Scientists studying the effects of Arctic melting have found themselves in what you could call the Polar Bear Wars.
On one side, a sizeable group of researchers have spent decades documenting the connection between the rapid melting of sea ice and declines in polar bear health and survival.
On the other, a handful of scientists who have observed polar bears eating nontraditional prey on land—like goose eggs and berries—have hypothesized that could mitigate the loss of their icy habitat as the globe warms. Studies from this side have spawned misleading headlines such as, "Polar Bears Just Might Outlive Us All."
A paper published today in the peer-reviewed journal Frontiers in Ecology and the Environment is the latest attempt to settle the score.
The review article, "Can Polar Bears Use Terrestrial Foods to Offset Lost Ice-Based Hunting Opportunities," concludes that even though some polar bears are eating on land, those instances are limited and the nutritional benefits of that food is in doubt.
"The intent was to clarify what we know now," said the paper's lead author Karyn Rode, an Alaska-based Research Wildlife Biologist with the U.S. Geological Survey. Rode said the research showed that eating on land may be helping a few bears, there is no evidence it is benefitting them on a population level.
Polar bears have evolved to survive on the sea ice, thriving on fat of the prey they hunt there, mostly ringed seals. Since the beginning of the satellite record in 1979, nearly 700,000 square miles of sea ice have vanished as the Arctic warms at a rate two to three times faster than the rest of the globe. Some polar bear populations, such as those in the Southern Beaufort Sea and the Western Hudson Bay, have shown declines in reproduction rates, health and survival as the bears are forced onto land for longer periods.
Recent work the paper cites has shown that polar bear genes differ from their ancestors, brown bears, because they evolved to metabolize fat. Brown bears live on food found on land, and those that live in the Arctic are some of the smallest in the species.
"The bottom line is that polar bears are really big," Rode said. "And they're big for a reason–they have the highest fat diet ... It's the size of polar bears that really causes them problems when they switch their diet."
This question of whether the bears will be able to adapt to longer periods on land has become significant because polar bears are the icons of climate change. Climate denialists have used healthy polar bear studies to weaken that icon status.
"We've established a threat in the loss of sea ice habitat, and basically the statement I made back in 2007 still seems to stand: As the sea ice goes, so goes the polar bear," said Steven Amstrup, former USGS biologist who is now the chief scientist for the conservation group Polar Bears International and who was a coauthor of the study. "If you want to deny global warming and if someone out there provides a little tidbit that you can interpret or reinterpret, then by proxy you can deny the threat of global warming."
Indeed, climate contrarian Wei-Hock "Willie" Soon, who was recently found to have failed to disclose the fossil fuel funding of some of his work, was a coauthor of a 2007 paper that claimed polar bears could adapt and start eating food on land. Soon received funds from the coal utility Southern Company to give a talk where he also argued that "too much ice is really bad for polar bears."
The authors of the new report write that the observations of polar bears eating food on land are limited, with studies documenting fewer than 30 polar bears in certain populations eating energy-dense foods like goose eggs. Though this may help some bears supplement their fat stores while on land, there is no evidence this makes a difference on a large scale.
"Warming-induced loss of sea ice remains the primary threat faced by polar bears," the report says.
The paper took issue with research by Robert Rockwell, a biologist with the American Museum of Natural History, and his colleague Linda Gormezano, a graduate student. His 2009 paper with Gormezano states, "The consumption of snow goose eggs could improve bears condition and stabilize or reverse the survival decline of this age class," and, "Such a reversal could at least temporarily increase the near-term growth rate of the western Hudson Bay population."
But Rockwell said he never hypothesized this would play a "major role" in polar bear adaptation—as is stated in the new study—and he, like the new study's authors, believe more research is needed on how many bears are helped by terrestrial feeding.
"If you'll look at our papers, you'll see that we say the same thing," Rockwell said.
Another study published late last month also focused on observations of polar bears eating goose eggs, berries, caribou and other species while on land. The paper examined polar bear predation on bird populations in Norway's Arctic archipelago and Greenland. The scientists found that in years where the bears arrived earlier, before the eggs had hatched, more than 90 percent of all nests were predated.
In this case, the study's conclusion was not that eating eggs and birds would offer alvation for the bears. Instead, it highlighted the damage to the bird species, calling that part of the "cascading effects of climate change."
"The number of polar bears exploiting the coastal habitat was small, and we suspect that there is little scope for further increase in numbers," the study's authors wrote. "This underlines the importance of sea ice habitat to support current population levels of polar bears."