Statistical power of presence-absence data to detect population declines

Population declines may be inferred from a decrease in the number of sites at which a species is detected. Although such presence-absence data often a re interpreted informally, it is simple to test the statistical significanc e of changes in the number of sites occupied by a species. I used simulatio ns to examine the statistical power (i.e. the probability of making the Typ e II error that no population decline has occurred when the population actu ally has declined) of presence-absence designs. Most presence-absence desig ns have low power to detect declines of <20-50% in populations but have ade quate power to detect steeper declines. Power was greater if the population disappeared entirely from a subset of formerly occupied sites than if it d eclined evenly over its entire range. Power also rose with (1) increases in the number of sites surveyed (2) increases in population density or sampli ng effort at a site; and (3) decreases in spatial variance in population de nsity. Because of potential problems with bias and inadequate power, presen ce-absence designs should be used and interpreted cautiously.