In the summer of 2014, 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. And every year, Deacy and Armstrong had gone to wait for the bears.

But in 2014, the bears were gone.

For years, Deacy and Armstrong, both ecologists from Oregon State University, had spent time at one particular stream on the island—three meters across, and only six inches deep. In July and August, its shallow waters turn red with around 60,000 sockeye salmon. “They’re bank to bank,” says Deacy. You’d have a hard time walking through the stream without catching one.”

The bears certainly have no problem. They kill around 70 percent of the spawning fish, and they focus on the most energy-rich organs. They’ll bite the humps off the backs of the males, and they’ll tear females open to get at their nutritious eggs. The signs of this massacre are evident. The tall grass on the stream’s banks gets so thoroughly flattened by the lumbering bears that it resembles a putting green. Gulls, scavenging from the cadavers, are so bloated that they can barely fly—and are liable to careen into human heads. And bits of shredded, half-eaten salmon float downstream.

“It’s usually terrifying to walk up the streams because you get a sense of carnage just around the next corner,” Armstrong says.

But in 2014: nothing. The grass was tall. The gulls were svelte. And the salmon were mostly untouched. Foxes, eagles, and wolves took their fair share, but without the bears, these lesser predators made the tiniest of dents in the salmon population. Most of the fish died naturally. “There’d be piles of dead salmon, just molding,” says Armstrong. “The bacteria were eating them instead of the bears.”

So where had the bears gone? Fortunately, at the beginning of the year, the team had collared around 15 of the animals to track their movements. As the data rolled in, the team noticed that the bears were up in the hillsides, far away from the streams. In particular, they were sticking to land that was covered in red elderberry bushes. And Bill Leacock, a team member from the U.S. Fish and Wildlife Service, noticed that all the bear droppings that summer were full of elderberry skins. Bear scat is a horrendous mess when the animals gorge on salmon, but this stuff was very different in color and consistency.

In most years, red elderberries only ripen from late August to early September, at the end of the salmon season. The two food sources don’t overlap, so the bears eat them in sequence, gorging on salmon before bingeing on berries. But, by looking at historical data, Deacy and Armstrong found that this natural timetable has changed. In Alaska, spring temperatures have increased and elderberries have been ripening earlier. In 2014, the berries ripened especially early, bringing them in sync with the spawning salmon. And it seems that whenever both items are on the menu simultaneously, the bears always choose berries.

Which makes no sense. Pound for pound, salmon contains twice as much energy as elderberries. If bears are looking to gain as much weight as possible, in anticipation of the coming winter, why would they pick the less calorific food? “All our conventional wisdom made it hard to believe that they were switching to these berries,” says Deacy.

He solved the mystery by talking to Charles Robbins, who runs the Bear Center at Washington State University. Robbins suggested that it’s not the total number of calories in these foods that matters, but the levels of different nutrients. In an earlier study, in which he offered captive bears a varied diet, he’d found that the animals mix and match their foods so they get around 17 percent of their energy from protein. That’s the level that allows them to gain weight most quickly. If they overload on protein, they actually lose weight.

Salmon are far too rich in protein—it accounts for about 84 percent of the energy in their flesh. But elderberries, by astonishing coincidence, comprise around 13 percent protein—far more than your typical berry, but almost exactly the optimal amount for a grizzly bear. By focusing on that single food, the bears can gain weight as fast as possible.

On average, red elderberries are ripening two and a half days earlier every decade. If that continues, they will regularly overlap with the salmon by 2070, and the unusual events that Deacy and Armstrong saw in 2014 will become the new normal.

Scientists have assumed that generalists like bears “will fare better under climate change because they have more dietary options,” says Stephanie Carlson, an ecologist from the University of California, Berkeley. That might not be the case. The bears, Carlson says, are used to “riding different resources waves,” bouncing from one high-quality foraging option to the next. If the berries ripen earlier, that might be a good thing, but once they’re done, the bears will have missed out on the earlier salmon pulse.

Then again, some of Kodiak’s salmon arrive much later than the main spawning horde. These fish stick to lakes and rivers instead of shallow streams, so they’re harder to catch. But bears could wait until they die and scavenge their corpses. “We’re speculating, but we think the bears are going to be fine,” Armstrong says. “There are a lot of backups on Kodiak Island.”

That might not be the case in the continental United States, where salmon runs are dominated by hatchery-raised fish that all tend to spawn at the same time. So far, no one knows if bears in other states are also abandoning salmon in favor of other time-shifted foods. But “we’re particularly worried about proposals for large mining operations and other human activities that would reduce the ability of bears to move across the landscape and make use of these backup resources,” says Armstrong.

Misty MacDuffee, from the Raincoast Conservation Foundation, also notes that a bear’s life isn’t just about finding food and gaining weight. “Many lines of evidence suggest that bears need the fat from salmon for successful hibernation, and delivery or nourishment of cubs inside the den,” she says. If they’re forgoing this fat for the easy carbs in the berries, are they inadvertently screwing themselves over?

And what about the other inhabitants of Kodiak Island? By killing salmon, bears make their flesh available to smaller fish and gulls. By dragging salmon corpses to river banks, bears move huge amounts of nutrients from the sea to the land, fertilising the soil and feeding small scavengers. In these forests, everything from trees to weasels depends on the bears’ annual slaughter of salmon. What happens if that slaughter ends?

Many scientists have shown that climate change is rescheduling nature. Warming temperatures are forcing birds to migrate sooner, insects to emerge earlier, and plants to bud and bloom before their time. These changes are disrupting many of the dances between species, forcing long-established partners to move to different rhythms. Flowers, for example, might bloom too early to catch a wave of pollinating insects.

But most examples of these “phenological shifts” involve asynchrony between two partners. The case of the Kodiak bears represents “an under-recognized phenomenon—that of increasing synchrony of [natural] events due to climate change,” says Nicole Rafferty, from the University of California, Riverside. “And the consequences of this shift in foraging behavior could be large with knock-on effects for the ecosystem as a whole.”

“Species that never lived together can now interact because we’re removing the barrier of time,” says Armstrong. “We’ll see these new combinations that we never thought about, and we’ll get strong responses that no one could have ever predicted.”