Pallid bat (Antrozous pallidus) COSEWIC assessment and status report: chapter 9

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 addresssing 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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