Evaluating Map Stability Across Repeated Entry-Light Surveys

Annual entry-light surveys can swing bat cluster counts by 22 percent at a stable site, and the swing has more to do with observer, lamp angle, and which ceiling landmark the observer picked than it does with bats. This post works through how to evaluate whether a multi-year quilt is actually stable, or whether its year-to-year differences are method drift.

USFS dual-frame sampling research on 82 bat-hibernaculum structures with five or more repeat counts documented up to 22 percent annual fluctuation in sites where the population was known to be stable. Some of that is real year-to-year variation in how many bats are present; much of it is method. The 2015 PLOS ONE paper on efficacy of visual surveys for WNS response showed that visual surveys have known detection limits, especially for small scattered clusters on complex karst ceilings.