Invader Removal Halts Long-term Population Declines of Northern Spotted Owls

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Invasive species can cause extinctions of native species and widespread biodiversity loss. Invader removal is a common management response, but the use of long-term field experiments to test the effectiveness of removals in benefitting impacted native species is rare.

USGS researchers and partners used a large-scale removal experiment between 2009-2019 in Washington, Oregon, and California to investigate the demographic response of a threatened native species, the northern spotted owl (Strix occidentalis caurina), to removal of an invasive competitor species, the barred owl (Strix varia). Removal of barred owls had a strong, positive effect on the survival of spotted owls, which arrested their long-term population declines. After removals, spotted owl population declines stabilized in areas with removals but continued to decrease sharply in areas without removals. The results demonstrate that long-term persistence of spotted owls will depend heavily on reducing the negative impacts of barred owls while simultaneously addressing other threats such as habitat loss.

Wiens, J.D., Dugger, K.M., Higley, J.M., Lesmeister, D.B., Franklin, A.B., Hamm, K.A., White, G.C., Dilione, K.E., Simon, D.C., Bown, R.R., Carlson, P.C., Yackulic, C.B., Nichols, J.D., Hines, J.E., Davis, R.J., Lamphear, D.W., McCafferty, C., McDonald, T.L., Sovern, S.G., 2021, Invader removal triggers competitive release in a threatened avian predator: Proceedings of the National Academy of Sciences of the United States of America (PNAS), v. 118, no. 31, e2102859118, p. 1-9.

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