Spotted bat (Euderma maculatum) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Because it is a solitary bat with distinctive search-phase echolocation calls that are audible to the unaided human ear up to 250 m, Spotted Bats can be censused by acoustic surveys at potential roost sites or foraging areas (Fenton et al. 1987; Wai-Ping and Fenton 1987). The most reliable population estimates are based on emergence counts made at dusk as Spotted Bats emerge from their day roosts to forage. Emergence counts are more reliable than counts of foraging bats because they avoid multiple counts of the same individual. Since its discovery in Canada in 1979, there have been at least nine acoustic inventories and estimates of the minimum population size for various parts of the Canadian range (Table 3). However, there has been no systematic inventory done throughout the Canadian range applying a standard methodology. Census data are also geographically biased for the southern Okanagan Valley. Inventories for the Cariboo-Chilcotin region are limited to two surveys done in 1992 and 1993 (Roberts and Roberts 1992, 1993). Although Sarell and Woodgate (1991) reported the first observations of Spotted Bats in the Thompson River valley in 1991, there has been no systematic inventory of potential roost sites throughout the North and South Thompson River valleys.

Table 3. Estimates of minimum population size for Spotted Bats (Euderma maculatum) from various regions of British Columbia. Estimates based on acoustic monitoring of emergence counts at roosts. Modified from Sarell and Haney (2000).
Year South Okanagan Similkameen Thompson Chilcotin-Cariboo Source
1981 50 - - - Leonard & Fenton (1983)
1986 29 - - - Fenton et al. (1986)
1987 34 - - - Wai-Ping & Fenton (1987)
1989 38 24 - - Collard et al. (1990)
1991 55 0 - - Chapman et al. (1994)
1992 29 - 241 192 Holroyd et al. (1994)1; Roberts & Roberts (1992)2
1993 - - - 62 Roberts & Roberts (1993)
2000 60 2 - - Sarell and Haney (2000)

1 includes observations for both the Thompson and Cariboo-Chilcotin
2 a partial census of the Chilcotin-Cariboo region

The Spotted Bat is possibly the only Canadian bat species with population estimates. Nevertheless, these population counts should be regarded with caution. There are inconsistencies in sampling periods among different studies, unequal sampling effort across the Canadian range, the possibility of multiple counts of foraging individuals in some studies, and a bias for only surveying high quality roosting habitat. Garcia et al. (1995) estimated the minimum population in Canada at less than 200 individuals. A similar estimate, presumably taken from Garcia et al. (1995), was given by Cannings et al. (1999). In the most recent survey, Sarell and Haney (2000) detected a minimum of 60 Spotted Bats in the south Okanagan. They estimated that the total population in this area was 100 to 200 animals and concluded that the entire Canadian population was less than 500. Assuming that the Cariboo-Chilcotin region supports a population similar to the south Okanagan as suggested by Roberts and Roberts (1992) and that the Thompson area also supports 100-200 animals, an estimate of less than 1,000 total Spotted Bats for Canada may be realistic. Because the Spotted Bat’s age at sexual maturity is unknown, the number of mature animals cannot be determined. But, given that volant young have been included in some counts, the number of mature animals is less than the total population count.

A comprehensive inventory of the entire Canadian range applying a standard methodology is required to obtain baseline data for monitoring long term population trends. The only region with a sequence of population estimates is the south Okanagan region (Table 3). Seven population counts that span about 20 years demonstrate no evidence for population declines.

Although the population status of Spotted Bats in Washington State is unknown, this species has been detected in Okanogan County near the British Columbia border (Sarell and McGuiness 1993; Johnson and Cassidy 1997). Trans-border movements are likely.

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