Climate Change Changes Bears' Choices; Bears Choose Berries

August 25, 2017 10:04 AM

Every year, William Deacy and Jonathan Armstrong returned to Kodiak Island, Alaska—a place where the world’s biggest grizzly bears gather to gorge themselves. Every year, hordes of sockeye salmon swim up from the ocean and fill the island’s streams in a spawning frenzy. Every year, the bears are waiting for them. But in 2014, the bears were gone.

Deacy et al. 2017. Phenological synchronization disrupts trophic interactions between Kodiak brown bears and salmon. PNAS doi: 10.1073/pnas.1705248114.

Climate change is altering the seasonal timing of life cycle events in organisms across the planet, but the magnitude of change often varies among taxa [Thackeray SJ, et al. (2016) Nature 535:241–245]. This can cause the temporal relationships among species to change, altering the strength of interaction. A large body of work has explored what happens when coevolved species shift out of sync, but virtually no studies have documented the effects of climate-induced synchronization, which could remove temporal barriers between species and create novel interactions. We explored how a predator, the Kodiak brown bear (Ursus arctos middendorffi), responded to asymmetric phenological shifts between its primary trophic resources, sockeye salmon (Oncorhynchus nerka) and red elderberry (Sambucus racemosa). In years with anomalously high spring air temperatures, elderberry fruited several weeks earlier and became available during the period when salmon spawned in tributary streams. Bears departed salmon spawning streams, where they typically kill 25–75% of the salmon [Quinn TP, Cunningham CJ, Wirsing AJ (2016) Oecologia 183:415–429], to forage on berries on adjacent hillsides. This prey switching behavior attenuated an iconic predator–prey interaction and likely altered the many ecological functions that result from bears foraging on salmon [Helfield JM, Naiman RJ (2006) Ecosystems 9:167–180]. We document how climate-induced shifts in resource phenology can alter food webs through a mechanism other than trophic mismatch. The current emphasis on singular consumer-resource interactions fails to capture how climate-altered phenologies reschedule resource availability and alter how energy flows through ecosystems.

Erlenbach et al. 2014. Macronutrient optimization and energy maximization determine diets of brown bears. American Journal of Mammalogy 95: 160-168.

Many animals consume mixed diets that maximize their fitness by optimizing macronutrient intake. We tested whether brown bears (Ursus arctos), generalist omnivores that hibernate, regulated their diet to a common nutrient target, achieved a nutrient target related to fitness, and selected a nutrient target that differed between seasons and from other species with differing life histories. When given unlimited access to 2 or 3 highly digestible foods containing primarily protein, carbohydrate, or lipid, brown bears selected mixed diets in which protein provided 17% ± 4% SD of the metabolizable energy and 22% ± 6% of the dry matter. This dietary protein content maximized the rate of gain per unit of energy consumed, is similar to the level preferred by other omnivores, and is less than that preferred by obligate carnivores. Between seasons, bears selected similar dietary protein levels, although the proportion of lipid was higher during the fall than during the spring. Bears strongly preferred lipids over carbohydrates, as did other carnivores, but they used lipids and carbohydrates with equal efficiency to produce a dietary protein content that maximized mass gain per unit of energy intake. Thus, dietary sources of lipids and carbohydrates play an interchangeable and important role in determining the productivity of bears that goes beyond their role in providing energy.

posted by ChuraChura (9 comments total)

For anyone wondering, as of the time of this comment, there are THREE bears just chilling at the base of the falls, hanging out, looking for food. Bears being bears.
posted by Fizz at 12:20 PM on August 25, 2017 [1 favorite]

Fizz: "there are THREE bears just chilling at the base of the falls, hanging out"

Paging Goldilocks, Goldilocks to the white courtesy telephone please.
posted by chavenet at 12:28 PM on August 25, 2017 [5 favorites]

Bears being bears.

Not until someone puts Cher on the decks, they're not.
posted by sonascope at 1:01 PM on August 25, 2017 [6 favorites]

Well, they can always trade up and decide to feed on quarterbacks...

(also, I've read "Zodiac bears", and imagined bears dressed in a back hood with a celtic cross attacking people on the wilderness because I'm really, really tired and that sounds like a very weird late 60s Disney comix parody)
posted by lmfsilva at 1:20 PM on August 25, 2017 [1 favorite]

The post is slightly misleading, conflating the famous bear gathering spot at Brooks Falls with the Kodiak Island feeding ground cited in the study. Brooks Falls is part of Katmai National Park, which is on the mainland just across from Kodiak Island. Even in 2014, bears still flocked to Brooks Falls. We're actually in the lull for Brooks Falls, most of the adult males are at Geographic Harbor and Moraine Creek right now, but they'll be back soon.
posted by Lame_username at 1:41 PM on August 25, 2017

Meanwhile, in another part of Alaska, a cold and rainy summer has produced an unusually small berry crop AND salmon returns are so low that the Alaska Department of Fish and Game has taken the shocking step of canceling the remainder of the 2017 king salmon fishery for both commercial and sport fishermen.

I'm seriously concerned about the bears (black bears around here, though the conditions are similar for brown bears on the nearby mainland and further north in SE AK's "ABC Islands") The berry crop is a small fraction of what it usually is and there are very few fish even in streams that are normally full of them. The combination of those two factors will be putting huge food stress on bears in the region and we're already seeing a spike in bear activity in my neighborhood in town. So far, except for some trash stealing, the downtown bear activity has been pretty benign(*) but as it increases and the bears get more stressed by the approaching end of the season it seems like a recipe for trouble.

--

(*) recent "living with bears" anecdote, since MeFites seem to enjoy them.. We've had a very cold and rainy summer with the exception of about two weeks ago, when we had sunshine and temperatures ranging into the 80s (F) -- unusually high for this area. My downstairs neighbor is an artist and was working on a project one evening with his front door open to take advantage of as much cross-breeze as he could get through his apartment. Around 11pm he stepped outside to take a break and found the large bruin who has been seen repeatedly around the neighborhood sitting on the porch. "Staring up at the stars" was what my neighbor swore the bear was doing. Neighbor decided to go back inside, no confrontation ensued, but I notice he's been keeping his door closed lately.
posted by Nerd of the North at 3:10 PM on August 25, 2017 [9 favorites]

I'm going hiking in bear country soon and after reading this I'm going to trade my berry-scented shampoo for salmon fragrance.
posted by AFABulous at 4:24 PM on August 25, 2017 [1 favorite]

I'm going hiking in bear country soon and after reading this I'm going to trade my berry-scented shampoo for salmon fragrance.

Gravlax for your locks?

Meanwhile, further south and kinda related, but with black bears:

Bear sightings down this summer in Sudbury: The MNRF says abundant natural food sources are keeping bears out of city limits
posted by mandolin conspiracy at 5:29 PM on August 25, 2017 [1 favorite]

Salmon runs are way down this year down here. For sport fishermen it's a mild disappointment, but for species that rely on the fish, they are pretty much hosed. The articles in the FPP are interesting, in how changing the timing of two key foods creates new complications.
posted by Dip Flash at 8:19 PM on August 25, 2017