The effects of mountain pine beetle outbreaks on avian communities in lodgepole pine forests across the greater Rocky Mountain region

Disturbance is an integral component of forest ecosystems. Therefore, contemporary changes to the frequency and severity of disturbances in forests may have lasting ecological consequences. In recent decades, widespread mountain pine beetle (MPB; Dendroctonus ponderosae)outbreaks have contributed more to total tree mortality than other disturbances such as wildfire. Past studies have used a broad range of measurements to evaluate the local effects of beetle outbreaks on birds (e.g., dead trees per acre versus percent mortality)and this has led to some challenges and inconsistencies in generalizing results across larger spatial scales. We sought to address these inconsistencies by using a single data source estimating outbreak-related tree mortality across the western United States to better understand the effect of beetle outbreaks on avian communities in lodgepole pine forests. This study uses five years of avian monitoring data (2008–2012)from the Integrated Monitoring in Bird Conservation Regions program, collected across Colorado, Idaho, Montana, and Wyoming. We developed a hierarchical multispecies occupancy model to quantify changes in the forest bird community following beetle outbreaks while accounting for the imperfect detection of species. Mean species richness (N)did not differ between MPB outbreak sites (N = 19.35, 95%CRI = 17.00, 22.01)and sites with no outbreak (N = 20.23, 95%CRI = 16.83, 24.25). Furthermore, we found weak evidence for a community-level response to increasing outbreak severity and time since outbreak, due in part to the variability among species within each of the four nesting guilds analyzed in this study (canopy, cavity, shrub, and ground nesting birds). However, we did find evidence of turnover in community composition with just over 50% of species (28 of 55)having statistically different occurrence rates following MPB outbreaks. Our study underscores the notion that species richness alone does not fully express changes in the forest bird community. Our results also suggest many species, particularly cavity nesters, utilize beetle-killed forests and persist at higher occurrence rates for up to 10 years post-outbreak. This period includes the timeframe when beetle-killed trees are most readily salvageable and emphasizes the need for continued evaluation of snag-retention policies prior to the implementation of post-outbreak management activities.