Recovery Plan for the Northern Spotted Owl - DRAFT

stable pair bonds because the number of recorded adult dispersals is low.Also, the conditions surrounding these observations of adult dispersal eventshave not been summarized.Demographic ProjectionsBecause spotted owls are long-lived animals, the status of their populations isdifficult to estimate. Thus, mathematical models are used to project populationtrends using estimates of the vital rates described earlier. Models can bedeterministic (linear projections based on the estimates of the vital rates) orstochastic (projections based on random variation of specific rates or conditions).Stochastic models generally are considered to be more sophisticatedbecause they are more complex, and they simulate variation that would beexpected in natural environments. Models of both kinds have been used toevaluate spotted owl population dynamics and dispersal (Boyce 1987, Marcotand Holthausen 1987, USDA 1988, Doak 1989, Lande 1988, Noon and Biles1990, Thomas et al. 1990, USDI 1990, Lutz 1992, Franklin In Press,Lamberson et al. In Press, LaHaye et al. In Press). In addition, Shaffer (1985)suggested that metapopulation models, in which species have populationsdiscontinuous in time and/or space, be used to evaluate spotted owl populationdynamics. Almost all modeling projections indicate that spotted owlpopulations are declining. However, Boyce (1987) criticized the first attempt touse a stochastic model for projecting population trends (USDA 1988) becausethe model did not incorporate density dependence. Density dependence is thefunctional response in survival probability and/or fecundity of a population tovariation in density. That is, as a population declines, the density declines.Presumably, the remaining individuals in the population have more resourcesavailable to them per capita (i.e., there is less competition) and these resourcesthen can be used by the survivors for reproduction and other life functions.Boyce (1987) argued that if a population declines numerically there should bea density-dependent response in the owl population, which would mitigate thelower density and serve to stabilize the population. In the case of the spottedowl, density has not been declining, only the abundance of owls, becausehabitat loss is the causative mechanism for the decline. Thus, when Thomaset al. (1990) incorporated density dependence into their metapopulation model,the projected population decline was more rapid. Most estimates of changes innorthern spotted owl populations indicate that populations are decliningthroughout their range (Appendix C).Models also can be spatially explicit. They can incorporate the influence oflandscape character on the underlying population dynamics (Lamberson et al.In Press, Lamberson and Brooks 1991). These models are useful for developinga more complete range of alternative hypotheses to account for observedphenomena. For example, the recent observations of abundant owls in theCalifornia Coast province could be a reflection of good habitat for owls, whichresults in high productivity and high survival among the owls. Or alternatively,the dynamics of these redwood zone, coastal owl populations could bethe result of immigration of owls from adjacent old-growth/mature forests innational forests in the Klamath province (Lamberson and Brooks 1991). Themodel illustrates the importance for recovery of the spotted owl throughout allof the provinces within its range (i.e., recovery of the owl in the CaliforniaKlamath province probably could not be achieved if there were not a concomitantrecovery in the California Coast province).29