For animals living in the woods of western Oregon, a log crossing a stream can act as the main street. Videos captured by a pair of Oregon State University (OSU) conservationists, released earlier this month in the review Biodiversity and Conservationshow dozens of species using logs from a stream restoration project for eating, preening and walking.
“We think this is a hidden role of the big wood,” says Ivan Arismendi, stream ecologist at OSU and co-author of the study. “Anyone who needs to cross will use them, so it creates a concentration of animals.”
Rivers in North America were once full of naturally felled trees. A traffic jam in Bellingham, Washington, removed by settlers in 1877, was three-quarters of a mile long, and was so old that trees grew above the fallen wood. For nearly a hundred years, logging companies, landowners, and even the army’s corps of engineers have pulled logs from rivers to speed up travel, or just because it looked messy.
[Related: America thrived by choking its rivers with dams. Now it’s time to undo the damage.]
But these newspapers also provided critical habitat for fish— especially young salmon, which need cold, sheltered water to survive their youth. So, starting in the 1980s, land managers began putting timber back into the rivers, hoping to restore some of this lost habitat.
OSU ecologists used motion-activated cameras to observe the world above 11 restored traffic jams and discovered that wood serves more than the aquatic community of a forest. Bobcats, cougars, bears, coyotes, and more roamed the logs, and otters, kingfishers, and eagles used the space to roost and hunt.
“In ecology, we use the idea of corridors,” says Arismendi. “You have certain structures that allow animals to be connected.” Forest animals can stick to scrubby riverbanks to move around the plains, while natural areas can serve as corridors through farmland. And as habitat has become fragmented by roads, suburbs and agriculture, conservationists have increasingly sought ways to connect ecosystems.
The cameras were set up at the site of a restoration project on Rock Creek, about an hour and a half south of Portland. In 2008, the nearby town of Corvallis, Oregon placed wooden poles in the creek to catch driftwood, creating new traffic jams. In the years that followed, these jams became covered in a thick layer of moss and began to trap dirt and grit.
Between June 2020 and June 2021, conservationists, led by Ezmie Trevarrow, an undergraduate student at OSU who is now at the University of Georgia, captured video of more than 2,000 animals. Of the approximately 40 species the researchers documented, most were relatively common, if not always easy to spot. The list included raccoons, mule deer, kingfishers and possums.
But the logs have also attracted rarer species, including a golden eagle, a giant bird of prey that normally lives in dry eastern Oregon. “It’s very unusual,” says Arismendi. “We consulted eagle experts from the US Geological Survey, and they were really excited. They were like, ‘what are they doing there?’
Meanwhile, smaller animals, including a single mouse, used the logs even when the stream was flowing high enough to completely cover the log, suggesting that it served as a crossing of last resort.
“They’re logs floating in a fast-flowing river, and they just come forward and cross there,” says Arismendi. “So one option is: they could escape predators. The other option is this: they can have a feeding habitat on one side and a nest on the other side. Researchers know something interesting is going on with risky crossings, but they don’t know what.
“[The logs] can connect habitat that was previously largely disconnected for many of these species,” says Arismendi. “We’re talking about very small rodents and small mammals that can now cross safely.”
five decades of ancient forest research demonstrated that it is the complexity of landscapes that makes them such rich habitats. And sometimes that complexity comes down to something as simple as having a place to cross a river.