Grizzly bears have stopped eating salmon in favour of elderberries after being forced to make a choice due to climate change. Warming temperatures meant that the berries are ripening earlier than usual, at exactly the same time as the freshwater streams on Alaska’s Kodiak Island are overflowing with sockeye salmon.

The island's brown bears typically feed first on salmon in early summer, followed by elderberries later in the season, in late August and September.

“What you have is a scrambling of the schedule," said William Deacy, a biologist at Oregon State University that studied the phenomenon.

"It's essentially like if breakfast and lunch were served at 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."

The study found that during the unusually warm summer of 2014, the bears, which would traditionally kill up to 75 percent of the salmon, were nowhere to be seen near the streams.

Instead, they were in the hills busy munching on berries, which contain less protein and therefore take less energy to break down, causing them to gain weight more quickly.

Biologists warned that changes caused by a warming planet were behind the bears' unusual behavior and could affect the entire ecosystem.

The researchers found that the forests around the streams suffered because the bears’ fish carcasses were no longer there to enrich the soil.

"Bears switched from eating salmon to elderberries, disrupting an ecological link that typically fertilizes terrestrial ecosystems and generates high mortality rates for salmon," the study said.

On average, red elderberries are said to be ripening two and a half days earlier every decade.

If the pattern continues, they will regularly overlap with the salmon by 2070.

The study was published in the Proceedings of the National Academy of Sciences.