Scientific research and the Spotted Owl (Strix occidentalis): Opportunities for major contributions to avian population ecology

Auk, The, Apr 2002 by Noon, Barry R, Franklin, Alan B

OVERVIEW

AMONG ALL avian research programs, results from investigations into the ecology, behavior, life history, and demography of Spotted Owls (Strix occidentalis) may have had the single greatest effect on land-use policy in the United States. The reasons for that are simple: in general, throughout their ranges, populations of all three subspecies of Spotted Owl-Northern, California, and Mexican-require large areas associated with large, old trees, that have high economic value (Simberloff 1987, Yaffee 1994). In truth, motivation for that long-term, intensive research program has been driven as much by legal and political concerns as scientific interest. Legal concerns stem from a mandate to comply with federal environmental laws such as the National Forest Management Act and the Endangered Species Act-legislation that compels federal land management agencies to maintain the viability of all vertebrate species. Intense political interest is an inescapable consequence of large-scale conservation programs with extensive economic effects.

Much has been written, from both positive and negative perspectives, on the conservation significance of Spotted Owl research findings (Noon and Murphy 1997). Our objectives in this article are not to review that literature or revisit the controversies, but rather to focus on the scientific importance of the research. In brief, Spotted Owl researchers have contributed to advances in the field of conservation biology in many ways including topics in reserve design (Murphy and Noon 1992, Lamberson et al. 1994, Noon and McKelvey 1996a), population viability analyses (Lande 1988, White 2000), metapopulation theory (Lande 1987, LaHaye et al. 1994, Noon and McKelvey 1996b), individual-based models (McKelvey et al. 1993, Dunning et al. 1995), analysis of space-use (Bingham and Noon 1997, Carey and Peeler 1995), trend analysis (Burnham et al. 1996, Franklin et al. 1999), and the interface between science and the formulation of public policy (Murphy and Noon 1991, Noon and Murphy 1997).

General ecology and life history.-Our discussion will only briefly summarize what is known of the ecology and life history of Spotted Owls because numerous federal management plans address Spotted Owls (e.g. Thomas et al. 1990; U.S. Department of Agriculture 1992; U.S. Department of Agriculture et al. 1993; U.S. Department of the Interior 1992, 1995; Verner et al. 1992) and thorough reviews of the owl's ecology and life history have been published (e.g. Gutierrez and Carey 1985, Gutierrez et al. 1995). Spotted Owls are mediumsized owls (450-850 g) occupying primarily coniferous forests in mountainous regions of western North America and Mexico (Gutierrez et al. 1995). In general, they are nonmigratory but there is some evidence for short-term altitudinal migrations in winter in some populations of California Spotted Owls (Verner et al. 1992). Pairs of Spotted Owls generally occupy large home ranges (up to 3500 ha for Northern Spotted Owls in Washington State), but homerange size varies widely within and among the three subspecies (Gutierrez et al. 1995). Variation in home-range size is associated with vegetation community type, amount of old-growth forest (Forsman et al. 1984, Carey et al. 1992), and variations in the primary prey species (Zabel et al. 1995). Even though there have apparently been large decreases in the abundance of Spotted Owl since the late 1800s, the current geographic distribution of all three subspecies appears to be similar to their historic distribution (Gutierrez 1994).

Most observations of habitat use for Spotted Owls are associated with some component of old-growth or late seral forest. In the vast majority of habitat-use studies, Spotted Owls have been found to use forests with greater structural complexity and older age than random sites for nesting and roosting behaviors (studies reviewed in Thomas et al. 1990; U.S. Department of the Interior 1992, 1995; Verner et al. 1992). Old-growth stands having large-diameter trees with cavities and broken tops are most often correlated with the location of nest sites. The most interesting geographic variation in habitat use is for California Spotted Owls on the western slopes of the Sierra Nevada where populations occupy mature oak woodlands at elevations below 1000 m (Verner et al. 1992) and for Mexican Spotted Owls that inhabit xeric desert canyons and older coniferous forest at higher elevations in the southwestern United States and Mexico (Ganey and Dick 1995).

Small and medium-sized mammals, primarily rodents, are the primary prey of Spotted Owls. In general, Northern Spotted Owls forage most intensely on northern flying squirrels (Glaucomys sabrinus) in Oregon and Washington (Carey et al. 1992) and at higher elevations in California (Verner et al. 1992). Woodrats (Neotoma spp.) are taken at lower elevations by both the Northern and California subspecies and at all elevations by Mexican Spotted Owls. There is some evidence that greater availability of woodrats is associated with smaller home range size in Northern and California spotted owls (label et al. 1995).