Pallid bat (Antrozous pallidus) COSEWIC assessment and status report: chapter 7

Habitat

In general terms pallid bat habitat is characterized as arid to semi-arid desert with mean annual rainfall of between 200 and 375 mm, and hot summer temperatures with daily maximums as high as 38° C (Vaughn and O’Shea 1976). In British Columbia this species seems restricted to valley bottoms with an elevational range of between 300-490 m (Nagorsen and Brigham 1993).

The availability and quality of pallid bat breeding and roosting habitat requirements for most of the south Okanagan has recently been modelled by Robertson (1998; Figure 4). The model ranks and distinguishes between both managed (e.g. livestock grazing areas; vineyards; golf courses) and un-managed (e.g. shrub-steppe; dry grassland, dry forest, wetlands) ecosystems in the region, in terms of various critical habitats and life requisites. Breeding and day roosting habitats (e.g. horizontal rock crevices of steep cliffs, canyon walls, rock outcrops, and talus), are considered virtually identical and of the highest priority for this species, while foraging habitat, although included in the model is considered less limiting (Robertson 1988).

For the purposes of this report, A. pallidus habitat requirements are considered in terms of 3 main categories: 1. foraging; 2. day roosting; and 3. night roosting. Hibernation habitat is another potential category omitted here because no data are available on hibernation by pallid bats in the Okanagan, nor do winter records exist for this species (Nagorsen and Brigham 1993). 

Foraging habitat requirements

By far the most common foraging strategy for A. pallidus involves using passive sound localization to listen for prey (usually terrestrial) and then gleaning prey items from surfaces and consuming them later, either in flight or at a night roost (Bell 1982, Fuzessery et al. 1993). This foraging strategy requires that pallid bats spend at least some time on the ground and, in captivity, they have been observed catching prey after lengthy terrestrial chases (Fuzessery et al. 1993). Spending time on the ground presents a range of problems for a bat, especially for a large one with relatively high wing loading (body mass / wing area) when it comes to taking off (Fenton 1990), and suggests a requirement of open, uncluttered foraging habitat. In fact, Bell (1982) reported a clear preference by foraging A. pallidus for open, sparsely vegetated terrain and, in the Okanagan, Chapman et al. (1994) found that pallid bats foraged mainly over open sagebrush or sparse grassland with scattered ponderosa pine. This type of habitat is common in those areas of the Okanagan which remain undisturbed but is increasingly at risk from agriculture and urban expansion (Bailey 1995, Sarell, personal communication). Because A. pallidus will not fly in cluttered or densely vegetated areas (Bell 1982) expanding fruit growing operations, as well as urban development, are likely to have a negative impact on this species by reducing available foraging area.

Livestock grazing, however, may not be completely detrimental to pallid bats because it tends to open up foraging habitat and increase the density of large beetles associated with dung (e.g. Family Silphidae; Chapman et al. 1994), which are common prey items for this species (Grindal et al. 1991). Until recently, livestock grazing has been the most common type of agriculture on the Inkaneep Reserve, which could help explain the relative abundance of A. pallidus there. On the other hand, grazing can reduce overall density and diversity of arthropods and, if trees are removed from grazing areas, night roosting habitat could be compromised (see below). Robertson’s (1998) habitat suitability model (Figure 3) suggests that high quality foraging habitat is much more abundant in the Okanagan than high quality breeding habitat. 

Diurnal roosting requirements

Recent work on both day and night roosting ecology of A. pallidus by Lewis (e.g. 1993, 1994, 1996) is of particular relevance to this report because it focused on pallid bat populations in Oregon. Intuitively, a Canadian population of pallid bats seems more likely to share habitat requirements, as well as genetic similarity, with animals from the northern United States, relative to southern populations, which have been the focus of most pallid bat research to date. 

Antrozous pallidus day roosts are usually found in cliff faces or rock crevices and often in buildings and under bridges (Vaughan and O’Shea 1976). Lewis (1996) found that pallid bats show low day roost fidelity and that short term roost switching is more strongly correlated with an individual bat’s ectoparasite load than with physical or climatic attributes of the roost. Over the long term, however, A. pallidus, prefers roosts behind thin slabs of rock during cooler months and deep in crevices during the hotter months of midsummer.

Only three pallid bat day roosts have been identified in the Okanagan Valley and to date no maternity roosts have been found. All three roosts were located by radio-telemtery within the Inkaneep Indian Reserve high up on cliff faces (Chapman et al. 1994). Of the three pallid bat habitat types, established day roosting sites may be relatively resistant to human disturbance in the Okanagan because these areas are inaccessible and this species does not seem highly sensitive to disturbance by human observers close to the day roost (Lewis 1996). Rock climbing, however, a growing recreational activity in the area, could pose a threat to day roosting habitat, because it can result in extremely close contact with roost openings.

Roost availability can be a natural limiting factor for a variety of bat species (Humphrey 1975) and, despite the relative security of established day roosts, Robertson’s habitat suitability model (1998; Figure 4) suggests that high quality day roosting habitat (i.e. breeding habitat) is very rare in the Okanagan and almost certainly the most limiting of the three habitat types. Incorporated into the model is the fact that pallid bats prefer roosting areas with close proximity to high quality foraging areas (i.e. within 8 km), an important consideration especially when foraging habitat close to potential roost sites is disturbed by development. When combined with the relative rarity of suitable maternity roost sites in the area, this means that day roosts are highly susceptible to human influence both at or near the roost itself and in nearby foraging sites. Radio telemetry studies are needed so that roosts can be located and protected.

Night roosting requirements

Night roosting requirements of A. pallidus, though not considered in the habitat suitability model, may be more important to this species than previously supposed. Lewis (1994) reported a remarkable degree of night roost fidelity by pallid bats in Oregon both within and between years, even after they were captured and recaptured at a specific roost. Night roosting locations in northern parts of A. pallidus range include bridges and rock overhangs (Lewis 1994), and a variety of human-made structures such as abandoned mines and open buildings are commonly used (Collard 1990). Only live ponderosa pine trees have been reported as night roosts in the Okanagan (Chapman et al. 1994). Night roosts observed by Lewis (1994) were most often under bridges and differed from day roosts in that they required no crawling by bats and were open enough to allow free flight in and out.

Recent evidence suggests that night roosts may serve a social function for this species. Lewis (1994) reported an interval between day roost emergence and night roost arrival (± 10 min) which was short enough to preclude foraging, and also that bats often arrived at, and left, the night roost in groups.  She speculated that sociality at the night roost could provide 2 potential benefits for pallid bats: 1. Information transfer with respect to productive foraging areas; and 2. Formation of foraging groups. Bell (1982) repeatedly observed as many as 15 pallid bats converging on common prey with no agonistic interaction, hinting at some form of group foraging which could be established or reinforced at the night roost.

If sociality established at the night roost improves A. pallidus foraging success, then night roost habitat could place some limits on this species in Canada, where all pallid bat night roosts have been found in live ponderosa pine trees (Chapman et al. 1994). Night roost fidelity tends to be very high for this species (Lewis 1994), which suggests that certain criteria make some potential night roosts more attractive to pallid bats than others. These roosts are also susceptible to human and natural disturbance in the Okanagan, because trees are easily removed by natural impacts (e.g. blowdowns, fire) and urban or agricultural development.

Pallid bat night roosts are easily identified by their accumulations of guano and unpalatable insect remains and, as such, may represent an important indicator for survey work in northern sections of A. pallidus range (Lewis 1994). Further study of night roosting by pallid bats in Canada employing radio telemetry should be considered a priority for this species.

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