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A shift in Kodiak brown bears' diets is having a larger impact on the ecosystem.

August 29, 2017

In the cartoon world, if bears want to ruin their dinner, they can always opt for stealing a pic-a-nic basket. But in the real world, bears in Alaska are facing a different man-made dietary concern: Climate change is causing red elderberries to ripen earlier than usual, leading Kodiak brown bears to forgo their normal diet of sockeye salmon and instead choose fruit—causing these bears to put on more weight than usual and disrupting the local ecosystem in the process.

In a study recently published the Proceedings of the National Academy of Science, Will Deacy, a post-doctoral researcher at Oregon State University, and his colleagues looked at how something seemingly as simple as warming temperatures causing berries to ripen earlier in the summer can lead to a much larger impact down the food chain and throughout an entire ecosystem. Specifically, the study looked at what happens when two sources of food—here, berries and salmon—go from being available at different times of the year to suddenly overlapping. "A large body of work has explored what happens when coevolved species shift out of sync," the paper states, "but virtually no studies have documented the effects of climate-induced synchronization, which could remove temporal barriers between species and create novel interactions."

In the past, Kodiak brown bears on Alaska's Kodiak Island would hit freshwater streams in early summer to chow down on spawning sockeye salmon and then switch to the freshly fruiting red elderberries found on the hillsides come late August and September. But in 2014, when temperature increases led to an early elderberry season, the bears (ostensibly deciding that fishing for salmon was a pain) avoided the rivers and ate the berries instead. "What you have is a scrambling of the schedule," explained Deacy. "It's essentially like if breakfast and lunch were served at the same time and then there is nothing to eat until dinner. You have to choose between breakfast and lunch because you can only eat so much at a time."

This change in diet caused the bears to put on more weight, switching from what Deacy described as a bear's version of the Atkins diet to a vegetarian diet. "Salmon that we eat in the store is fatty because it is caught in the ocean. In contrast, spawning salmon have used all of their fat reserves to migrate upstream," he wrote Food & Wine via email. "It turns out that it is very inefficient for bears to digest very high protein foods. (They lose a lot of the energy just breaking down the protein and storing it as fat.) Red elderberries have almost the perfect amount of protein for maximum efficiency. The outcome: salmon have almost twice the energy as red elderberries, but the bears probably gain more weight eating elderberries."

A larger concern, however, is how these bears' new vegetarian lifestyle affects the ecosystem. Typically, these bears hunt about 25 to 75 percent of the spawning salmon. The bears' change in behavior "likely altered the many ecological functions that result from bears foraging on salmon," the study states, including the ability of leftover salmon carcasses to enrich the nearby soil which helps sustain the surrounding forest.

However, though this phenomenon may be changing the bears' diets, humans' diets—even further down the food chain—probably won't suffer. Sockeye salmon fishing will likely go unaffected. "Mortality of stream spawning salmon almost certainly decreased when they switched to berries, but that won't necessarily result in more adults returning to spawn in later generations," Deacy writes. "There are many other salmon spawning in other habitats (rivers, lakes), and many other opportunities for salmon to die before they are able to return again as adults. The result is that even dramatic changes in mortality at one point in their life doesn't reliably result in more or less adult returning salmon."

For now, it appears the only effect a bear's appetite will have on our food is if it leads them to snatch that aforementioned pic-a-nic basket.