July 26, 2012

Wild Speculation on Climate and Polar Bears

Filed under: Adaptation, Animals, Polar

Here is another big one from PNAS.

For those who don’t know, PNAS stands for the Proceedings of the National Academy of Sciences and it has gained the unfortunate reputation for publishing scientific research articles that regularly get knocked out of the park within hours of their release. The lack of rigor stems from its rather unique “peer-review” process in which National Academy members can submit articles for publication that the authors themselves have had “peer-reviewed”—that is, they passed the article by a couple of friends of theirs for comments. It’s more like “pal review.”

It is hard to imagine many papers being rejected under this system, although it can happen. For example, a contributed article by National Academy member Dr. Richard Lindzen that argued that the climate sensitivity to anthropogenic greenhouse gas emissions isn’t as large as commonly thought was rejected by the PNAS editor in change, overruling the recommendations of the reviewers chosen by Lindzen. But such occurrences are quite rare.

Instead, papers with rather speculative conclusions can be regularly found in the pages of PNAS as we have documented on several occasions (see here and here for example).

A new paper has just appeared which should be added to this list in the form of a contribution by National Academy foreign associate and molecular biologist Dr. Luis Herrera-Estrella on the subject of polar bears, evolution, and climate influences.

While the paper employs cutting edge genetic analysis to try to better establish the evolutionary tree of the polar bear species, when it comes to tying climate changes to the branches of that tree the analysis reverts to visual association (and a selective one at that).

Genetically speaking, the research team compared the DNA make-up of today’s polar bears with those of brown bears and black bears, with that of a single polar bear from ~120,000 years ago. The differences among these genetic codes are clues to the when these animals diverged into separate color morphs*. Additionally, the authors were able to extract some information as to the size of the population of each “species” over time.

The main gist of the findings is that polar bears became a separate “species” some 4- 5 million years ago, although some interbreeding with brown bears occurred on and off throughout the period (see our footnote on the nature of separate species).

This result lends further evidence (which we have highlighted previously, here for example) that polar bears as a species have survived many interglacial warm periods and thus are less “fragile” to climate warming than the more “concerned” among us would have us believe.

But, curiously, the authors of the new PNAS paper arrive at a somewhat different conclusion based upon what we consider to be a less than thorough analysis of the climate data. Here is how they describe their take on the situation:

[T]he marked increase in [effective polar bear population] between 800 and 600 kya [Figure 2(top)], possibly facilitated by Middle Pleistocene [the era of ice ages] cooling, is approximately bounded by Marine Isotope Stage 11 (420–360 kya), the longest and possibly warmest interglacial interval of the past 500,000 y and a potential analogue for the current and future climate. Although [polar bear effective population] remains low thereafter, a small recovery roughly coincident with the [brown] bear–[polar bear] maternal split could be associated with post-Eemian cooling, although this could also indicate an increase in population structure. The very recent, slight increase in [effective population] during the Holocene [the current interglacial] might reflect cooling during the Last Glacial Maximum, although genomic signatures of such recent events are known to have less power. Overall, this analysis strongly suggests that although [polar bear effective population] might have been considerably larger in the past, it appears to have experienced a prolonged and drastic decline for the past 500,000 y, being significantly smaller than brown bear [effective population], and perhaps explaining the observed lower genetic diversity in [polar bears] compared with brown bears. Taken together, our results strongly indicate that key climatic events have played a significant formative role in bear effective population size. [emphasis added –eds.]

And further:

If modern [polar bear] populations result from Holocene range expansions from a few
small, contracted populations in Middle-Late Pleistocene refugia, this may explain the observed low genetic diversity in [polar bears] today, and possibly leave modern [polar bear] populations even more vulnerable to future climatic and other environmental disturbances. [emphasis added –eds.]

Basically, the authors contend that even though polar bears have been a separate “species “ for some 4 to 5 million years and have survived repeated interglacial warm periods, it is vulnerable to warming. It survived during a a period of 2 million years (from 3 million to 5 million years ago) with an average temperature about was as warm as today. Somehow this evolutionary history has made polar bears of today “even more vulnerable to future climatic…disturbances.”

Right. PNAS yet again has fallen victim to it’s “pal review” process.

That is certainly some creative interpretation of the data at hand!

Here is why.

Consider the temporal population data for the bear species derived by the authors compared with the relatively-well established general climate history of the earth. Figure 1 shows the full 5 million year record and Figure 2 shows just the last million years.


Figure 1 (top): Estimates of the effective population size over the past 5 million years of the different bear species studied; BLK—black bears, GRZ—brown bears, ABC1—a different kind of brown bear, PB7—polar bears (source: Miller et al., 2012). Figure 1 (bottom): General climate history of the earth for the past 5 million years as derived from a collection of ocean sediments (source: Lisiecki and Raymo, 2005).

Figure 2 (top): Estimates of the effective population size over the past 1 million years of the different bear species studied (as in Figure 1). The larger gray-shaded area on the right refers to the Early Pleistocene, and the other gray areas (from right to left) refer to the interglacial Marine Isotope Stages (MIS) 15, 13, and 11, and the Eemian, respectively. The arrows point to major events in bear population history discussed by the authors . H, Holocene epoch. (source: Miller et al., 2012). Figure 2 (bottom): General climate history of the earth for the past 1 million years as derived from a collection of ocean core sediments. The numbers are the various MIS (source: Lisiecki and Raymo, 2005).

While there is some bit of character to the population data, largely it is varies rather slowly considering the timescales involved. The character of the climate record is vastly different. The climate record is dominated by the repeated pulse of interglacial warm periods within ice age conditions. About a million years ago, the variance increased and the time between warm periods increased to about 100,000 years (from ~ 41,000 years characteristic of the previous 4 million+ years). During the past million years, there have been 11ish warm events during which time the temperature approached the average temperature of the period from about 3 to 5 million years ago. However, during the majority of the past million years, the temperatures were much colder than those experienced in the period spanning 3 to 5 million years ago.

Now, perhaps you could make a case that as the temperature variance increased about a million years ago and most notably the coldest periods got colder that this had a general negative impact on brown bear (ABC1 and GRZ in Figure 1) populations.

But it is hard to know what to make of the polar bear population (PB7 in Figure 1) trace. It basically bears no resemblance at all to the climate signal—a strong indication that the environmental pressures on the polar bear populations arose from a non-climate origin.

To us, the authors conclusion that climate variation played a strong role in the evolutionary history of the polar bear over the past 5 (or even 1) million years derives from a reasoning (described in the block quote above) that just doesn’t jive with the climatological record. Assuming that the paleoclimate record and the paleo-polar bear population record are fair representations of what actually transpired over the past million years (and there is some questions about the reliability of the latter), to us it seems that the polar bear populations fluctuated over time largely independently of the climate variations.

If, as the authors assert, that interglacial warm periods were warm enough to reduce the polar bear population down to only a few bears in climate refugia thus setting the stage for enhanced vulnerability to climate change as a result of low genetic diversity, then any of the past 3-4 interglacial warm periods could have pushed them to extinction. Clearly they did not. And, further, it seems rather than extinction, what a warmer climate leads to is an increase in interbreeding with brown bears—something which apparently took place with some regularity over the bears’ history, even more so in warmer times. So perhaps in extended warm periods, the polar bear becomes a bit browner—and takes on characteristics which are better suited for a warmer climate, only to re-emerge as the great white bear of the north when glacial conditions return.

Certainly this is just speculation on our part, and perhaps is incompatible with the genetic data. But the genetic methodologies applied in the paper are very young and the sampling of bears analyzed is pretty sparse. Consequently, these first results are liable to be much less than robust as are any conclusions derived from them—especially those related to the specific details of the climate.

But one thing that is undeniable is that the polar bears have survived a score or more climate swings over the past 5 million years, including extended periods as warm as today. If climate were the only stressor on polar bear populations, these new findings should bode well. But as it is not, polar bears will almost certainly face a challenging future. But in discussing and planning their future, focusing on climate change would be off the mark—a story that is told through the findings of the PNAS paper, but not so much by the authors.

*Note that the brown bear and the polar bear are not separate species, at least in the classic sense. Mate the two and you get viable cubs that are reproductively competent. That’s the definition of what comprises a species.

References:

Lisiecki, L., and M. Raymo, 2005. A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records. Paleoceanography, 20, PA1003, do:10.1029/2004PA001071.

Miller, w., et al., 2012. Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change. Proceedings of the National Academy of Sciences, www.pnas.org/cgi/doi/10.1073/pnas.1210506109




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