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COSEWIC Assessment and Update Status Report on the Pallid Bat Antrozous pallidus in Canada – 2000

Illustration of a Pallid Bat Antrozous Pallidus.

Threatened – 2000

Table of Contents

Document Information

List of figures


Document Information

COSEWIC - Committee on the Status of Endangered Wildlife in Canada

COSEWIC status reports are working documents used in assigning the status of wildlife species suspected of being at risk. Anyone wishing to quote or cite information contained in this status report may do so provided that both the author and COSEWIC are credited. This report may be cited as follows:

Please note: Persons wishing to cite data in the report should refer to the report (and cite the author(s)); persons wishing to cite the COSEWIC status will refer to the assessment (and cite COSEWIC). A production note will be provided if additional information on the status report history is required.

COSEWIC. 2000. COSEWIC assessment and update status report on the Pallid Bat Antrozous pallidus in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. v + 18 pp.

Willis, C.K.R. and M.L. Bast. 2000. Update COSEWIC status report on the Pallid Bat Antrozous pallidus in Canada, in COSEWIC assessment and update status report on the Pallid Bat Antrozous pallidus in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. 1–17 pp.

Previous report(s):

Balcombe, J.P. 1988. COSEWIC status report on the Pallid Bat Antrozous pallidus in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. 16 pp.

For additional copies contact:

COSEWIC Secretariat
c/o Canadian Wildlife Service
Environment Canada
Ottawa, ON
K1A 0H3

Tel.: (819) 997–4991 / (819) 953–3215
Fax: (819) 994–3684
E-mail
Website

Cover illustration/photo:
Pallid Bat -- Judie Shore, Richmond Hill, Ontario.

© Her Majesty the Queen in Right of Canada, 2011.
Catalogue No. CW69–14/243–2002E–IN
ISBN 0–662–32935–x

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COSEWIC
Assessment Summary

Assessment Summary – May 2000

Common name
Pallid Bat

Scientific name
Antrozous pallidus

Status
Threatened

Reason for designation
Very small distribution; much of critical foraging habitat has been destroyed by agricultural and urban development, and habitat loss is continuing. Rescue effect is possible from adjacent populations in Washington state.

Occurrence
British Columbia

Status history
Designated Special Concern in April 1988. Status re–examined and designated Threatened in May 2000.

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COSEWIC
Executive Summary

Pallid Bat
Antrozous pallidus

The Pallid Bat (Antrozous pallidus) in Canada is found exclusively in the extreme south of the Okanagan Valley in British Columbia. Although common and widespread in arid and semi–arid regions of the south–western United States, this distinctive species is rare in Canada due in part to the fact that the southern Okanagan represents its northern range limit where climate, foraging habitat and roosting habitat may be sub–optimal. Human disturbances, however, which could include habitat degradation and possibly pesticide bioaccumulation may contribute to this species’ rarity in Canada, as well. Because very little land in the southern Okanagan Valley is protected in parks or ecological reserves, anthropogenic factors have the potential to impose dramatic impacts on this species. Relatively little is known about A. pallidus in Canada and further study is required before a management or protection plancan be initiated. Based on recent evidence, however, the presence of a breeding population currently seems much more likely than it did at the time of preparation of the original COSEWIC status report (Balcombe 1988).

COSEWIC History
The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC Mandate
The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) determines the national status of wild species, subspecies, varieties, and nationally significant populations that are considered to be at risk in Canada. Designations are made on all native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fish, lepidopterans, molluscs, vascular plants, lichens, and mosses.

COSEWIC Membership
COSEWIC comprises representatives from each provincial and territorial government wildlife agency, four federal agencies (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biosystematic Partnership), three nonjurisdictional members and the co-chairs of the species specialist groups. The committee meets to consider status reports on candidate species.

Definitions (2010)

Wildlife Species
A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)
A wildlife species that no longer exists.

Extirpated (XT)
A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)
A wildlife species facing imminent extirpation or extinction.

Threatened (T)
A wildlife species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)*
A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR)**
A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data Deficient (DD)***
A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction.

* Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.
** Formerly described as “Not In Any Category”, or “No Designation Required.”
*** Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994.

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.

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Update COSEWIC Status Report on the Pallid Bat Antrozous pallidus in Canada – 2000.

Introduction

The Pallid Bat (Antrozous pallidus, Chiroptera: Vespertilionidae) is found throughout arid and semi–arid regions of western North America from Mexico to the Okanagan Valley in south–central British Columbia. This distinctive species is a large bat by North American standards, and Canada’s largest, at 12.0–24.3 g with a 310–370 mm wingspan (Nagorsen and Brigham 1993). It is characterized by large ears and eyes, and short pale fur which fades from yellowish–brown dorsally to a cream colour ventrally. In contrast to most North American bats, A. pallidus typically gleans arthropods from surfaces and often travels to a night roost before feeding, where unpalatable parts can be removed. This species feeds on a range of arthropods, including scorpions (Edwards 1974), beetles and moths (Hermanson and O’Shea 1983) but may consume non–arthropods and has even been reported taking a 7–10 gram pocketmouse (Perognathus flavus) (Bell 1982).

Balcombe (1988) suggested a COSEWIC designation of vulnerable[1] for A. pallidus based on its sporadic occurence in Canada, which he attributed primarily to the fact that the Okanagan Valley represents the northern range boundary for this species, but also to possible anthropogenic limitations. Balcombe (1988) suggested agriculture and logging, particularly, could put an Okanagan population of A. pallidus at direct risk due to habitat disturbance, and indirect risk due to bio–accumulation of the pesticides widely used in fruit farming, the most common type of agriculture in the region. Logging, however, may have little impact on Pallid Bats because it occurs at relatively high elevations, above the 300–490 m within which this species has been observed in B.C (Nagorsen and Brigham 1993, Brigham, personal communication).

Prior to 1988 A. pallidus had been recorded from the southern Okanagan Valley only six times (e.g. Racey 1933, Anderson 1946, Fenton 1980) and a lack of success locating it in 1986 (Fenton et al. 1987) led Balcombe (1988) to suggest the possibility that previous accounts may have involved stray representatives of a population resident south of the Canada–U.S. border. Compounding uncertainty regarding a Canadian population of the species was the fact that even in the southwestern United States, where A. pallidus is abundant, population sizes were poorly estimated as of 1988.

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Distribution

No evidence has been presented since 1988 to suggest that A. pallidus range in Canada or the United States has changed since that time, although field study in the Okanagan Valley by Collard and Grindal, Chapman and McGuiness, and Barclay and his students after 1988 provides convincing evidence that a resident population exists in south–central B.C. No evidence is available to suggest that these animals are isolated  from Pallid Bat populations in Washington State to the south and the remainder of the species’ North American range, which is presented in Figure 1.

Figure 1.  North American distribution of the Pallid Bat (Antrozous pallidus) (from Grindal et al. 1991).

Map of the North American distribution of the Pallid Bat.

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Locations of recorded occurrences of A. pallidus are provided in Figure 2. A. palliduscaptures or recovered specimens have been confirmed only 28 times since the species was first reported in the southern Okanagan Valley in 1931. The size of the distributional area in Canada is difficult to estimate because some areas have received more survey attention than others, but is likely between 150 and 500 km².

Figure 2.  Locations of recorded occurrences of Antrozous pallidus in Canada. Each white dot represents a single captured female bat and each grey dot a single captured male. The heavy, solid black line represents the boundary of the Inkaneep Indian Reserve. Dots are placed as closely as possible to capture sites but are not precisely placed because of limited space. Re–captures are included as individual dots (modified from Chapman et al. 1994).

Map showing locations  where the Pallid Bat has been recorded in Canada.

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The majority of Pallid Bat captures have occurred between Vaseux Lake and Osoyoos, especially on the Inkaneep Indian Reserve which may be due to the fact that, traditionally, the Reserve has been subject to less development than the Okanagan region as a whole (Chapman et al. 1994), though no direct evidence exists to support this hypothesis. Based on compilations of visual observation, directive calls (which are audible to human observers and serve as social contact calls between individuals (Brown 1976)), and captures (6 reproductively active males and 1 non–reproductive male) in 1991, Chapman et al. (1994) estimated that a minimum of 12 animals were resident on the Inkaneep Reserve. Interestingly, their surveys of the neighbouring Similkameen River valley for spotted bats (Euderma maculatum) yielded no captures, sightings, or directive call counts for A. pallidus, although this work is likely insufficient to verify, conclusively, the absence of Pallid Bats from the Similkameen Valley (Brigham, personal communication). The absence of high quality foraging and breeding area in the Similkameen, as determined by a recent B.C. environment habitat evaluation model (Robertson 1998; Figure 3; and see below), however, may be a contributing factor.

Figure 3.  Habitat suitability map for Antrozous pallidus in the South Okanagan. (Robertson 1998).

Map showing habitat suitability for the Pallid Bat in the South Okanagan.

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Protection

Despite its rarity and extremely localized distribution in British Columbia, A. pallidus remains common throughout much of its range, especially in the southwestern United States. Only 1 of 28 Pallid Bat captures has occurred on protected lands (the Vaseux Lake Wildlife Reserve; Robertson 1998), and only 1% of the Okanagan region is afforded legal protection in parks or ecological reserves (Durance 1992). The Inkaneep plateau, where capture rates of the animal have been highest, is located on the Inkaneep Reserve where the risk of habitat degradation due to development has, in the past, been lower than in the region as a whole (Chapman et al. 1994). A recent trend for converting pasture land into vineyards on the reserve, however, will likely reduce available Pallid Bat foraging area (Brigham, personal communication), while a proposed hotel–casino, condominium development (Bailey 1995) could also disturb Pallid Bat habitat (Nagorsen, personal communication).

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Population Size and Trend

In the southwestern United States, where this species is known to be abundant, no positive or negative changes in Pallid Bat population biology have been reported since 1988. Direct data addressing population structure and dynamics of A. pallidus in Canada also remain limited. The British Columbia Conservation Data Centre reports their abundance as fewer than 1000 individuals per 800 hectares, or 15 km streamlength, and even this may be a generous estimate. Based on recent field work it seems unlikely that Balcombe’s (1998) hypothesis, namely that Canadian records for this species represented stray individuals from a population south of the U.S. border, is true. Survey work in the summers of 1990, 1991, and 1993 yielded a number of captures, visual and auditory observations, locations of both day (Figure 4) and night roosts, and preferred foraging areas of Pallid Bats in the South Okanagan (Collard et al. 1990, Grindal et al. 1991, Chapman et al. 1994, Barclay, unpublished data). Grindal et al. (1991) and Chapman et al. (1994) attribute this increased observation, not to any change in population size, but to improvements in sampling effort and ability.

Although the presence of a Pallid Bat population in the Okanagan Valley now seems assured, the population’s breeding status remains open to some question. Almost 67% (21 males, 7 females) of Pallid Bats captured to date have been males, suggesting that breeding may be limited in Canada. Relatively recent evidence, however, supports the existence of a breeding population. First of all, the worn teeth of some of both male and female captured bats (Sarell, unpublished data; Barclay unpublished data) suggest that these individuals are relatively old and therefore not dispersing juvenile or yearling bats from south of the border (Barclay, personal communication). Second, and more compelling, in the summer of 1990, Grindal et al. (1991) captured a male with testes enlarged, a juvenile female, and a lactating female prompting them to suggest that a breeding population does exist. Moreover, these captures all occurred at distances greater than 10 km from each other, leading Grindal et al. (1991) to hypothesize that at least 3 maternity colony sites could exist in the South Okanagan. Finally and more recently, two lactating females were captured at Gallagher Lake in July of 1997 and represent the most recent captures of Pallid Bats in Canada (Sarell, personal communication). Based on available data, then, it now seems probable that females and breeding do occur in Canada though it appears that females are less plentiful than males. Because the South Okanagan represents the northern range limit for this species conditions might naturally lead to fewer females without completely prohibiting them. Therefore, a male–biased sex ratio could be a natural consequence for a breeding population living at the boundaries of its tolerance.

Figure 4.  Diurnal roosts used by radio–tagged, male Antrozous pallidus in the Okanagan Valley, during the summer of 1991 represented as grey dots. The solid black line represents the Inkaneep Indian Reserve Boundary (modified from Chapman et al. 1994).

Map showing diurnal roosts used by  radio-tagged male Pallid Bats in the Okanagan Valley during the summer of 1991.

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One explanation for the observed male biased sex ratio is sexual segregation, a behavioural phenomenon which has been observed in some Pallid Bat populations (Nagorsen and Brigham 1993). In a study of A. pallidus in Oregon, males and females were not captured in the same areas suggesting, that in northern parts of their range, sexual segregation by Pallid Bats can occur (Lewis, personal communication) and this may be the case in British Columbia. One hundred percent (14/14) of the Pallid Bats captured at or near Waterdog Lake and on the Inkaneep plateau, for example, were male, while 100% (3/3) of those captured at Gallagher Lake, to the north, and 50% (6/12) of those captured at, or north of, the Reserve boundary were female (see Figure 2). Hypothetically, females could be restricted to specific locations because of the energetic requirements of rearing young, while males reduce competition with females by generally avoiding those areas. The habitat suitability model (Robertson 1988; Figure 3; see Habitat section below) supports this idea and highlights the relative availability of high quality breeding habitat in close proximity to Vaseux and Gallagher Lakes.

Particularly interesting is the fact that all but one of the female captures have occurred in the northern half of the Pallid Bat’s known Okanagan range. Intuitively, one might expect females to be restricted to southern areas, even south of the U.S. border, where warmer temperatures would better facilitate rearing young, while males would be more tolerant of conditions at the extreme range boundary. This discrepancy suggests the possibility that factors other than climate alone, most likely maternity roost availability, influence the distribution of females in the area, and that perhaps maternity roosts are most available at, and north of, Gallagher Lake. Again, the habitat suitability model (Robertson 1998; Figure 4) supports this idea with high quality breeding areas being relatively abundant in the vicinity of Vaseux, Gallagher and Skaha Lakes. Areas with habitat suitable for Pallid Bats on the Inkaneep Reserve, especially near Waterdog Lake, have been the focus of greater sampling effort than the Okanagan at large (Chapman et al. 1994), so a sampling effect may account in part for the observed male biased sex ratio. Much more survey work employing radiotelemetry is required, particularly around Gallagher Lake, Vaseux Canyon, and other areas outside of the Inkaneep Reserve, to shed further light on the breeding status of A. pallidus in Canada.

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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).

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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.

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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–telemetery 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.

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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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Biology

In addition to roosting ecology, one important aspect of A. pallidus biology has been well investigated since the original COSEWIC status report for this species was prepared in 1988. Particularly relevant to northern populations of the species is the work done, once again by Lewis (1993) in Oregon, addressing the effects of climatic variation on aspects of reproduction. Prior to this research it was unknown whether a ground foraging bat faced the same energetic trade–offs as aerial insectivores with respect to the use of heterothermy (i.e. torpor) during pregnancy and lactation. By comparing Pallid Bats captured in a relatively cool spring and those captured during a normal spring, Lewis (1993) found that during a cooler year females exhibited a delay of nearly a month in parturition dates, less synchrony of parturition, lower body mass when lactating and a higher percentage of non–reproductivity. This has important implications for the stability of a Canadian population of A. pallidus, where bats very likely face regular exposure to cold temperatures near the limits of their tolerance.

Relatively low temperatures and inter–season climatic variation may account for the low and sporadic capture rate of reproductively active, and female Pallid Bats in the Okanagan. Because bats, generally, are very long lived mammals, however, inter–year variation in reproductive success does not necessarily preclude a stable breeding population. A female bat who fails to reproduce in a cool year may still have a relatively high probability of surviving to reproduce successfully in a warmer year.

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Limiting Factors

Pallid Bats may be limited by several natural factors in Canada. Because the Okanagan Valley represents the northern tip of this species’ range, low temperatures may limit an A. pallidus population, or contribute to the observed male–biased sex ratio discussed above, especially in light of Lewis’ (1993) work addressing effects of climatic variation on reproduction. Further study of climatic influences on A. pallidus reproduction and population stability in Canada is especially important. Night roosts in the Okanagan have been found mainly in live ponderosa pine trees, which are susceptible to both natural and human impacts, so Pallid Bats may be limited by the availability of these roosts, especially if some form of sociality established at the night roost significantly enhances foraging success. A terrestrial foraging strategy may also expose Pallid Bats to high predation rates (e.g. by owls or snakes) in the Okanagan, another potential natural limiting factor. It seems likely, though, that day roosting habitat is the most limiting natural factor for A. pallidus in Canada, based on the relative rarity of suitable breeding habitat (Robertson 1998; Figure 4).

By far the most pressing anthropogenic limitation facing A. pallidus in the Okanagan is habitat loss. The Okanagan Valley’s human population is now growing almost exponentially and swells further during the summer months because of a robust tourist industry (Northcote 1996). Population projections suggest that by 2020, 1 million people will live in the area and over 2 million tourists will visit per year. Even current population levels, of approximately 100 000 residents and 750 000 tourists annually, have left the Okanagan, especially at the low elevations to which Pallid Bats are restricted, in an extremely fragile state (Northcote 1996; Durance 1992). Most of the land area has been disturbed to some level and the majority of the endemic bunchgrass, wetland, and riparian habitats have already been lost (Durance 1992).

Potential roosting and foraging habitat continues to be lost to, or disturbed by, residential / commercial development, recreational use (e.g. golf, rock climbing) and agriculture (e.g. grazing, fruit growing) (Durance 1992, Bailey 1995). Roosting habitat suitable for maternity colonies faces a double threat from development in that, ideally, foraging habitat must be in close proximity to high quality roosting areas (Robertson 1998). This means that agricultural or urban development in open sagebrush or sparse grassland areas may not simply reduce foraging opportunities for Pallid Bats but may force them to abandon high quality maternity roosts, as well. Less than 1% of Okanagan land is protected in parks or reserves and rates of development in the unprotected areas are increasing (Bailey 1995).

Livestock grazing may have both positive and negative impacts on the species because it creates open foraging habitat and attracts large dung beetles (Chapman et al. 1994) on which A. pallidus are known to feed (Grindal et al. 1991), although it may reduce overall prey density and diversity (Chapman et al. 1994), as well as the availability of night roosts if trees are removed from grazing areas.

Pallid Bats may also be at risk from the bioaccumulation of pesticides, the use of which is generally intensive for fruit growing, a common and expanding industry in the Okanagan (Watson 1997). Chemical pesticides accumulate mainly in the fat tissue of mammals (Fenton 1983) so a northern population of temperate bats, especially one like A. pallidus known to feed on agricultural pests (Chapman et al. 1994), could face a considerable threat from pesticide accumulation via the metabolism of fat reserves during torpor and hibernation. Using temperature sensitive radio tags to evaluate the use of torpor by Pallid Bats in the Okanagan would help determine the severity of this risk.

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Evaluation and Status Recommendation

Antrozous pallidus is afforded legal protection in British Columbia under the Wildlife Act and is currently red–listed by the British Columbia Conservation Data Centre with an S1 provincial ranking, as a species considered "critically imperiled" in the province. It is also classified as a priority 1 species by the South Okanagan Conservation Strategy, as a species native to British Columbia found "only in the South Okanagan" (Chapman et al. 1994). Pallid Bats are not considered rare in the Western United States, however, and since 1989, the species has been removed from the Washington State Department of Fish and Wildlife Species of Concern list and enjoys a global ranking of G5 (i.e."secure"). There is no evidence to suggest that the Canadian population of Pallid Bats is isolated from the species’ range as a whole and, in 1989, Pallid Bats were reported just 50 km south of the U.S. border (Grindal et al. 1991). Clearly A. pallidus is not at risk in the majority of its range.

Despite the secure status of this species elsewhere, the Pallid Bat is facing a number of threats in its limited Canadian range. It now seems virtually certain that a population of A. pallidus is resident in the Okanagan Valley. Capture records date back almost 70 years and recent data refute Balcombe’s (1988) suggestion that the species may be extinct in Canada. Furthermore, there now exists good evidence supporting the existence of resident females and a breeding population in the region. Sexual segregation may put this species at additional risk because it means a wider range of conservation issues need to be addressed in order to protect the habitat requirements of both males and females, though clearly protection of female requirements should be the priority.
A number of factors place Pallid Bats in Canada at risk. First, all three of the habitat categories required to sustain A. pallidus (i.e. foraging, day roosting and night roosting) are at considerable risk in the Okanagan because of extremely rapid population growth. In the absence of adequate protection in parks and reserves, population growth has already led to considerable habitat destruction because of urban, recreational and agricultural expansion, especially in the low elevations to which Pallid Bats are confined (Durance 1992, Bailey 1995). Day roosting habitat suitable for maternity colonies, likely the most important of the three habitat types for sustaining a population, faces particular risk because it is generally rare (Robertson 1998; Figure 4), because cliff and rock face roosts, although relatively secure, can be destroyed or disturbed by development and recreation, and because Pallid Bats have a strong preference for roosting areas in close proximity to foraging sites and vice versa (Robertson 1998). This means that loss of foraging habitat near potential roosts will jeopardize both roosting and foraging opportunities.

Second, and related to habitat loss, Robertson (1998) points out in the guidelines of her habitat evaluation model that 2 of the 28 records for this species were the result of cat kills and a further two bats were found dead or dying from unexplained causes in residential areas. As residential development continues to expand into open sagebrush habitat at lower elevations in the valley, not only will foraging and roosting habitat be disturbed or destroyed, but Pallid Bats could face increased mortality as a direct result of properties inherent to the residential community. Examples include increased predation by domestic cats (to which Pallid Bats will be especially sensitive because of their terrestrial foraging strategy), bioaccumulation of domestic garden pesticides, and road kills.

Third, although the effects of intensive pesticide use, common for fruit farming, have not been addressed adequately for this species, it is quite possible that due to increased torpor use, and therefore fat reserve use, in a relatively cool section of their range, Pallid Bats in Canada face increased mortality as a result of pesticide bioaccumulation. This hypothesis should be a priority for future research in the Okanagan.

Fourth, and finally, fringe populations are of particular interest to both ecologists and conservationists because it is in marginal habitat that selection pressures can be most pronounced and where diversification and speciation can occur. In addition, conservation efforts must increasingly focus on preserving populations already forced into sub–optimal habitat by human development. Studying the ecology of species living at the limits of their tolerance, then, is important to developing effective conservation strategies (Chapman et al. 1994).

The Pallid Bat meets COSEWIC criteria for an endangered designation because of its small population size and multiple threats to its habitat. However, because of the reasonably high likelihood of immigration from more stable Pallid Bat populations in the United States, the most appropriate status would be that of a threatened species in Canada.

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Acknowledgements

We wish to thank Dave Nagorsen, Mark Brigham, Susan Lewis, Marco Festa‑Bianchet, Jan Murie, Chuck Dauphine, and Rod Willis for providing helpful comments. Robert Barclay, Mike Sarell, the British Columbia Conservation Data Centre and Ted Lea at B.C. Environment forwarded records of Pallid Bat occurrences in the Okanagan and Orville Dyer at B.C. Environment in Penticton provided the habitat suitability map and helpful contact information. This report was coordinated by the Committee on the Status of Endangered Wildlife in Canada and funded by the Canadian Wildlife Service.

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Literature Cited

Anderson, R. M. 1946. Catalogue of Canadian recent mammals. National Museum of Canada Bulletin 102, 237 pp.

Bailey, P. 1995. Little Sahara. Nature Canada 24:27–32.

Balcombe, J. P. 1988. Status report on the Pallid Bat (Antrozous pallidus) in Canada. Committee on the Status of Endangered Wildlife in Canada, 13 pp.

Bell, G. P. 1982. Behavioral and ecological aspects of gleaning by a desert insectivorous bat, Antrozous pallidus (Chiroptera: Vespertilionidae). Behavioral Ecology and Sociobiology 10:217–223.

Brown, P. 1976. Vocal communication in the Pallid Bat, Antrozous pallidus. Z. Tierpsychol. 41: 34–54.

Chapman, K., K. McGuiness, and R. M. Brigham. 1994. Status of the Pallid Bat in British Columbia. Ministry of Environment, Lands & Parks, Victoria, B.C., Wildlife Working Report No. WR–61, 26 pp.

Collard, T. S. 1991. Identification of the status and critical habitats of the Pallid Bat (Antrozous pallidus) in the South Okanagan, British Columbia. Report for the B.C. Ministry of Environment, Lands and Parks, Penticton, B.C. 20 pp.

Collard, T. S., S. D. Grindall, R. M. Brigham, and R. M. R. Barclay. 1990. Identification of the status and critical habitats of spotted bats (Euderma maculatum), Pallid Bat (Antrozous pallidus), and fringed bat (Myotis thysanodes) in the South Okanagan and Similkameen valleys, British Columbia. A report for World Wildlife Fund, B. C. Ministry of Environment and B.C. Habitat Conservation Fund. 27 pp.

Cowan, I. McT., and C. J. Guiguet. 1965. The mammals of British Columbia. B.C. Provincial Museum Handbook 11, 414 pp.

Davis, R. 1969a. Wing loading in Pallid Bats. Journal of Mammalogy 50:140–144.

_______ 1969b. Growth and development of young Pallid Bats, Antrozous pallidus. Journal of Mammalogy 50:729–736.

Durance, E. 1992. Vanishing desert of the Okanagan. Borealis 11:16–21.

Edwards, S. 1974. The Cover: Bat Research News 15:21.

Fenton, M. B. 1983. Just Bats. University of Toronto Press. 165 pp.

Fenton, M. B. 1990. The foraging behaviour and ecology of animal–eating bats. Canadian Journal of Zoology 68:411–422.

Fenton, M. B., C. G. vanZyll de Jong, G. P. Bell, D. B. Campbell, and M. Laplante. 1980. Distribution, parturition dates, and feeding of bats in south–central British Columbia. Canadian Field–Naturalist 94:416–420.

Fenton, M. B., V. Wai Ping, and M. G. Stoneman. 1987. The status of spotted bats (Euderma maculatum) in the Okanagan Valley, British Columbia. Report for World Wildlife Canada and the British Columbia Nature Trust.

Fuzessery, Z. M., P. Buttenhoff, B. Andrews, and J. M. Kennedy. 1993. Passive sound localization of prey by the Pallid Bat (Antrozous pallidus). Journal of Comparative Physiology A 171:767–777.

Herrera, L. G., T. H. Fleming, and J. S. Findley. 1993. Geographic variation in carbon composition of the Pallid Bat, Antrozous pallidus, and its dietary implications. Journal of Mammalogy 74:601–606.

Humphrey, S. R. 1975. Nursery roosts and community diversity of nearctic bats. Journal of Mammalogy. 56: 321–46.

Lewis, S. E. 1993. Effect of climatic variation on reproduction by Pallid Bats (Antrozous pallidus). Canadian Journal of Zoology 71:1429–1433.

Lewis, S. E. 1994. Night roosting ecology of Pallid Bats (Antrozous pallidus) in Oregon. American Midland Naturalist 132:219–226.

Lewis, S. E. 1996. Low roost–site fidelity in Pallid Bats: associated factors and effect on group stability. Behavioral Ecology and Sociobiology 39:335–344.

Nagorsen, D. W. and Brigham, R. M. 1993. Bats of British Columbia. Royal British Columbia Museum Handbook, University of British Columbia Press.

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Robertson, S. 1988. Pallid Bat (Antrozous pallidus) habitat capability and suitability values. British Columbia Environment.

van Zyll de Jong, C. G. 1985. Handbook of Canadian Mammals. 2. Bats. National Museums of Canada, Ottawa.

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The Authors

Craig Willis received his B.Sc. Honours in Biology from Queen’s University and his M.Sc. from the Department of Clinical Studies at the University of Guelph. His interest in bats began in the Okanagan Valley during a field course led by Brock Fenton, Robert Barclay and Mark Brigham, three of Canada’s foremost bat researchers. He is currently conducting doctoral research in Dr. Brigham’s laboratory at the University of Regina addressing the use of torpor by tree–cavity–roosting temperate bats.

Marcy Bast obtained her B.Sc. in Biology from the University of Regina and her M.Sc. from the Department of Botany at the University of Guelph where she studied growth patterns of Black Spruce in the boreal forests of Northern Ontario. Her interest in bats also began under the guidance of Drs Fenton, Barclay and Brigham in the Okanagan Valley. She is currently working with the Government of the Northwest Territories, Transportation Department as the Manager of Environmental Affairs.


1 Until 1990 the COSEWIC designation “rare” referred to a species that “because of its biological characteristics, or because it occurs at the fringe of its range, or for some other reason, exists in low numbers or in very restricted areas in Canada but is not a threatened species”. This designation has since been changed to “vulnerable” in 1990 and more recently in 2000 to “Special Concern”.