Empirical data collection
We collected data on roosting structure at eight sites in south-east
Queensland and north-east New South Wales (Figure 1). These sites were
chosen to represent a gradient of habitats utilised by flying-foxes,
ranging from metropolitan areas of Brisbane and the Gold Coast, to
roosts in peri-urban and rural areas (Figure 1, Table 1). All sites were
previously documented as having a continuous population of grey-headed
or black flying-foxes. Little red flying-foxes visited some roost sites
intermittently, however no roost sites occurred within the distribution
of spectacled flying-foxes (National Flying-Fox Monitoring Program
2017).
We mapped the spatial arrangement of all overstory, canopy and midstory
trees in a grid network of 10 stratified random subplots (20 x 20 meters
each) per roost site. Trees were mapped and tagged using tree survey
methods described in the “Ausplots Forest Monitoring Network, Large
Tree Survey Protocol” (Wood et al. 2015). To evaluate
spatio-temporal patterns in roosting, we revisited all tagged trees and
scored the extent of species occupancy using the following tree
abundance index: 0= zero bats; 1= 1-5 bats; 2=6-10 bats; 3=11-20 bats;
4=21-50 bats; 5=51-100 bats, 6=101-200 bats, 7= >200 bats.
For a subset of trees (N=60 per site, consistent through time) absolute
counts and minimum/maximum roosting heights of each species were taken.
Overall roost perimeter (perimeter of area occupied) was mapped with GPS
(accurate to 10 meters) immediately after the tree survey to estimate
perimeter length and roost area. Total abundance at each roost was also
estimated with a census count of bats where feasible (i.e. where total
abundance was predicted to be <5,000 individuals), or by
counting bats as they emerged in the evening from their roosts
(“fly-out”), as per recommendations in Westcott et al. (2011).
If these counts could not be conducted, population counts from local
councils (conducted within ~a week of the bat surveys)
were used, as total abundance of roosts are generally stable over short
timeframes (Nelson 1965b). Because roost estimates become more
unreliable with increasing abundance we also converted the total
estimated abundance into an index estimate for use in analyses, as per
values used by the National Flying-Fox Monitoring Program (2017). Census
index categories were as follows: 1 = 1-499 bats; 2 = 500-2,499 bats; 3
= 2,500 - 4,999 bats; 4 = 5,000 - 9,999 bats; 5 = 10,000 - 15,999 bats;
6 = 16,000 - 49,999 bats; and 7 = > 50,000 bats. Roosting
surveys were repeated once a month for 13 months (August 2018 - August
2019). More detailed methods of empirical data collection can be found
in Appendix S1 in the Supporting Information.