“‘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.