 Encroaching Competitor Adds to Spotted Owl's Struggle
The U.S. Fish and Wildlife Service has identified competition from barred owls as one of two main threats to the northern spotted owl’s continued survival (habitat loss is the other). We are currently proposing an experiment to test the effects of removing barred owls from certain areas of spotted owl habitat to see if it would benefit spotted owls.
Barred owls are native to eastern North America. It is believed they began moving west of the Mississippi River around the turn of the 20th century. This could have been a natural range expansion or human-caused, or a combination of both. The most common theory is that their westward movement was caused by changes to the environment in the Great Plains as people increasingly settled there and dramatically altered the landscape. This may have removed natural barriers that previously inhibited the barred owl’s cross-country migration and settlement into new areas.
Barred owls now outnumber spotted owls in many portions of the latter’s range. Researchers have seen strong evidence that spotted owl population declines are more pronounced in areas where barred owls have moved into their range. Declines are greatest where barred owls have been present the longest. The U.S. Fish and Wildlife Service is concerned that the spotted owl is likely to go extinct in some parts of its range without barred owl population management.
Barred owls are larger, more aggressive, and more adaptable than northern spotted owls. They displace spotted owls, disrupt their nesting, and compete with them for food. Researchers also have seen a few instances of barred owls interbreeding with or killing spotted owls. Because the spotted owl is already struggling due to its reduced habitat, the effect of the barred owl’s presence is like “adding insult to injury.” An already vulnerable population has a much more difficult time withstanding dramatic changes in the ecosystem such as the encroachment of a competitor. A healthy population, on the other hand, has more flexibility to adapt to changes.
Scientific Research Papers
The following is a list (more references are included in a draft Environmental Impact Statement on experimental removal of barred owls in support of northern spotted owl recovery). Also see this timeline on the evolution of the barred owl threat, which summarizes the significance of this key research.
Anthony, R.G., E.D. Forsman, A.B. Franklin, D.R. Anderson, K.P. Burnham, G.C. White, C.J. Schwarz, J. Nichols, J.E. Hines, G.S. Olson, S.H. Ackers, S. Andrews, B.L. Biswell, P.C. Carlson, L.V. Diller, K.M. Dugger, K.E. Fehring, T.L. Fleming, R.P. Gerhardt, S.A. Gremel, R.J. Gutiérrez, P.J. Happe, D.R. Herter, J.M. Higley, R.B. Horn, L.L. Irwin, P.J. Loschl, J.A. Reid, and S.G. Sovern. 2006. Status and trends in demography of northern spotted owls, 1985–2003. Wildlife Monograph No. 163.
Buchanan, J.B., R.J. Gutierrez, R.G. Anthony, T. Cullinan, L.V. Diller, E.D. Forsman, and A.B, Franklin. 2007. A synopsis of suggested approaches to address potential competitive interactions between barred owls and spotted owls. Biol. Invasions 9:679-691
Courtney, S.P., J. A. Blakesley, R.E. Bigley, M. L. Cody, J. P. Dunbacher, R.C. Fleischer, A. B. Franklin, J. F. Franklin, R.J. Gutiérrez, L. M. Marzluff, and L. Sztukowoski. 2004. Scientific evaluation of the status of the northern spotted owl. Sustainable Ecosystems Institute, Portland, OR, U.S.A.
Dugger, K.M., R.G. Anthony, and L.S. Andrews. 2011. Transient dynamics of invasive competition: barred owls, spotted owls, and the demons of competition present. Ecological Applications.
Forsman, E.D., R.G. Anthony, K.M. Dugger, E.M. Glenn, A.B. Franklin, G.C. White, C.J. Schwarz, K.P. Burnham, D.R. Anderson, J.D. Nichols, J.E. Hines, J.B. Lint, R.J. Davis, S.H. Ackers, L.S. Andrews, B.L. Biswell, P.C. Carlson, L.V. Diller, S.A. Gremel, D.R. Herter, J.M. Higley, R.B. Horn, J.A. Reid, J. Rockweit, J. Schaberl, T.J. Snetsinger, and S.G. Sovern. 2011. Population demography of northern spotted owls: 1985–2008. Studies in Avian Biology 40. Cooper Ornithological Society.
Gutiérrez, R.J., M. Cody, S. Courtney and A.B. Franklin. 2007. The invasion of barred owls and its potential effect on the spotted owl: a conservation conundrum. Biological Invasions 9:181–196.
Hamer, T.E. 1988. Home range size of the northern barred owl and northern spotted owl in western Washington. M.S. thesis, Western Washington University, Bellingham, WA.
Note: the citation for the published version of the above is:
Hamer, T. E., E. D. Forsman, and E. M. Glenn. 2007. Home range attributes and habitat selection of barred owls and spotted owls in an area of sympatry. Condor 109:750–768.
Kelly, E.G., E.D. Forsman, and R.G. Anthony. 2003. Are barred owls displacing spotted owls? Condor 105:45–53.
Olson, G.S., R.G. Anthony, E.D. Forsman, S.H. Ackers, P.J. Loschl, J.A. Reid, K.M Dugger, E.M. Glenn, and W.J. Ripple. 2005. Modeling of site occupancy dynamics for northern spotted owls, with emphasis on the effects of barred owls. Journal of Wildlife Management 69:918–932.
Singleton, P.H., J.F. Lehmkuhl, W.L. Gaines, and S.A. Graham. 2010. Barred owl space use and habitat selection in the eastern Cascades, Washington. Journal of Wildlife Management 74:285–294.
USFWS. 1989. Status review supplement for the northern spotted owl.
Wiens, J.D., R.G. Anthony, and E.D. Forsman. 2011. Barred owl occupancy surveys within the range of the northern spotted owl. Journal of Wildlife Management 75(3): 531-583.
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