A prospective power analysis and review of habitat characteristics used in studies of tree-roosting bats
We identified 25 studies published between 1988 and 2001 that measured characteristics of roosting sites of tree-roosting bats, and where measures were compared to characteristics of random or available locations. The most frequently measured habitat characteristics were roost-tree diameter (n = 23), roost-tree height (21), roost-tree canopy cover (16), roost height (14), and slope (10). Habitat characteristics of the roost tree itself were measured more frequently than stand or landscape characteristics; a total of 31 different habitat characteristics was used to describe stand or landscape conditions as opposed to 23 different habitat characteristics used to describe features of the roost tree. The overall mean (± SE) number of habitat characteristics examined per study was 8.0 ± 1.1, with an average of 4.2 ± 0.7 characteristics reported to be significant (P < 0.05). Mean estimated effect size, or the absolute value of the difference between means divided by the population standard deviation, of habitat characteristics ranged from 0.83 to 1.52. A sample size of 11 radio-tagged bats was sufficient to achieve acceptable power, i.e., 0.80, for all habitat characteristics examined when only using the upper limit of the 95% confidence intervals for estimated effect sizes. In contrast, a sample size of 39 radio-tagged bats was sufficient in achieving the same level of power for only 50% of the habitat characteristics evaluated at the lower end of the 95% confidence intervals. We encourage researchers to conduct pilot studies, and estimate effect sizes and variances to assess the level of sampling effort required to evaluate habitat characteristics in studies of tree-roosting bats.
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