Review of the effects of barred owls on spotted owls

Long, Linda L.; Wolfe, Jared D.

doi:10.1002/jwmg.21715

Cited by 21 publications

(17 citation statements)

References 80 publications

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“…As barred owls invaded the geographic range of northern spotted owls, they displaced and subsequently excluded spotted owls from their breeding territories via interspecific territoriality ( 16 ). Barred owls also exhibit a high degree of ecological overlap with spotted owls in terms of space use, habitat selection, and diets ( 33 ). This combination of interference and exploitation competition led to decreased survival and recruitment of territorial spotted owls, thereby exacerbating historical population declines associated with habitat loss.…”

Section: Discussionmentioning

confidence: 99%

Invader removal triggers competitive release in a threatened avian predator

Wiens

Dugger

Higley

et al. 2021

Proc. Natl. Acad. Sci. U.S.A.

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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.

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Section: Discussionmentioning

confidence: 99%

Invader removal triggers competitive release in a threatened avian predator

Wiens

Dugger

Higley

et al. 2021

Proc. Natl. Acad. Sci. U.S.A.

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“…Continued population declines have underscored the increasing vulnerability of spotted owls to extirpation. The cause of declines has been at least partially attributed to habitat loss through forest alteration, and, more recently, competition with barred owls (Strix varia), a congener that has rapidly expanded into the range of the northern spotted owl from its historical range in eastern North America (see reviews by Lesmeister et al [2018], Long and Wolfe [2019]). Resource partitioning can alleviate congeneric interactions, but such behavioral adaptations may not keep pace with increased competitive pressure resulting from rapid increases in invader populations (Schoener 1974, Wiens et al 2014, Lesmeister et al 2015, Jenkins et al 2019b.…”

Section: Introductionmentioning

confidence: 99%

Conspecific and congeneric interactions shape increasing rates of breeding dispersal of northern spotted owls

Jenkins

Lesmeister

Forsman

et al. 2021

Ecological Applications

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Breeding dispersal, the movement from one breeding territory to another, is rare for philopatric species that evolved within relatively stable environments, such as the old-growth coniferous forests of the Pacific Northwest. While dispersal is not inherently maladaptive, the consequences of increased dispersal on population dynamics in populations whose historical dispersal rates are low could be significant, particularly for a declining species. We examined rates and possible causes of breeding dispersal based on a sample of 4,118 northern spotted owls (Strix occidentalis caurina) monitored in seven study areas over 28 years, 1990 to 2017, in Oregon and Washington, USA. Using a multistate mark-resight analysis, we investigated the potential impacts of an emergent congeneric competitor (barred owl Strix varia) and forest alteration (extrinsic factors), and social and individual conditions (intrinsic factors) on 408 successive and 1372 non-successive dispersal events between years. The annual probability of breeding dispersal increased for individual owls that had also dispersed in the previous year and decreased for owls on territories with historically high levels of reproduction.Intrinsic factors including pair status, prior reproductive success, and experience at a site, were also associated with breeding dispersal movements. The percent of monitored owls dispersing each year increased from ~7% early in the study to ~25% at the end of the study, which coincided with a rapid increase in numbers of invasive and competitively dominant barred owls. We suggest that the results presented here can inform spotted owl conservation efforts as we identify factors contributing to changing rates of demographic parameters including site fidelity and breeding dispersal. Our study further shows that increasing rates of breeding dispersal associated with population declines contribute to population instability and vulnerability of northern spotted owls to extinction, and the prognosis is unlikely to change unless active management interventions are undertaken.

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“…Recent evidence suggests that barred owls suffer less exposure to rodenticides than spotted owls [8], further increasing their competitive advantage. Together, these factors threaten the continued viability of northern spotted owl populations in the region [40].…”

Section: Overlap Of Cultivation Site Likelihood With Species Of Concernmentioning

confidence: 99%

Distribution of trespass cannabis cultivation and its risk to sensitive forest predators in California and Southern Oregon

Wengert¹,

Higley²,

Gabriel³

et al. 2021

PLoS ONE

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Illegal cannabis cultivation on public lands has emerged as a major threat to wildlife in California and southern Oregon due to the rampant use of pesticides, habitat destruction, and water diversions associated with trespass grow sites. The spatial distribution of cultivation sites, and the factors influencing where they are placed, remain largely unknown due to covert siting practices and limited surveillance funding. We obtained cannabis grow-site locality data from law enforcement agencies and used them to model the potential distribution of cultivation sites in forested regions of California and southern Oregon using maximum entropy (MaxEnt) methods. We mapped the likely distribution of trespass cannabis cultivation sites and identified environmental variables influencing where growers establish their plots to better understand the cumulative impacts of trespass cannabis cultivation on wildlife. We overlaid the resulting grow-site risk maps with habitat distribution maps for three forest species of conservation concern: Pacific fisher (Pekania pennanti), Humboldt marten (Martes caurina humboldtensis), and northern spotted owl (Strix occidentalis caurina). Results indicate that cannabis cultivation is fairly predictably distributed on public lands in low to mid-elevation (~800-1600m) forests and on moderate slopes (~30–60%). Somewhat paradoxically, results also suggest that growers either preferred sites inside of recently disturbed vegetation (especially those burned 8–12 years prior to cultivation) or well outside (>500m) of recent disturbance, perhaps indicating avoidance of open edges. We ground-truthed the model by surveying randomly selected stream courses for cultivation site presence in subsets of the modeling region and found previously undiscovered sites mostly within areas with predicted high likelihood of grow-site occurrence. Moderate to high-likelihood areas of trespass cultivation overlapped with 40 to 48% of modeled habitats of the three sensitive species. For the endangered southern Sierra Nevada fisher population, moderate-high likelihood growing areas overlapped with over 37% of modeled fisher denning habitat and with 100% of annual female fisher home ranges (mean overlap = 48.0% + 27.0 SD; n = 134) in two intensively studied populations on the Sierra National Forest. Locating and reclaiming contaminated cannabis grow sites by removing all environmental contaminants should be a high priority for resource managers.

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Review of the effects of barred owls on spotted owls

Cited by 21 publications

References 80 publications

Invader removal triggers competitive release in a threatened avian predator

Invader removal triggers competitive release in a threatened avian predator

Conspecific and congeneric interactions shape increasing rates of breeding dispersal of northern spotted owls

Distribution of trespass cannabis cultivation and its risk to sensitive forest predators in California and Southern Oregon

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