Amelia J. Koch

The use of hollow-bearing trees by vertebrate fauna in wet and dry Eucalyptus obliqua forest, Tasmania

The loss of hollow-bearing trees in production forest areas can have large impacts on animal populations that rely on them for shelter. This study facilitates the selection of appropriate trees for retention by examining the proportion and type of trees that were used by vertebrate fauna in mature wet and dry Eucalyptus obliqua forest in Tasmania. Felled trees were searched for hollows and secondary evidence of use by fauna. Classification Trees and Bayesian logistic regression modelling were used to examine the site and tree attributes that best explained the use of a tree by fauna. We did two separate analyses: one using attributes expected to be causally related to hollow use, and a second using attributes that might be correlated with hollow use and could be easily assessed in standing trees. In all, 28% of hollow-bearing trees examined showed evidence of use, which is at the lower end of the scale found in other areas of Australia. The variables most strongly related to the use of a tree were hollow abundance, tree size and senescence. Random Forest modelling indicated that the likelihood of a hollow being used increased with hollow size, particularly hollow depth. Counting the number of hollows in standing trees was the best way to identify a tree that is likely to be used by fauna and this was particularly important for younger and healthier trees. It was recommended that trees to be retained should contain at least one large hollow. It was estimated that 8–15 trees per hectare were used by hollow-using fauna in mature wet and dry E. obliqua forest in Tasmania.

Hollow-using vertebrate fauna of Tasmania: distribution, hollow requirements and conservation status

In Tasmania, a considerable proportion of the forested landscape is available for land clearance and production forestry, which has and will continue to result in a decline in hollow availability unless managed appropriately. All hollow-using species are listed as having priority status under the Tasmanian Regional Forest Agreement. To ensure the habitat for hollow-using fauna is managed effectively, we first have to understand the requirements of the species involved. This paper is a review of the distribution, hollow requirements and conservation status of the five species of arboreal marsupials, eight species of bats and 29 bird species that use hollows in Tasmania. The number of species that use hollows is lower than in many other areas of Australia, but these species represent a large proportion of the vertebrate fauna of Tasmania. Three of these species and nine subspecies are endemic to Tasmania and seven are exotic. Four bird species are listed as Threatened at the state and/or national level. Twenty-five of Tasmania’s hollow-using species are capable of using small hollows, 14 can use medium-sized hollows and three bird species are limited to using large hollows. Current records indicate that the highest diversity of hollow-using species is associated with dry forest areas.

The efficacy of common species as indicators: avian responses to disturbance in British Columbia, Canada

Common species can be major drivers of species richness patterns and make major contributions to biomass and ecosystem function, and thus should be important targets for conservation efforts. However, it is unclear how common species respond to disturbance, because the underlying reasons for their commonness may buffer or amplify their responses to disturbance. To assess how well common species reflect changes in their community (and thus function as indicator species), we studied 58 bird species in 19 mixed conifer patches in northern British Columbia, Canada, between 1998 and 2010. During this time period two disturbance events occurred, stand level timber harvest and a regional-scale bark beetle outbreak. We examined relationships among densities of individual species, total bird density and overall species richness, correlations in abundance among species, and responses to disturbance events. We found three broad patterns. First, densities of common species corresponded more strongly with changes in total bird density and overall species richness than rare species. These patterns were non-linear and species with intermediate-high commonness showed similar or better correspondence than the most common species. Second, common species tended to be more strongly correlated with abundances of all other species in the community than less-common species, although on average correlations among species were weak. Third, ecological traits (foraging guild, migratory status) were better predictors of responses to disturbance than species commonness. These results suggest that common species can collectively be used to reflect changes in the overall community, but that whenever possible monitoring programs should be extended to include species of intermediate-high commonness and representatives from different ecological guilds.

Estimating the accuracy of tree ageing methods in mature Eucalyptus obliqua forest, Tasmania

Summary Estimates of tree age are important for making management decisions on the tree hollow resource because hollows suitable for fauna occur with greater frequency in older trees. The methods used to age trees vary with the practicalities of obtaining wood samples and the quality of the wood samples available. Ring counting is commonly used on smaller sample sizes when complete wood samples are available. When wood samples are incomplete, a combination of ring counting and extrapolation is often used. When no wood samples are obtained, growth models are generally used to estimate tree age. This paper uses all three methods, including three different growth models, to examine the accuracy of ageing trees. Simple regressions between tree age and diameter at breast height (over bark, dbh) provided the most accurate growth models at a site. Age estimates obtained from such models, however, had unacceptably large errors when few trees were used or when variable growth rates occurred. Under these circumstances, smaller errors margins were obtained from using a model based on tree dbh and site attributes than when averaging growth rates across sites. The estimated error in tree age estimates when using ring counting and extrapolation was about 10% of the tree age. The error of extrapolation increased with the amount of wood sample that was missing. Error margins were large for the oldest trees (average ±42.4 y for trees >350 y old) but less than ± 15 y for most (73%) of the trees estimated to be 100–300 y of age. These middle-aged trees are often the most useful to study when examining the rate of hollow production in eucalypts. Therefore, age estimates acquired in this way are generally accurate enough to be useful for making management decisions regarding the tree-hollow resource in production forests.

The relationship between environmental conditions and activity of the giant barred frog (Mixophyes iteratus) on the Coomera River, south-east Queensland

Determining the population density of ectotherms is often confounded by individual activity levels, which are highly dependent on ambient climatic conditions. In this study we used radio-telemetry and streamside surveys to examine the influence of local climatic conditions on individual activity levels (detectability) and streamside density of a population of endangered giant barred frog (Mixophyes iteratus) along the Coomera river in south-east Queensland. Temperature was the most important climatic variable influencing the behaviour and hence detectability of M. iteratus. The results indicated that males bury under the leaf litter during cold conditions (<18°C) so fewer were detected during surveys. Although females were also found to bury under the leaf litter in cold weather, no significant relationship between exposure and streamside density was detected. This is likely to be due to the lower number of females detected during surveys. The streamside density of juveniles was significantly related to temperature and rainfall, but little of the variance in the data was explained by climatic conditions, despite greater numbers of juveniles being found than adults. These results indicate that, for increased efficiency, surveys of Mixophyes iteratus should be undertaken when temperatures exceed 18°C.

Bird use of native trees retained in young eucalypt plantations: species richness and use of hollows

Context. The retention of mature eucalypts in plantation areas is expected to have biodiversity benefits, especially for animals with specific requirements such as hollow-using species. Aims. The aim of the current study was to examine the degree to which retained trees embedded in young (≤3 years old) eucalypt plantation provide habitat for birds, whether as a perching/foraging resource for all bird species or as a potential shelter/breeding site for hollow-using species. Methods. We examined bird use of remnant trees in young eucalypt plantations at five sites in northern Tasmania. Four 20-min surveys were done on 214 trees (101 of these had visible hollows) over two breeding seasons (2007 and 2008). Bird activity at a hollow was recorded during all four surveys, whereas the number of bird species observed in a tree was only recorded during two surveys in the first breeding season. The relationship between both species richness and hollow use with site and tree variables was explored using hierarchical partitioning. Key results. Bird species richness at a tree was most strongly related to which of the five properties the tree was located on. This may be due to differences in soil fertility and/or the amount of forested area within and around the property. Birds were observed investigating 50 hollows on 36 trees. More than one hollow was used in 10 trees and hollow use was most strongly related to hollow abundance in a tree. High re-use of particular trees (n = 15) and hollows (n = 16) indicates that retained trees vary in the type and quality of habitat they provide for hollow-using birds. The most common hollow users observed were the common starling (Sturnus vulgaris) and the striated pardalote (Pardalotus striatus). Conclusions and implications. Retaining trees in plantation areas, particularly trees with special features such as hollows, should be encouraged to provide habitat for birds and help contribute to the maintenance of bird diversity in an area.