In short – no, not in the Mountain Ash forests of the Central Highlands, a region that constitutes the core habitat of the critically endangered possum.
So what is the issue?
The large old trees that Leadbeater’s Possum use to den in are critically important for their survival, but are being lost to fire and forestry. These old trees are so vital that we need to protect each and every one across the possum’s range. Investing time, money, resources and energy into installing nest boxes to secure the possum’s survival is a theory that has already been tried, tested and disproved as a management tool. Ultimately, it is a distraction from the main game of habitat identification and protection.
Leadbeater’s Possum has always had to contend with the natural decay and collapse of stags (hollow-bearing trees) as a major threatening process. Over the last 100 years however, the combined effects of frequent high intensity wildfire and intensive timber harvesting have accelerated this process, increasing the pressure on this otherwise natural process.
The use of nest boxes to create more denning sites therefore has been a constant issue raised by land managers. And a good question too; nest boxes have had some good successes in many and varied environments, the world over.
To better understand the use of nest boxes by Leadbeater’s Possum, in 1998 Professor David Lindenmayer and his research team set up a large nest box study in the Mountain Ash forest east of Melbourne. We used a box design that had been successful with Leadbeater’s on the floodplain at Yellingbo (marine ply, small entrance hole, same internal dimensions), and installed 96 boxes, in groups of 4, at different heights, in different age classes of forest, in different regions, and adjacent to known records of the critically endangered possum. The neat experimental design was guided by statisticians, our own knowledge of Leadbeater’s ecology and other experts.
A huge amount of hard work, time and money went into building the boxes, choosing the sites and setting up the infrastructure. Then the annual monitoring took a keen researcher over 2 weeks to check the boxes and the experiment ran for 10 years.
This experiment showed that the use of nest boxes as a surrogate for natural hollows was an unviable management option. Here’s why.
After several years of monitoring, maintenance and repair, just 14 % of the 96 boxes showed signs of use by Leadbeater’s Possum, and this use represented just two possum colonies. They were predominantly in younger regrowth stands, not older 1939 aged sites.
Following these results, we tried different box materials and designs, and added them in larger clusters of 40 boxes in two separate regions.
We found the same negative results.
During the nest box surveys an underestimated task was the amount of maintenance that was required. Bees colonised boxes, falling trees or limbs knocked boxes off, or damaged them badly. This was especially an issue in regrowth forest, where limb shedding, tree thinning and tree fall is a constant problem. It does not matter what your nest box is made out of; if a branch falls from 30m above it, it will be damaged. Over the 10 years of our study, 50 % were damaged or collapsed. We calculated the life span of each nest box in Mountain Ash to be 5 years, even with all our intervention and maintenance.
Since our study, nest boxes continue to be used with some success for Leadbeater’s at the Yellingbo Reserve, in the snow gum forests at Lake Mountain and new installations at Mount Baw Baw and Mount Bullfight. Parks Victoria have been managing ‘Project Possum’ which targets areas with known colonies of Leadbeater’s for new box installation. They are recycled plastic, thermally better and a more solid design than the timber style we used in our study. One of the most knowledgeable Leadbeater’s Possum experts, Dan Harley surveys these areas to the tree-level for box placement. Over 400 boxes have been placed across the known Leadbeater’s range including over 160 in montane ash sites. Dan found 19% of these montane ash nest boxes have nests with multiple boxes again used by single colonies. This is comparable to our own study, even though much more time and thought was put into Project Possum box placement.
As with other studies, the Project possum boxes have been much more successful in snow gum forest, where 30% of boxes have nesting material.
There are even new trials to mechanically cut nesting holes into living trees. Tree climbers have used chainsaws to cut 72 of these mechanical nest holes, and the first round of surveys found 9 nests.
All of these trials and experiments have provided very similar results to our own study, but they have all cost time and money and continue to do so. We have not contributed significantly to the knowledge of Leadbeater’s Possum and nest box use since even Malcolm McFarlane was using recycled sewerage pipes up in the Mount Margaret area in the 1980’s.
During Since this time, our knowledge on the rate of decay, collapse and the slow pace of recruitment of large old trees has vastly increased. We can productively use this knowledge to help save the possum.
Right now the large old trees Leadbeater’s Possum uses for natural den sites are at a critical point. In 1939, a massive wildfire burnt a huge proportion of the possum’s habitat, killing the stands of old forest it lived in. This created a pulse of large old dead stags, the preferred habitat of Leadbeater’s Possum.
In 1983, another wildfire burnt substantial parts of important habitat. Then in 2009, 50 % of the Leadbeater’s Possum’s range was burnt, including around 40% of the reserve system set up to protect the possum. Much of the forest burnt in 2009 was younger regrowth forest, so the pulse of large standing dead stags created after 1939 will not occur in areas burnt in 2009. How long these smaller diameter dead poles will stand is something we are continuing to investigate.
Now, the old growth forest is just 1.16 % of this forest estate, spread around in small patches. With fire predicted to increase in severity and intensity with climate change, it is likely we will see another large fire reduce even this small percentage of remaining old growth.
We have been carefully tracking the decay and collapse of these large old trees since the 1980’s at nearly 200 long term monitoring sites. We measure and monitor nearly 2000 trees and the situation is grim. In the 33 years since we started monitoring, 41 % of the trees have collapsed. These sites are representative of the entire forest estate, from young forest to old, steep to flat, high elevation to foothill forest, water catchment and National Park to State Forests.
Models predict that by 2065, 80 % of the sites we study will have no hollow trees left. 2065 is the estimated date that the trees regrowing from the 1939 fire will be old enough to begin to form suitable hollows for Leadbeater’s Possum.
So, what do we do about this habitat loss?
Currently, the protection of large old trees in state forests is woefully inadequate. Although harvesting the large old trees is not permitted, following logging activities, they become more vulnerable. There is no protection from the coupe regeneration burn, (where the logging waste in a harvested area is burnt to prepare the soil for seed to be spread). The regeneration burn often kills retained trees, but if they survive the fire, they then need to compete for nutrients and water with thousands of vigorous seedlings. In addition, they are exposed to wind-throw and heavy soil disturbance. The old dead and decaying trees (the kind that the possum currently prefers to den in) have even less protection than the large old live trees. They are either knocked over during the logging process or consumed by the regeneration burn.
We know we have a large old tree crisis, yet we still persist with clearfelling and burning practices which are not only destroying current habitat, they are also limiting future habitat for Leadbeater’s Possum. Instead of adequately protecting existing large old trees, land managers are persisting with nest boxes in the face of decades of research which very clearly shows that investing large amounts of time, money, resources and expertise into installing these boxes is not a good way to spend tax payers money and is unlikely to provide the effective supplementary habitat elements the possum requires.
Nest boxes have a very small impact in certain conditions. With hollow availability at such a critical level, there may be some areas where nest boxes may help for a time, but nest box lifespan is so small! Regrowth from the 1939 fires is predicted to be five decades away from being old enough to start forming hollows. It is unfathomable to attempt to install and maintain nest boxes across the central highlands for fifty years, given a box lifespan of 5 years.
We need to protect every hollow tree right now to bridge this half century gap.
With only 1.16% old growth forest spread out in small patches across the central highlands, the whole ecosystem is at a stage where forest restoration is the only path forward if we want to save Victoria’s faunal emblem. This means not only protecting every large tree now, but allowing the “next old growth” to develop. The 1939 regrowth is now that next old growth; but it is also the age of forest in which the most timber harvesting occurs.
Currently the Leadbeater’s Possum Recovery Plan is out for review (public submissions close on May 20th). In this proposed plan, nest boxes are still being pushed as a good option and worthy of experimenting with. At the same time, large old trees in logging areas have been given no increased protection.
We need to have a long term vision for these forests and this animal, and ideas like nest boxes are a short term distraction.
References / Suggested Reading:
Lindenmayer, D. B., Cunningham, R. B., Tanton, M. T., & Smith, A. P. (1990). The conservation of arboreal marsupials in the montane ash forests of the Central Highlands of Victoria, south-east Australia: II. The loss of trees with hollows and its implications for the conservation of Leadbeater's possum Gymnobelideus leadbeateri McCoy (Marsupialia: Petauridae).Biological Conservation, 54(2), 133-145.
Lindenmayer, D. B., MacGregor, C. I., Cunningham, R. B., Incoll, R. D., Crane, M., Rawlins, D., & Michael, D. R. (2003). The use of nest boxes by arboreal marsupials in the forests of the Central Highlands of Victoria. Wildlife Research, 30(3), 259-264.
Lindenmayer, D.B., Welsh, A., Donnelly, C., Crane, M., Michael, D., Macgregor, C., McBurney, L., Montague-Drake, R. and Gibbons, P., 2009. Are nest boxes a viable alternative source of cavities for hollow-dependent animals? Long-term monitoring of nest box occupancy, pest use and attrition. Biological Conservation, 142(1), pp.33-42.
Lindenmayer, D. B., Tanton, M. T., & Cunningham, R. B. (1991). A critique of the use of nest boxes for the conservation of Leadbeater's possum, Gymnobelideus leadbeateri McCoy. Wildlife Research, 18(5), 619-623.
Lindenmayer, D.B., Blanchard, W., Blair, D., McBurney, L. and Banks, S.C., 2016. Environmental and human drivers influencing large old tree abundance in Australian wet forests. Forest Ecology and Management, 372, pp.226-235.
Lindenmayer, D. B., MacGregor, C., & Gibbons, P. (2002). Comment Economics of a nest-box program for the conservation of an endangered species: a re-appraisal. Canadian Journal of Forest Research, 32(12), 2244-2247.