In the gateway to the Arctic, fat, ice and polar bears are crucial. All three are in trouble.

ON HUDSON BAY (AP) — Searching for polar bears where the Churchill River dumps into Canada’s massive Hudson Bay, biologist Geoff York scans a region that’s on a low fat, low ice diet because of climate change. And it’s getting lower on polar bears. There are now about 600 polar bears in the Western Hudson Bay, one of the most threatened of the 20 populations of the white beasts.

That’s about half the number of 40 years ago, says York, director of research at Polar Bear International. His latest study, with a team of scientists from various fields, shows that if the world doesn’t cut back more on emissions of heat-trapping gases “we could lose this population entirely by the end of the century,” he says. More than polar bears are threatened in this changing gateway to the Arctic, where warmer waters melt sea ice earlier in the year and the open ocean lingers longer.

For what grows, lives and especially eats in this region, it’s like a house’s foundation shifting. “The whole marine ecosystem is tied to the seasonality of that sea ice cover,” University of Manitoba sea ice scientist Julienne Stroeve said. When the sea ice melts earlier it warms the overall water temperature and it changes algae that blooms, which changes the plankton that feed on the algae, which changes the fish, all the way up the food web to beluga whales, seals and polar bears, scientists say.

“What we’re seeing is a transformation of an Arctic ecosystem into more of a southern op.