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

Approximately 200 alien grass species introduced with imported wool have been recorded in the British Isles (Hubbard 1968)
and 25 species of Austrostipa, Stipa and Nassella have been recorded there as casuals regarded as mainly “wool-aliens” that have
entered the country as seed contaminants of raw wool (Stace 1997). Some exotic stipoid populations in France may also have
this origin (Verloove 2005). Noting European records near wool factories and tanneries, Bourdôt and Hurrell (1987a 1989a)
suggested that that N. neesiana was likely to have reached New Zealand from South America in the wool and hides of grazing
animals. In 19th century Europe the treatments applied in wool processing, including scouring in alkali and acid baths, dry
heating and crushing through rollers (Vines 2006), were intended to remove all such contaminants, which were nevertheless
dispersed in waste streams from the treatment plants, including water discharges and in wool waste or “shoddy” which was used
as garden fertiliser in Scotland (Hayward and Druce 1919, Vines 2006). Installation of a sewage treatment plant at Galashiels,
Scotland, soon eliminated viable seed discharge to waterways (Vines 2006). Dispersal in wool has been a major method of
introduction of weeds to Australia, and has possibly been the most important overall dispersal method within the country, after
their arrival (Carr 1993). Movement of wool in bales after shearing in Australia is possibly responsible for some N. neesiana
spread. Seed was probably able to penetrate and move through hessian or jute bale bags (which probably went out of use in
Australia in the early 1990s), but appear to be unable to penetrate the densely woven high density polyethylene or nylon fabrics
used in modern bales bags (personal observations).
Movement of livestock between farms is the most likely cause of intermediate range expansion in New Zealand (Connor et al.
1993). Slay (2002c) stated that stems bearing seed can be walked across tracks by livestock. Seeds are unlikely to attach to the
pelts of cattle but could be moved in mud on hooves (Gardener et al. 2003a). They may also adhere to other animals including
kangaroos and rabbits (Gardener and Sindel 1998). Ens (2002a) suggested dog and rabbit dispersal as possibilities at a number of
Sydney sites. Bruce (2001) found patches of the plant in Macropus giganteus Shaw daytime rest areas under trees, and suggested
that kangaroos may disperse seed. Liebert (1996 p. 8) implied that dispersal by native animals was unknown, however Bedggood
and Moerkerk (2002 p. 6) stated that “dogs, humans and wild animals such as kangaroos and rabbits spread the seed”,
presumably on the basis of general experience of presence in areas where wild animal movements are concentrated, and other
informed speculation. Peart (1979 p. 860) however noted the absence of any awned grasses seeds in the fur of “some 100
carcasses of wild marsupials in Australia”. Slay (2002c p. 16) mentioned “birds” and “vermin” as probable dispersal agents in
New Zealand and stated that stems bearing seed can be walked across tracks by livestock. No documented records of carriage on
animals other than sheep in Australia appears to be available.
Both panicle and stem seeds of N. neesiana can be distributed and remain viable after ingestion by livestock, but usually a high
proportion of ingested plant seeds are digested, and the viability of the seeds that survive gut passage is significantly reduced
(Stanton 2006). Gardener et al. (2003a) found that an average of 1.7% of N. neesiana panicle seeds and 5.3% of cleistogenes fed
to Angus cattle (Bos taurus) were voided in dung within 4 days, mostly within 1-2 days. Less than half the voided seeds
remained viable and no viable seed was passed after 4 days. Endozoochorous dispersal was considered to be less likely by sheep,
which digest a higher proportion of seed (Gardener et al. 2003a), probably because they chew their food more thoroughly
(Stanton 2006). Sheep, but not cattle, fed with a range of pasture seed digest a greater proportion of long seeds than short
(Stanton 2006). Rabbits also void a wide range of seed in their dung (Bloomfield and McPhee 2006) but would be unlikely to eat
N. neesiana seed.
Even at high stocking rate livestock avoid eating N. neesiana once the reproductive stage is reached (Grech 2007), and the
extremely sharp callus and rough texture of the panicle seed assist in making the panicle unpalatable. These seeds evidently are
adapted to avoid being eaten.
Cleistogene dispersal
Gardener et al. (2003a p. 614) thought that cleistogenes “have no obvious dispersal mechanism” but that ingestion by grazing
animals was one possibility. Barkworth and Everett (1986) suggested that stipoid seeds adapted to zoochory by animal ingestion
may have short, deciduous awns, globose florets and obtuse calluses, a set of characteristics possessed increasingly by N.
neesiana stem cleistogenes from upper to basal.
Cleistogenes can develop even if the flowering tiller is damaged, and are important in maintaining the species during climatic
extremes (Gardener and Sindel 1998) and under conditions of heavy grazing or fire (Dyksterhuis 1945). They are better
protected from some predators than panicle seeds (Gardener and Sindel 1998) being tightly covered by leaf sheaths during their
formation and after maturity, and remain available for dispersal from the parent for as long as 6 months on standing dry culms
(Gardener et al. 1999) e.g. in hay, and for much longer if attached on basal nodes. According to Connor et al. (1993), stem
cleistogenes are released when the leaf sheaths weaken or rupture, so any dispersal of released cleistogenes requires tiller
breakdown. According to Slay (2001 p. 38) the tiller dies after panicle seed is produced and its roots decay over a period of up to
two years, “eventually releasing the cleistogenes and or producing the ideal germination conditions for their establishment, as
evidenced by the seedlings that grow out of decayed clumps”. He found old, rotted root/stem areas “up to three tiers deep,
suggesting plants die and re-establish ... on top of each other” (p. 42), and that at least the basal cleistogenes are adapted to not
disperse.
In terms of large grazing mammals, the tightly attached covering leaf sheath and stem node segment can be conceived of as an
attractive ‘fruit’ containing the cleistogene ‘seed’, and when ingested this ‘fruit’ may enable better survival of the seed and faster
passage through the gut (Davidse 1986). Lllamas and alpacas reportedly eat the straw and possibly disperse cleistogenes in their
guts (Colin Hocking, 26 October 2006).
Cleistogenes are dispersed by cultivation machinery (Connor et al. 1993) and in decayed tussocks or sods (Slay 2002a). Old and
broken culms bearing cleistogenes can be carried by livestock (Slay 2001). Bourdôt (1989) noted that basal cleistogenes in
particular are likely to survive fire in situ, and have probably evolved not to disperse, but rather to replace the parent plant should
it die.
Grasses that are ‘herbivore exploiters’ are palatable, recover well after grazing, and have seeds adapted for dispersal by grazing
mammals (Milton 2004). N. neesiana appears to have a mixed dispersal strategy, probably being an exploiter of large grazers via
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