Abstract
The Anthropocene is defined by rapid environmental changes such as biological invasions and shifting disturbance regimes that threaten native species. Understanding the drivers of endangerment for species facing multiple simultaneous threats is challenging without experimental methods. Here, we examined the relative and combined effects of severe wildfires and an early-stage barred owl (Strix varia) invasion on an assemblage of three native forest owl species in the Sierra Nevada, California, USA, leveraging manipulative (lethal barred owl removals) and natural (severe wildfires) experiments and a regional passive acoustic monitoring program from 2018 to 2023. Wildfires reduced flammulated owl (Psiloscops flammeolus) occupancy by 71% in severely burned areas (sites experiencing near-complete high-severity fire) for at least 3 years postfire but did not affect great horned (Bubo virginianus) or northern pygmy owl (Glaucidium californicum) occupancy. Because flammulated owls have small home ranges and an insectivorous diet that depends on nearby mature forest foraging habitat and secondary-cavity nest sites, they showed a strong negative response to extensive high-severity burn areas that eliminate these resources. Flammulated owl occupancy increased approximately twofold from 0.09 (85% CI: 0.03, 0.20) to 0.18 (85% CI: 0.07, 0.36) following lethal barred owl removals (with only 4% posterior distribution overlap), but removals did not affect the other two native species. Despite evidence of habitat segregation between barred owls and the native species, where barred owls typically occupied intermediate-to-late seral forests in flatter, lower elevation areas, this niche partitioning was insufficient to prevent nonconsumptive or predatory effects on flammulated owls. In contrast, the resilience of great horned and pygmy owls may have stemmed from their larger body size and diurnal activity, respectively, suggesting that life history mediates forest owl vulnerability to invasive barred owls. The negative effects of barred owls on flammulated owls, even during the early invasion stage, coupled with well-documented effects on other small, nocturnal forest owl species in regions with high barred owl densities, reinforce the conservation value of proactive invasive species management. Our study demonstrates the power of regional-scale experimentation, facilitated by bioacoustic monitoring, for understanding biological community responses-mediated by species' life history-to rapid environmental changes.