High in the Sierra Nevada, a spotted owl soundlessly touches down in a massive red fir. As its ears scan the forest for the faintest flying squirrel noises, the moon arcs overhead. A flood of hostile, feathered invaders is the furthest thing from its mind, but new research suggests that’s precisely what’s coming.
Invasive barred owls waged an asymmetrical “owl war” against northern spotted owls in the Pacific Northwest starting in the late 1960s, reaching a fever pitch in the 1980s and 1990s. Now, the invasive birds are poised to do the same to the Californian spotted owl subspecies. In doing so, barred owls could simultaneously upend the region’s forest food webs and jeopardise many unique, already struggling species.
“[The barred owl] is larger and more aggressive so it can directly out-compete spotted owls,” Connor Wood, a conservation biologist at the University of Wisconsin-Madison, told Earther. “And they are also more flexible with what they eat and where they live, so the landscape can support more barred owls.”
Barred owls bridged the continent likely starting in the late 1800s by following veins of forested riversides from the east, a feat made possible when tree-obliterating fires and buffalo herds disappeared from the Great Plains. They first spread into the Northern Rockies and British Columbia. Over the past 40 years or so, barred owls have spread through the northern spotted owl’s Pacific Northwest territory, leaving ecological chaos in their wake.
“Barred owls have basically been very effective at kicking spotted owls out of their preferred nesting areas,” Wood told Gizmodo. He added that the bullying has already contributed to serious declines in spotted owl populations.
Barred owls have been seen sporadically in the northern Sierras since 1989, Wood said, slowly trickling in from the Cascade Range via Mount Shasta. As sightings steadily increased in the Golden State, anxiety about the risk of a full-fledged invasion is rising among conservation researchers concerned about the impact on the native California spotted owl. But very little data existed about exactly where the barred owls were, and how their population was changing.
To find out, Wood and his colleagues trekked into the Lassen and Plumas National Forests during the summers of 2017 and 2018, and placed passive recording units throughout the region. These lunchbox-sized devices recorded the sounds of the forest for a week and then were moved to another site, including both barred and spotted owl calls. Searching for signs of both owl species this way is efficient, allowing researchers to cover a study area of 2,300 square miles (5,956 square kilometres) or roughly twice the size of Rhode Island.
Barred and spotted owls are close relatives and look similar to each other, with their respective names describing their slightly differing plumage patterns. But their calls are so dissimilar they can be easily distinguished on an audio recording.
“A spotted owl sounds sort of like hoh-hoo OOH,” Wood demonstrated, “and a barred owl goes oh oh oh-oh, oh oh oh-OOH.”
The team’s results published last month in The Condor: Ornithological Applications reveal trouble on the horizon for the California spotted owl. The barred owl population ballooned between 2017 and 2018, growing by 2.6 times. Worse, the invasive owls were more likely to show up in old-growth forest, the favoured habitat of spotted owls.
But why were the barred owls exploding in numbers now, after 30 years in the region? Wood said this is common in invasive species: a prolonged “lag phase” of gradual population growth, followed by an abrupt jump. This isn’t likely caused by any single environmental event, but is simply the consequence of reaching a threshold in population size where further growth becomes far easier than it was when there were much fewer owls, resulting in a feedback loop. It’s this type of explosive growth that’s concerning Wood and his team since it means the next phase of Californian invasion is probably starting.
“The barred owl has the potential to drive the spotted owl to extinction and also to cause a lot of other cascading effects on the ecosystem,” Wood warned. “They’re replacing the spotted owl, but they’re eating more than just what the spotted owls used to eat. So it’s not just the question of spotted owls, but a lot of other species in the Sierra Nevada ecosystem.”
The influx of alien predators can potentially disrupt these forests’ food webs. The invasion threatens to substantially reduce numbers of small mammals like northern flying squirrels and red tree voles, the latter of which is already threatened by habitat loss. Forest rodents have key roles in spreading tree seeds and churning soils, so barred owls might actually hobble tree reproduction and growth. Barred owls are known to even exploit aquatic prey, like amphibians. Oregon and California forests are host to several rare salamander species, including the endangered Siskiyou Mountain salamander, found only on the bleeding edge of the owl’s invasion front. With forests already contending with a host of other threats from human development to climate change, the barred owl would only ratchet up the pressure even further.
Because so much is at stake, Wood and his colleagues argue that now is the time to start experimental removal of barred owls from the northern Sierras. Previous studies have shown that removing barred owls from forests can be incredibly helpful for spotted owls, Wood said, noting that “the population declines of spotted owls will slow down and potentially even stop if barred owls are removed from the landscape.”
Even though knowledge of the barred owl situation there is incomplete, Wood says that following the “precautionary principle” is appropriate.
“Sometimes, the cost of waiting until you have the absolute certain answer is too dangerous.” Wood said.
The California spotted owl may still have a real shot, thanks to the early detection of the uptick in invading owls.
“[The finding] was also a cause for optimism, because usually biological invasions are not identified quickly, and in this case we were able to do that, which means the management community is able to develop a response in a much more timely way,” Wood said.