Queensland, most of the population occurring in the Northern Territory (Churchill et al. ... used by sympatric species of cave bats; (5) R. aurantius is limited by a ...
Wildl. Res., 1991, 18, 343-53
Distribution, Abundance and Roast Selection of the Orange Horseshoe-bat, Rhinonycteris aurantius, a Tropical Cave-dweller
Sue K. Churchill Conservation Commission of the Northern Territory, P.O. Box 496, Palmerston, N.T. 0831, Australia.
Abstract Temperature and humidity were recorded from roost sites used by nine species of cave bats in northern Australia. The 10 sites containing R. aurantius exhibited the narrowest range of roost conditions of any species, this species having a strong preference for hot and humid roosts (28-32°C and 85-100% RH). R. aurantius colony sizes ranged from 20 to 25 000, and varied seasonally, almost all colonies abandoning their cave roosts during the wet season. Colony size was strongly related to mean minimum monthly temperature and rainfall, populations being greatest during the coolest and driest period of the year. Other sympatric species also exhibited preferences for specific roost conditions, indicating interspecific partitioning of roost resources. Species that utilised a broad range of roost humidity occupied a larger geographic range than those with more specific requirements.
Introduction The orange horseshoe-bat, Rhinonycteris aurantius, is a tropical cave-dwelling species that ranges across northern Australia from the Pilbara of Western Australia to western Queensland, most of the population occurring in the Northern Territory (Churchill et al. 1988). Rhinonycteris, along with the genera Aselliscus, Coelops, Anthops, Cloeotis and Triaenops, is an early derivative from the main line of Hipposideridae (S. Hand, personal communication). Rhinonycteris is represented in the fossil record from Riversleigh, Qld, about 3-5 million years ago (Archer et al. 1989) with Rhinonycteris-like taxa as o;d as 15-25 million years (Hand et ~1.1989). R. aurantius is known from only 10 caves and mines throughout its range (Churchill et al. 1988; Jolly 1988) and is classified as a rare and vulnerable species (Ride and Wilson 1982). Rabinowitz et al. (1986) defined seven types of rarity based on combinations of three major traits: geographic distribution, habitat specificity and local population size. In this paper I examine the reasons why R. aurantius is rare, and suggest that this is due to highly specialised habitat requirements within a climatically limited geographical range (Rabinowitz et al. 1986). Several hypotheses are examined: (1) the distribution of R , aurantius is limited by climatological conditions; (2) environmental conditions in caves used by R. aurantius differ from those in other caves within the range; (3) R. aurantius select specific roost microclimates within a cave; (4) roost microclimates selected by R. aurantius differ from those used by sympatric species of cave bats; (5) R. aurantius is limited by a combination of local climate and cave microclimate.
S. K. Churchill
Most published data on roost selection are concerned with conditions required for hibernation by heterothermic temperate species (Twente 1955; Daan and Wichers 1967; Gaisler 1970; Van der Merwe 1973; Raesley and Gates 1987). Other data are concerned with the use of caves as maternity sites (Dwyer and Hamilton-Smith 1965; Dwyer and Harris 1972; Hall et al. 1975). Published data on the microclimate of diurnal roost sites used by tropical cave bats are scarce. Comparisons are made of roost microclimate selection and geographic range among seven sympatric species of tropical cave bats. Materials and Methods Between May 1987 and November 1988, a total of 204 caves and mines were searched for colonies of R. aurantius. Areas of limestone outcrops and cliff lines were searched on foot. In mining areas, adits (horizontal tunnels) were searched and shafts (vertical tunnels) were mist-netted.
Counting Techniques Populations were estimated by a variety of methods. In small accessible colonies it was possible to count the bats at the roost site. Larger colonies were counted as the bats left the cave during the evening exodus. In the early stages of the study, low-intensity lights were placed on each side of the entrance, so that they were not visible from inside the cave, and the bats counted as they flew through (Helman and Churchill 1986). The large numbers of bats at Tolmer Falls Cave made counting of individuals impossible; instead, groups of 10 (a number easily visualised) were estimated and the number of groups counted. From April 1988 an electronic counter was used at Tolmer Falls Cave and Cutta Cutta Cave. This (built by the Northern Territory University) consisted of a series of vertical infrared beams that, when interrupted by a bat flying through, triggered a counter (Churchill and Lowe, unpublished). This method did not require the use of lights and proved to be much more effective in obtaining an accurate count of the population. A calibration factor was used to standardise the results obtained by the different techniques. Two study sites, Tolmer Falls Cave (13°13'S.,130045'E.), in Litchfield Park, and Cutta Cutta Cave (14°35'S.,132028'E.), near Katherine, were selected for monitoring seasonal fluctuations in population size. At each of these sites the population was counted for three consecutive nights every month for 18 months, during the week of the new moon to avoid the effect of bright moonlight on bat behaviour. The number of bats leaving the cave was recorded every 15 min, to obtain an indication of activity patterns. Bats were counted from sunset until they began returning to the cave after foraging.
Table 1. Climatic variables used to predict the geographic distribution of R. aurantius
Values calculated by
B I ~ C L I M(Nix
Variable Temperature ( T ) Mean annual Annual maximum Annual minimum Annual range Mean, hottest month Mean, coldest month Monthly range Isotherm Minimum, coldest month Maximum, hottest month Maximum range Mean, wettest quarter
1986) after imputing latitude, longitude and altitude of each of the 17 localities where roosts have been recorded Mean t s.d.
Variable Mean, driest quarter Mean, hottest quarter Mean, coldest quarter Precipitation (mm) Mean annual Wettest month Driest month Monthly range Seasonality Wettest quarter Driest quarter Hottest quarter Coldest quarter
Meants.d.
Roost Selection by Rhinonycteris aurantius
Geographic Range and Measurement of Microclimates Distributional records of R, aurantius were collected and analysed by BIOCLIM, a predictive model that produces specific climatic profiles from distribution records (Nix 1986). These profiles were used to identify climatically similar regions on a 0.5tgrid. The climatic region defined by BIOCLIM was used as the theoretical distribution for R, aurantius, and the climatic requirements of R. aurantius, as defined by BIOCLIM, are shown in Table 1. Caves within the geographic range. Data on temperature and humidity were collected from caves and mines in the geographic area determined by BIOCLIM, both at every site where bats were found roosting and at a number of others, to characterise the full range of microclimate conditions available. Temperature and humidity were measured with a Bacharach swing psychrometer. Wet and dry bulb readings were recorded to the nearest 0.5"C and later used to calculate vapour density. This was used in the statistical analysis because it provided a better measure of water vapour present than the temperature-dependant measure of relative humidity. Caves used by R. aurantius. The microclimates of caves that contained and did not contain colonies of R. aurantius were compared. Temperatures and vapour densities recorded throughout the caves were used in this analysis. The climatic condition in the two R. aurantius study sites were sampled monthly for 18 months. Temperature and humidity were recorded from six sites within Tolmer Falls Cave, and 15 sites distributed at 50-m intervals throughout Cutta Cutta Cave. Analysis of variance was used to determine the effects of season and distance from cave entrance on cave temperature and humidity. Temperature and humidity were used as dependent variables in relation to season, distance from cave entrance, and the interaction of both factors. Roost Microclimate and its Selection Choice of microclimate. To determine whether R. aurantius selects specific roost microclimates, and whether these vary throughout the year, conditions at R. aurantius roost sites were compared to those elsewhere in the same cave. Analysis of variance was used to determine the effect of season on cave and roost microclimate. At both Cutta Cutta and Tolmer Falls Caves, temperature and vapour
Table 2. Summary of caves and mines reported to contain populations of R, aurantius and their current population status Cave or mine
Population
Northern Territory Tolmer Falls Cave Tolmer Falls Cave 10 Figtree Falls Cave Cutta Cutta Cave Mathison Creek Cave Jensens Adit Mt Wells Mine Spring Hill Mine Echo Gorge Cave Virginia Mine Queensland Kalkadoon Cave Lawn Hill Gorge Cave Western Australia Geikie Bat Cave Fairfield Station Cave Cave Spring Tunnel Creek Klondike Queen Mine Current known population
Status
Protected Protected Protected Protected Not threatened Threatened Threatened Destroyed Temporary Temporary Protected Temporary Protected Not threatened Abandoned Abandoned Abandoned 35 570
S. K. Churchill
density were compared seasonally for roost sites used by R. aurantius and for other sites. Temperature and vapour density were dependent variables in relation to season, presence of R. aurantius, and their interaction. The year was divided into: wet season, from November to February; early dry season, from March to June; late dry season, from July to October. During the wet season most R. aurantius leave known cave roosts. Microclimatic conditions of R. aurantius roost sites at this time were compared (ANOVA) with the early and late dry seasons.
Local climatic conditions and population size. Fluctuations in R. aurantius population at Cutta Cutta Cave and Tolmer Falls Cave were investigated in relation to external environmental factors, by an analysis of variance. Variation in population size between sites (Table 2) was examined in relation to bioclimatic variables predicted by BIOCLIM (Nix 1986). Roost selection, geographic range and body size. Stepwise discriminant analysis (SAS Institute Inc. 1985) was used to determine the microclimate selected by R. aurantius and each of six species sympatric with it. The analysis used the variables temperature and vapour density. Variables were entered stepwise on the basis of minimal values of Wilk's lambda (minimum F to enter, 0.15; minimum F to remove, 0.15). Spearman's Rank Correlation Coefficients were used to examine the relationships of microclimate preference, bat body size, and size of geographical range for each species. The geographical range of Eptesicus caurinus included those of E. caurinus and a newly described sibling species, E. findlaysoni (Kitchener et al. 1987). These are morphologically and ecologically very similar and cannot be readily identified in the field.
Fig. 1. Distribution of R , aurantius. 0 Colonies. 0 Areas where caves and mines were searched. Shaded areas, distribution predicted by BIOCLIM.
Results
Geographic Range and Caves Used by R. aurantius The geographic range of R. aurantius (Fig. 1) although widely scattered, is well distributed across the climatic range predicted by BIOCLIM. The 204 caves and mines examined exhibited a broad range of environmental conditions. Cave temperatures generally approximated the mean ( t s.d.) annual ambient temperature (Dwyer 1971) of 27 t 0 . 7 " C . The average temperature of all the caves measured was 28.3"C. Available cave temperatures ranged from 23.5" to 33°C. Vapour density ranged from 4 . 1 to 30.36 g m - 3 with a mean of 20.5 g m-3. A total of 137 caves and mines contained colonies of bats, as follows: Rhinonycteris aurantius Hipposideros ater Macroderma gigas
10 53 42
Miniopterus schreibersii Myotis adversus Taphozous georgianus
21 6 55
Eptesicus caurinus Caves without bats Caves searched
46 67 204
Roost Selection by Rhinonycteris aurantius
Table 3. Temporal variation in temperature and vapour density at Cutta Cutta Cave in relation to distance from cave entrance
Degrees of freedom
Factor
Sum of squares
F
P
Temperature Season Depth of site Season x depth Vapour density Season Depth of site Season x depth
Fig. 2. Regression lines showing the seasonal relationship of temperature to distance from the cave entrance at Cutta Cutta Cave (r2=0.695, n = 189, P
