Invasive barred owls select old-forest structure: Functional habitat loss for spotted owls and a threat to old-forest fauna

Invasive predators are leading drivers of native species decline, and their impact is heaviest where they concentrate in the habitats native species most depend on. The barred owl ( Strix varia ) is a generalist predator invading old forests across western North America. Using GPS telemetry of 56 barred owls across the range of the northern spotted owl ( Strix occidentalis caurina ) in northwestern California, we found consistent selection for an old-forest axis integrating high canopy cover, tall stands, large trees, diverse tree sizes, and old age. Because these attributes define old forest, selection concentrated owls in stands sustaining a distinctive old-forest fauna, and disproportionately in habitat of the spotted owl, an umbrella species for this forest, peaking in its highest-quality nesting and roosting forest. Each owl also appropriated a large area, roughly 28 times the home range recorded in its native range, magnifying the invasion's per-individual footprint. Because habitat is defined by the conditions permitting occupancy, pervasive barred owl presence converts structurally intact forest into functional non-habitat for the spotted owl, a biotic habitat loss invisible to structure-based monitoring, while concentrating novel predation on naive old-forest prey. Because the forest persists, removing barred owls can restore this habitat: we estimate each territorial pair removed frees several square kilometers, though gains require sustained control and are not spatially additive. More broadly, where an invader co-opts the structure defining a specialist's habitat, protecting that structure is necessary but not sufficient: the specialist becomes conservation-reliant, dependent on managing the invader itself.