The echolocation calls, habitat relationships, foraging niches and communities of Pilbara microbats

WA Museum Records and Supplements | Updated 7 years ago

ABSTRACT – Between 2004 and 2007, we systematically surveyed microbats across the Pilbara region in Western Australia, and collected data on species’ foraging ecology. Here we report the results of the echolocation survey of 69 sites dispersed among 24 survey areas covering the 179,000 km2 region. Echolocation call sequences were identified using a library of known calls accumulated during field work. In combination, the frequency maintained for the greatest number of cycles (FpeakC) and the bandwidth ratio of this peak (Q) identified search-mode echolocation calls by 13 of the 17 species comprising the microbat fauna of the Pilbara bioregion. These variables did not separate Taphozous georgianus from T. hilli calls, Chalinolobus gouldii from Mormopterus loriae, and allopatric pairs of Nyctophilus species. Even so, the spectral characters provided an ecologically informative, viable and non-intrusive survey tool.

The survey revealed two compositionally distinct communities. One comprised 14 species and occupied landward environments, while the other comprised 9 species and occupied mangroves. Three members of the mangrove community were confined to mangroves (M. loriae, Nyctophilus arnhemensis and N. geoffroyi pallescens), being replaced by allopatric congenerics in the region’s landward environments (Mormopterus beccarii, Nyctophilus bifax daedalus and N. g. geoffroyi). In both communities, the searchmode calls of syntopic species were dispersed in spectral space, showed only peripheral overlap in their spectral variables (Q and FpeakC) and were arrayed according to differences in foraging niche determined from empirical data on species’ flight capabilities and foraging behaviours. These observations imply a niche-assembly model of metacommunity structure.

However, on its own, this model was insuffi cient to explain the composition of the Pilbara microbat assemblages. Nestedness was observed in assemblage composition that could be explained by environmental factors, implying the influence of environmental controls. The richest microbat assemblages were recorded in well-developed riparian environments with complex vegetation structures and permanent pools that were set in cavernous landscapes. Two species (Nyctophilus bifax and Chalinolobus morio) were restricted to these productive riparian environments, while two others (Macroderma gigas and Rhinonicteris aurantia) were found to be more common than previously supposed despite detectability constraints caused by their cryptic calls. The widespread occurrence of M. gigas and R. aurantia is reasonable because caves and mines are common in the ranges of the Pilbara region, and offer physiologically favourable day-roosts to these otherwise mesic tropical species. Proximity of cavernous landscapes also explained the presence or absence of other obligate cave/rock-crevice roosting species in assemblages (Taphozous spp., and Vespadelus finlaysoni).

Comparison with surrounding regions revealed a diversity-productivity model of faunal structure, with an organisation that conforms to the specialisation hypothesis in which co-occurring species occupy different foraging niches. Three riparian sites (Turee Creek, Cattle Gorge and Weeli Wolli) and four mangrove stands are identified for reservation.