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Realized population change for long-term monitoring: California spotted owl case studyAuthor(s): Mary M. Conner; John J. Keane; Claire V. Gallagher; Gretchen Jehle; Thomas E. Munton; Paula A. Shaklee; Ross A. Gerrard
Source: Journal of Widlife Management 77(7):1449-1458
Publication Series: Scientific Journal (JRNL)
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DescriptionThe annual rate of population change (λt) is a good metric for evaluating population performance because it summarizes survival and recruitment rates and can be used for open populations. Another measure of population performance, realized population change (Δt) is an encompassing metric of population trend over a period of time; it is the ratio of population size at an end time period relative to the initial population size. Our first goal was to compare mean λ and Δt as summaries of population change over time. Our second goal was to evaluate different methods for estimating these parameters; specifically we wished to compare the value of estimates from fixed effects models, random effects estimates from mixed effects models, and Bayesian MCMC methods. Our final goal was to evaluate the use of the posterior distribution of Δt as a means for estimating the probability of population decline retrospectively. To meet these goals, we used California spotted owl (Strix occidentalis occidentalis; CSO) data collected on 3 study areas from 1990-2011 as a case study. The estimated MCMC median λ for 2 of the study areas were 0.986 and 0.993, indicating declining populations, while median λ was 1.014 for the third study area, indicating an increasing population. For 2 of the study areas, estimated MCMC median Δt over the 18-year monitoring period was 0.78 and 0.89, suggesting 21 and 11% declines in population size, while the third study area was 1.22 suggesting a 22% increase. Results from Δt highlight that small differences in mean λ from 1.0 (stationary) can result in large differences in population size over a longer time period; these temporal impacts are better depicted by Δt. Fixed effects, random effects, and MCMC estimates of mean and median λ and of Δt were similar (≤ relative difference). The estimate of temporal process variance was larger for MCMC than the random effects estimates. Results from a Bayesian approach using MCMC simulations indicated that the probabilities of a ≥15% decline over 18 years were 0.69, 0.40, and 0.04 for the 3 study areas, while the probabilities the populations were stationary or increasing were 0.07, 0.22, and 0.82. For retrospective analyses of monitored populations, using Bayesian MCMC methods to generate a posterior distribution of Δt is a valuable conservation and management tool for robustly estimating probabilities of specified declines of interest.
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CitationConner, Mary M.; Keane, John J.; Gallagher, Claire V.; Jehle, Gretchen; Munton, Thomas E.; Shaklee, Paula A.; Gerrard, Ross A. 2013. Realized population change for long-term monitoring: California spotted owl case study. Journal of Widlife Management 77(7):1449-1458.
KeywordsBayesian MCMC approach, California, California spotted owl, monitoring, Pradel¡¦s temporal symmetry model, random effects estimator, rate of population change, realized population change, Strix occidentalis occidentalis
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