Invader removal triggers competitive release in a threatened avian predator

Abstract

Changes in the distribution and abundance of invasive species can have far-reaching ecological consequences. Programs to control invaders are common but gauging the effectiveness of such programs using carefully controlled, large-scale field experiments is rare, especially at higher trophic levels. Experimental manipulations coupled with long-term demographic monitoring can reveal the mechanistic underpinnings of interspecific competition among apex predators and suggest mitigation options for invasive species. We used a large-scale before–after control–impact removal experiment to investigate the effects of an invasive competitor, the barred owl (Strix varia), on the population dynamics of an iconic old-forest native species, the northern spotted owl (Strix occidentalis caurina). Removal of barred owls had a strong, positive effect on survival of sympatric spotted owls and a weaker but positive effect on spotted owl dispersal and recruitment. After removals, the estimated mean annual rate of population change for spotted owls stabilized in areas with removals (0.2% decline per year), but continued to decline sharply in areas without removals (12.1% decline per year). The results demonstrated that the most substantial changes in population dynamics of northern spotted owls over the past two decades were associated with the invasion, population expansion, and subsequent removal of barred owls. Our study provides experimental evidence of the demographic consequences of competitive release, where a threatened avian predator was freed from restrictions imposed on its population dynamics with the removal of a competitively dominant invasive species.

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.…”

“…Invasive barred owls and native spotted owls have been previously shown to occupy similar forest habitats and dietary niches, and the more dominant barred owl displaces territorial spotted owls via both exploitative (direct) and interference (indirect) competition (Hamer et al, 2001; Wiens et al, 2014; Wood et al, 2021). 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).…”

“…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).…”

Feather corticosterone reveals developmental challenges in a long‐term study of juvenile northern spotted owls

“…In addition to shrinking mature forests, the range expansion of the congeneric barred owl S. varia also threatens spotted owl persistence-contributing to decreased occupancy, survival, reproduction and disrupted dispersal dynamics of spotted owls (Franklin et al, 2021;Jenkins et al, 2021;Wiens et al, 2021;Yackulic et al, 2019). Barred owls are a larger, more aggressive generalist species that exist on the landscape at much higher densities than spotted owls (Lesmeister et al, 2018;Singleton et al, 2010).…”