286 Monitoring Threatened Species and Ecological Communities
Second, for target species that are likely to be continuously present at a site, the
type of monitoring should be able to detect the organisms across a range of
densities. Many species of vertebrates show density-dependent shifts in behaviour
and movements, and this can make them more or less difficult to find. For
example, some desert rodents become more social and sedentary as their numbers
increase after rainfall, thus increasing the chance that they will be live-captured in
pitfall traps (Dickman et al. 2010) (Fig. 21.4). Conversely, female small mammals,
both rodents and dasyurids, may become aversive to new objects such as traps
when they are pregnant or lactating, thus reducing the chance that they will be
detected. For species that exhibit labile behaviours, monitoring methods should be
used that produce reliable detections, especially at low densities. Remote cameras
may serve this purpose for many such species, with occupancy modelling based on
camera detections used to estimate the probability of species’ presence (MacKenzie
and Royle 2005).
Third, for target species that are likely to be continuously present within a
region, but can move from site to site depending on conditions, monitoring may
need to be expansive and cover multiple sites. Some small mammals provide good
Fig. 21.4. Checking pitfall traps at Ethabuka, in the Simpson Desert. Photo: C. Dickman/Desert Ecology
Research Group, University of Sydney.