Barred Owls (BAOW; Strix varia varia), which expanded their range into BC in the 1940?s from eastern North America, have been identified as a significant potential threat to the endangered Northern Spotted Owl (SPOW; Strix occidentalis caurina; Courtney et al. 2004, Chutter et al. 2004) ? a Species at Risk, which has a small and declining population. The BAOW?s generalist use of habitat and prey, including all habitats and prey used by SPOW, coupled with its great dispersal ability, high rate of productivity, and similarity in size to the SPOW, make it a potentially serious threat to the SPOW. In general there is a lack of understanding on BAOW ecology and demography within the range of the SPOW in both BC and the US. Limited information from the US suggests spatial separation of the two species in some areas, with BAOWs occupying lower elevation forests closer to rivers, while SPOWs occur upslope at higher elevations (Pearson and Livezey 2003), but spatial overlap occurs in many other landscapes (Courtney et al. 2004). Recent evidence from Oregon indicates that where habitat use by BAOWs and SPOWs overlaps, BAOWs may have an effect on SPOWs; in areas where BAOWs occur within 0.8 km of a SPOW nest site there is a greater likelihood that the SPOW site will not be occupied by SPOW compared to SPOW sites with more distant BAOWs (Kelly et al. 2003). Anthony et al. (2005) found evidence that BAOW presence negatively influenced SPOW survival. In addition, Olson et al. (2005) found that SPOW response rates declined when BAOW were in the vicinity. We believe it is important to understand the influence of BAOW on SPOW, because a competitive relationship that favors the BAOW may compromise land management strategies for recovery of the SPOW, and thus make the strategies ineffective, and point to the need for other management actions. Discussions are underway in the US to fund research on BAOW/SPOW interactions. In collaboration with the CSORT (Canadian Spotted Owl Recovery Team), Cortex Consultants Inc. and Gowlland Technologies Ltd. have developed an integrated modeling framework (landscape projection, spatial habitat supply and population models) to evaluate the influence of land management strategies on SPOW population dynamics (Sutherland et al. in review). This model framework can be used to assess the potential threat of BAOW on SPOW, and thus SPOW potential for recovery under this threat, by testing mechanisms of interaction between the two species. Proposed tests include manipulating model parameters including probabilities of pair separation (displacement), and reduced survival or fecundity rates; estimates of the amount of functional habitat (i.e., as would occur under competitive exclusion or interference), and reduced efficiency (measured by an existing cost function) of dispersal by juveniles. Preliminary experiments testing displacement probabilities suggest that BAOW can have a significant negative effect on modeled SPOW populations. For this project we propose to expand upon this preliminary analysis and 1) produce spatial probability map(s) that depict the likelihood of BAOW occurrences over the SPOW range based on available BAOW inventory data (usually collected incidental to SPOW inventory) and reported habitat use findings, 2) spatially model the key mechanisms of interaction (e.g., displacement and competition) between the two species using the BAOW likelihood of occurrence map, 3) design and run simulation experiments to quantify the potential effects of BAOW on SPOW population trajectories (e.g., assess for thresholds and slope of trajectories), and 4) interpret the results to help inform proposed Recovery options for this species.
O'Brien, Daniel T., Sutherland, Glenn D.; Waterhouse, F. Louise; Buchanan, Joseph B.; Hobbs, Jared; Harestad, Alton S.; Smith, Jason. 2007. Evaluating the Potential Threat of Barred Owls on Northern Spotted Owl Population Recovery and Habitat Management Strategies. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2007MR281
Topic: FLNRORD Research Program
Keywords: Forest, Investment, Account, (FIA), Northern, spotted, owl, British, Columbia
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