Take-home quote from a new polar bear paper by Todd Atwood and colleagues (2016):
“…there is no causal link between the patterns in polar bear vital rates and increased use of terrestrial habitat…”
In other words, there was no information to link the increased time polar bears spent onshore with either an increase or a decrease in body condition, survival or cub production. The authors did find that polar bears were strongly attracted to the bone piles that accumulated in the fall from 2010-2013 after Inuit bowhead whale (Balaena mysticetus) hunting at Barrow, Cross Island, and Kaktovik. Isn’t that a surprise?
The results also appear to confirm previous work that showed terrestrial (land-based) foods are not important to polar bears – a conclusion I totally agree with and which I discussed last year here. No wonder there was no press release issued by USGS about this study. It’s only “news” because someone the Anchorage Daily News interviewed lead-author Atwood yesterday as a way of promoting the International Bear Conference (see previous post here, now updated with a link to the Talk of Alaska radio podcast). Atwood implied there could be advantages to bears from feeding on the bone piles but admitted he had no data to support that assumption.
Take-home facts from the paper: The average percentage of the Southern Beaufort polar bear population in Alaska that spent time on land increased from 5.8% (1986-1999) to 20% (2000-2014) based on surveys conducted in 2010-2013 (compared to previous studies). It turns out that 2010-2013 was the period when fall hunting of bowhead whales resulted in bone piles accumulating onshore during the open-water season. As I said, no big surprise.
Caveat: Reader beware. Despite using percentage of the population onshore as a critical metric, the authors never once mention the changes in size of the population over time (which has been much publicized and even mentioned in the paper), nor are the actual numbers of bears onshore in any given year provided. That is, percentage of bears onshore each year was calculated as number of bears divided by the total populations size but those numbers are not provided (not even in the Supplementary Information). This is a trick used by scientists to make data appear more significant than it really is – and in this case, likely glosses over the fact that the actual numbers of polar bears onshore did not increase over time. It may also be a means of hiding the estimated size of the population in 2013, since it very likely increased over the amount estimated at 2010.
The amount of time polar bears spent on land also increased over this period (from 20 days to 56 days). The greatest percentage of bears that spent time on shore occurred in 2013, the last year of the 2010-2013 study. But, as noted above, whether that actually meant more bears remains to be seen. As the map from the study below shows, most of the bears were found near bone piles of bowhead whales at Cross Island and Kaktovik.
Atwood, T.C., Peacock, E., McKinney, M.A., Lillie, K., Wilson, R., Douglas, D.C., and others. 2016. Rapid Environmental Change Drives Increased Land Use by an Arctic Marine Predator. PLoS ONE 11(6): e0155932. doi:10.1371/journal.pone.0155932 pdf here.
In the Arctic Ocean’s southern Beaufort Sea (SB), the length of the sea ice melt season (i.e., period between the onset of sea ice break-up in summer and freeze-up in fall) has increased substantially since the late 1990s. Historically, polar bears (Ursus maritimus) of the SB have mostly remained on the sea ice year-round (except for those that came ashore to den), but recent changes in the extent and phenology of sea ice habitat have coincided with evidence that use of terrestrial habitat is increasing. We characterized the spatial behavior of polar bears spending summer and fall on land along Alaska’s north coast to better understand the nexus between rapid environmental change and increased use of terrestrial habitat. We found that the percentage of radiocollared adult females from the SB subpopulation coming ashore has tripled over 15 years. Moreover, we detected trends of earlier arrival on shore, increased length of stay, and later departure back to sea ice, all of which were related to declines in the availability of sea ice habitat over the continental shelf and changes to sea ice phenology. Since the late 1990s, the mean duration of the open-water season in the SB increased by 36 days, and the mean length of stay on shore increased by 31 days. While on shore, the distribution of polar bears was influenced by the availability of scavenge subsidies in the form of subsistence-harvested bowhead whale (Balaena mysticetus) remains aggregated at sites along the coast. The declining spatio-temporal availability of sea ice habitat and increased availability of human-provisioned resources are likely to result in increased use of land. Increased residency on land is cause for concern given that, while there, bears may be exposed to a greater array of risk factors including those associated with increased human activities.