A paper published last week in the journal Science, written by a team of biologists and atmospheric scientists, expounds on a possible dire future for a range of Arctic animals. It’s called, “Ecological consequences of sea-ice decline” and surprisingly, polar bears are discussed only briefly.
However, with the inclusion of one short sentence, the paper manages to perpetuate misinformation on grizzly/polar bear hybridization that first appeared in a commentary essay three years ago in Nature (Kelly et al. 2010)1. The Post et al. 2013 missive contains this astonishing statement (repeated by a Canadian Press news report):
“Hybridization between polar bears and grizzly bears may be the result of increasing inland presence of polar bears as a result of a prolonged ice-free season.”
Lead author of the paper, Professor of Biology Eric Post, is quoted extensively in the press release issued by his employer (Penn State University, pdf here). In it, Post re-states the above sentence in simpler terms, removing any doubt of its intended interpretation:
“… polar and grizzly bears already have been observed to have hybridized because polar bears now are spending more time on land, where they have contact with grizzlies.”
Both statements are patently false. All recent hybridization events documented (2006-2013) occurred because a few male grizzlies traveled over the sea ice into polar bear territory and found themselves a polar bear female to impregnate (see news items here and here, Fig. 1 below). These events did not occur on land during the ice-free season (which is late summer/early fall), but on the sea ice in spring (March-May).
Grizzlies have been documented wandering over the sea ice of the western Arctic since at least 1885 (Doupe et al. 2007; Fig. 2, below) and the presence in this region of hybrid grizzly/polar bear offspring is not an indicator of declining summer sea ice, whether due to global warming or natural causes, or some combination thereof.
Leading polar bear specialist Ian Stirling knows this. He said as much in an interview with National Geographic News reporter John Roach when the first hybrid was shot back in 2006 (May 16, 2006), pdf here:
“Stirling said grizzly bears have been showing up in the Canada’s western Arctic as far north as Banks Island and Victoria Island in the province of Nunavut (see these islands in the upper left of our Nunavut map) periodically for the past 50 years.
The hybrid, he said, is “definitely not” a sign of climate change.” [my bold]
Ian Stirling is not only a co-author on the Post et al. Science paper but he is the only polar bear specialist on the team. He may not have written this erroneous sentence; his less-experienced co-authors may have composed it. If so, he failed to correct it or insist it be removed. The responsibility for the inclusion of this blatant misrepresentation of fact in the Post et al. paper lies with Stirling, since he knows it’s not true.2
Figure 1. Sea ice extent in late spring in the western Arctic (May 23, 2013). Note that there is solid ice between the mainland and Banks and Victoria islands at this time. Some grizzly males emerge from their winter dens long before grizzly females and go wandering in search of food and potential mates: they can find both out on the sea ice. See Fig. 2 for a history of grizzly excursions onto the sea ice and the western Arctic islands, which have taken then even further north than Victoria Island. Click to enlarge.