California spotted owl population dynamics in the central Sierra Nevada: an assessment using multiple types of data

Abstract

The California spotted owl (<italic>Strix occidentalis occidentalis</italic>) is a focal management species in the Sierra Nevada because it uses late-seral forests for roosting and nesting. Thus, obtaining accurate and precise estimates of population trends is necessary to reliably assess the effects of management actions and habitat change on this species. To demonstrate how this objective can be better met, I used multiple data types from a long-term population study in the central Sierra Nevada, as opposed to the typical approach of using only mark-recapture data. First, I compared population trends estimated from occupancy and mark-recapture data. Occupancy surveys are more cost-effective than mark-recapture studies for monitoring territorial species over large spatial extents. I found that the realized change in territory occupancy from 1993-2010 (&delta_t = 0.702, 95% CI 0.552-0.852) closely matched the realized population change estimated from mark-recapture data (&delta_t = 0.725, 95% CI 0.445-1.004). This suggested that occupancy can provide reliable inferences on population trends, especially when funds preclude more intensive mark-recapture studies. I recognize, however, that mark-recapture studies provide important demographic information not provided by occupancy studies, which may allow the identification of life-history stages that are limiting a population. Second, I developed an integrated population model (IPM) to obtain estimates of population change for my study population from 1990-2012 because IPMs may improve the precision of parameter estimates. My IPM incorporated count, reproductive, and mark-recapture data. I observed a significant population decline, as evidenced by the geometric mean of the finite rate of population change (&lambda_t = 0.969, 95% CRI 0.957-0.980) and the resulting realized population change (proportion of the initial population present in 2012; &delta₂₀₁₂ = 0.501, 95% CRI 0.383-0.641). My IPM provided more precise estimates of realized population change than either the occupancy or mark-recapture analyses, but I did not account for covariance among the demographic rates in my IPM, which may have resulted in "false" precision (i.e., underestimation of the true variance). If covariances are incorporated into the IPM, they have excellent potential as a tool for assessing the status of species of conservation or management concern. My results also suggested that continued monitoring of this population and reconsideration of the California spotted owl's status under the U. S. Endangered Species Act may be warranted.