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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neesiana appears to be a habitat generalist and <strong>Australia</strong> evidently <strong>of</strong>fers abundant suitable habitat, but the reasons for the<br />
success <strong>of</strong> this species in <strong>Australia</strong> remain poorly understood.<br />
Management measures for N. neesiana in agricultural areas are currently focused on maximising utilisation by livestock and<br />
minimising seed production using herbicides or grazing (e.g. Grech 2007a), and in natural areas on control <strong>of</strong> new outbreaks and<br />
some serious infestations with herbicides, managing the rate <strong>of</strong> mineralisation <strong>of</strong> nitrogen to favour C 3 <strong>grass</strong>es (Groves and<br />
Whalley 2002, Hocking 2005b 2007).<br />
Carr (1993 p. 278) observed that it is “a general tenet <strong>of</strong> <strong>Australia</strong>n weed ecology that disturbance is a prerequisite for invasion”.<br />
It is also widely suggested that <strong>Australia</strong> is more subject to invasion than Northern Hemisphere biomes (Crosby 1986, New 1994<br />
citing Di Castri 1990), because <strong>of</strong> its relative biogeographical isolation over a long period (McIntyre and Lavorel 1994a).<br />
Disturbances <strong>of</strong> various types operate at all spatial and temporal scales and may have individualistic effects on each organism in<br />
a community and potential new entrants to that community. A community or ecosystem is the consequence <strong>of</strong> all disturbances<br />
that have acted over the period in which it has been assembled. Natural disturbances are essential to maintain native vegetation<br />
(Hobbs 1991). To understand the role <strong>of</strong> disturbance it is therefore critical to distinguish between perturbations that have been<br />
formative factors over evolutionary, biogeographical and ecological time (the endogenous disturbances <strong>of</strong> Fox and Fox 1986),<br />
and those perturbations that are <strong>of</strong> new types or are extraordinary and contribute to community destruction (usually exogenous,<br />
human induced disturbances, but also geological and cosmological) (McIntyre and Lavorel 1994a, Lockwood et al. 2007).<br />
Distinguishing between such disturbance regimes is difficult in systems, such as <strong>Australia</strong>n temperate <strong>grass</strong>lands, that are poorly<br />
understood historically (Adair 1995). Palaeoecological knowledge <strong>of</strong> ecosystems has a crucial role in understanding biological<br />
invasions (Froyd and Willis 2008), but appears to be fragmentory, at best, for these <strong>grass</strong>lands. Fire and grazing are two <strong>of</strong> the<br />
most important disturbance factors that have operated both formatively and destructively. Much <strong>of</strong> the focus <strong>of</strong> <strong>grass</strong>land<br />
management has consisted <strong>of</strong> attempts to reinstitute or imitate supposed natural disturbance regimes that are poorly understood,<br />
in a new context in which invasive exotic species and exogenous human disturbances (fragmentation, N enrichment, global<br />
warming, etc.) have historically had pr<strong>of</strong>ound influence and continue to have pervasive effects. Thus there is an ongoing<br />
requirement to assess contemporary, ‘managed’, disturbance regimes in terms <strong>of</strong> their new effects.<br />
Disturbance is <strong>of</strong>ten necessary in <strong>Australia</strong>n temperate <strong>grass</strong>land to maintain the canopy gaps between the tussocks <strong>of</strong> the<br />
dominant <strong>grass</strong>es in which most <strong>of</strong> the plant diversity <strong>of</strong> the system exists. But the same sorts <strong>of</strong> disturbances also promote exotic<br />
plants, which comprise the bulk <strong>of</strong> the soil seed bank (Lunt 1990 etc.). Colonisation by novel plants is itself a disturbance factor.<br />
Hobbs (1991) found that disturbances increase invasion if they increase the availability <strong>of</strong> a resource that limited the invader<br />
prior to disturbance and are accompanied by propagule pressure. Identification <strong>of</strong> the characteristic <strong>of</strong> disturbance regimes that<br />
favour particular weeds and suites <strong>of</strong> weeds, and the management <strong>of</strong> these disturbances is one <strong>of</strong> the most critical tasks in<br />
minimising environmental weed impact (Adair 1995).<br />
Gardener and Sindel (1998) advocated quantitative studies to evaluate the biodiversity impacts <strong>of</strong> N. neesiana, compare the<br />
impacts resulting from general degradation, and evaluate the effects <strong>of</strong> N. neesiana management techniques on the promotion or<br />
inhibition <strong>of</strong> biodiversity. Grice (2004a) concurred with the need for such studies, noting that monitoring <strong>of</strong> biodiversity can be<br />
an important tool in evaluating a weed management strategy.<br />
The success <strong>of</strong> a plant invader depends on its biological attributes, the attributes <strong>of</strong> the communities and ecosytems that are<br />
potentially invasible, and the effects <strong>of</strong> human interference. Bourdôt and Hurrell (1989a p. 415) considered the invasiveness <strong>of</strong><br />
N. neesiana in sheep pastures to be due to “adaptations that enable the plant to survive the hazards <strong>of</strong> semi-arid, low-fertility<br />
environments, rather than to high competitive ability”.<br />
In the <strong>review</strong> which follows, the invasion <strong>of</strong> <strong>Australia</strong>n <strong>grass</strong>lands by N. neesiana is first contextualised within the frameworks<br />
<strong>of</strong> current biological invasion theories and hypotheses. The biological and ecological attributes <strong>of</strong> the plant are then examined in<br />
detail, along with its history, impacts and management in <strong>Australia</strong>. Next, the concept <strong>of</strong> biodiversity is discussed. The<br />
components <strong>of</strong> biodiversity and the ways they may be assessed are examined, along with the nature <strong>of</strong> weed impacts on the<br />
various components <strong>of</strong> native ecosystems. In section 4, knowledge <strong>of</strong> the properties <strong>of</strong> the invaded <strong>grass</strong>lands, including their<br />
components and dynamics are <strong>review</strong>ed. Finally an attempt is made to synthesise this knowledge into a complete picture <strong>of</strong> what<br />
is and is not known about the impact <strong>of</strong> N. neesiana on biodiversity in <strong>Australia</strong>’s indigenous <strong>grass</strong>lands.<br />
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