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

D’Antonio 1998 p. 195). Alterations to fire regimes are the best documented (Versfeld and van Wilgen 1986, D’Antonio and
Vitousek 1992, Grice 2006), along with invasions by woody weeds (Adair and Groves 1998).
Altered disturbance regimes are less likely when the invasive species differs little from the natives, but subtly different
organisms can produce subtle changes (Mack and D’Antonio 1998). Permanent changes to disturbance regimes and successional
pathways eventually result in conversion of the system to a new state.
Ecosystem function and ecosytem services
Weeds that significantly modify ecosystem function affect the living conditions of all the other species in the system, and thus
have the most profound effects on biodiversity (Adair and Groves 1998). Most studies of these impacts have involved
comparison of invaded and uninvaded areas and require cautious interpretation because the mechanisms that enable one system
to be invaded and not the other are usually poorly understood (Levine et al. 2003).
Woody weed invasions of South African fynbos have resulted in major declines in stream flows and water yield (Versfeld and
van Wilgen 1986, Versfeld et al. 1998). Mechanisms of impact include increased interception of rainfall, increased transpiration
and changes in infiltration and erosion rates (Versfeld and van Wilgen 1986). Invasive species impacts can also result in
reductions in water use (Levine et al. 2003) and thus increased flooding and habitat change. The replacement of native perennial
bunchgrasses by annual exotic grasses in California grasslands has decreased the amount of C they store (see references in
Seabloom et al. 2003), and consequently the amount of atmospheric CO 2 and the extent of global warming. Impacts on nutrient
cycling have been widely investigated with a focus on nitrogen (particularly in grasslands) and leguminous plants (Levine et al.
2003).
In the most extreme cases weeds threaten whole ecosystems or ecosystem units, either by creating a new vegetation stratum or
by altering major ecosystem properties (Adair and Groves 1998). Acacia nilotica (L.) Delile thickets threaten Mitchell Grass
grasslands in the Northern Territory and northern Queensland by creating a dense overstorey, with over 1 million ha infested by
1992 (Parsons and Cuthbertson 1992). The grass-fire feedback cycle (D’Antonio and Vitousek 1992), where the biomass of an
invasive grass enhances fire (see below) is transforming ecosystems around the world.
Impacts on the delivery of ecosystem goods and services may be evident, but few studies have quantified the effects at regional
or wider scales (Richardson and van Wilgen 2004).
Management regimes
Management activities are targetted at significant weeds and often have unintended consequences. Attempts to reduce the
prevalence of Nassella trichotoma in Victoria by establishment of trees have attracted criticism that they will also eliminate the
native grassland remnants in which the weed is growing. Similarly, herbicidal control of exotic stipoid grasses in Melbourne area
grasslands has been criticised for its severe impact on native plants.
Invasive grasses - impacts and threats
Useful grasses have been widely introduced as forage plants for livestock and for other purposes, and many grasses of less forage
value have dispersed widely without deliberate human intervention. In many cases they have replaced native grasses, being the
agents, the beneficiaries, or both agents and beneficiaries of ecosystem transformation. These transformations have taken place
in many of the major temperate grassland regions of the world. North American grasslands have been subjected to massive
changes through the deliberate introduction of a wide range of grasses considered superior for livestock production including
Bothriochloa spp. (Schmidt et al. 2008). The grasslands of the Llanos, Venezuela, and savannah-forest in the Cerrado, Brazil,
both supported cattle grazing, but the Spanish and Portuguese immigrants considered the native grasses inferior and by the late
18th century had intoduced African species such as Brachiaria mutica (Forrsk.) Stapf, Melinis minutiflora P. Beauv. and
Panicum maximum Jacq., which are now the dominant species in huge areas of tropical and subtropical Latin America (Mack
and Lonsdale 2001). Similar transformations have occurred in Australia.
Poaceae is one of few plant families that consistently provides a high proportion of invasive species relative to its total taxa
(Rejmánek 2000). Poaceae is probably the dominant weed family in Australia in terms of areas occupied and species diversity.
Poaceae represent about 14% (375 spp.) of all naturalised vascular plant species in Australia, and 141 spp. (37.6%) are
considered major weeds (Grice 2004b). There are more species and infraspecific taxa of Poaceae in the exotic flora of Victoria
than any other family (Carr 1993).
Invasion of native plant communities by exotic perennial grasses is listed as a key threatening process under the NSW
Threatened Species Conservation Act 1995, based on the impact of five species, Hyparrhenia hirta, Cenchrus ciliaris, Eragrostic
curvula, N. trichotoma and N. neesiana (NSW Scientific Committee 2003, Downey in Virtue et al. 2004). Perennial grasses are
one of the major groups threatening biodiversity in NSW (Downey and Coutts-Smith 2006). N. neesiana, N. trichotoma and E.
curvula are among the major species recognised as threats in temperate grasslands (Kirkpatrick et al. 1995, Groves 2004). More
cautiously, Adair and Groves (1998 p. 9) suggested that N. neesiana invasion of temperate Themeda grasslands is “perhaps” an
example of simple species displacement, causing no significant functional changes in the ecosystem.
McArdle et al. (2004) investigated the impact of H. hirta by comparison of the botanical composition of matched invaded and
uninvaded areas in Kwiambal National Park, northern NSW, and demonstrated reduced native plant richness and projective
foliar cover in the ground strata of invaded areas. Exotic components of the system were not affected. The tendency of this plant
to dominate was quantified, with infested sites being more homogeneous. Chejara et al. (2006) further investigated the impact on
vascular plant diversity of this species on a travelling stock route near Manilla NSW. Where it was present it was the dominant
species. Invaded areas had native plant species richness significantly reduced by half or more, as determined by spring and
autumn surveys in 2003 and 2005, and native cover was significantly less in infested plots. A major fault with this study was that
the areas lacking H. hirta had been spot sprayed with glyphosate to control the plant from 2001 to 2004. These plots were found
to contain significantly greater numbers of exotic weed species in 2005. Another problem was that the invaded and uninvaded
areas were 1 km apart and were “similar in respect to soil, landform, drainage and apparent disturbance history”, but there was
no way to tell whether the vegetation prior to invasion had been similar (Chejara et al. 2006 p. 208)
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