Suchergebnisse

Context
Global climate change will lead to increased climate variability, including more frequent drought and heatwaves, in many areas of the world. This will affect the distribution and numbers of wildlife populations. In south-west Queensland, anecdotal reports indicated that a low density but significant koala population had been impacted by drought from 2001–2009, in accord with the predicted effects of climate change.

Aims
The study aimed to compare koala distribution and numbers in south-west Queensland in 2009 with pre-drought estimates from 1995–1997.

Methods
Community surveys and faecal pellet surveys were used to assess koala distribution. Population densities were estimated using the Faecal Standing Crop Method. From these densities, koala abundance in 10 habitat units was interpolated across the study region. Bootstrapping was used to estimate standard error. Climate data and land clearing were examined as possible explanations for changes in koala distribution and numbers between the two time periods.

Key results
Although there was only a minor change in distribution, there was an 80% decline in koala numbers across the study region, from a mean population of 59 000 in 1995 to 11 600 in 2009. Most summers between 2002 and 2007 were hotter and drier than average. Vegetation clearance was greatest in the eastern third of the study region, with the majority of clearing being in mixed eucalypt/acacia ecosystems and vegetation on elevated residuals.

Conclusions
Changes in the area of occupancy and numbers of koalas allowed us to conclude that drought significantly reduced koala populations and that they contracted to critical riparian habitats. Land clearing in the eastern part of the region may reduce the ability of koalas to move between habitats.

Implications
The increase in hotter and drier conditions expected with climate change will adversely affect koala populations in south-west Queensland and may be similar in other wildlife species in arid and semiarid regions. The effect of climate change on trailing edge populations may interact with habitat loss and fragmentation to increase extinction risks. Monitoring wildlife population dynamics at the margins of their geographic ranges will help to manage the impacts of climate change.

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.