Long-term effects of grating derelict mines on bat emergence activity, abundance and behaviour

In Australia, there are at least 50 000 derelict mines, many of which provide habitat for cave-roosting bats. Grating of derelict mines, be it horizontal (adits) or vertical (shafts) drives, is commonly undertaken to prevent human access, though longer-term responses of bats are largely unknown. We assessed the long-term (2–20 years) effects of grating on bats by documenting trends in emergence activity and bat abundance at grated and ungrated derelict mines and quantified behavioural responses of bats in autumn and winter. Emergence activity was dominated by the eastern horseshoe bat (Rhinolophus megaphyllus) with limited activity of other less manoeuvrable species. Both emergence activity and minimum colony size at horizontal adits were 8–9 times greater than at vertical shafts, with bats observed emerging from only 2 of 13 shafts. Emergence activity and minimum colony size were 7–10 times greater at adits with ‘bat friendly’ grating (horizontal bars with spacing >125 mm) than at other treatments (ungrated adits and adits with standard grating). In winter, there were 4–11 times more aborted exit attempts per bat at adits with ‘bat friendly’ grating compared with other treatments, which corresponded to greater emergence activity. Emergence activity and minimum colony size were not related to spacing between bars or time since grating, indicating rapid habituation by R. megaphyllus. However, circling at grates continued for many years and bentwing bats (Miniopterus spp.) made little use of these sites. Bat-friendly grates appear to be an effective management option for R. megaphyllus, but alternatives need to be trialled for other species.