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Biologically Significant Population Declines and Statistical Power

The preservation of a species is intimately tied to accurate assessments of its population status. Errors in determining population status can profoundly affect management decisions and may ultimately determine whether or not a species will persist. For example, if the populations of a species are erroneously determined to be robust, when in fact they are in decline, the species might not receive the necessary help it needs to survive. On the other hand, if populations of a species are determined to be in decline, when in fact they are robust or exhibiting natural population fluctuations, mitigation measures might waste resources, conservation biologists might risk a loss in credibility, and there is a potential to compromise future conservation efforts (Blaustein et al. 1994). Retrospective power analysis is one statistical tool that can be used to help assess whether or not a population is in decline. Statistical power is the probability that a test will yield statistically significant results-a significant trend or confidence in a lack of trend (Cohen 1988). Assessing whether or not a population is in decline has been the subject of recent intense debate with regard to amphibians (for recent reviews and discussion see Pechmann & Wilbur 1994; Blaustein & Wake 1995; Stebbins & Cohen 1995). We used retrospective power analysis on nine amphibian populations for which no decline in population was asserted (Reed & Blaustein 1995). Our analysis showed that statistical power was below the required confidence to conclude that populations were not declining. Our analysis was reevaluated and expanded upon by Hayes and Steidl (1997) and by Thomas (1997) who correctly pointed out that doing a retrospective statistical power analysis on non-signifi-