Power of track surveys to detect changes in cougar populations

Beier, Paul; Cunningham, Stanley

Little is known about the ability, or statistical power, of track surveys to detect a change in abundance of cougars (Puma concolor). We examined monitoring schemes that would have 80% power to detect a 30% or 50% change in track abundance between 2 survey periods. We used data from track transects in southeastern Arizona to evaluate survey designs for 8-km transects in first- and second-order dry washes. Track density (number of 0.5-km segments with tracks along an 8-km transect) followed a Poisson distribution, with no serial correlation between consecutive surveys of a given transect. We used simulated Poisson data to determine how power varied in response to number of 8-km transects, risk of Type I error, direction of change (increase or decrease), magnitude of change (30% or 50%), and whether track density between surveys changed uniformly or patchily across transects. Power decreased only slightly when change in track density was patchy. Track transects had low power to detect increases in track density (e.g., about 190 transects would be needed to detect a 30% increase with 80% power and alpha = 0.05), but somewhat more power to detect decreases (about 140 transects would detect a 30% decrease with 80% power at alpha = 0.05). Managers san increase the power of surveys (or decrease the number of transects) if a 10-20% risk of Type I error is acceptable, i.e., about 140 transects would be needed to detect a 30% decrease in track density with 80% power at alpha = 0.05, 110 transects at alpha = 0.10 and 85 transects at alpha = 0.20. If surveys need to detect only large decreases (50%), track surveys are more powerful, with only about 50 transects needed for 80% power at alpha = 0.05, and 30 transects at alpha = 0.20. Thus, track surveys usually will not detect small annual changes, but may reveal large changes more efficiently than other methods.

Power of track surveys to detect changes in cougar populations

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