Last week, Damian Carrington (May 28, 2014) at The Guardian offered a scary-sounding polar bear story, based on the work of Jon Aars and colleagues from the Norwegian Polar Institute (“Fewer polar bear cubs are being born in the Arctic islands, survey finds”). As often is the case however, once you see the scientific data, you will sleep better.
[Dr Aars also gave a radio interview with CBC Canada (May 29): “Is climate change the cause of lower polar bear birth rates in Norway?”; audio available]
[Update June 24, 2014 — see below]
Female polar bear with cubs. (U.S. Fish & Wildlife Service)
The Guardian article said:
“The proportion of polar bear females around the Arctic islands of Svalbard who gave birth to cubs crashed to just 10% in 2014, according to a small scientific survey of the animals. It follows a series of warm years and poor sea ice.
The Barents Sea population of a few thousand polar bears is one of the biggest in the world. But global warming is rapidly reducing the extent of sea ice on which the bears hunt seals, their main food.
The annual survey undertaken by Jon Aars and his colleagues at the Norwegian Polar Institute was conducted in April, just after cubs and mothers leave their dens. They discovered that just three of the 29 adult females they tracked and examined had a cub born that year.
“This is a lower number than we would have expected,” he told the Guardian. “Typically one third or more of the adult females have cubs from that year.” But even this higher level is in long-term decline: annual records dating back two decades show that about half of adult females in Svalbard had cubs in the mid-1990s.
“Maybe this [year’s low number] was because we have had mild years recently with worse ice conditions or maybe it was just a bad year,” said Aars, who says it is too early to conclude the population is collapsing. “It is alarming but it is quite a small sample, so we have to be careful. But if this is something that repeats itself in coming years then it will be a concern.”
…
Winter ice cover in the Arctic fell to its fifth lowest extent on record in 2014. This continues a long-term trend of decline which is occurring more rapidly than scientists expected and the ice cap could vanish in summer within decades.” [my bold]
But look at the data, which Jon Aars and his colleague Magnus Andersen posted online last year (some of which I discussed previously, here and here), to which we can compare this year’s data.
From their preliminary research on “Proportion of females with cubs” from 1993 to 2013 (reposted here as Figure 1, below) it turns out that two years (1994 and 1996) of high cub production in the mid-1990s pushed the average for that decade to about half of females having new cubs – otherwise, the average would have been closer to 40%.
And to my eye, the very slight declining trend from 1993 to 2013 is due primarily to those two especially high production years of 1994 and 1996 (note that 1994 also had the smallest sample size of the entire study).
Figure 1. Data posted by Aars and Andersen of the Norwegian Polar Institute, original caption: “Proportion of females with cubs of the year – COYs based on data from the annual capture-recapture program 1993-2013.”
In fact, cub production in their study was ~30% or higher for 16 of the last 21 years (including the 2014 provided in the Guardian article) – five times over that period, the proportion of females with new cubs was below that level.
Notice also that the proportion of females with cubs was as low in 2010 as in 2014 but rose to more than 50% the following year – oddly, no mention was made of that 2010 “crash” in Carrington’s Guardian story. Not only did cub production recover nicely the year following that 2010 decline but 2011 had the highest proportion of mothers with cubs recorded since 1996.
The US National Snow and Ice Data Center (NSIDC) has already put to rest Carrington’s (and the CBC’s) suggestion that the low ice coverage in the Barents Sea in recent years was due to global warming. Earlier this year, NSIDC stated that Barents Sea ice is particularly affected by variations in the state of the Atlantic Multidecadal Oscillation (AMO) in spring. Low ice in 2014 was due primarily to the state of the AMO, as I discussed a few months ago here (“Barents Sea polar bear condition varies with AMO and spring sea ice conditions”).
In fact, Aars and Anderson acknowledged the effect of the AMO on Barents Sea bears on the website that contained their research results. This is what they said about their ‘females with cubs’ data:
“The figure shows the number of cubs of the year (COYs) pr adult female based on data from the annual capture-recapture program 1993-2013. The dotted line shows a significant decreasing trend over time (p = 0.049). A major part of the interannual variation is explained by variations in the Arctic Oscillation (AO) in spring (Apr-Jun) the preceding year. Higher values of AO correlate with lower cub production the year after (p < 0.01).” [my bold]
Finally, I’m disappointed that Aars failed to mention (or Carrington failed to report) the other possible reason for the slight decline in cub production in the Barents Sea. Biologists long ago suspected that Barents Sea bears had recovered so well from the over-hunting of the 1950s and 1960s that they were experiencing a natural decline in reproduction after years of higher than usual birth rates (so-called ‘density-dependent responses’).
Update June 24, 2014 I forgot to say, all that said, that biologists working in the area already suspect that females around Svalbard are moving their traditional denning areas in response to recent low ice levels, see previous post here, rather than just sit around and suffer.
In other words, the decline could have nothing to do with sea ice changes (or global warming). In an earlier post on that topic (here), I said made the following comment:
Andrew Derocher (2005), in his report on his study of Svalbard area polar bears between 1988 and 2002, says this:
“…estimates derived in the 1970s-1980s, when the population was thought to be recovering from over-harvest (Larsen 1986), were of questionable reliability when produced and with the passage of 20 years, have little relevance for the current state of the population.”
…Further research in the population is needed to determine whether the changes in the population are part of longer-term fluctuations or directional changes associated with density-dependence or climate change.
…given that the population may be showing density-dependent responses, it is not possible to differentiate the climatic effects from population effects.”
My conclusion regarding this story is that the low production of polar bear cubs this year in the Barents Sea is not an indicator of anything in particular. It would be a different story if cub production had been as low reported here (or lower) consistently over ten years. However, so far, low numbers of cubs have been one-year affairs with recovery the following season — in other words, natural variation within an ecosystem that varies naturally.
References
Derocher 2005. Population ecology of polar bears at Svalbard, Norway. Population Ecology 47:267-275. http://link.springer.com/article/10.1007%2Fs10144-005-0231-2
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