Each summer, the shallow freshwater streams of Kodiak Island, Alaska, are so thick with sockeye salmon, you literally cannot cross the waterways without stepping on the brightly colored fish. With the salmon come brown bears, often dozens of grizzlies per stream, hauling the fish onto nearby banks for an easy meal.
During an unusually warm summer in 2014, however, no bears could be found. At the peak of the annual salmon run, as the fish made their way upstream to spawn, the roughly 1,000-pound bears were busy feasting on berries instead, according to a study published today in the Proceedings of the National Academy of Sciences. A similar phenomenon is believed to have also occurred in 2016, after the study period ended, but the bears were not closely monitored to confirm their feeding behavior.
Biologists who study Alaska's iconic omnivores say changes in seasonal phenomenon caused by a warming planet were behind the bears' unusual behavior, which could affect the entire ecosystem.
Different species are responding to climate change in different ways, "so what you have is a scrambling of the schedule," said William Deacy, a biologist at Oregon State University in Corvallis and lead author of the study.
The island's brown bears typically feed first on salmon, followed by elderberries later in the season. An earlier-than-usual ripening of red elderberries, however, forced the bears to make a choice.
"It's essentially like if breakfast and lunch were served at same time and then there is nothing to eat until dinner," Deacy said. "You have to choose between breakfast and lunch because you can only eat so much at a time."
When given a choice between salmon and elderberries, grizzlies, a common name for brown bears across North America, actually prefer the latter. Salmon provide twice the energy density, but elderberries have a better nutrient profile that allows grizzlies to grow more quickly. The berries are 13-14 percent protein, close to the 17 percent that biologists say allows them to grow most quickly, whereas spawning salmon are roughly 85 percent protein, which require more energy for the bears to break down.
For brown bears on an island rich with food options, having to choose between two abundant meals won't likely have a negative effect, Deacy said. In other locations, however, like the Pacific Northwest, where bears are already stressed by a number of ecological factors including fewer food options, changes in the timing of food availability could pose a bigger problem.
The earlier ripening of elderberries could also have significant implications for the rest of Kodiak Island and the salmon that spawn there. Bears typically kill up to 75 percent of the salmon, roughly half of which are killed before they are able to spawn. The reduction in salmon deaths would likely benefit the salmon population but could imperil other parts of the island's ecosystem that derive nutrients from the salmon carcasses that are dragged onto shore by the bears.
Researchers looking at the effects climate change has on changes in plant and animal phenomenon typically focus on interactions between two species that are closely linked but then go out of sync due to climate change.
The current study shows you can also have strong, and potentially negative, effects from species going into synch, Deacy said. "It's a new way to look at how climate change can affect these species' interactions."