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Biology and conservation ecology of selected saproxylic beetles in Tasmania's southern forests

BELINDA YAXLEY

PhD thesis, University of Tasmania

ABSTRACT

Many saproxylic beetle species (those associated with dead wood) are considered to be threatened by intensive forest management, yet few have been subjected to autecological studies that might help in their conservation. For this thesis, two lines of research were followed to investigate the autecology of six species, selected because of their perceived vulnerability to intensive forestry.  The study was carried out in the wet-eucalypt production forests of southern Tasmania.  The first line of research comprised (a) an observational study investigating larval and sexual development and life-history strategies; and (b) a molecular study exploring the extent of one species’ dietary dependence on a particular rotten-wood type.  The second comprised a log-, site- and landscape-scale study to identify the habitat requirements of all six species, using predictive habitat models. In study 1a, some seasonality patterns in larval and sexual development were noted, but these were partially superimposed, suggesting the concurrent existence of multiple age-cohorts and hence multi-year development cycles. In terms of their life-history strategies, all study-species appeared to possess traits that would lead to low intrinsic rates of population increase, but the range of variation in these traits suggested that some species would be more vulnerable to local extinction in managed forest landscapes than others. In study 1b, numerous fungal species were detected in the gut contents of adult and larval Prostomis atkinsoni but few were found in both; furthermore, there was little overlap in the mycota of beetles’guts and the ‘gingerbread rot’ that this species inhabits. Strangely, species of basidiomycete known to be wood-rotters were found in the guts but not in the gingerbread rot itself.  In study 2, models derived from log- (and/or sample-) level variables performed well in relating the probability of occurrence of each of the six beetle species to the presence or absence of particular suites of rotten-wood types.  The addition of site-level variables did not greatly enhance the models’ abilities to predict occurrence, but at this level, forest stand structure and age of regeneration generally contributed the most explanatory power. At the landscape scale geology, aspect, and time since last fire were good predictors of occurrence across the study-area, although predictive ability may have been inflated by over-fitting and autocorrelation. Overall, the models indicate that in unharvested forest these beetles tend to occur preferentially (in their chosen rotten-wood types) in multi-aged stands with a mature eucalypt element, while their continued occurrence in forest regenerating following clearfelling may be explained by their occupation of  ‘legacy’ coarse woody debris derived from the previous unharvested stand. The findings from both lines of research in this thesis will enable further exploration and modelling of species persistence in production-forest landscapes and could help guide management practices accordingly.

BIOBUZZ ISSUE FIFTEEN, DECEMBER 2011