Climate, habitat quality, and fitness in Northern Spotted Owl populations in northwestern California

A controversy exists in the Pacific Northwest of the United States between logging of old-growth coniferous forests and conservation of Northern Spotted Owl (Strix occidentalis caurina) populations. This species has a strong association with old-growth forests that also have economic value as timber. Research questions relevant to conservation of this species include how temporal trends in Northern Spotted Owl populations are influenced and how spatial configuration of old-growth forests affects these populations. To address these questions, we studied a population of marked Northern Spotted Owls on 95 territories in northwestern California from 1985 through 1994. We examined the magnitude of temporal and spatial variation in life history traits (survival, reproductive output, and recruitment), the effects of climate and landscape characteristics on temporal and spatial variation in these traits, respectively, and how this variation affected aspects of population dynamics. We used a components-of-variation analysis to partition sampling from process variation, and a model selection approach to estimate life history traits using capture–recapture and random-effects models. Climate explained most of the temporal variation in life history traits. Annual survival varied the least over time, whereas recruitment rate varied the most, suggesting a “bet-hedging” life history strategy for the owl. A forecast of annual rates of population change (λ), estimated from life history traits, suggested that Northern Spotted Owl populations may change solely due to climate influences, even with unchanging habitat conditions. In terms of spatial variation, annual survival on territories was positively associated both with amounts of interior old-growth forest and with length of edge between those forests and other vegetation types. Reproductive output was negatively associated with interior forest, but positively associated with edge between mature and old-growth conifer forest and other vegetation types. A gradient existed in territory-specific estimates of fitness derived from these life history estimates. This gradient suggested that a mosaic of older forest interspersed with other vegetation types promoted high fitness in Northern Spotted Owls. Habitat quality, as defined by fitness, appeared to buffer variation in annual survival but did not buffer reproductive output. We postulated that the magnitude of λ was determined by habitat quality, whereas variation of λ was influenced by recruitment and reproductive output. As habitat quality declines, variation in λ should become more pronounced.