Reply to the comment by Lindenmayer et al. on "Economics of a nest-box program for the conservation of an endangered species: a re-appraisal"

Abstract

Spring et al. (2001) and Lindenmayer et al. (2002) discuss the use of nest boxes to conserve Leadbeater’s possum (Gymnobelideus leadbeateri McCoy) populations, which are predicted to decline by 90% over the next 40 years because of a shortage of tree cavities (Smith et al. 1985). As a consequence of past fires, most notably a wildfire in 1939, more than 80% of the forests inhabited by the possum are ecologically young regeneration containing few living trees with cavities (Macfarlane et al. 1995; van der Ree and Loyn 2002). Given that trees possessing cavities suitable as den sites for the possum are typically at least 190 years old (Smith and Lindenmayer 1988), it will be more than a century before adequate replacement cavities develop. However, the possum still inhabits many sites, denning in cavities within dead trees that were killed by the 1939 fire but remained standing (typically referred to as “snags”) (Lindenmayer et al. 1991). We share the concerns of Lindenmayer et al. (2002) about current rates and methods of timber harvesting and its impact on the abundance of tree cavities within Leadbeater’s possum’s range. However, they fail to discuss another key factor contributing to the scarcity of tree hollows: snag collapse. Over the past decade, snags have been collapsing at about 4% per annum, a result of damage sustained during the 1939 wildfire and subsequent decay (Lindenmayer et al. 1990, 1997). Snag decay and collapse is predicted to continue, irrespective of habitat protection measures or the extent of timber harvesting. Another large fire during the next century could further exacerbate this situation. This means that habitat protection in reserves, while a necessary and essential conservation strategy to ensure the development and provision of tree cavities in the long term (i.e., >100 years), will not prevent the imminent hollow scarcity that threatens Leadbeater’s possum. Hence, habitat protection alone is unlikely to avert the serious population decline that the possum is predicted to undergo in the short term. This is a key point that was not acknowledged by Lindenmayer et al. (2002) and one that can only be addressed by finding a more immediate means of increasing den site availability for the species. At Yellingbo, we installed 150 nest boxes and subsequently monitored them between 1995 and 2002. During this time, 29 different Leadbeater’s possum colonies used the boxes (one colony for every 5.4 nest boxes installed). Boxes were initially spaced at 100-m intervals in a survey phase (i.e., to locate colonies). Then, to maximize occupancy, several additional boxes were positioned in the territories of the colonies that had been located. Over the 7-year period that the 150 nest boxes have been monitored, 113 (75%) show signs of use by Leadbeater’s possum (i.e., either nesting material or animals were observed). This value increases to 85% if we exclude those nest boxes positioned in forest types not occupied by the species at Yellingbo. Of the 29 colonies detected, 17 (59%) were observed denning in the nest boxes on more than five occasions and 10 (34%) were recorded on more than 15 occasions, indicating that colonies continued to occupy nest boxes on a regular basis over time. These nest box occupancy results are markedly different from those reported by Lindenmayer et al. (2002). The objectives of a nest-box project will determine the study design that is adopted (e.g., placement strategies). Lindenmayer et al. (2002) examined the nest-box preferences of several different species by installing boxes of two different size classes at two different heights. In the case of our research, which investigated Leadbeater’s possums’ population dynamics, we relied on nest boxes as a capture technique and thus attempted to maximize occupancy through time by concentrating boxes around established possum colonies. Given the differing aims, it is not particularly surprising that the two studies have recorded very different results. The significant point is that the Yellingbo work demonstrates that high rates of occupancy can be achieved for this species. We would also emphasize that a project deliberately setting out to install nest boxes as a conservation measure would