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Hundreds of species rely on tree-hollows for shelter and breeding, however land-clearing has reduced their availability worldwide. While nest-boxes are deployed extensively in hollow-deficient habitats, their thermal value for arboreal marsupials compared to tree-hollows is unclear, particularly in temperate environments. We analysed thermal regimes in nest-box and tree-hollow pairs during summer and winter environmental conditions. Using a biophysical model, we quantified the relative suitability of den-sites for several marsupial species, estimating the impact of microclimates (and ambient conditions) on predicted heat-production and heat-loss. Nest-box temperatures were strongly influenced by ambient temperatures and solar radiation, whereas tree-hollows buffered external temperature fluctuations. On average, nest-boxes reached maximum temperatures 8 °C higher than tree-hollows in summer, and 3 °C higher in winter, with maximum temperatures of 52 °C recorded in nest-boxes, compared to 38 °C in tree-hollows. During summer, estimated heat-loss required by marsupials was 1.5–2.4 times higher in nest-boxes than tree-hollows. Conversely, predicted winter heat-production requirements were slightly lower in nest-boxes (0.95–0.97 of hollow requirements). Our study emphasises the importance of retaining tree-hollows as thermal refuges for hollow-dependent marsupials in temperate zones to reduce thermoregulatory costs during heat-events. Current nest-box designs are likely of limited value during high temperatures and solar radiation loads if they consistently reach temperatures exceeding species upper critical temperatures, however may provide suitable microclimates during winter. With increasing and more prolonged heat-events predicted under climate change, future conservation-management programs should focus on improving nest-box thermal properties to enhance suitability for wildlife.

Surrogates are commonly used for monitoring biodiversity under a wide range of scenarios. However, surrogates are not often evaluated under diverse ecological conditions, and this hinders the identification of spatial and temporal boundaries of a surrogate’s relationship with its biodiversity metric, including whether a surrogate can predict biodiversity responses to environmental change. We adapted a causal framework from the medical literature and applied this framework to investigate the consistency of a well-established habitat surrogate of arboreal marsupials: hollow-bearing trees. We tested the consistency of the relationship between hollow-bearing trees and arboreal marsupials across four long-term studies (>10 years) covering different habitat types and environmental disturbance. We also tested the ability of the change in hollow-bearing trees over time to predict the change in arboreal marsupials over time. We found a somewhat consistent relationship between hollow-bearing trees and relative abundance of arboreal marsupials, but the mechanistic details of this relationship varied both by location and by species of arboreal marsupial. Similarly, the surrogate approach was not able to predict trends over time, a result likely due to differences in natural temporal variation between the surrogate and target species of interest. Our investigation demonstrates that habitat surrogates can be very useful for certain aspects of monitoring programs, but that serious limitations prevail when trying to monitor changes over time, or if information on species-specific responses is required. Our new framework can be readily applied to any biodiversity surrogate with an established mechanistic link between the surrogate and biodiversity metric of interest.

Habitat requirements of arboreal reptiles may determine their vulnerability to anthropogenic disturbance, but have attracted little research. We studied habitat use by the broad-headed snake Hoplocephalus bungaroides, a threatened species from southeastern Australia. Intensive radio-tracking of 22 broad-headed snakes (from 1992 to 1995 in Morton National Park, 160 km south of Sydney) provided detailed information on the habitat requirements of these animals. During spring, broad-headed snakes were sedentary and used rocks and crevices on exposed cliff edges as diurnal retreat sites. These results are consistent with the widespread view that this species is restricted to rocky outcrops. However, > 80% of our telemetered snakes moved away from the rock outcrops to open woodland during the summer. Radio-tracked snakes used one to nine trees each summer and spent long periods (up to 48 days) sequestered inside tree-hollows. Tree use was highly non-random; the snakes actively selected dead rather than live trees, large rather than small trees and trees with many branches and hollows. The selection of trees with hollow branches may reflect the thermo-regulatory opportunities provided by this microhabitat, and/or the abundance of potential prey (arboreal mammals) in tree-hollows. Our radio-tracked snakes actively selected grey gums Eucalyptus punctata and Sydney peppermints E. piperita, but avoided the most common tree species on our study sites (turpentine Syncarpia glomulifera). Our study shows that persistence of broad-headed snakes in an area may depend crucially not only on suitable rocky habitat, but on adjacent forests. Hence, forestry practices should be designed to ensure that suitable ‘habitat trees’ are retained in forested areas near rocky cliffs.

The breeding densities of many bird-species which nest in tree cavities are in some areas limited by shortage of sites. This is evident from circumstantial evidence in which the numbers of breeding pairs in different areas correlate with the numbers of local nest sites, or where changes in the numbers of nest sites resulting from natural processes or human action are followed by changes in the numbers of pairs. It is also shown experimentally, where nest site provision or removal has been followed by a corresponding change in breeding density. When nest boxes were provided, they were often occupied in the same year, leading to an immediate rise in breeding density. This implied that surplus birds were available in the vicinity and able to take them up. In future years, pair numbers often increased further but eventually they levelled off. This implied that, at this higher level, other limiting factors came into play. Experiments on nest site provision thus provide examples of different factors acting successively to limit breeding density at different levels. By constraining breeding density, shortage of nest sites not only prevents some birds from breeding, but must also limit the total numbers of breeders and non-breeders, because no population can increase beyond a certain point, when the production of young is limited. Once all available nest sites are occupied, reproduction is inevitably density-dependent with respect to the total population of breeders and non-breeders. Because several bird species can use the same type of site, competition is frequent, and the numbers of dominant species can affect the numbers and distribution of others. In extreme situations, a species may be totally absent from areas where all suitable nest sites are occupied by dominant competitors. In less extreme situations, the numbers of a subordinate species may vary from year to year or from place to place in inverse relation to the numbers of a dominant species. Both these situations can be changed by the provision of extra sites. In managed woods and modern tree plantations, most hole-nesting bird species are excluded or kept at extremely low levels by shortage of nest sites. This is a direct result of forestry practice, which usually entails removal of the old and dying trees that normally provide the sites. The problem can be rectified by allowing a number of old and dead trees to remain in managed forests, or (on a more local scale) by the provision of nest boxes. By appropriate design and positioning of the boxes, the most desired species can be favoured.

Biodiversity is fundamental for ecosystem functioning, but little is known about how function responds to biodiversity loss following habitat disturbance in natural systems. Due to the global decay of veteran trees, many associated saproxylic (i.e. deadwood-dependent) insects are considered threatened. Nevertheless, the role of habitat spatial configuration on saproxylic insect biodiversity and dead wood decomposition is poorly understood. We performed a six-year landscape-scale colonization experiment on saproxylic beetles inhabiting hollow oaks, using boxes filled with wood mould as standardized habitat patches. We placed boxes either on a hollow tree or on another tree 61–324 m from the hollows, thereby creating two habitat isolation levels. We quantified wood mould decay and biodiversity in the boxes, measuring species richness, total abundances and community-weighted mean of body mass (CWM) as an index of community functional composition. Isolation had a persistent negative effect on primary consumer biodiversity, but it only impaired decay at the beginning of the experiment. All effects were independent of landscape-level (500-m radius) habitat amount surrounding the boxes. Wood mould decay was mediated by CWM of primary consumers. Therefore function was driven by the body masses of the dominant primary consumer species but not by species numbers (richness) or individual numbers (abundance). Our experiment shows that small-scale habitat isolation leads to biodiversity loss and reduced function and indicates that habitats created by conservation efforts will be used by more saproxylic species if located within sites with a high density of veteran trees.