Tag Archives: Jim Steele

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? Continue reading

Guest post: How ‘science’ counts bears

This essay by Dr. Jim Steele, professor emeritus, San Francisco State University, is reblogged from a July 3 2013 post at WUWT post, with Dr. Steele’s permission. I am not a field biologist and have never done a mark-recapture study but Dr. Steele has. His perspective on the way polar bear biologists count bears and estimate survival in the Southern Beaufort is a perfect companion to yesterday’s post, a related post that I’ll put up later this week, and this one from December, among others. I’ve added links to the references cited in this essay where they are available, as is my custom. See the original post for Jim’s responses to comments and questions.

———————————————————————
Guest post by Jim Steele   “How ‘science’ counts bears”

The Inuit claim “it is the time of the most polar bears.” By synthesizing their community’s observations they have demonstrated a greater accuracy counting Bowhead whales and polar bears than the models of credentialed scientists. To estimate correctly, it takes a village. In contrast the “mark and recapture” study, which claimed the polar bears along South Beaufort Sea were victims of catastrophic global warming and threatened with extinction, relied on the subjective decisions of a handful of modelers.

In mark and recapture studies, the estimate of population abundance is skewed by the estimate of survival. For example, acknowledging the great uncertainty in his calculations of survival, in his earlier studies polar beat expert Steven Amstrup reported three different population estimates for bears along the South Beaufort Sea. If he assumed the adult bears had an 82% chance of surviving into the next year, the models calculated there were 1301 bears. If survivorship was 88%, the abundance climbed to 1776 bears. If he estimated survivorship at a more robust 94%, then polar bear abundance climbed to 2490.1 Thus depending on estimated survival rates, a mark-and-recapture study may conclude that the population has doubled, or that it has suddenly crashed.

Here are the simplified basics of estimating survival.

Continue reading