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

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