Recovery Plan for the Northern Spotted Owl - DRAFT

Demographic AnalysisAs noted earlier, one problem with estimating population trends based oncounts of owls is that nonterritorial owls are essentially undetectable, so theoverall population could be declining even if the counts did not indicate anydecline. One way around this problem is to use estimates of birth and deathrates to deduce whether the population is increasing or decreasing. Thisapproach may avoid the problem of not being able to detect floaters, and it hasthe added advantage of providing insight into what factors might be responsiblefor population declines. For example, in comparing two areas, one with astable population and one with a declining population, we might, by estimatingdemographic rates, learn that owl populations in the two areas had similarrates of productivity and juvenile survivorship, but that the declining populationhad a lower adult survival rate. This provides more information than wewould obtain simply by monitoring territorial populations.As a simple example of the approach, suppose we consider two consecutiveyears, beginning at the start of the breeding season (i.e., before any eggs havebeen laid). Let N. = the number of birds alive at the start of year 1, and N2 =the number of birds alive at the start of year 2. Then, we may write,N 2 = NiSa + Nifsi(M)where sa= proportion of the N. birds that survive until the start of year 2f = average number of young fledged per bird alive at the start of year 1s, = proportion of the young fledged during year 1 that survive until thestart of year 2.Eq. (1) contains no assumptions; it is simply a way of expressing the empiricalrelationship between N2 and N.. Given eq. (1), we can also writeN2IN, = Sa + fsJ (1)so if we could estimate sa, f, and sY, we could estimate N 2 /N,, the proportionalamount by which population size changed between year 1 and 2 (e.g., if N 2 /N.= 0.95, then the population declined 5 percent between the start of year 1 andthe start of year 2).Now suppose we wish to estimate the average change for several pairs of years.We may write eq. (1) in the more general form(t indicates year), and therefore,NIVl = Nts + Nftsj,Nt+, =s5t+frSflThe average annual change (with n pairs of years) is thus1 E Nt =n) (2)= S;+- I ,ftst264