Arresting the spread of invasive species in continental systems

Abstract

Invasive species are a primary threat to biodiversity and are challenging to manage once populations become established in previously unoccupied areas. But removing them is further complicated when invasions occur in continental, mixed‐ownership systems. We demonstrate a rare conservation success: the regional‐scale removal of an invasive predator – the barred owl (Strix varia) – to benefit the spotted owl (Strix occidentalis) in California. Barred owl site occupancy declined sixfold, from 0.19 to 0.03, following 1 year of removals, and site extinction (0.92) far exceeded colonization (0.02). Spotted owls recolonized 56% of formerly occupied territories within 1 year, contrasting starkly with removals conducted after barred owls achieved high densities in the Pacific Northwest. Our study therefore averted the otherwise likely extirpation of California spotted owls (Strix occidentalis occidentalis) by barred owl competition. Collectively, leveraging technological advances in population monitoring, early intervention, targeting defensible biogeographic areas, and fostering public–private partnerships will reduce invasive species‐driven extinction of native fauna in continental systems.

DNA metabarcoding reveals the threat of rapidly expanding barred owl populations to native wildlife in western North America

“…Experimental studies conducted in the Pacific Northwest and Sierra Nevada have demonstrated that lethal removals can substantially reduce barred owl densities and alleviate well-documented competitive pressures on native spotted owls (Diller et al, 2014; Wiens et al, 2021; Hofstadter et al, 2022). Our results suggest that removals conducted to promote the recovery of spotted owls may also benefit a broad range of additional vertebrate species consumed by invasive barred owls.…”

“…Our results suggest that removals conducted to promote the recovery of spotted owls may also benefit a broad range of additional vertebrate species consumed by invasive barred owls. While barred owl populations have been reduced to very low densities throughout the range of the California spotted owl in the Sierra Nevada (Hofstadter et al, 2022), removal areas only encompass a small portion of the range of the northern spotted owl in California, Oregon, and Washington (Wiens et al, 2021). Without expanding current efforts, barred owl populations will inevitably continue to grow outside of the limited footprints of these areas.…”

“…This approach would involve continued removals in the range of the California spotted owls, as well as additional proactive removals in the few remaining areas of low barred owl densities within the range of the northern spotted owl. Such proactive efforts could prevent potential effects on biological communities before they occur, requiring less effort and resources to curb barred owl populations than in areas where populations have become established (Diller et al, 2014; Hofstadter et al 2022). Removals in areas where barred owl populations have achieved high densities, however, will require substantially more effort and resources, but may be needed to reverse impacts to native species.…”

“…Indeed, barred owls are now considered an existential threat to spotted owls, with formerly large populations of spotted owls functionally extirpated (Gutiérrez et al, 2007; Wiens et al, 2021). In response, experimental removals of barred owls have been conducted at local scales in the range of the northern spotted owl (Diller et al, 2014; Wiens et al, 2021), and region-wide experimental removals have effectively reduced the population of barred owls in the Sierra Nevada (Hofstadter et al, 2022).…”

DNA metabarcoding reveals the threat of rapidly expanding barred owl populations to native wildlife in western North America

“…Indeed, barred owls are now considered an existential threat to spotted owls, with formerly large populations of spotted owls functionally extirpated (Gutiérrez et al, 2007;Wiens et al, 2021). In response, experimental removals of barred owls have been conducted at local scales in the range of the northern spotted owl (Diller et al, 2014;Wiens et al, 2021), and region-wide experimental removals have effectively reduced the population of barred owls in the Sierra Nevada (Hofstadter et al, 2022).…”