, which ultimately suppresses seedling establishment by most native species in some of the
ecosystems it occupies, including canopy trees (Groves 2004). In Australia, Mimosa pigra has replaced native sedgelands with
tall shurbland, Annona glabra L. has replaced wet grassland with closed forest, Acacia nilotica (L.) Willd.ex Del. has replaced
dry grassland with tall shrubland, rainforest has been converted to vine thickets by Thunbergia grandiflora Roxb., Mcfadyena
unguis-cati (L.) A. Gentry and Andredera cordifolia (Ten.) Steenis in Queensland (Panetta and Lane 1996), and Tamarix aphylla
(L.) H. Karst. Had replaces Eucalyptus camaldulensis woodland in the inland (Groves 2002). Amongst the invasive Poaceae,
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