New Paper: Predictions Of Polar Bear Population Crash Based On Flawed Assumption

  • Date: 19/01/17
  • Dr. Susan J. Crockford, University of Victoria, Canada

Has recent summer sea ice loss caused polar bear populations to crash?

A paper published today finds that predictions of polar bear population crashes due to summer sea ice loss are based on a scientifically unfounded assumption. Specifically, the paper addresses the basic premise upon which predicted population declines linked to modeled habitat loss were made by polar bear specialists back in 2006 and 2008 (by the International Union for the Conservation of Nature, IUCN, and the US Fish and Wildlife Service, USFWS), and concludes that when assessed as a testable hypothesis against data collected since then, it must be rejected.


Photo credit: US Fish and Wildlife Service (2007).

Those mid-2000s survival assessments predicted significant population declines of polar bears would result by mid-century as a consequence of summer sea ice extent reaching approximately 3-5 mkm2 on a regular basis: in 2006, the IUCN predicted a >30% decline in total population would occur, while in 2008, the USFWS predicted the global population would decline by an astonishing 67%. Most shocking, perhaps, was the USFWS prediction that ten subpopulations within two vulnerable sea ice ecoregions would very likely disappear entirely (all purple and green areas shown in the map below) when summer sea ice routinely reached levels of 3-5 mkm2.


Image credit: US Geological Survey.

But summer sea ice declined much faster than anyone expected. In fact, those low ice levels of 3-5 mkm2 that were not expected until mid-century have occurred regularly since 2007. By 2015, polar bears had been living through the grim reality of their predicted future for almost 10 years, as the sea ice graphic below shows. This early realization of predicted sea ice levels meant the ‘sea ice decline = population decline’ assumption for polar bears could be tested against actual survival data (i.e., observations).


Image credit: Figures 2 and 5 from Crockford 2017.

As it turns, data collected between 2007 and 2015 by field biologists reveal that polar bear numbers have not declined as predicted and no subpopulation has been extirpated. Several subpopulations expected to be at high risk of decline have remained stable and at least one showed a marked increase in population size over the entire period, despite marked sea ice loss. Another at-risk subpopulation was not counted but showed marked improvement in reproductive parameters and body condition with less summer ice – the hallmarks of a stable or increasing population.

The hypothesis that repeated summer sea ice levels of below 5 mkm2 will cause significant population declines in polar bears must be rejected. This result indicates the USFWS and IUCN judgments to list polar bears as threatened or vulnerable based on future risks of habitat loss back in 2006 and 2008 were scientifically unfounded and suggests that similar dire predictions for Arctic seals and walrus may be likewise flawed. Ultimately, the lack of a demonstrable ‘sea ice decline = population decline’ relationship for polar bears almost certainly invalidates recent survival models that predict catastrophic population declines should the Arctic become ice-free in summer.

The publication forum for this paper is PeerJ Preprints, which I found while looking for recent research papers about ringed seals. I discovered that Canadian ringed seal biologist Dr. Steven Ferguson (Dept. Fisheries and Oceans) recently used this non-peer review publication service:

Ferguson et al. 2016. Demographic, ecological and physiological responses of ringed seals to an abrupt decline in sea ice availability. DOI:10.7287/peerj.preprints.2309v1 Pdf here.

This publication service is free to use and free to download for readers (open access). It archives reviewer comments on each paper and an assigned DOI means the article will show up on Google and Google Scholar searches. I decided that if this publication forum was good enough for Ferguson and his Arctic research community, it was good enough for me.

Since my paper addresses a controversial topic, I considered publication in a peer-reviewed journal to be a long shot – but I tried. Before submission to PeerJ Preprints, this paper had been through two rounds of peer-review at two different scientific journals but was ultimately rejected. A third journal rejected the paper without review. This wasn’t a big surprise but it was still rather discouraging.

So, I have incorporated all pertinent reviewer comments and suggestions, added a few recent references, and present the final result as this PeerJ Preprint so that all readers may evaluate the argument for themselves without gate-keeper interference. See what you think.

Article: Crockford, S.J. 2017. Testing the hypothesis that routine sea ice coverage of 3-5 mkm2 results in a greater than 30% decline in population size of polar bears (Ursus maritimus). PeerJ Preprints 19 January 2017. Doi: 10.7287/peerj.preprints.2737v1 Open access.

Link to the article

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