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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Determining cause and effect in relation to N.neesiana invasion is complicated by this range <strong>of</strong> compounded factors. Changes in<br />
community composition and biodiversity coinciding with invasion by an alien <strong>grass</strong> may be the result <strong>of</strong> the anthropogenic<br />
disturbances, rather than the invader. This situation is exemplified by the natural <strong>grass</strong>lands and subsequent ‘invaded’ pastures <strong>of</strong><br />
<strong>Australia</strong>n and North America (see Mack 1989), where it is not possible in the historical context to separate the effects <strong>of</strong><br />
introduced grazing animals from that <strong>of</strong> the invasive plants (Woods 1997). Distinguishing between direct effects <strong>of</strong> disturbance<br />
on native plants and the competitive effects <strong>of</strong> invasive species has generally been difficult, as has determination <strong>of</strong> the<br />
interaction between disturbance and the invader (McIntyre 1993). Morgan (1998c p. 153) stressed that “conservation <strong>of</strong> many<br />
perennial native plants <strong>of</strong> species-rich <strong>grass</strong>lands would appear to be critically dependent on the conditions that maintain the<br />
standing [native] flora (i.e. low levels <strong>of</strong> canopy shading by the dominant <strong>grass</strong>es); that maintain flowering ... and that maintain<br />
opportunities for occasional successful recruitment, i.e. large canopy gaps”. The extent to which N. neesiana invasions prejudice<br />
these objectives are a critical issue in restricting its biodiversity impact.<br />
N. neesiana responds positively to anthropogenic disturbance. It appears to invade when there is nutrient enrichment with N and<br />
P, particularly the former, a condition that occurs when the major nutrient pool <strong>of</strong> the <strong>grass</strong>land system, contained in the roots<br />
and crowns <strong>of</strong> the dominant <strong>grass</strong>es, is mobilised by the death <strong>of</strong> these <strong>grass</strong>es. Exogenous nutrient inputs <strong>of</strong> diverse kinds,<br />
including water enrichment, may also be driving invasions. Although the dominant native Poaceae generally survive fire,<br />
burning appears to promote exotic annual weeds, and possibly releases smaller labile nutrient pools which N. neesiana may<br />
utilise in initial colonisation. Burning, grazing and herbicide application can all create bare ground which N. neesiana seedlings<br />
require to establish. Best practice mangement <strong>of</strong> T. triandra <strong>grass</strong>lands in Victoria generally includes regular burning to reduce<br />
T. triandra cover. Areas that are not burned rapidly loose their vascular plant diversity through loss <strong>of</strong> forbs, which generally<br />
have very small short-lived soil seed banks. N. neesiana invades T. triandra <strong>grass</strong>lands that are not burnt regularly or otherwise<br />
managed to reduce their biomass. T. triandra becomes senescent and dies under such conditions, resulting in liberation <strong>of</strong> the<br />
major nutrient stores in the crowns and roots. Other types <strong>of</strong> native <strong>grass</strong>land in south-eastern <strong>Australia</strong> are usually managed by<br />
low intensity livestock grazing to reduce <strong>grass</strong> biomass and maintain plant diversity. Periodical harvest <strong>of</strong> the livestock removes<br />
nutrients from the system and is probably a significant factor in protecting them from invasion.<br />
Native vascular plant diversity, overwhelmingly comprised <strong>of</strong> species that grow in the inter-tussocks spaces, also responds<br />
positively to disturbance. Regular biomass reduction is required to maintain open areas in T. triandra <strong>grass</strong>lands. The<br />
germination requirements <strong>of</strong> inter-tussock forbs are poorly understood and current poor recruitment levels are probably linked to<br />
the loss <strong>of</strong> vertebrate biodiversity. It is not clear whether the disturbances that facilititate invasion <strong>of</strong> N. neesiana and other<br />
weeds are <strong>of</strong> the same magnitude and type that are required to maintain existing native plant diversity,and it is not clear whether<br />
N. neesiana invasion is a cause <strong>of</strong> plant biodiversity loss, or a consequence <strong>of</strong> it.<br />
N. neesiana seed appears to be widely and commonly dispersed by human agencies. Propagule pressure is a pre-requisite for any<br />
plant invasion. Seed dispersal processes and rates are difficult to significantly reduce in the highly developed landscapes in<br />
which most remnant <strong>grass</strong>lands exist. All remnants are located in the cultural steppe or urban contexts and most have high<br />
perimeter : area ratios, allowing propagule pressure to be more or less relentless while the <strong>grass</strong> remains uncontrolled on<br />
roadsides, in pastures and in neglected areas. Protection <strong>of</strong> native <strong>grass</strong>land therefore requires greater emphasis on post-dispersal<br />
processes and minimisation <strong>of</strong> significant disturbance.<br />
Occupation <strong>of</strong> a site by an exotic perennial <strong>grass</strong> can effectively be permanent if there is no managment intervention. Heavily<br />
invaded, high nutrient <strong>grass</strong>lands constitute a metastable state that is very difficult to shift. Extensification <strong>of</strong> native pasture and<br />
more highly developed <strong>grass</strong>land tracts, and the restoration <strong>of</strong> native <strong>grass</strong>lands from degraded enriched <strong>grass</strong>lands require a<br />
similar set <strong>of</strong> techniques to reduce their nutrient levels and establish a large range <strong>of</strong> native plants. These techniques are poorly<br />
developed in <strong>Australia</strong> and have limited effectiveness.<br />
Given the trajectories <strong>of</strong> these factors, what is the prognosis for the impact <strong>of</strong> N. neesiana on biodiversity? Previous studies <strong>of</strong><br />
invasive <strong>grass</strong> impacts on biodiversity in <strong>Australia</strong> have correlated presence <strong>of</strong> the <strong>grass</strong> with reduced native vascular plant<br />
diversity (e.g. McArdle et al. 2004) and alterations in invertebrate communities (Ens 2002a), but have generally failed to<br />
demonstrate cause and effect relatioships: ‘matched’ invaded and uninvaded areas are selected, compared, found to be similar,<br />
and the <strong>grass</strong> is ultimately assumed to be the cause <strong>of</strong> the impacts detected. Prediction <strong>of</strong> impact requires an understanding <strong>of</strong> the<br />
causes and mechanisms <strong>of</strong> invasion, many <strong>of</strong> which are undoubtedly disturbance-related, and require historical ecological<br />
understanding <strong>of</strong> the area invaded. The prognosis is continued invasion, as management failures accumulate across the <strong>grass</strong>land<br />
realm, to the climatic limit. New biiotic constraints on N. neesiana will develop as predators, parasites and competing plants<br />
evolve. There is some evidence that N.neesiana is used by native organisms and has begun to accumulate predators. General<br />
considerations suggest that whatever native invertebrates utilise Austrostipa spp. may have some preadaptations that enable them<br />
to exploit N. neesiana or to host shift on to it, because <strong>of</strong> the close generic relationship <strong>of</strong> the hosts.<br />
N. neesiana is expected to support a small fauna <strong>of</strong> generalist phytophagous insects and may according to biotic resistance theory<br />
be favoured over a native <strong>grass</strong>es by some <strong>of</strong> the polyphagous generalist invertebrate herbivores. Native herbivores mammals<br />
should favour N. neesiana over native <strong>grass</strong>es. Where it displaces native <strong>grass</strong>es, specialist <strong>grass</strong> feeders with a narrow host<br />
range can be expected to be displaced. What this fauna might be is largely unknown. Detritovores dependent on <strong>grass</strong> litter may<br />
be little affected unless there are major differences in the nutrient content or indigestible components. N. neesiana has a different<br />
seasonal growth pattern which may restrict its utilisation by <strong>grass</strong>-feeding insects that have lifecycles synchronised with the<br />
native <strong>grass</strong>es. Little specificity is expected for insects with larvae that feed on <strong>grass</strong> roots, and a number <strong>of</strong> these probably<br />
exploit N. neesiana. Post dispersal seed feeders may be little affected or advantaged due to the similarilty <strong>of</strong> N. neesiana and<br />
Austrostipa seeds and seeding patterns. Some specialist Austrostipa feeders may be able to shift hosts to N. neesiana given the<br />
close relationship between the genera. The balance <strong>of</strong> evolved and adaptive changes might be expected to reduce the dominance<br />
<strong>of</strong> N. neesiana, but it too will evolve. Some evolved changes may happen rapidly and reduce the significance <strong>of</strong> N. neesiana as a<br />
weed, but predicting outcomes is far beyond our capabilities.<br />
There are many similarities in the biology and ecology <strong>of</strong> N. neesiana and dominant native caespitose <strong>grass</strong>es it displaces, and<br />
some differences. Both N. neesiana and T. triandra resprout post-fire and retain dead leaf material that promotes fire frequency.<br />
Both have large, awned seeds, probably adapted for dispersal over longer distances mainly by animals, but which usually fall<br />
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