Inbred Spotted Owls Doomed By Their Own Genes?

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Sunday, July 22nd, 2018

Northern spotted owls in the Pacific Northwest are facing a new threat: decreasing populations and a lack of suitable mates are forcing the owls to breed with their own parents or siblings. This may lead to an “extinction vortex,” where each new inbred generation further amplifies harmful genes from already-inbred parents, resulting in weaker and weaker offspring until a population goes entirely extinct. Once caught in this downward spiral, recovery is difficult without human intervention, like capture-and-translocate programs that shuffle owls between areas to improve genetic diversity.

A recent study published by the American Ornithological Society examined 14,000 owls in Washington, Oregon, and California over a 30-year period, finding that up to 15% of the owls are inbreeding. Stressed by habitat loss and competition from the larger barred owl species, spotted owls populations are shrinking by approximately 4% per year, with some populations already down to a third of their 1985 levels. As these geographically separated populations become smaller and more isolated, inbreeding gets worse and populations become more vulnerable.

Using long-term surveys and new statistical models, the researchers found owls to be most at risk in the Washington Cascades, where an estimated one in eight owls were breeding with siblings or parents. In addition to current conservation programs, the researchers recommend transplanting owls from California to Washington to introduce new mates into the most inbred populations.

Like most animals, spotted owls prefer healthy, unrelated mates. This avoidance of inbreeding is an evolved behavior, common to many species, that helps prevent harmful recessive genes from accumulating and weakening a population. However, this can only take place when there are a sufficient number of healthy mates available. Spotted owls are typically monogamous, and face dangers from logging, barred owl competition, climate change, wildfires, and toxic rodenticides. Together, these threats are decreasing the number of viable mates with every generation, causing population bottlenecks and potentially a “mutational meltdown,” where a critical number of bad genes become “fixed” into the shrinking gene pool, rendering subsequent generations unfit to survive or breed.

EPIC uses diverse tactics to help fight for the owls, including public commentary and legal action. We monitor timber logging plans on public and private lands to ensure operations do not encroach upon Spotted Owl habitat, and take violators to court. We petition state and federal regulators to “up-list” the spotted owl conservation status from “threatened” to “endangered” in the hopes of increasing their protections under the California and federal Endangered Species Acts. For more information, please see EPIC’s Spotted Owl Self Defense Campaign page.

This article was contributed by Roger Tuan, EPIC Intern.