“Roost abundance peaks in March” & “Intra- and inter-annual variations in abundance can be extreme”
March was identified in some management documents as being the time for peak abundance in flying-fox roosts (e.g. State of Queensland Department of Environment and Science 2020). However, studies on P. poliocephalus and P. alecto identify a typical pattern of increasing abundance from September-October (when females give birth) until a peak in January-February (when the season’s young are able to fly independently) (Nelson 1965b; Eby 1991; Eby & Palmer 1991; Parry‐Jones & Augee 2001). Roost sizes then decrease during March-April (the period of mating) to low winter counts in continuously occupied/overwintering roosts, or zero winter counts in seasonally occupied summer roosts (Nelson 1965b; Eby 1991; Eby & Palmer 1991). These studies note that cyclical patterns of occupation are driven by reproductive factors (i.e. timing of birth and independent flight), but highlight that irregular, local dynamics of food availability can superimpose variability into these patterns of abundance (Parry-Jones & Augee 1992). Indeed, many studies note high intra- and inter-annual variability in abundance. Parry‐Jones and Augee (2001), for example, note that animals from their study roost appeared to migrate away and decrease in abundance in response to a blossoming event, presumably to move to a roost in closer proximity to the blossoming.
In our study, some roosts showed patterns consistent with a total roost abundance peak towards March (e.g. ‘Redcliffe’, ‘Canungra’ and ‘Clunes’). Others showed either no considerable fluctuation in abundance (‘Burleigh’) or peaks at other times (‘Toowoomba’, ‘Sunnybank’, ‘Avondale’, ‘Lismore’). Drivers of peaks were variable between roosts. For the ‘Redcliffe’ roost, seasonal migration of little red flying-foxes from ~January 2019 contributed to a peak in abundance around March (see species abundance plots in
Appendix S2). For the ‘Lismore’ roost, a blossoming event in winter 2018 triggered an influx of nomadic bats into the population, driving the peak observed in August 2018. Dynamics observed in other roosts were likely the result of local dynamics of food availability.
We note also that estimates of abundance from our study were much smaller than those of historical estimates. Ratcliffe (1931) describes ‘small’ roosts as ~5,000-10,000 animals, ‘medium’ as 10,000-50,000, and ‘large’ as anything over this size. Ratcliffe (1931) also report roosts in northern Queensland with bats ‘into the millions’ (Red River) and ‘exceeded a quarter of a million, possibly considerably’ (Burnett River). Likewise, Lunney and Moon (1997) report historical observations of flying-foxes in the Richmond Valley (1870’s) as into the millions. The maximum roost site observed in this current study was ~ 95,000, recorded at the Lismore roost in August 2018 in response to a local eucalyptus flowering event. Roost sizes of <5,000 were more common for the roost sites surveyed and, extending from the sizes in Ratcliffe (1931), may constitute a new category of ‘very small’. Local management areas should expect that local conditions can change substantially and rapidly for flying-fox populations, resulting in population changes outside of times predicted by demographic driven dynamics alone. An understanding of the timing and productivity of flower resources within the feeding range of roosts is likely to be of greater importance to forecasting and interpreting large population fluctuations than are reproductive considerations.