Mathematical Demography of Spotted Owls in the Pacific Northwest

Abstract

We examined the mathematical demography of northern spotted owls (Strix occidentalis caurina) using simple deterministic population models. Our goals were to gain insights into the life history strategy, to determine demographic attributes most affecting changes in population size, and to provide guidelines for effective management of spotted owl populations. The spotted owl apparently has evolved high adult survival rates associated with irregular and unpredictable reproduction. The finite rate of population change (X) in this subspecies is most sensitive to variation in adult survival rate and relatively insensitive to variation in fecundity and age at first reproduction. However, rates of population change are strongly affected by reproductive senescence if it occurs before 15 years of age. Sound management practices should include efforts to control factors that adversely affect the survival rate of adult females. J. WILDL. MANAGE. 54(1):18-27 Considerable interest has developed over the harvesting of old-growth forests in the Pacific Northwest and impacts on the long-term survival of the spotted owl. Current information suggests that the northern subspecies (S. o. caurina) is restricted to coniferous forests in mature and old-growth age classes (Forsman et al. 1984, see Gutierrez and Carey 1985) and that continued loss and fragmentation of the remaining old-growth forests may threaten the viability of the subspecies. The possible endangerment of the subspecies and the need to retain large areas of suitable habitat have generated heated debate because of perceived economic costs to the timber industry associated with setting aside large areas of older forests for the spotted owl (Dixon and Juelson 1987, Salwasser 1987, Simberloff 1987). In a search for solutions to this dilemma, the Forest Service (U.S. For. Serv. [USFS] 1986) and the academic community (Dawson et al. 1986) identified various aspects of the owl's biology and life history for further investigation and proposed guidelines for spotted owl research. An aspect of the owl's biology that was addressed in detail by both groups was the species' demographic life history. The apparent decline of the northern subspecies may be exacerbated by low preadult survivorship, low adult fecundity, and delayed age at first reproduction. These were features identified as primary parameters that should be estimated more precisely. Neither report gave a high priority to the annual adult survival rate even though adult survival rate significantly affects the rate of population change of both the California condor (Gymnogyps californianus) (Mertz 1971, Verner 1978) and the snail kite (formerly the Everglade kite [Rostrhamus sociabilis]) (Nichols et al. 1980), species with demographic characteristics similar to those of the spotted owl. The conclusions by Dawson et al. (1986) and the USFS (1986) concerning the relative importance of various demographic parameters of the spotted owl have recently been criticized by Lande (1988). Our goal was to characterize the life history structure of the spotted owl and to determine demographic parameters that most influence rates of population change. Detailed, analytical analyses of a species' life history structure are not only a necessary prerequisite to understanding its population dynamics, but also suggest direction for future research. We analyzed the available demographic data of the northern spotted owl using life history and demography in an approach similar to Mertz (1971) and Nichols et al. (1980). We also assessed the direction of current spotted owl research and suggest future priorities indicated by our demographic analyses. The owl's basic demographic life history includes several parameters; however, accurate and precise estimates of all parameters are not available and these parameters might exhibit substantial geographic variation throughout the range of the northern spotted owl. It is thus of considerable interest to examine the effects of parameter variation on projected population