Pallid bat (Antrozous pallidus) COSEWIC assessment and status report: chapter 6
Population Sizes and Trends
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areas of pallid bats in the South Okanagan (Collard et al. 1990, Grindal et al. 1991, Chapman et al. 1994, Barclay, unpublished data).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 First of all, the worn teeth of some of both male and female captured bats (Sarell, unpublished data; Barclay unpublished data) suggests that these individuals are relatively old and therefore not dispersing juvenile or yearling bats from south of the border (Barclay, personal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 female 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).
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) 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.