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 Spott ed 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 t emporal trends in Northern Spotted Owl populations are influenced and how s patial configuration of old-growth forests affects these populations. To ad dress these questions, we studied a population of marked Northern Spotted O wls on 95 territories in northwestern California from 1985 through 1994. We examined the magnitude of temporal and spatial variation in life history t raits (survival, reproductive output, and recruitment), the effects of clim ate and landscape characteristics on temporal and spatial variation in thes e traits, respectively, and how this variation affected aspects of populati on dynamics. We used a components-of-variation analysis to partition sampli ng from process variation, and a model selection approach to estimate life history traits using capture-recapture and random-effects models. Climate e xplained most of the temporal Variation in life history traits. Annual surv ival varied the least over time, whereas recruitment rate varied the most s uggesting a "bet-hedging" life history strategy for the owl. A forecast of annual rates of population change (lambda), estimated from life history tra its, 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 associate d both with amounts of interior old-growth forest and with length of edge b etween those forests and other vegetation types. Reproductive output was ne gatively associated with interior forest, but positively associated with ed ge 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 old er 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 lambda was determined by habitat qual ity, whereas variation of lambda was influenced by recruitment and reproduc tive output. As habitat quality declines, variation in X-should become more pronounced.