Multi-species hierarchical Bayesian occupancy models for examining avian community responses to Sierra Nevada National Forest management

Background/Question/Methods

The composition and structure of Sierra Nevada and southern Cascade forests have been altered by timber harvest and fire suppression over the past century. Along with the loss of late seral forest there has been an increase in tree densities and loss of shade intolerant plant assemblages including chaparral. The focus of Sierra Nevada National Forest management is now largely towards reducing disturbances to protect habitat for California spotted owl (Strix occidentalis occidentalis) and other mature forest associated wildlife species. As a result, and combined with a lack of natural disturbance, tree density and canopy cover are increasing across the landscape. To appreciate the implications of these habitat changes on the broader avian community we estimated occupancy and species richness on 557 points inside and 608 points outside of spotted owl home range management boundaries (“Core Areas”) in the Lassen and Plumas National Forests in northeastern California. To generate these estimates we used a hierarchical Bayesian occupancy model that incorporates the detection history of 53 species over a two-year time period (2005-2006) assuming that probability of occurrence and detection are random effects that vary by species, but are governed by community-level hyper-parameters, and are functions of habitat and survey covariates. 

Results/Conclusions

Our study sites inside the core areas have significantly higher tree cover, tree height, and basal area than study sites outside the core areas, while shrub cover and herbaceous cover are both higher outside the core areas. Our occupancy estimates revealed that roughly twice as many species had higher occupancy outside the Core Areas than inside. The magnitude of differences in occupancy inside vs. outside the core areas was much larger on average for the species that are more prevalent outside.  Estimated species richness per point was also significantly higher outside the core areas. A larger proportion of the species more prevalent outside Core Areas are experiencing regional population declines according to BBS. The majority of the species negatively associated with spotted owl Core Areas are associated with early successional and other disturbance-dependent habitats. If these species are to be protected, forest heterogeneity including early successional and edge habitats should be retained as a management goal to ensure the long-term persistence of biological diversity. The multi-species modeling approach we used corrects for imperfect detection and allows us to examine management effects on both abundant and rare species together, facilitating true community-level investigations.