Invader removal triggers competitive release in a threatened avian predator

Authors

J. David Wiens, USGS Forest and Rangeland Ecosystem Science CenterFollow
Katie M. Dugger, Oregon State UniversityFollow
J. Mark Higley, Hoopa Tribal Forestry
Damon B. Lesmeister, USDA ARS Corvallis Forestry Sciences Laboratory
Alan B. Franklin, USDA APHIS National Wildlife Research CenterFollow
Keith A. Hamm, Green Diamond Resource Company
Gary C. White, Colorado State UniversityFollow
Krista E. Dilione, USGS Forest and Rangeland Ecosystem Science Center
David C. Simon, USGS Forest and Rangeland Ecosystem Science Center
Robin R. Bown, Oregon State Office
Peter C. Carlson, Colorado State UniversityFollow
Charles B. Yackulic, United States Geological SurveyFollow
James D. Nichols, Patuxent Wildlife Research CenterFollow
James E. Hines, Patuxent Wildlife Research CenterFollow
Raymond J. Davis, USDA ARS Corvallis Forestry Sciences Laboratory
David W. Lamphear, Colorado State University
Christopher McCafferty, USDA ARS Corvallis Forestry Sciences LaboratoryFollow
Trent L. McDonald, LLC
Stan G. Sovern, Oregon State UniversityFollow

ORCID IDs

J. David Wiens https://orcid.org/0000-0002-2020-038X

Katie M. Dugger https://orcid.org/0000-0002-4148-246X

Alan B. Franklin https://orcid.org/0000-0001-9491-795X

Keith A. Hamm https://orcid.org/0000-0003-3646-3982

David W. Lamphear https://orcid.org/0000-0002-1248-5987

Document Type

Article

Date of this Version

8-3-2021

Citation

PNAS 2021 Vol. 118 No. 31 e2102859118

https://doi.org/10.1073/pnas.2102859118

Comments

Creative Commons Attribution License 4.0 (CC BY).

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.