Bears play a well-known role in fertilizing Alaska’s temperate forests. They catch, carry, and digest fish, spreading nutrients through the undergrowth.
But two scientists using remote control cameras near Haines show that bears are making contributions on a different front.
They’re not just fertilizing seeds. They’re planting them.
It started when the scientists were overwhelmed by the number of berries in a Southeast forest near Haines. Salmonberries, high bush cranberries, soapberries, blueberries, devil’s club berries and more — not just in one or two places, but all throughout the forest.
Others might reach for a pie pan. These two thought these plants must have a really successful reproductive strategy.
Laurie Harrer is a biology instructor at Central Oregon Community College and Oregon State University, Cascades.
“There are just a ton of berry-producing plants in Southeast Alaska, all throughout the understory,” Harrer said. “In particular, Devil’s Club we noticed was everywhere. If it was that prolific, and all over the place, it must have a very successful method of dispersing its seeds around.”
So, what was it? Taal Levi is an assistant professor of wildlife at Oregon State University. He says they had a guess.
“These fruit in Southeast Alaska appear to be morphologically adapted for birds, because they’re small berries,” Levi said. “In the case of Devil’s Club, small red berries.”
So the two set up a study. Over two summers, they identified more than 400 clusters of berries in a stretch of forest about 30 miles north of Haines. They calculated an average number of berries per plant. They trained motion-triggered cameras on the plants, to see what was eating them.
And what they found was a surprise.
“A bird would fly in and maybe peck and eat anywhere between two and five berries at a time,” Harrer said. “But when a bear would visit these clusters of berries, they would just demolish the entire area. But it wasn’t until we finally crunched the numbers that we realized how huge of an effect bears were having. ”
Bears weren’t just eating more berries; they were eating magnitudes more berries. The two scientists estimate a single bear could eat over 100,000 berries an hour. Robins and thrushes couldn’t begin to keep up.
Harrer said bears were like cargo carriers, taking seeds in their stomachs far away from the original berry patch.
“The whole reason they want their seeds to be dispersed in the first place is so you don’t have the parent plant competing with its offspring, growing up in the exact same area competing for resources and sunlight,” Harrer said.
Bears were helping plants fulfill their biological imperative to reproduce, without sacrificing their own access to resources. And dispersed berries had a better chance than those that just dropped from the bush, Levi says when bears finally released the seeds, they left something to help them grow.
“They’re deposited in a place that was just fertilized by bear scat and has good moisture retention,” Levi said. “It’s like this little patch of potting soil that this seed is left in.”
When Levi and Harrer went back to count how many berries were left behind in their patches, they realized they had a unique opportunity to increase their data.
“[Bears] are placing their whole mouths over the thing,” Harrer said. ”So they’re probably leaving behind a lot of saliva all over that central stalk.”
They started swabbing the stalks for DNA, to see exactly what kind of bears they were dealing with, even if they couldn’t catch them on camera. All the bears — black, brown, male, female — ate berries. Brown bears ate early in the season, allowing black bears into the patch later in the summer. The shift happened when the sockeye started to run.
“Brown bears start to dominate these devil’s club patches as soon as the fruit ripens,” Levi said. “And then a lot of those animals move to feed on salmon primarily, opening that space up for black bears.”
Based on their research, Levi and Harrer believe bears are the most important seed dispersers in the region for berries. While jungle plants often depend on mammals to eat their fruit and carry their seeds to new places, the scientists say it’s the first known temperate example of plants relying on a mammal’s digestive tract as their primary method to travel.
“Because there are so many bears, if they really, truly are the most important seed dispersers, it’s understandable, then, why you have so many more berries in Alaska,” Harrer said.
But bears aren’t the only animals contributing to better berry hauls, the scientists say. Salmon are, too. Dependable salmons runs help Alaska maintain its high density of large bears, compared to places on the East and West Coast where bear populations have dwindled and salmon no longer return.
“A large population of bears spreading berry seeds is helping those understory plants grow,” Harrer said. “Salmon supporting that bear population is vital for bears to help assist all those berry-producing plants.”
Levi says it’s likely that losing salmon could have changed the growth of berrying plants down south — they could no longer travel in the bellies of the bears.
You can find Harrer and Levi’s full study on bears and berries here.