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

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