Population Regulation in Northern Spotted Owls: Theoretical Implications for Management

A marked population of northern spotted owls was examined within a bounded, 292-km2 study area in northwestern California over a six-year period (1985-1990). Observed and predicted finite rates of population change (λ) for male spotted owls were significantly stable. Predicted λ, for females indicated a significant decline even though observed λ indicated stability. Observed stability in numbers of territorial males was maintained by recruitment, whereas stability in numbers of females was maintained by immigration. Most recruits did not become territory holders until several years after their birth. I hypothesized that the study area population was regulated by territorial behavior. Under this mechanism, spotted owl populations may be declining even though observed numbers of territorial birds appear to be stable. Using a computer model, I examined the effects of“floaters”on the stability of territory holders, and suggest warning signals which may predict imminent instability for the population.

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