08 Jul 2010
For Hudson Bay Polar Bears, The End is Already in Sight
The polar bear has long been a symbol of the damage wrought by global warming, but now biologist Andrew Derocher and his colleagues have calculated how long one southerly population can hold out. Their answer? No more than a few decades, as the bears’ decline closely tracks that of the Arctic’s disappearing sea ice.
No polar bears have been more closely studied than Canada’s western Hudson Bay population. In recent decades, biologists such as Andrew E. Derocher of the University of Alberta have compiled an impressive store of data on everything from the weight of females at denning, to the body mass of bears of all sexes, to the length of time the bears spend annually on the shores of Hudson Bay, to the decline of sea ice in the bay itself.
Now, Derocher, working with Peter K. Molnar and other colleagues from the University of Alberta, has marshaled that data to forecast how long it will be before western Hudson Bay’s polar bears disappear. The calculation is not overly complex, since the health of polar bears is directly tied to the amount of time they spend on sea ice hunting seals.
The basic facts are as follows: The region’s polar bears have been forced to spend an extra week per decade onshore; the bears have been losing, on average, more than 20 pounds per decade; the body mass of the bears has been steadily declining; females have lost 10 percent of their body length; and the population has dropped from 1,200 to 900 in three decades, with much of the decline coming in the last 10 years.
Looking at projected sea ice declines, Derocher and his colleagues estimated in a recent paper in Biological Conservation
that western Hudson Bay’s polar bear population could well die out in 25 to 30 years. Indeed, in an interview with Yale Environment 360
senior editor Fen Montaigne, Derocher said that the population — one of 19 in the Arctic — could be gone within a decade. All it would take is several straight years of low sea ice conditions — such as the current year — which could force the bears onshore for more than five months a year, leading to a sharp decline in the bears' physical condition and the inability of females to gestate cubs. “One of the things we found was that the changes in this population could happen very dramatically,” says Derocher. “And a lot of the change could come within a single year if you just ended up with an earlier melt of sea ice.”
Yale Environment 360:
You predict that the western Hudson Bay polar bear population, which is one of the most southerly, could reach a point within three decades where there are too few animals to sustain a breeding population. Could you summarize for our readers how you reached that conclusion?
Sure. If you look at polar bears in the global context we’ve actually got 19 different subpopulations. Now, they’re reasonably distinct, but the interesting thing about the western Hudson Bay population is that it’s actually one of the most accessible and it’s certainly by far the most studied population that we have. So that’s one of the reasons that we’re focusing our attention on the western Hudson Bay population. But
The changes that could come in this population could happen very dramatically.”
more importantly, we have a lot of the background information on the workings of this population. So we understand very well things like how fat a bear has to be to produce a certain number of cubs, we know a lot about how much energy these bears are burning during the period of time over the summer that they’re forced ashore when the sea ice melts. We also have a very good understanding of how the sea ice has changed in this part of the world. So really, what this is is kind of a model system that’s giving us some early indications about what one of the more southern populations is doing relative to the issue of climate change.
So what we did in the study, led by Peter Molnar, is that Peter is a mathematical biologist and was able to create models that take the inner workings of a bear and put it into mathematical context. And from here, it’s really not much more than sort of an accounting process — a fancy one — but you can account for changes in the sea ice and how the bears respond in terms of how much fat they’ve been able to store. And from there it’s fairly easy to run out various scenarios of sea ice change to look at when, basically, the bears’ fat stores run out, and when that happens the bears, of course, subsequently die. So it’s not that complicated, but the implications of the results are quite dramatic.
One of the major issues we’ve found is that when we’ve looked through the empirical data we can see there’s been a gradual decline in body condition that dates right back into the 1980s. And we can now correlate that very nicely with the loss of sea ice in this ecosystem. And one of the things we found was that the changes that could come in this population could happen very dramatically and a lot of the change could come within a single year if you just ended up with an earlier melt of sea ice.
In the 1980s the bears were on the ice around eight months a year and ashore and fasting around four months a year. Is that true?
That’s exactly it. And what we’ve seen is that over the last three decades we’re getting about one week extra onshore time for this population every decade. That doesn’t sound like a lot, but you have to remember that these bears are burning almost two pounds of body fat for every day that they’re ashore. So basically what’s happening is that we’re
Photo by Andrew E. Derocher
An underweight adult female with two 10-month-old cubs waiting for the ice to re-form on Hudson Bay in November.
pushing them at both ends of the spectrum, in that we force them off the ice earlier in the springtime. And that’s actually the best feeding period for the bears. So we’re taking them away from their food sooner. And because we’re putting them on shore sooner, it starts the period of time that they have to rely on their body fat sooner. And then, on the flip side, the freeze-up in the fall, which allows them to get back out and start hunting again, we’re pushing them at that end of the situation, as well. This has always been a population that’s sort of sitting on the edge — we don’t see polar bears any further south except for the Ontario population. So if we extend this ice-free period too long, we predict that the population really won’t sustain itself. We’re already seeing early indications of this, but the real concern is that the change could come very quickly.
Let’s talk about the early indications of this decline in physical condition and body mass.
One of the things that’s very clear is that their overall body mass has been declining slowly and quite steadily. It really amounts to several pounds per year per bear — it’s really an indication of how much energy the bears have been able to store while they’re out on the sea ice. And it’s this energy that really drives what the bears can do. So pregnant females coming ashore in July or June — we actually had bears ashore this year as early as June 11, which is almost unheard of for bears in the Hudson Bay area — but the ice was very poor this year. So the bears come ashore, and it’s the amount of fat that they’ve been able to store — especially these pregnant females, because they won’t actually get to see another seal until around February or March next year — so they’re relying totally on their stored fat to produce cubs. And we know that fat females produce more twins and triplets, whereas skinny females either give up the whole reproductive attempt or give birth to smaller cubs, and those smaller cubs have subsequently lower survival rates.
So the cumulative effect has been that the decline in body conditions has resulted in fewer cubs being produced, meaning there’s less recruitment in the population, and that’s caused a decline in the number of bears in
If we extend this ice-free period too long, we predict that the population really won’t sustain itself.”
western Hudson Bay. On top of this, there is a harvest of bears that occurs not in Manitoba, where the bears summer, but further north in the territory of Nunavut, and those bears that migrate northward on the very first-forming ice in the fall are harvested there. So the harvest didn’t come down in numbers fast enough for the population to stabilize. So the combination of lower recruitment and the harvest has caused this population to decline well over 20 percent over the last decade or so.
The population now is about how many?
It’s just a little over 900.
And 30 years ago it was eleven or twelve hundred?
That’s correct, yes. A little over twelve hundred.
And are you saying that the bears, over the last few decades, have been losing on the order of 20 or 30 pounds per decade?
That’s about right, yes. And particularly it shows up in the females. The females have lost about 10 percent of their body length. Females used to grow longer — we measure them from the tip of the tail to the tip of the nose — and they’ve shrunk about 10 percent over the last three decades. So not only are the bears skinnier but also smaller, so we’ve got the famous shrinking bears of Hudson Bay.
It’s my understanding that the lightest female that, in your western Hudson Bay population, has ever been measured, at den entry, was 189 kilograms?
And below that — is that kind of a threshold beyond which it’s difficult for them to reproduce?
Click to enlarge
Polar Bear Specialist Group
The 19 distinct polar bear populations across the Arctic.
That’s exactly what’s happening. The bears are being pushed, and they just don’t have enough stored fat on their bodies to successfully reproduce. And, of course, one of the consequences of this extended fasting period is that the females have to make a physiological decision whether to continue with a pregnancy or just basically to abandon the reproductive attempt. It’s clear that there’s been a massive reduction in how many females are able to carry off a reproductive attempt. A lot of females try to initiate the denning and then their fat stores run out and they return to the sea ice without cubs, or they try to return to the sea ice with cubs that haven’t developed far enough along to be viable out on the sea ice, and then we see problems there.
In the 1980s, when you had a typical time off the ice of, say, four months, there was a three to six percent mortality of bears, but if the sea ice continues to decline and you’ve got polar bears off the ice for, say, six months a year, you could see mortality of 28 to 48 percent. Is that correct?
In this paper we actually looked at the most conservative scenario. Adult males are probably [more] able to deal with this fasting period than any group of bears out there, because when they hit the shore of Hudson Bay they basically just flop down on the beaches, curl up, and go to sleep for the ice-free period. So they really don’t do much with those energy stores, and when you compare them with a sub-adult bear that’s actively growing or a female that has offspring with her, she’s going to be nursing those cubs all through this ice-free period. So the energetic demands on the females are actually quite a bit higher. So we looked at these adult males and projected forward in time based on how long this ice-free period was and how they would basically do with this extended fasting period. Now, 150 or 180 days is a much longer ice-free period than we’ve normally seen, but this year, for example, it seems like we’re easily going to top 150 days. It just depends on when the ice freezes in the fall. So, basically, we have an early meltout... and clearly they’re already going to be pushed because last year was a late freeze-up, so they didn’t get out early and we’ve taken them off very early this year. So we may be looking at a record-breaking scenario for this ice-free period.
Because the bears are distributed in a bell-shaped curve, there’s always been a little bit of mortality. We always have a few males barely hanging on in the fall, and sometimes they don’t make it back to the ice and die on shore. So the concern is, because you’re going up the rise of the bell you can very quickly shift to more and more of the population that doesn’t have the energy reserves to make it back onto the sea ice. So our prediction is increasing mortality, but the mortality may increase very quickly over time as you go up this bell curve, and that’s our real concern. When it occurs is very difficult to say. The problem is, we can end up with this sort of scenario, going to 150 or 160 or 180 days of ice-free period at any time.
And at that point you could be seeing annual mortality rates of adult males of 30 percent?
That’s exactly what would happen. And that’s the concern, that because the males are one of the most robust elements in the population, if we see 30 percent mortality in the adult males we will also see, probably, much higher mortality in sub-adult animals and we can pretty much be certain that we won’t see many females able to nurse their cubs long
The number of observed cannibalism events in the last couple of years in this population has been skyrocketing.”
enough to get through this ice-free period. So the real rub here is that there’s not much in the way of terrestrial resources for these bears to pull on during the ice-free period — there’s a few berries and a bit of grass, but that’s certainly not enough to sustain a polar bear population. So the challenge is, you push them, there’s nothing on shore, they run out of energy, and then the real catastrophe is that you’ll also see things we’re beginning to see on land like increased cases of infanticide and cannibalism. The number of observed cannibalism events in the last couple of years in this population has just been skyrocketing. That’s what happens when you get desperate animals trying to survive — anything becomes a viable option and cannibalism is one that we’ve seen increasing dramatically.
So under a scenario in which you would have bears on shore for half the year, if you’re losing 25 to 30 percent of the population per year and have females unable to produce cubs, you could see a population of polar bears go extinct in a matter of a decade once this cascade starts to unfold?
It’s totally dependent on the sea ice changes. The bears will respond in lockstep with the sea ice conditions. And certainly everything we’re seeing in the Hudson Bay ecosystem is of great concern.
In the western Hudson Bay region, how much sooner is the ice melting out in the spring and how much later is it forming in the fall?
It’s about one week per decade. This year I’d say we’ve lost at least two weeks in terms of sea ice cover in the Hudson Bay system. Some of the bears are already ashore. I mean, the bears don’t want to leave the sea ice — it is their primary habitat — but you can see them lined up there trying to find the last few seals. I mean, a lucky bear that gets one or two seals late in the ice-cover season will do much better than a bear who happened to miss a couple of seals. It’s a very fine balance that these bears are on, and of course one of the problems is that as the ice breaks up it becomes more energetically expensive for the bears to move through these habitats.
And when you say more energetically [expensive], you mean they’ve got to swim instead of walk on the ice?
That’s exactly the problem. And you can sort of think about it in the hunting success that the bears have. It’s not a lot of fun to be in and out of water all the time, and it’s a rather inefficient way for the bears to move across large distances, and these bears are moving several hundred miles over a single year. So what happens is that it’s just less energetically
Even the most optimistic projections don’t allow the polar bears to adapt to terrestrial resources.”
effective. So you’re basically burning more energy just to find your meal. And the other challenge is that when you’ve got very high ice cover there are still cracks where the seals have to maintain access to air. So what happens is as the ice cover drops down it becomes much more difficult for the bears to predict where the seals are actually going to be, just because there are so many more places where they can come up for a breath of air that it becomes much more difficult for the bears to get access to the seals to make a successful kill. So in that context it really becomes a challenge for the animals to get themselves at the right place at the right time. So we think that their hunting efficiency drops dramatically as sea ice conditions break up.
And what about suggestions that polar bears will adapt to a life onshore and eat goose eggs.
Well, even the most optimistic projections don’t allow the polar bears to adapt to terrestrial resources. We’ve actually looked at goose eggs and blueberries, which are two things that polar bears do eat periodically. If they ate pretty much all of the goose eggs from the snow goose colony in the Churchill area, it could probably help them out for about one or two days of lost time out on the sea ice. Of course, once you eat all of the goose eggs there is no more goose colony, so it’s kind of a nonissue. So there’s no way that it’s a viable food resource.
Another key factor in your paper is the impact of declining sea ice and larger areas of open water on the ability of males and females to find one another and mate.
What happens is the females come into estrus or breeding condition in the springtime and they’re basically going about their business. They seem more intent on trying to find seals and to improve their body condition. Because usually these females have just weaned 2 ½-year-old cubs that have really depleted their body reserves, so the females are mostly looking for things to eat. The males are much more driven to find females, especially early in the spring. So what they do is, they
The worst-case scenarios are that this population could be gone within the decade.”
basically take off on these long journeys, they’re just running a straight line, and every time they come across another set of tracks they veer off for two or three steps. And if it’s not a female that’s in breeding condition, they just get back on their bearing and they continue on in their search. If it is a female that’s in reproductive condition, then it’s basically like a magnet. The challenge is that historically the sea ice has always been fairly well connected, and so it’s fairly [easy] to get on those tracks — and these bears can follow them for several days trying to find the female. The challenge becomes if the sea ice starts to break up the bear is no longer able to find the female as efficiently in these broken ice areas. We project that there will be a decrease in the searching efficiency of males for finding females. And if that happens, basically, the pregnancy rate of females drops.
And when you put all these factors together in your paper, with what degree of certainty did you project that within several decades the western Hudson Bay population might no longer be viable?
The worst-case scenarios are that this population could be gone within the decade. A more optimistic scenario would say that we’ll bounce between good years and bad years for several decades to come. Everything that we can see about the sea ice in western Hudson Bay suggests that it’s going to disappear sooner rather than later... The question is, will we get a really bad year that knocks the wind out of this population sooner than later? Or will it just sort of bounce along on this long-term decline?
Do you think, given the sea ice trends throughout the Arctic, that what is happening with the western Hudson Bay population is really a harbinger of what’s to come with more northerly populations?
That’s exactly the concern that we have. We have, as I mentioned, 19 different populations. Many of them are doing quite well right now, and they will continue to do well for periods to come. Just by the nature of the way the sea ice is in these areas, they’re much less vulnerable to warming. It’s really those populations that are in those most dynamic habitats along the ice edge that we’re most concerned about. So in
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that context we can look at the bears in the Hudson Bay ecosystem, we can look in the bears in the Beaufort Sea, and then the population that’s shared between Alaska and Russia in the Chukchi Sea — that population looks also extremely vulnerable to climate change. And then if we go to the population that’s right between Canada and Greenland in the Davis Strait area, that area is under huge pressure from loss of sea ice. It was actually quite a robust population until quite recently. And then if we go further east into the Barents Sea between Norway and Russia — that population also looks to be extremely vulnerable to ongoing warming. So we’re going to see different scenarios played out in different populations, but what we’re seeing is sort of this progression of loss of body condition that results in lower reproduction and then subsequent increases in mortality. That seems to be the chain of events that is showing up in more and more populations as we get better information.
It’s not a lot of fun for somebody who’s spent over 30 years studying polar bears. The first paper I coauthored about this came out in 1993 and at that time I was still under the impression that even though climate change was a concern it was really going to be for the next generation of biologists — or perhaps even the one after that — to deal with the issue. And I’ve been really shocked at the rate of change, and I’ve probably been even more shocked at the lack of concern of political bodies to deal with this... It’s been quite disheartening to watch this lack of interest, and I think it’s really unfortunate that people don’t understand that we have a limited time to deal with this issue if we want to save the polar bears.
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