Currently there are attempts to demonstrate pro or contra the attribution of blame on possums for some cases of tree damage. Relatedly, there has also been discussion about the value of introduced predators in keeping the numbers of possums down. Currently Mt Eliza Council has hired Ecology Australia to study this matter. Their final results will be published at the completion of their study early in 2014. This article reviews the results of completed studies regarding tree dieoff and possum predators.
Earlier studies have not even mentioned native animals as having a role in causing tree damage, yet appear satisfied with a wide range of other causes.
"Patches of dead and dying eucalypts have been recorded in Victoria for many years and may be caused by both biotic agents (fungi, insects etc) and/or abiotic agents (drought, water logging, fire, herbicides etc.)." See /files/Survey of tree dieback on the Mornington Peninsula Smith Smith and Clements 2006.pdf (Draft copy).
Below we reproduce the results of a number of scat [droppings] content studies for owls and foxes. These show that possum numbers are much less affected by fox presence than the presence of owls, their natural predators. Owl scats carried signs of digested possum in 90% of cases; foxes only in 11% of cases. The effectiveness of fox predation on possums shows that foxes make only a minimal impact on possum numbers. Cats prey less on Brush-tail possums than foxes but possibly more on ringtail possums than foxes. The table below shows the results of fox predation on mammals in various reserves, based on the percentage of scats containing possum remains.
Considering that one possum would produce several scats, the above figures appear relatively too high.
The other 82% of mammalian remains in the scats consist of Sugar Gliders, Bandicoots, Swamp Rats, Bush Rats, Marsupial Mice, Dunnarts and introduced species. This scenario can be compared with the cane toad which does much more damage to other species than to the intendant ones. It is therefor unacceptable to relay on foxes and cats to reduce possum numbers. Beside mammals, both foxes and cats are also serious predators on birds, lizards,frogs and insects.
The ideal predator on possums may well be the powerful owl. This species is declared endangered in Victoria and it feeds almost entirely on both species of possums. Owl pellets collected less that 200m from a residential area at Hastings contained a high 90% of possum remains.
Probably the most drastic reduction in possum numbers occurs during the more and more frequent heat waves that we experience where thousands of possums die from heat exhaustion. We may ask, is there a need for the reduction of possums?
Then comes the big question, what are all the other reasons that are the cause of tree dieback? In an article:
“Survey of tree dieback on the Mornington Peninsula, Victoria” by I.W. Smith, D.I. Smith and P.A. Clements of Melbourne University, several other other reasons were given that cause tree dieback.
- Root disease (Phytophtora sinnamomi)
- Armilla luteobubalina (fungal pathogen)
- Defoliaters (Bell miners associated dieback)
- Other insect defoliaters (6 species)
- Mycpherella Leaf disease (several mycpherella species)
- Vertebrates (Possums and Koalas)
- Bark and wood borers (Longicorn beetles)
- Abiotic (Mundulla Yellows)
- Salt (Salt spray)
In their recommendations for each reserve, possum control (by tree banding) was only once mentioned and this was in the Mt. Martha reserve. Are possums really such a problem?
** Finally, why don’t we remove the thousands of pines and replace them with eucalypts?
These zoological studies reflect situations when fewer humans impacted on the areas in question and there were more trees. Arguably they therefore represent a more normal environment than today's.
Sherbrook Forest Park:
“Fox Scat Analysis in a Forest park in South-eastern Australia”
Hans Brunner, John W. Lloyd and Brian J. Coman, 1975, Aust. Wildl. Reseach
“The Vertebrate fauna of Dandenong Valley Metropolitan Park”
Robert R.L. Wallis, Peter R. Brown, Hans Brunner and Anna M. Andrasek.
A Report Prepared for the Melbourne and metropolitan Board of Works, Waterways and
Park Division, May, 1990
“Diet of Red Foxes and Cats;” The impact on Fauna Living in Parks Near Melbourne.
R.L. Wallis, H. Brunner and J. H. Seebeck. Vic. Nat. Vol. 113, 1996
One Tree Hill Reserve:
Vic. Nat. Vol. 113, 1996
Vic,. Nat. Vol. 113, 1996
Flora and Fauna Surveys of Seaford Wetlands by Hans Brunner & Bev Courtney,.
Report to Frankston Council, 1999
Vic. Nat. Vol. 113, 1996
Dartmouth Reservoir A&B:
“The diet of Dingoes and foxes in the Dartmouth Reservoir Area” Technical Report
based on”The Use of Predator Scat Analysis in a Mammal Survey at Dartmouth in
Northern Victoria” H. Brunner, R. L. Amor and P.L. Steven, Aust. Wilsl. Res. 1976