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

Perunga grasshopper Perunga ochracea (Sjöstedt)
The Perunga grasshopper Perunga ochracea (Sjöstedt) (Acrididae: Catantopinae) is a medium sized, flightless (Farrow 1999),
almost wingless grasshopper with a narrow distribution in the ACT and neighbouring areas of NSW (Rentz et al. 2003). Farrow
(1999) found the species at 6 of 11 grasslands surveyed by sweep netting in the ACT, but only in spring, and considered it to be
unusual amongst the grassland Orthoptera in being winter-spring active. It occurs in T. triandra (Farrow 1999), Austrostipa and
Austrodanthonia grasslands, feeds on forbs, overwinters as a nymph and is present as adults during spring and summer (Rentz et
al. 2003). In the ACT it has disappeared from areas where it was once common, possibly as a result of “encroachment of dense
cover of introduced grasses” (Rentz et al. 2003). P. ochracea is officially declared vulnerable in the ACT (ACT Government
2005).
Lewis’s Laxabilla, Laxabilla smaragdina Sjöstedt
Lewis’s Laxabilla, Laxabilla smaragdina Sjöstedt (Acrididae: Oxyinae) is a small grasshopper with wingless females and fully
winged or brachypterous males, found in grasslands and “open savannah” from southern NSW to Mackay, Queensland (Rentz et
al. 2003). Farrow (1999) noted that it had not been recorded in the ACT for 20 years.
Impact of N. neesiana on invertebrates
Weed invasion can eliminate native host plants and may enhance the spread of exotic invertebrates (Yen 1995).
Ens (2002a) conducted the only study to date of the effects of N. neesiana on invertebrates. She studied two endangered
ecological communities in New South Wales: the edge of remnant Cumberland Plain Woodland (grassy woodland) at St Clair
and much altered Sydney Coastal River-flat Forest (a coastal swamp forest) at Mt Annan. Pitfall trap and vacuum sampling were
undertaken to enable comparison of areas dominated by N. neesiana and relatively devoid of native ground cover species, and
native areas relatively free of N. neesiana. Sites had similar distrubance history, geology, topography and proximity to water.
Point quadrats were assessed to quantify basal cover of N. neesiana, other exotic plants, native plants, bare ground, Eucalyptus
litter, grass litter and sticks. Tree canopy cover was estimated using charts. Vegetation community structure was assessed by a
point-height method with 4 height classes. Temperature and light were also measured above and below foliage, as well as
distance to the nearest tree. Ens (op. cit.) reported significant quantitative impact, with a negative effect of N. neesiana on
Formicidae and 3 Formicidae spp., reportedly “by altering the ground cover composition”, and on mean abundance of
Thysanoptera and Cicadidae moults, but a beneficial effect (“significant habitat”) on Blattodea and two unidentified Coleoptera
spp. Abundance of Collembola, Hemiptera, Gastropoda, Lepidoptera larvae and Araneae was significantly reduced in invaded
areas. These results were attributed to the altered habitat structure and “change in plant architecture” i.e. the scale, complexity
and heterogeneity of plants in the invaded community, and “indirect effects on the trophic heirarchy”. Ens (2005) summarised
her results as reduced ant abundance and alteration of “the entire invertebrate community composition”.
However the higher proportion of bare ground in the native vegetation explained the effects on Formicidae at one site and an
increased cover of Eucalyptus bark at the other, neither necessarily related to N. neesiana effects. The effects on one ant species
was best explained by the higher weed richness in the native vegetation. Multiple regression analysis failed to reveal a sensible
cause for decreased Thysanoptera or inreased Blattodea. The abundance of cicada moults was explained by Eucalyptus bark
cover, suggesting the native plots were closer to trees, the roots of which some Cicadidae nymphs feed upon, however the
reduction in bark cover was attributed as an effect of N. neesiana (Ens 2002a p. 67) and there was no correlation with the
variable ‘distance to nearest tree’. Some cicadas are grass feeders, so this may be a host plant influence. Identification of the
species would have helped resolve this question. A number of correlations between environmental variables or higher taxa and
other taxa with significantly different abundance in the N. neesiana areas do not make much biological sense e.g. Hemiptera
were more abundant in greater litter depths of the native areas (?protection from predation), gastropods were more abundant in
areas with more sticks (?protected from predation), Araneae with abundance of larvae (which they don’t consume) but not larvae
with abundance of Araneae, but these perhaps await fuller explanation. No trophic cascades or indirect effects on the trophic
heirarchy are clear. No attempt was made to distinguish exotic and native invertebrates, pest or beneficial species, or widespread
versus rare taxa and no trophic links to N. neesiana were identified.
The main effects were attributed to “changes in habitat parameters, cascade effects to higher trophic levels, changes to
invertebrate community structure … decreases [in] ground temperature and ground incident light ... [and a] thick layer of foliage
10-20 cm above ground when a thick monoculture” (Ens 2005). These however were correlations and may not represent true
causative relationships. Dense growth of T. triandra appears likely to produce a very similar set of effects and alter community
structure in a similar way.
Grassland restoration
Restoration of degraded, weed-invaded grasslands is difficult. Corbin et al. (2004) advocated an integrated approach utilising all
available tools, including traditional weed management techniques, fire, grazing, and reduction of soil N availability, along with
measures that increase the abundance of native seeds and seedlings. Reintroduction of the disturbance regimes to which the
systems are adapted is usually the first step in restoration (MacDougall and Turkington 2007). But the original functioning of the
system may not be properly understood, the disturbances may function in different ways because the degraded systems differ
from the original ones and there may be substantial risks, especially related to small species populations, and costs (MacDougall
and Turkington 2007). “The more degraded the site, the less likely the recovery by native species other than those already
present. Supplemental measures targeting dispersal and the survival of juveniles are needed in conjunction with treatments that
reduce the disturbance-sensitive competitive dominants” (MacDougall andTurkington 2007 p. 270).
Preservation of native grassland remnants has been both politically and ecologically challenging. Simultaneous limited
understanding of ecology. A critical turning point where most of the saveable remnants have been saved or their value
appreciated and are being managed conservatively to preserve or enhance their biodiveristy. Future challenge of widespread
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