The sole infestation of Paterson's curse in the Aranda Bushland
is in an area about 50 m in diameter in the extreme south west
corner. Members of Friends of the Aranda Bushland have been aware
of it for some years, and have hand pulled plants there every
year since. It has not spread significantly in that time, and it
does not extend as far as the watercourse that drains the Cook
horse paddocks.
In the nineteenth and early twentieth centuries there would have been significant horse traffic along the road that follows the southern margin of the Aranda Bushland. The infestation of Paterson's curse in the south west corner of the Aranda Bushland is close to this road.
In the same time span working horses were probably used in the Bushland, the western half of which contains signs of long past ringbarking, tree felling and other European activities.
More recently there was a low level of recreational equestrian
use until 1993.
If equestrians are allowed to use the Aranda Bushland they may be required to keep to designated sections of existing trails. Most of the currently proposed sections are parts of graded fire trails (1).
Experience in other parts of Canberra Nature Park is that horses are not always restrained to designated trails.
Movement of Paterson's curse seeds in water is an important means of dispersal, particularly in hilly country where run-off water carries them to lower areas (2). Paterson's curse seeds deposited on the surface of a graded fire trail, whether in mud, in faeces, or directly, may well be washed into the gutter by rain. In addition, horses may well shed seeds straight into the gutter. The gutters of graded fire trails tend to be slightly below the surface of neighbouring undisturbed ground, to be slightly moister, and to provide a favourable environment for the germination and establishment of weeds. In the Aranda Bushland the weeds paspalum and fleabane are often found in fire trail gutters. Nutrients from horse urine or faeces would make the gutter an even more favourable environment for establishment of weeds. Paterson's curse established in a gutter could be spread by horses to off-trail areas visited by them.
Several species of ants (Pheidole megacephala Fab., Iridomyrmex discors Forel., Prolasius sp.) have been observed to collect seeds of E. plantagineum and store them in granaries above and below ground, where many later germinate (4). The first two species are common in the local area and could well disperse seeds from trails or elsewhere.
Paterson's curse appears to have been spread along the horse trails at Cook chiefly by horses.
It should neither be assumed that all seeds dropped in the Aranda
Bushland by horses would be on horse trails, nor that seeds
dropped on horse trails would be unlikely to become established.
Paterson's curse is found on a wide range of soil types (2). An infestation to the south of Wybalena Grove is on a type of soil that is common in the Aranda Bushland.
Horses appear to be the weed's most important vector in the local area, and to have been chiefly responsible for its spread along horse trails at Cook. It has already travelled about half of the 800 m from the paddocks to the proposed equestrian entry to the Aranda Bushland.
The long life of Paterson's curse seeds, the high stocking rate in the horse paddocks, and the failure of previous efforts to control the weed there indicate that it would be very difficult to eradicate it from the paddocks and surrounding areas. Even if it were eradicated, reintroduction by means of seeds in hay would be likely in times of drought.
Horses from Cook would enter the Aranda Bushland if the current ban is lifted, and their entry would pose a significant threat of Paterson's curse becoming established in parts of the Bushland now free of it.