Previous year's reproductive state affects Spotted Owl site occupancy and reproduction responses to natural and anthropogenic disturbances

Abstract

Understanding interactions among site occupancy, reproduction, vegetation, and disturbance for threatened species can improve conservation measures, because important aspects of vegetation and disturbances may be identified and managed. We used 9 yr of survey data collected at 168 sites to investigate dynamic site occupancy and reproduction in a declining population of California Spotted Owls (Strix occidentalis occidentalis) in southern California, USA. We used multistate models to examine the relationship among owl site occupancy, reproduction, high-severity wildland fire, and postfire logging, while accounting for variation in vegetation characteristics and variation in detectability. Both occupancy and reproduction were positively correlated with successful reproduction in the previous year. Tree cover (ha) in a site's 203-ha core area also was positively correlated with both occupancy and reproduction. We detected no effect of disturbance covariates on reproduction, given that a site was occupied. Fire and logging covariates were both negatively correlated with the probability of site occupancy, and the effect sizes of these disturbances were large in sites that were occupied by owls that were nonreproductive the previous year (reduced 0.19 by fire and 0.26 by post-fire logging), but small in sites that were occupied by owls that were reproductive the previous year (reduced 0.02 by fire and 0.03 by postfire logging). This study illustrates the important contribution of consistently occupied and productive breeding sites to this population of Spotted Owls, and demonstrates that both occupancy and reproduction at these productive sites exhibited negligible effects from disturbances. Our results suggest that sites with recent owl reproduction and sites with more tree cover in this study area should receive enhanced protection from management actions that modify vegetation utilized by Spotted Owls.