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

Competition with native plants
Competition with other plants is a widespread general expectation for invasive grasses (Evans and Young 1972, Newsome and
Noble 1986, D’Antonio and Vitousek 1992, Lonsdale 1999, Grice 2004). ‘Unequal competition’ is widely assumed where
grasses are invasive, but the mechanisms by which it operates are rarely demonstrated (Seabloom et al. 2003). Release from
natural enemies is one contributing factor often suggested (e.g. Schmidt et al. 2008). Phalaris aquatica and Ehrharta calycina
J.E. Sm., for example, are associated with major habitat degradation on roadsides on Kangaroo Island, threatening a range of
endangered plants, imputedly the result of their superior competitive abilities (Vidler 2004, citing Davies 1996 and Taylor 2003).
Competitive superiority of invasive species was demonstrated for Old World Bothriochloa species over native Kansas
bunchgrasses by Schmidt et al. (2008). In pot experiments, the exotics reduced one or more of three productivity attributes of
native species, either vegetative tiller height or above or below ground biomass, while two of three native species tested failed to
inhibit growth of the exotics.
Demonstable competition has been recorded where the invasive grass has higher rates of N uptake and higher N use efficiency
(Rossiter et al. 2006). This is thought to be the case with exotic Bothriochloa inavsion of native tallgrass prairie in the USA
(Schmidt et al. 2008). Increasing N levels in one American grassland created dramatic shifts in grass species dominance, with
Bromus hordeaceus becoming a superior competitor to grasses including Aira caryophyllea and Briza minor, in the presence of
adequate P. B. mollis has also been demonstrated to be an inferior competitor to Erodium botrys (Cav.) Bertol. (Geraniaceae)
when the S status of soil is low, E. botrys having more rapid root growth. But when nutritional conditions were favourable, B.
mollis outcompeted the herb for light because of its greater size and more erect habit (Evans and Young 1972). Competitive
shifts in the flora resulting from changed nutrient status of the soil may be altered by a range of other environmental factors,
including weather, grazing regime and the interaction between rainfall, temperature and grazing (Evans and Young 1972). N
enrichment of soils in Californian grassland in dry years resulted in the almost complete elimination of the native bunchgrass
Agropyron intermedium (Host) Beauv. by Bromus tectorum L., depletion of soil moisture by the superior competitor being
thought to be the main mechanism (Evans and Young 1972).
Monocultures of invasive Bothriochloa species in the USA are more structurally homogeneous than native grassland and have
reduced forb species richness (Schmidt et al. 2008).
There are relatively few studies that document the effects of exotic grasses on species diversity in Australia in any detail
(Chejara et al. 2006), but numerous introduced species are implicated in decline of native vegetation via competition. These
include Cortaderia spp. (Harradine 1991). Threats to small populations of endangered plants by competition from introduced
grasses, include Nassella spp. on Amphibromus pithogastrus S.W.L. Jacobs and Lapinpuro “by reducing potential bare areas for
establishment of seedlings” (Ashton and Morcom 2004 p. 2), and Phalaris aquatica L. on Prasophyllum fosteri D.L.Jones
(Coates 2003a), P. sp. aff. suaveolens (Western Basalt Plains) (Coates 2003b) and Thelymitra gregaria D.L. Jones and M.A.
Clem. (Coates 2003c).
In Queensland, Brachiaria mutica Para grass and H.amplexicaulis are a threat to the aquatic Aponogeton queenslandicus
H.Bruggen (Vidler 2004 citing Williams pers. comm.). Kikuyu, Pennisetum clandestinum, is a threat to Pimelea spicata R.Br. in
NSW (Vidler 2004 citing Groves and Willis 1999). Ehrharta calycina is a threat to Blue Gum woodlands (Eucalyptus leucoxylon
F. Muell.) and Metallic Sun Orchid Thelymitra epipactoides F. Muell. in South Australia (Vidler 2004 citing Mercer pers.
comm.) and to Eucalyptus incrassata and E. fasiculosa woodland associations and the Sandhill Greenhood Pterostylis arenicola
(Virtue and Melland 2003). E. calycina frequently establishes on bare ground. Native vegetation “subject to disturbances such as
livestock grazing, fire or soil movement [is] particularly prone to invasion” although “certain ‘naturally open’ vegetation types
on sandy soils appear susceptible to invasion in the absence of major disturbance” (Virtue and Melland 2003 p. 111, citing pers.
comms. of B.Bartel and D. Ancell) or the plant “may be mainly establishing in gaps (e.g., on lichen crusts) where there is no
competing vegetation” (Virtue and Melland 2003 p. 112). E. calycina “can have a major effect on the diversity and regeneration
of native plants, particularly understorey species” (Virtue and Melland 2003 p. 112) and can form 100% groundcover (G.Carr
pers. comm. cited by Virtue and Melland 2003).
Grasses collectively, or particular categories of grasses, have also been regularly listed as threats to particular native plants.
Introduced grasses are a threat to Shiny Peppercress Lepidium aschersonii Thell. in NSW (Vidler 2004 citing Ayers et al. 1996).
In NSW exotic annual grasses are a threat to Red Darling Pea Swainsona plagiotropis F. Muell. while exotic grasses are a threat
to Swainsona recta A.T. Lee (Vidler 2004 citing Ayers et al. 1996). Grasses are a threat to Ironstone Grevillea Grevillea
elongata Olde and Marriott in WA (Vidler 2004 citing Stack and English 2003a). Annual grasses are a threat to Acacia aprica
Maslin and A.R.O. Chapm. blunt wattle in WA (Vidler 2004 citing Bayliss 2003). Wild Oats Avena fatua and other “grasses” are
a threat to Pinnate-leaved Eremophila Eremophila pinnatifida Chinnock MS in WA (Vidler 2004 citing Stack and Brown 2003)
and introduced grasses are a threat to Spreading Grevillea Grevillea humifusa Benth. in WA (Vidler 2004 citing Stack and
English 2003b). African Lovegrass Eragrostis curvula (Schrad.) Nees is a threat to Narrow-petalled Featherflower Verticordia
plumosa (Desf.) Druce var. pleiobotrya A.S. George in WA (Vidler 2004 citing Phillimore and Evans 2003). Weeds associated
with irrigation crops are a threat to Menindee Nightshade Solanum karsense Symon, in NSW (Vidler 2004 citing Ayers et al.
1996). Quaking Grass. Tall wheatgrass Thinopyrum ponticum (Podp.) Z.-W. Liu and R.R.-C. Wang “has been observed invading
native Themeda triandra, Austrostipa and Austrodanthonia grasslands” in Victoria (Virtue and Melland 2003 p. 127) but is most
successful at sites with high soil moisture levels and water tables and saline or alkaline soils, particularly winter-wet, saline sites
(Virtue and Melland 2003).
Competition from grasses have been listed as a threat to a wide range of plants in basalt plains grasslands, including, introduced
grasses on Carex tasmanica Kuk. (Morcom 2004) and Comesperma polygaloides F. Muell. (McIntyre et al. 2004), annual
grasses on Senecio macrocarpus Belcher (Hills and Boekel 1996 2003) and ‘dense grasses’ on Rutidosis leptorrhynchoides
(Humphries and Webster 2003), although this probably refers to T. triandra more than exotic species (cf. Morgan 1995a).
89