Are there habitat thresholds in koala occupancy in the semiarid landscapes of the Mulgalands Bioregion?
Andrew G. Smith A D , Clive McAlpine A , Jonathan Rhodes A , Leonie Seabrook A , Daniel Lunney B C and Greg Baxter A
+ Author Affiliations
- Author Affiliations
A The University of Queensland, Landscape Ecology and Conservation Group, School of Geography, Planning and Environmental Management, Brisbane, Qld 4072, Australia.
B Office of the Environment and Heritage, PO Box 1967, Hurstville, NSW 2220, Australia.
C School of Biological Sciences and Biotechnology, Murdoch University, Murdoch, WA 6150, Australia.
D Corresponding author. Email: andrew.g.smith@uq.edu.au
Wildlife Research 40(5) 413-426 https://doi.org/10.1071/WR13010
Submitted: 17 January 2013 Accepted: 15 August 2013 Published: 9 September 2013
Abstract
Context: Habitat thresholds are the critical point(s), below which the probability of occurrence of a species declines. Identifying thresholds assists land managers to decide how much habitat is needed to conserve a species. However, for any given species, a threshold may not exist, or might occur at one scale but not at others, and it may differ across regions. The use of critical habitat thresholds can negatively affect populations if simplified conservation targets for habitat retention are prescribed. This problem is relevant to the koalas where there is evidence of habitat thresholds in mesic regions, but no studies of thresholds in semiarid regions.
Aims: The aim of the present study was to investigate whether a threshold exists between the occupancy of a site by koalas and habitat variables at both the site and at four landscape scales in the semiarid Mulgalands Bioregion of Queensland, Australia.
Methods: We modelled habitat relationships using standard and piece-wise logistic regression, and an information-theoretic approach, to determine whether the best model that explained the occupancy–habitat relationships was linear or had a distinct threshold. The site-scale variable was the percentage of primary eucalypt species. The landscape-scale variables included the amount of primary and secondary habitat, and an interaction between them.
Key findings: There was a threshold relationship between the occurrence of koalas and the percentage of primary trees at the site scale. At the landscape scale, most threshold models failed to converge, and evidence pointed to a linear relationship between habitat amount and koala occupancy.
Conclusions: Conservation actions for koalas in the Mulgalands Bioregion should concentrate on protecting the primary tree resource for koalas, namely, river red gums (E. camaldulensis). However, the maintenance or restoration of primary and secondary habitat to distances of 1000 m from the creek is important because of the linear relationship between koala presence and habitat amount.
Implications: As habitat is lost in the semiarid Mulgalands Bioregion, koala occupancy declines. If known thresholds from mesic regions are used to define a minimum amount of habitat to be retained for koalas, conservation of local koala populations may well fail.
Additional keywords: piece-wise logistic regression, presence/absence data, river red gum, species distributions.
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