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.