High-speed climate change can be especially rough for specialist animals, whose focus on specific foods may backfire as seasons shift. Some migratory birds, for example, now show up too late or too early for their normal springtime feasts.
That's less of a problem for generalists like grizzly bears, who have learned to exploit a wider range of plants and prey. But what if, instead of missing one seasonal food source, they must choose between two that normally appear at different times?
Alaska's Kodiak bears — a bulky subspecies of brown bear, also known as grizzlies — have recently given up their famous salmon hunts due to climate change, according to a new study, but not because salmon are scarce. Warmer weather led a different food source to overlap with the annual salmon run, presenting the bears with an unusual choice between two of their favorite foods at the same time.
And while they love salmon, these omnivores seem to want the other food even more. When it made an early debut, they left the salmon streams — where they typically kill 25 to 75 percent of the salmon — and moved onto nearby hillsides.
What could lure grizzlies away from all that fish? Elderberries, apparently.
Respect your elders
Published this week in the Proceedings of the National Academy of Sciences, the study looked at why bears abandoned their salmon hunts on Alaska's Kodiak Archipelago in the summer of 2014. That July and August, the islands' freshwater streams filled up as usual with the yearly salmon run. This bonanza is normally raided by bears, but as Ed Yong explains in the Atlantic, that didn't happen in 2014.
Other predators barely made a dent, study co-author Jonathan Armstrong tells Yong. "There'd be piles of dead salmon, just molding," says Armstrong, an ecologist at Oregon State University (OSU). "The bacteria were eating them instead of the bears."
Data from tracking collars showed the bears were on nearby hills instead of fishing in streams. Hills with red elderberry seemed most popular, and a survey of local bear droppings revealed lots of elderberry skins and little sign of salmon.
Kodiak bears are already big elderberry fans, but the berries usually ripen in late August and early September — the end of salmon season. The bears are used to eating these foods in order, switching to elderberries after the salmon are gone. But using historical temperature data, the study's authors found that rising temperatures have been helping Kodiak elderberries move up their schedule.
In years with especially warm spring weather, like 2014, red elderberry "fruited several weeks earlier," the researchers write, "and became available during the period when salmon spawned in tributary streams." As co-author William Deacy tells Phil McKenna of InsideClimate News, this forced the bears to make a decision.
"It's essentially like if breakfast and lunch were served at same time, and then there is nothing to eat until dinner," says Deacy, a biologist at OSU. "You have to choose between breakfast and lunch because you can only eat so much at a time."
The bears chose berries, a seemingly bad decision since salmon offers twice the energy density. But research has shown that elderberries have a better nutrient profile for helping brown bears gain mass quickly — a key part of their preparations for winter. Their berries contain 13 to 14 percent protein, close to the 17 percent identified as optimal for brown bears in a 2014 study. Spawning salmon are about 85 percent protein, McKenna notes, and require more energy to break down.
Kodiak bears might adapt well to this change, the researchers say, given their rich habitats and diverse diets. Yet there are places in North America where grizzlies enjoy much less food security, so they may be more vulnerable to a shift in phenology, or the timing of biological events like migration, blooming and breeding.
And this shift could still cause trouble for the Kodiak ecosystem, too. Because the bears normally kill so much salmon — up to 75 percent, including many before they spawn — this is a big change for the islands' wildlife. It may be good news for salmon, but as the researchers point out, many other land animals normally get valuable nutrients from all the salmon left on land by the bears' feasts.
"Bears switched from eating salmon to elderberries, disrupting an ecological link that typically fertilizes terrestrial ecosystems and generates high mortality rates for salmon," they write. "These results demonstrate an underappreciated mechanism by which climate-altered phenologies can alter food webs."