Modelling koala density using incidental koala sightings in South East
Queensland, Australia (1997-2013)
Abstract
The koala, Phascolarctos cinereus, is an iconic Australian wildlife
species, but faces rapid decline in South-East Queensland (SEQLD). For
conservation planning, estimating koala populations is crucial.
Systematic surveys are the most common approach to estimate koala
populations, but such surveys are restricted to small geographic areas,
they are costly and conducted infrequently. Public interest and
participation in the collection of koala sightings is increasing in
popularity, but such data is generally not used for population
estimation. We used incidental sightings of koalas reported by members
of the public from 1997-2013 in SEQLD to estimate the yearly
spatio-temporal koala sightings density. For this, a spatio-temporal
point process model was developed accounting for observed koala density,
spatio-temporal detection bias and clustering. The density of koalas
varied throughout the study period due to the heterogeneous nature of
koala habitat in SEQLD, with density estimates ranging between 0.005 to
8.9 koalas per km2. The percentage of land areas with very low sightings
densities (0-0.25 koalas per km2) remained similar throughout the study
period representing in average (SD) 68.3% (0.06) of the total study
area. However, land areas with more koalas per km2 showed larger annual
variations, with koala mean (SD) densities of 0.25-0.5, 0.5-1, 1-2, 2-5
and > 5 koalas per km2 representing 16.8% (0.21), 13.8%
(0.25), 0.7% (0.20), 0.3% (0.13), and 0.2% (0.1) of the study area in
South-East Queensland, respectively.We did find that clustering of koala
sightings was not prominently different between the mating and
non-mating seasons of koalas. While acknowledging the limitations
associated sightings data, we developed a statistical model that
addressed the spatio-temporal bias associated with observed koala
sightings and provided long-term relative koala density estimates for
one of the largest koala populations of Australia.