The anatomy of a failed offset

Abstract

Biodiversity offsetting is widely applied but its effectiveness is rarely assessed. We evaluated the effectiveness of a nest box program intended to offset clearing of hollow-bearing trees associated with a freeway the upgrade in southern Australia. The offset targeted three threatened vertebrates: squirrel glider (Petaurus norfolcensis), brown treecreeper (Climacteris picumnus) and superb parrot (Polytelis swainsonii). Clearing led to the loss of 587 tree hollows and the offset was the placement of an equivalent number of nest boxes in nearby woodland (1:1 ratio). Of these, we monitored 324 nest boxes in six sample periods between 2010 and 2013, yielding 2485 individual checks of nest boxes. For the three target species, we found: (1) no records of nest box use by the superb parrot, (2) two records of the Brown Treecreeper (0-0.76% of accessible nest boxes used per survey period), and (3) seven records of use of nest boxes by the Squirrel Glider (0-2.1% of accessible nest boxes used per survey period). Rates of nest box use by the Superb Parrot and Squirrel Glider were markedly lower than rates of use of hollow-bearing trees observed in other investigations. Low levels of use by target species coupled with the extent of nest box attrition suggest the offset program was unlikely to have counterbalanced the loss of the hollow-bearing trees. We make suggestions for improving future offset programs including a greater emphasis on: (1) avoiding impacts on hollow-bearing trees; (2) offset effectiveness as a measure of compliance; and (3) using realistic offset ratios.

Chainsaw-Carved Cavities Better Mimic the Thermal Properties of Natural Tree Hollows than Nest Boxes and Log Hollows

“…For example, long-term monitoring of bat boxes in Melbourne, Australia has shown that, despite the provision of boxes constructed from materials with a range of insulative properties (ranging from 12 mm plywood to 90 mm pine), box use was dominated by one widespread, urban-adapted species, Gould's wattled bat Chalinolobus gouldii (Gray, 1841) [71]. Further research is required to investigate whether there is a link between the thermal profiles in nest boxes (compared to tree hollows) deployed in disturbed landscapes and their disproportionate level of use by widespread, highly adaptable species of birds [29,93,94] and mammals [21,71,95,96]. Field-based studies examining rates of occupation of chainsaw hollows and log hollows by target species versus undesirable exotic species would be of great interest, particularly in urban and peri-urban landscapes where nest box programs are often undertaken by land managers and conservation-focused community groups [4,97].…”

“…Nest boxes are a popular tool for providing supplementary habitat for hollow-dependent fauna in disturbed landscapes where natural hollows are limited [4,21]. However, traditional plywood or timber nest boxes are not effective at replicating the thermal conditions that occur within natural tree hollows [28].…”

“…When assessing the efficacy of providing supplementary habitats for hollow-dependent fauna, consideration should be given to both the ecological value and the financial costs involved in making and maintaining the artificial habitats [21]. In our study, the primary expense was for highly skilled arborists, with the appropriate training and safety accreditation, and precision chainsaw work, to create the structures: it took approximately one hour to carve a glider chainsaw hollow (estimated cost = $238 AUD), 20 min to carve a bat chainsaw hollow (estimated cost = $79 AUD), and 30 min to carve a log hollow (estimated cost = $120 AUD).…”

“…The glider box dimensions were similar to commercially available nest boxes for small marsupial gliders that are commonly used to supplement natural hollows in management programs and to compensate for the loss of natural hollows in biodiversity offset programs in southeastern Australia [12,16,21]. Glider boxes were painted with three coats of exterior acrylic paint (Wattyl Solagard Low Sheen); we used a dark green colour, consistent with common practice in southeastern Australia [39,40].…”

“…If traditional nest box designs provide a different microclimate to that of natural tree hollows, they may not be an effective substitute for those hollows in terms of conserving the community of hollow-dependent fauna that rely on these habitat structures [40]. As nest boxes are frequently used in landscape restoration and biodiversity offset programs to compensate for the loss of natural hollows [4,21], our study provides a timely impetus for policy makers and land managers to consider ways in which novel habitat-creation techniques could be incorporated into these programs to provide artificial hollows that effectively buffer wildlife from hot or cold weather extremes.…”