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