DEPENDENCE OF NORTHERN SPOTTED OWLS STRIX-OCCIDENTALIS-CAURINA ON OLD-GROWTH FORESTS IN THE WESTERN USA

Habitat requirements of northern spotted owls Strix occidentalis caurina have become the focus of a major controversy over how much old-growth forest in the western United States should be preserved. Analysis of three large data sets showed that the subspecies was rare or absent in areas with little older (i.e. >80-year-old) forest but with extensive stands nearing harvest age. The owls were also rare in areas with the small amounts of old-growth typically left after harvest operations. Old-growth stands in Wilderness Areas supported sparse populations of northern spotted owls, and their reproductive success was only about half that of owls outside Wilderness Areas. The results indicate that timber harvest operations, as currently practiced, lead to declines in numbers of northern spotted owls, and that currently protected old-growth stands are unlikely to provide enough high-quality habitat to maintain self-supporting populations of northern spotted owls.