Study area
The area of interest for this study comprised 15 local government areas
(LGAs) located in South-East Queensland (Figure 1). South-East
Queensland has a higher population of koalas compared with other areas
of Queensland (Dique, Preece, Thompson and Villiers 2004), but has also
a high urban development. The point locations of koala sighting events
(details provided below) were plotted on a map of South-East Queensland,
indicating relatively few sightings in the North and to the far West of
Queensland. To develop an observation window that was tractable for
modelling we constructed a convex hull around all koala sighting
locations and then dilated the convex hull by 3 kilometers to
accommodate the home range of koalas sighted on the border of the
observation window (de Oliveira, Murray, de Villiers, & Baxter, 2014).
Refinement of the study area in this way reduced the number of
constituent LGAs from 15 to 11 and included parts of 3 remaining LGAs.
The selected LGAs were in the Eastern and Central part of South-East
Queensland. The northern LGA of the Fraser Coast was excluded due to an
absence of koala sighting records. This reduced the study from 57,800
square kilometers to 30,500 square kilometers.
Figure 1. Local Government Areas (LGA) of South-East
Queensland, Australia (N=15). The LGAs for which koala population
densities were calculated are shown in the rectangle (Green areas
represents the main koala conservation in South-East Queensland).
Koala sighting
data
Incidental koala sightings reported in South-East Queensland between
1997 and 2013 (N = 14, 250 sightings) were retrieved from
KoalaBASE, a database of clinical admission and sightings data of
koalas, which developed by the University of Queensland’s School of
Veterinary Science
(www.koalabase.com.au). The dataset
comprised of the date of the sighting as well as the longitude and
latitude of the location, where the koala was observed. We assumed that
if two koalas were reported in the same area in the same month, these
could be repeated sightings of the same animal. The term ‘area’ in this
context represents the estimated home range for a koala, which was
assumed to be circular with an average size of 0.35
km2 (de Oliveira, et al. 2014). Thus, the minimum
distance between two koalas was given by\(d=2\ \times\ \sqrt{A\ \div\ \pi}\), or 666 metres. Figure 2a
shows a selection of koala sighting locations for month 1, which are
minimum of 666 metres apart and excluded other sightings for the month,
while Figure 2b shows a selection of koala sightings for month 2.
Because home ranges of koalas can overlap (Ellis, et al. 2009), Figure
2c shows the two monthly data sets combined. In Figure 2c sightings are
permitted to be closer than the minimum distance set for individual
monthly observations, acknowledging that koala home ranges overlap here.
Following this approach, monthly sightings observations for each year
over the 17 years period were compiled, resulting in a total of 17 years
× 12 months = 204 selections. By adopting this approach, we permitted a
koala (i.e. the same koala or a different koala) to be seen in the same
area at a maximum of 12 times per year.
Figure 2. Illustration of the method used to select incidental
koala sightings to estimate koala sightings density. Circles represent
an average home range area for a koala. Figure 2a shows a selection of
koala sighting locations for month 1, while Figure 2b shows a selection
of koala sightings for month 2. Figure 2c shows the two monthly data
sets combined. Following this approach, monthly sightings observations
for each year over the 17 years period were compiled and used a subset
of sightings to estimate koala sightings density.