Southern Beaufort polar bear ‘decline’ & reduced cub survival touted in 2008 was invalid, PBSG now admits

It is now clear that the phenomenon of bears moving across Southern Beaufort Seapbsg logo subpopulation boundaries compromised the US decision to list polar bears as ‘threatened’ and the IUCN Polar Bear Specialist Group (PBSG) knows that was the case.

As I pointed out last week, the PBSG has admitted in their 2013 status table update (pdf here) that bears move around so much between the Chukchi Sea (CS), the Southern Beaufort (SB), and the Northern Beaufort (NB) subpopulations that major changes in the boundaries of the SB subpopulation are necessary (see Fig. 1 below).

Figure 1. From the paper by Amstrup and colleagues (2005) describing the effect that movement of bears across subpopulation boundaries has on setting harvest quotas – and population estimates. Southern Beaufort boundary is solid red, Chukchi Sea is dashed yellow and Northern Beaufort is dotted light blue. “Point Barrow” is Barrow, AK (well inside the SB boundary). Click to enlarge.

Figure 1. From the paper by Amstrup and colleagues (2005) describing the effect that movement of bears across subpopulation boundaries has on setting harvest quotas and population estimates. Southern Beaufort (SB) boundary is solid red, Chukchi Sea (CS) is dashed yellow and Northern Beaufort (NB) is dotted light blue. “Point Barrow” is Barrow, AK (well inside the SB boundary). Click to enlarge.

Well, that’s not really news — changes to the SB boundaries were promised by the PBSG back in 2009 (Obbard et al. 2010), based on research by Steven Amstrup and colleagues published in 2001 and 2005. But now, in an astonishing admission, the PBSG have acknowledged that the last population survey for the SB (Regehr, Amstrup and Stirling, 2006), which appeared to register a decline in population size and reduced cub survival over time, did not take known movements of bears into account as it should have done.

In other words, that 2006 study almost certainly did not indicate bears dying due to reduced summer sea ice in the SB, as biologists said at the time — and which they presented as evidence that polar bears should be listed by the ESA as ‘threatened’ — but reflected capture of bears that were never part of the SB subpopulation and so moved out of the region.

As the PBSG said about the 2006 estimate:

“…it is important to note that there is the potential for un-modeled spatial heterogeneity in mark-recapture sampling that could bias survival and abundance estimates.” [my emphasis]

Spatial heterogeneity” means that the sampled bears could have come from more than one population, a possibility which violates a critical requirement of the statistics used to generate the population and survival estimates. “Un-modeled” means that the ‘movement of bears’ problem was not factored into the mathematical models that generated the 2006 population size and survival estimates as it should have been.

Ecologist Jim Steele pointed some of this out in his book and his guest post last year, so it’s not news that this was done.

What’s shocking is that the PBSG have now admitted that the ‘movement of bears’ issue essentially invalidates the 2006 population estimate and the much-touted ‘reduced survival of cubs.’ The reduced survival of cubs data from that SB study was a critical component of the argument that US bears were already being negatively impacted by global warming and thus, should be listed as ‘threatened’ under the ESA (US Fish & Wildlife Service 2008).

Since the population decline and reduced survival is now acknowledged to be unfounded — and perhaps deliberately so — I ask you this: will a new SB survey — soon to be released by the same lead author (Eric Regehr) — undo the broken trust in US and PBSG polar bear biologists?

From the 2013 status update posted February 14, 2014 on the IUCN PBSG website, here is the above PBSG quote in context:

Subsequent analyses of the 2001-2006 data using multistate and demographic models [Regehr, Amstrup and Stirling, 2006] indicated that the survival and breeding of polar bears during this period were affected by sea ice conditions, and that population growth rate was strongly negative in years with long ice-free seasons, such as 2005 when Arctic sea ice extent reached a record low (Hunter et al. 2007, Regehr et al. 2010). However, it is important to note that there is the potential for un-modeled spatial heterogeneity in mark-recapture sampling that could bias survival and abundance estimates. A thorough re-assessment of survival and abundance is underway and a final result is anticipated in late 2014.” [my emphasis]

The middle sentence in the above quote (in bold) was not included in the 2009 PBSG status report (Obbard et al. 2009) – this is a new admission.

The justification for this statement was explained as follows (see Fig. 1 above for locations mentioned in bold):

Analyses of more recent satellite relocations using probabilistic models indicate that, rather than exhibiting distinct boundaries, there are areas of overlap between the SB and adjacent subpopulations (Amstrup et al. 2004; Amstrup et al. 2005). At Barrow, Alaska, in the west, it is estimated that 50% of polar bears are from the SB subpopulation and 50% are from the Chukchi Sea (CS) subpopulation. At Tuktoyaktuk, Northwest Territories, Canada, in the east, there is a 50% probability of polar bears being either from the SB or the northern Beaufort Sea (NB) subpopulation. Based on this analysis, most polar bears in the vicinity of the current eastern boundary near Pearce Point, Northwest Territories, are probably members of the NB subpopulation. To address this issue, user groups, scientists and resource managers are discussing a western shift of the SB-NB boundary. One proposal has been to move it west to 133° W longitude (due north of Tuktoyaktuk, Northwest Territories, Canada) but a line further east is also under consideration. A similar boundary shift, or a change in the way harvest is allocated among subpopulations, may be required on the western side of the SB subpopulation where it borders the CS subpopulation (Amstrup et al. 2005).” [my bold]

In the 2009 PBSG report (Obbard et al. 2009: 52) included the above text but was followed by this assurance about the boundary change issue:

“A decision on the potential boundary shift is expected in 2010, at which time the abundance and status of the SB and NB subpopulations will be re-evaluated. [my bold]

The 2013 PBSG status report makes no mention of an abundance and status update for NB — in fact, in the NB section, it suggests the SB/NB boundary shift is needed because of recent sea ice changes caused by global warming, despite the fact that the phenomenon of bear movement across this boundary was first identified in the 1980s and the Amstrup et al. papers they cite placed no blame on sea ice changes:

“Analyses using data from satellite tracking of female polar bears and spatial modeling techniques suggest that the boundary between NB and SB may need to be moved somewhat to the west of its current eastern limit at Pearce Point, in response to changing patterns of breakup and freeze-up resulting from climate warming (Amstrup et al. 2004, Amstrup et al. 2005).” [my bold]

While the Amstrup et al. (2005) paper said that 50% of bears captured at Barrow were Chukchi bears, this wasn’t news — Amstrup had known this fact for years, well before the 2001-2006 research project began.1

Amstrup et al. (2005:253) had this to say:

Just as the Agreement [for sharing harvest quotas between the US and Canada] was based upon the assumption that hunters share one population throughout the SBS [Southern Beaufort Sea], early population estimates were based upon the same assumption.

Therefore, the inability to appreciate overlap among members of adjacent populations has restricted the accuracy of past population estimates in the Beaufort Sea and elsewhere (Amstrup et al. 2001b). [my bold]

They continued:

“In the past, we did not know whether a captured bear represented the population we were trying to estimate or not.

The current annual harvest quota of 82 polar bears for the SBS is based on the assumption that all bears taken between Wainwright, Alaska and Pearce Point, Northwest Territories, belong to the SBS population. Clearly this paradigm is not supported by the most recent quantitative data. Hence, harvest management for polar bears in the Chukchi and Beaufort seas can no longer be based on the previously established population boundaries. [my bold]

Not only did Amstrup et al. (2004, 2005) identify the overlap of SB with CS and NB population, they came up with a statistical solution for allocating quotas.

Now we get to the nitty-gritty. Clearly, this issue was identified in 2004-2005 (on both SB boundaries) and a potential statistical solution proposed. However, it appears that no adjustment was applied to the population and survival estimates completed in 2006 to support the petition to have polar bears listed by the ESA as ‘threatened.’

The 2001-2006 data referred to by the PBSG (Regehr et al. 2006:3) came from bears captured from locations from one end of the SB region to the other: from beyond Barrow in the west and almost as far east as Pearce Point (Fig. 1). That broad capture coverage was considered a major strength of the study – but now, it’s apparent this was a major weakness. Amstrup and colleagues (2005:257) found that only bears captured between 1350W and 1530W (the central half of the SB region) could be assumed with high confidence to be SB bears (Fig. 2).

Figure 2. From the paper by Amstrup and colleagues (2005: Fig. 7) showing the restricted region in the central Southern Beaufort (135 to 153 degrees W) where any bear captured can be assumed with confidence to be a Southern Beaufort bear.

Figure 2. From the paper by Amstrup and colleagues (2005: Fig. 7) showing the restricted region in the central Southern Beaufort (135 to 153 degrees W), well east of Barrow and far to the east of Pearce Point and Tukoyaktuk, where any bear captured can be assumed with confidence to be a Southern Beaufort bear.

In their discussion, Regher et al. (2006:11) admitted:

the declines we observed in model-averaged survival rates may reflect an increase in the number of “emigrants” toward the end of the study, and not an actual decrease in biological survival. [my bold]

But that was as far as it went — the ‘movement of bears’ problem was not taken into account when the population and survival estimates were made.

Ecologist Jim Steele discussed this issue in his book (Steele 2013: 222-227). In his guest post here last year, How science counts bears, he said this about the Regehr et al. (2006) population estimate:

They chose to dismiss the high percentage of bears migrating out of the study area and subjectively chose to argue fewer captured bears meant more dead bears.
.. .

The dramatic drop in survival meant 400 bears suddenly died but there were no carcasses.” [my bold]

So, in 2006, Regehr and colleagues apparently got away with ignoring the ‘movement of bears’ issue and the effect this would have had on their population and survival estimates. Now, six years after the ESA declared the polar bear ‘threatened’ based substantially on the Regehr-led study, the PBSG has acknowledged that the estimate was invalid but suggests that a new SB study, led again by Eric Regehr, will resolve the issue.

The question is, knowing these people misrepresented data, arguably to support a desired outcome — and that this was was not caught by reviewers of their papers — is there any reason to trust the upcoming Regehr et al. report? Why should we not assume something similar will happen again?

Footnote 1. Amstrup and colleagues’ 2001 paper, submitted in 2000 (Amstrup et al. 2001:230), stated that the probability of capturing Chukchi bears around Barrow in the western SB was 80-90%. Accounting for this bias in their SB population models gave them an estimate of more than 2500 bears, up significantly from the late 1970s estimate of 1,800. They concluded (Amstrup et al. 2001:233) that this amount of increase was unreasonable and suggested a “conservative” estimate of <2000 bears (later stated as “1,800”) was more useful for management purposes.

Apparently, this is how conservation biologists do science. We saw the same practice, of spending enormous amounts of money on a mark-recapture study to generate data for a ‘scientific’ model and then ignoring the results, in the Northern (aka Eastern) Beaufort population study (previous post here).. In that case, because the researchers realized their model had not accounted for bears moving out of the region (generating an “unreasonable” decline over one year), they bumped the estimate upwards. Amstrup and Stirling were involved in almost all of the studies, while Regehr was common to the most recent ones only.

Related posts:
Did polar bear numbers in E Beaufort fluctuate each decade due to thick ice years? July 2 2013

Did the PBSG game the polar bear listing process? December 26, 2012

Ten out of ten polar bears being tracked this summer in the Beaufort Sea are on the ice August 29, 2013

Tracking polar bears in the Beaufort Sea: November map December 5, 2013

Guest post: How ‘science’ counts bears July 3, 2013

References
Amstrup, S. C., McDonald, T. L. and Stirling, I. 2001. Polar bears in the Beaufort Sea: A 30-year mark-recapture case history. Journal of Agricultural, Biological, and Environmental Statistics 6:221-234. http://link.springer.com/article/10.1198/108571101750524562

Amstrup, S. C., York, G., McDonald, T. L., Nielson, R. and Simac, K. 2004. Detecting denning polar bears with forward-looking infrared (FLIR) imagery. Bioscience 54:337-344.

Amstrup, S.C., Durner, G. M., Stirling, I. and McDonald, T. L. 2005. Allocating harvests among polar bear stocks in the Beaufort Sea. Arctic 58:247-259. http://arctic.journalhosting.ucalgary.ca/arctic/index.php/arctic/article/view/426
Pdf here.

Obbard, M.E., Theimann, G.W., Peacock, E. and DeBryn, T.D. (eds.) 2010. Polar Bears: Proceedings of the 15th meeting of the Polar Bear Specialists Group IUCN/SSC, 29 June-3 July, 2009, Copenhagen, Denmark. Gland, Switzerland and Cambridge UK, IUCN.

Regehr, E. V., S. C. Amstrup, and I. Stirling. 2006. Polar bear population status in the southern Beaufort Sea. U.S. Geological Survey Administrative Report, U.S. Department of the Interior. Reston, Virginia, USA. 20 pp. pdf here.

Steele, James. 2013. Landscapes and Cycles: An Environmentalist’s Journey into Skepticism.James Steele. http://landscapesandcycles.net/

US Fish and Wildlife Service. 2008. Determination of threatened status for the polar bear (Ursus maritimus) throughout its range. Federal Register 73(95):28212-28303. Pdf here.

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