Two studies have deepened the ethical dilemma faced by biologists tasked with saving the controversial, threatened Northern Spotted Owl.
The first, an analysis of data collected between 1985 and 2013 at 11 study areas in northern California, Oregon, and Washington, indicates that federal listing in 1990 and reductions in timber harvests have failed to halt the subspecies’ decline, calculated to be four percent annually.
The only exception is a small area in northwestern California, where wildlife biologist Lowell V. Diller and colleagues conducted the second study: a four-year pilot experiment to determine if Spotted Owl numbers would increase if invasive Barred Owls were removed.
The researchers published the results in The Journal of Wildlife Management: Where Barred Owls were shot, Spotted Owl occupancy rates went up. In areas where no Barred Owls were removed, Spotted Owls continued to decline.
The experiment was conducted between 2009 and 2013 on commercially managed timberlands where Spotted Owls had been monitored since 1990. Diller and his team were authorized to shoot 20 Barred Owls in 2009 and up to 70 more over the following three years.
In areas where Barred Owls were removed, Spotted Owl extinction rates became comparable to sites where Barred Owls had never been present, providing “compelling evidence,” Diller concluded, not only that Barred Owls were responsible for increases in Spotted Owl extinction rates but also that removal efforts were effective.
That Spotted Owls rapidly re-colonized four sites after Barred Owls were removed suggested to the researcher that Spotted Owls remain in the vicinity of, or regularly investigate, their former territories for years after being displaced by Barred Owls. “These results also suggest that Barred Owls are not simply colonizing areas vacated by declining Spotted Owl populations, but rather that Barred Owls are actively displacing Spotted Owls.”
Read the abstract
Lowell V. Diller, Keith A. Hamm, Desiree A. Early, David W. Lamphear, Katie M. Dugger, Charles B. Yackulic, Carl J. Schwarz, Peter C. Carlson, and Trent L. McDonald (2016) Demographic Response of Northern Spotted Owls to Barred Owl Removal, The Journal of Wildlife Management. Abstract.
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