Strayer, 1999

Strayer, D. L. 1999. Statistical power of presence-absence data to detect population declines. Conservation Biology 13:1034-1038.

Population declines may be inferredfrom a decrease in the number of sites at which a species is detected. Although such presence-absence data often are interpreted informally, it is simple to test the statistical significance of changes in the number of sites occupied by a species. I used simulations to examine the statisticalpower (i.e., the probability of making the Type II error that no population decline has occurred when the population actually has declined) ofpresence-absence designs. Most presence-absence designs have low power to detect declines of <20-50% in populations but have adequate power to detect steeper declines. Power was greater if the population disappeared entirely from a subset of formerly occupied sites than if it declined evenly over its entire range. Power also rose with (1) increases in the number of sites surveyed; (2) increases in population density or sampling effort at a site; and (3) decreases in spatial variance in population density. Because of potential problems with bias and inadequate power, presence-absence designs should be used and
interpreted cautiously.