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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Nematodes are mostly minute animals that can be extremely abundant in soils and on plants, particularly on the roots. Numerous<br />
species are associated with <strong>grass</strong>es in <strong>Australia</strong> and several have been recorded from native Stipeae. These are discussed in an<br />
Appendix to this literature <strong>review</strong>.<br />
Exotic invertebrates are <strong>of</strong>ten widespread and common in native <strong>grass</strong>lands, particularly in urban fringe areas (personal<br />
observations). Collembola is one group with a substantial proportion <strong>of</strong> exotics, however many higher taxa including various<br />
Coleoptera families and Formicidae are largely or almost entirely dominated by native species (New 2000). Common exotics<br />
present in <strong>grass</strong>lands include molluscs (slugs and snails), red legged earth mites Halotydeus destructor (Tucker), insects such as<br />
the aforementioned aphids, the African black beetle Heteronychus arator (Scarabeidae: Dynastinae), the weevils Graphognathus<br />
leucoloma (Boheman) and Sitona discoideus (Gyllenhal) (Gregg 1997, Hill et al. 1997),the European honeybee Apis mellifera,<br />
and possibly earthworms (Annelida: Haplotaxida). Morgan (1995b) found that H. destructor grazed seedlings <strong>of</strong> the endangered<br />
Rudidosis leptorrhynchoides but did not significantly affect their survival, and that grazing by unidentified slugs occurred mostly<br />
on very young seedlings and caused little damage. Exotic Chriothrips spp. (Thysanoptera: Thripidae) are found on <strong>grass</strong>es in<br />
south-eastern <strong>Australia</strong> (Mound and Palmer 1972). The <strong>grass</strong>land daisy Senecio macrocarupus is subject to attack by red-legged<br />
earthmites Halotydeus destructor (Tucker) and aphids (Hills and Boekel 2003). H. destructor and blue oat mites Penthaleus<br />
major (Dugès) attack a wide range <strong>of</strong> native <strong>grass</strong>land forbs including species <strong>of</strong> Craspedia, Podolepis and Senecio (Robinson<br />
2005). The Portuguese Black Millipede Ommatoiulus moreletti (Lucas) (Julidae) has spread into Victorian basalt plains<br />
<strong>grass</strong>lands, and although there is no evidence <strong>of</strong> competition with native millipedes, it may influence litter decay rates (Yen<br />
1995). Many <strong>of</strong> the invasive insect species probably have significant effects on <strong>grass</strong>land plants and the ecological functioning <strong>of</strong><br />
the system, but there appear to be no studies that have directly addressed particular impacts. The agricultural literature, although<br />
restricted to few species, contains some valuable data about invertebrate impact on ecosystem processes, particularly effects on<br />
plant biomass production (Gregg 1997) and soils.<br />
A range <strong>of</strong> exotic molluscs are considered pests <strong>of</strong> pastures (Smith and Kershaw 1979, Kershaw 1991) and have been suggested<br />
to be important predators <strong>of</strong> sensitive native plants such as orchids (Sydes 1994, Daniell 1994). Exotic slugs “appear to be highly<br />
invasive <strong>of</strong> native <strong>grass</strong>lands” and were found to be equally abundant in <strong>grass</strong>lands dominated by exotic and native <strong>grass</strong>es in the<br />
ACT (Melbourne et al. 1997 p. 366). Daniell (1994) recorded no or few species <strong>of</strong> native molluscs at four Melbourne <strong>grass</strong>lands,<br />
and observed that exotic slugs can occur at very high densities in native <strong>grass</strong>lands. Holland et al. (2007) found only exotic<br />
molluscs, three snail and five slug species, in extensive surveys in the Victorian volcanic plains. A few mollusc species have<br />
been identified as threats to particular native taxa, e.g. unidentified slugs and snails are considered a threat to Diuris<br />
fragrantissima (Webster et al. 2004), the Black-keeled Slug Milax gagates (Draparnaud) defoliates Rutidosis leptorrhynchoides<br />
(Daniell 1994), slug predation has caused the loss <strong>of</strong> a Pterostylis nutans R.Br. (Orchidaceae) colony (Sydes 1994), while<br />
unidentified slugs and possibly Common Garden Snails Helix aspersa (Müller) caused mortality <strong>of</strong> Thesium australe planted at<br />
Lake Omeo (Scarlett et al. 2003). Lenz et al. (2003 p. 30) applied molluscide in a low-diversity, weedy native <strong>grass</strong>land with a<br />
high population <strong>of</strong> the white snail Cernuella virgata da Costa and found that snail exclusion had no effect on vascular plant<br />
species composition over 9 months. The dietary preferences <strong>of</strong> introduced herbivorous molluscs in regard to native plants do not<br />
appear to have been investigated (Lenz et al. 2003) except for M. gagates which was found by Holland et al. (2007) to prefer<br />
some native plants over others, with the two native <strong>grass</strong>es tested having relatively low palatability.<br />
Grass-feeding insects<br />
Various insect groups that avoid plants with toxic metabolites are restricted to Poaceae including some Acrididae (Orthoptera)<br />
and Auchenorrhyncha (Hemiptera). Theoretical considerations suggest that the general absence <strong>of</strong> toxic principles should result<br />
in a lower ratio <strong>of</strong> endophagous to ectophagous feeders in <strong>grass</strong>es than in dicotyledons, and that appears to be the case, with<br />
external feeders being much more common than gall-makers, borers and miners. Ratios <strong>of</strong> c.30:10 and 30:1 have been recorded<br />
in six <strong>grass</strong> species (Tscharntke and Greiler 1995). Dominance <strong>of</strong> ectophages is also apparent in Sporobolus spp. (Witt and<br />
McConnachie 2004). Veldtman and McGeoch (2003) found five <strong>grass</strong> species with insect galls, caused by five gall-forming<br />
insect species, in a broad survey in South Africa. Poaceae had one <strong>of</strong> the highest number <strong>of</strong> galled species compared with other<br />
plant families. However, herbaceous plants generally have fewer gall-forming species than woody plants, possibly in part<br />
because lignified material is more long-lasting, and is thus ‘safer’ for gall formation (Veldtman and McGeoch 2003). Most <strong>of</strong> the<br />
chewing ectophages on <strong>grass</strong>es are oligophagous, whereas amongst the main ‘sucking’ groups (Hemiptera) Delphacidae are<br />
monophagous while Cicadellidae has a higher proportion <strong>of</strong> oligophages (Tscharntke and Greiler 1995).<br />
As with vertebrates, the nutritional quality <strong>of</strong> <strong>grass</strong> foliage, particularly silicate and lignin content, are important determinants <strong>of</strong><br />
palatability for invertebrates. Juvenile foliage has a reduced content <strong>of</strong> these structural polymers but has better chemical defenses<br />
against herbivores. C 3 species have higher protein contents than C 4 species, so may be preferred by herbivores. Large abundant<br />
<strong>grass</strong>es generally have larger faunas (5-12x) than rare, small species. Two variables, shoot length and life-cycle dichotomy<br />
(annual or perennial) explain a high proportion <strong>of</strong> variance in the species richness <strong>of</strong> a particular <strong>grass</strong>. Unpredictably in the<br />
spatial and temporal distribution <strong>of</strong> annuals, rather than any difference in the number <strong>of</strong> possible niches they <strong>of</strong>fer appears to<br />
explain this (Tscharntke and Greiler 1995).<br />
Data on the <strong>grass</strong> food plants <strong>of</strong> insects in <strong>Australia</strong>, as elsewhere in the world, is very fragmentory. The literature appears to be<br />
devoid <strong>of</strong> studies <strong>of</strong> whole faunas associated with <strong>Australia</strong>n native <strong>grass</strong> species. According to Wapshere (1993) no arthropods<br />
had been recorded from Nassella trichotoma in <strong>Australia</strong>, despite its long presence in the country, and nothing was known <strong>of</strong> the<br />
invertebrate faunas <strong>of</strong> Austrostipa species (although this is no longer correct for these <strong>grass</strong>es). At attempt has been made to<br />
draw together some <strong>of</strong> the scattered information sources on <strong>grass</strong> invertbrates in an Appendix to this <strong>Literature</strong> Review. Taxa <strong>of</strong><br />
Gondwanan origin such as Austrodanthonia may harbour a larger range <strong>of</strong> endemic coevolved invertebrates than species such as<br />
T. triandra which have colonised <strong>Australia</strong> more recently from the north (E.D. Edwards cited by Driscoll 1994).<br />
Lawton and Schroder (1977 p.137) compiled data on the insects associated with species <strong>of</strong> British plants but excluded Poaceae<br />
“because the insect data appeared to be particularly unreliable”, the entomological literature frequently recording “<strong>grass</strong>”, rather<br />
than particular <strong>grass</strong> species as food plants. However they found that the monocots studied had the fewest insect species<br />
associated with them compared with shrub, perennial herb, ‘weeds and annuals’, and aquatic dicot herb groups investigated.<br />
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