The Hidden Seed-Spreading Services of Salmon-Loving Bears

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Where salmon run plentifully, any resident bears tend to live high on the hog. Brown bears inhabiting salmon ecosystems grow larger, rear more young and live in higher densities than their counterparts elsewhere, while American black bears also prosper where spawning salmon provide reliable and bountiful pulses of energy. And bears’ enthusiasm for salmon-eating reverberates ecologically: The scattering of partly eaten fish carcasses and fishy scat that bears leave in the woods along spawning streams provides a significant nutrient input to the terrestrial ecosystem. But what sorts of other ecological effects result from the higher densities of bears salmon runs support?

That’s a question partly answered by a new study out of southeastern Alaska, which looked into a dense, salmon-boosted bear population’s influence on the local plant community. Here, brown and black bears turn out to be much more significant seed-dispersers for one of the area’s defining shrubs than birds, which were previously thought of as primary spreaders of seed.

Setting the Scene

The study, led by two Oregon State University researchers and published in Ecosphere in January 2018, took place in a spruce-hemlock forest near salmon-plied Chilkat Lake and Klehini river, some 30 miles north of the town of Haines, Alaska. The researchers chose to analyze seed dispersal by focusing on the predominant understory plant of the ecosystem: devil’s club, a gnarly shrub that can reach heights of 10 feet, brandishes maple-like leaves better than a foot across and comes nastily barbed on both stalks and foliage. Despite this impressive armor, devil’s-club berries make prized eating for both bears and birds.

Brown and American black bears coexist here in southeastern Alaska as they once did northwestern North America’s temperate rainforest, and were along with songbirds the berry-eating critters the researchers kept tabs on in the summers of 2014 and 2015.

In the study area, devil’s club ripens in late July and August, while the local salmon run peaked around August 19. The researchers trained motion-sensor video cameras on devil’s-club thickets in the study area to capture footage of local frugivores (aka fruit-eaters) and to monitor the schedule, or phenology, of berry-ripening. They also swabbed berry stalks fed upon by bears to collect DNA from saliva for the purposes of distinguishing between the two species as well as between male bears (boars) and females (sows).

The Relative Berry-Scarfing Performance of Bears and Birds

Several kinds of thrush – Swainson’s, hermit and varied thrushes as well as the American robin – fed upon devil’s-club berries at the study site, but at nothing like the levels brown and black bears did. The scientists estimated bears consumed more than 16,000 of the monitored devil’s-club berries during the study’s two fruiting seasons, while birds likely ate just shy of 700 – no contest. Bears almost inhale the cone-shaped berry clusters, while thrushes pluck off a few berries per visit.

The researchers estimate that black and brown bears can ingest some 100,000 berries in an hour of focused munching and, collectively, spread about 200,000 devil’s-club seeds per square kilometer per hour. Those seeds remain viable after passage through the bear’s gut and may benefit from the natural fertilization of the scat they’re deposited in. There’s a secondary dispersal pathway at work, too: Rodents tend to hoard and bury the seeds they pilfer from bear scat, which disseminates the seeds more widely yet.

“Devil’s club is extremely abundant in northern southeastern Alaska, so it didn’t seem plausible that birds were dispersing all this fruit,” OSU’s Taal Levi, who co-authored the study with Laurie Harrer, said in a press release. “Bears are essentially like farmers. By planting seeds everywhere, they promote a vegetation community that feeds them.”

The researchers also found that, even with all of this frugivorous feasting going on, a majority of devil’s-club berries went uneaten by the end of the fruiting season. This suggests that bears and birds aren’t really competing for the resource, and that the substantial seed-dispersal services bears render aren’t likely to be easily replaced by another berry-eating animal.

Interwoven Patterns of Bears, Berries and Salmon

That bears outshine birds as seed-spreaders is only part of the story. Brown bears apparently consume more berries, and thus disperse more seeds, than black bears. The two species also primarily fed on berries at different intervals of the fruiting season: Brown bears began at the end of July and phased out after mid-August, which is when black bears began eating berries. When the salmon run starts, brown bears appear to switch over to fish, while black bears – excluded from that finned bounty by bigger, competitively dominant brown bears – then move into the devil’s-club patches.

The end result is an extended period of bear seed dispersal when devil’s club ripens, with brown bears fulfilling the role first followed by black bears when spawning salmon occupy the former's energies.

The Implications

Historically, salmon fed brown bears across large reaches of the Northern Hemisphere, as well as some populations of American black bear. Major declines in both salmon stocks and bear populations mean such “salmon-bear ecosystems,” as Harrer and Levi term them, are a rarer phenomenon these days – mainly restricted to the North Pacific basin of northeastern Eurasia and northwestern North America.

This study suggests that the loss of bears, salmon or both could impact vegetation communities in ways ecologists haven’t before fully appreciated. Removing an animal that can spread a couple hundred thousand seeds in a square kilometer per hour is likely going to have an effect on the plant in question. So would taking salmon out of the equation, given the resulting decrease in bear density – and associated decline in seed-dispersal – likely to follow.

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