“‘Core areas’ are more densely occupied than ‘peripheral areas”’
Existing empirical data broadly supported this statement (Table 2). In our study, peripheral areas (those occupied less than 80% of the time) generally were less densely occupied than core areas, though density varied substantially across roost site, subplot and session (all contributed substantially as random effects). Here, lower density refers to both a lower number of bats per subplot in peripheral subplots (-0.581 ± 0.177, p= 0.001, Figure 3), and a lower proportion of occupied trees (-0.222 ± 0.078, p= 0.005,
Appendix S2). Within subplots, we also note that some trees were more consistently used than others, including trees that were occupied in 100% of surveys where bats were present at the roost (Appendix S1). The number of bats per tree in irregularly occupied trees (occupied less than 80% of the time) were typically lower than for regularly occupied trees (-0.606 ± 0.034, p< 0.001).
We observed negative relationships between bat occupation metrics and distance from the roost centre, including in the number of bats per occupied subplot (-1.639 ± 0.016, p< 0.001, Figure 4) and proportion of occupied trees per subplot (-0.315 ± 0.034, p< 0.001, Appendix S2). This decline with distance from the centre of subplot was largely driven by little red flying-foxes (Figure 4). Roost site, subplot and session also all contributed substantially as random effects (Appendix S2).
“Roost area fluctuates with total abundance”
Studies have previously reported changes to total roosting area, but none to date have formally quantified the relationship between area and abundance (Table 2). From our data, we observed substantial fluctuations in total roost area within some roost sites across monthly surveys, and overall, a positive relationship with total roost abundance. The extent of variation was variable across roosts, however (Figure 5). We note that relationships between total abundance and area were likely masked in many roosts by the large span of population values in some index categories (e.g. index 6 spans 16,000 - 49,999 bats). It is probable that data of finer resolution may have detected this relationship more strongly for roosts in this size range, but are not available in this dataset.