The Back Paddock is the central southern part of the park which is enclosed by a predator proof fence. See area 7 on Aerial Photo Map. It is now officially called the Bandicoot Sanctuary. Note that the fence is in the process of being replaced (or soon will be), and the new back paddock will be smaller and moved to the west.
The purpose and history of the Back Paddock and its predator fence is described on the Bandicoot page. In particular that page describes the problems with management of Kangaroos. Since 2016 there are no Kangaroos in the back paddock, and as a result there is plenty of long grass which is good for Bandicoots. Unfortunately it is not so good for the health of native grasslands dominated by Kangaroo Grass. If ungrazed, Kangaroo Grass needs to be burnt every 2 or 3 years to prevent it choking out other herbaceous plants and eventually dying off itself. There has been no burning in the back paddock since well before the Kangaroos were eliminated in 2016.
These main vegetation types can be identified in the back paddock:
- Red Gum woodland at the south end of Greenvale Creek and the south end of the eastern gully.
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Grey Box woodland or forest, which consitutes nearly all the tree cover in the eastern half of the back paddock. Except where the density of old trees is high, these areas are choked with Grey Box saplings. The Restoration Plan recommended they be thinned. Grass has recovered in some of the Grey Box areas since the removal of Kangaroos, but there are still many patches of dense saplings where little or no grass survives.
There is a smaller patch of Grey Box south of the gate in the western fence. Here the trees are smaller, there are fewer saplings, and it is now well grassed.
- Highly degraded native grassland dominated by Kangaroo Grass, both sides of Greenvale Creek. On the eastern side wattles have colonised it significantly since about 2010. There is also some areas of Kangaroo grass in the cleared areas within the eastern Grey Box forest.
- Very weedy grassland in the cleared areas east and west of the ruins of Cumberland Homestead.
- Open to very open woodland of Red Gum and Yellow Box, west of greenvale creek and north of the Cumberland pastures. Spear grasses and Weeping Grass dominate in this area, although there are scattered sparse patches of Kangaroo Grass. There are also patches of Chilean Needle Grass and Phalaris. The Sheokes in the centre of this section are the result of revegetation efforts in the early years of the park.
The Rabbit Fences
There are two rabbit fences running from north to south through the back paddock. These were constructed some time last decade as part of efforts to control or eradicate rabbits. I suspect it was in imitation of the strategy used at Mount Rothwell Biodiversity Interpretation Centre, where an extensive network of internal fences has been used to gradually expand the rabbit free area. However the individual enclosures at Mt Rothwell are smaller, and no attempt has been made to eradicate rabbits in rocky areas or in woodland, which would be much harder since warrens cannot be dug out if they are under trees or rocks. This is perhaps why the rabbit fences are not in use in the Back Paddock, although maybe there will be a renewed effort after the new predator fence has been constructed. In any case, rabbits don't like long grass so they should be less of a problem now that Kangaroos are not present.
Swamp Wallabies
A small number of Swamp Wallabies were introduced into the Back Paddock soon after it's construction (National Parks Service 1988), despite the fact that there appears to have been no suitable habitat except the scrub on Gellibrand Hill. Gellibrand Hill is no longer in the Back Paddock but the Wallabies still are. The population took a long time to increase, but some time after 2010 it became obvious that they were severely overpopulated. Of their preferred foods, the only ones present in the Back Paddock are a few shrubs of Tree Violet and perhaps some fungus in the cooler months. By 2016 the Tree Violets had almost no foliage left, and the Wallabies were eating the bark instead. They were also eating Red Gum leaves, a food supply which was not yet exhausted but which they may only be able to tolerate a limited amount of given its toxicity. Another food available in winter was the weed Bridal Creeper, which covered several hectares in the north-west corner of the back paddock. The Bridal Creeper grows to about two metres high on Gellibrand Hill just outside the back paddock, but inside the fence was only a few centimetres high.
In summer the Wallabies were eating the bark from Sweet Bursaria. All Bursarias I inspected in April 2016 had obviously had their bark eaten off in at least two previous years, and many were already completely dead.
The Wallabies pose a threat to themselves and to the vegetation, but unlike the Kangaroos they are unlikely to be a threat to the Bandicoots. They were heavily culled in 2019(?), although a few remain.
Weeds
Paterson's Curse is much less apparent now that overgrazing has been stopped, but is still seen in bare patches.
One weed which has multiplied considerably in the Back Paddock in recent years is St. Johns Wort, which persists even in dense grass.
There are many patches of Cape Tulip in bare areas and along tracks.
There are large areas of Chilean Needle Grass particularly near Cumberland Ruins, and smaller patches elsewhere.
Serrated Tussock was common in most parts of the back paddock in 2019. The advent of COVID and the federal governments Jobkeeper payments made it possible for Conservation Volunteers Australia staff to make big inroads into controlling it, although unfortunately the pandemic did not last long enough for this to be completed. Spot spraying of Serrated Tussock is much more difficult in long grass, and the best time to do it is after a burn.
The Bridal Creeper patch in the north-east corner of the Back Paddock was referred to above. This weed does not appear to be spreading as fast in the Back Paddock as it is outside, perhaps because the longer grass is inhibiting germination.
At least woody weeds are not a problem in the back paddock, thanks to CVA volunteers.
Gully Erosion
There are three gullies or drainage lines running from north to south through the back paddock.
Cumberland Gully
The drainage line at the eastern end of the back paddock is the shortest of the three and the least eroded. Most of the water is captured by Cumberland Dam.
Greenvale Creek
The central gully is glorified by the name Greenvale Creek, although like the other gullies it rarely flows. It drains a large area of the former Greenvale Sanatorium land as well some roads and part of Greenvale Reserve, before entering the back paddock. Since the invasion by Europeans it has undergone several sudden changes in the amount of water that flows down it, as well as more gradual changes due to alterations in vegetation and soil. The first sudden change was the construction of a dam which we call the Sanatorium Dam even though it was constructed well before the sanatorium. It was enlarged in 1923 as described on the page about the Sanatorium water supply.
Another sudden change occurred in 1929 when the Sanatorium was connected to mains water. From then on there was a more or less continuous flow of drainage and sewage effluent down the creek, increasing when the complex was enlarged in the 1950s and again in 1974. An Environment Protection Authority licence issued in 1981 allowed the discharge of up to 9000 l/h of treated effluent. This would have come to an abrupt halt in 1998 when the hospital was closed.
Enlargement of the complex would also have resulted in increased rainfall run-off due to all the paved roads and rooftops, but this would have flowed through the Back Paddock only when the dam overflowed.
Erosion of the creek probably started long before the Sanatorium was built. The continuous flow of effluent down it may have supported vegetation growth which would have decreased the erosion. Erosion starts upstream of the dam, at the culvert under the Sanatorium access road, but the worst erosion occurs just below the track crossing constructed by Parks Victoria in the centre of the back paddock.
Eastern Gully
The eastern gully starts just inside the north-east corner of the back paddock. About half way down is a dam which is the only one in the park that has never been known to dry out. Most of the water flowing into it seems to come from the erosion zone south of Gellibrand Hill, which accounts for the muddy colour from suspended clay.
Dams contribute to erosion because water flows less often downstream of them. The Eastern gully is indeed eroded downstream of the dam, and the erosion is worse downstream of a second dam which rarely fills.
Another thing to observe about the eastern gully is that except at the lowest end, the Red Gums along it are all dead or dying. There are only a couple of patches of live Red Gum saplings, one of which is dependant on the lower dam.