Amstrup & colleages can’t refute my critique of their 2007 polar bear survival model, Part 2

Polar bear specialists Andrew Derocher and Steven Amstrup recently spent inordinate energy trying to refute the opinion piece I’d written for the Financial Post in celebration of International Polar Bear Day last month, ignoring my fully referenced State of the Polar Bear Report for 2017 that was released the same day (Crockford 2018) and the scientific manuscript I’d posted last year at PeerJ Preprints (Crockford 2017).

polar_bear_USFWS_fat Chukchi Sea bear

Their responses use misdirection and strawman arguments to make points. Such an approach would not work with the scientific community in a public review of my paper at PeerJ, but it’s perfect spin for the self-proclaimed “fact-checking” organization called Climate Feedback. The result is a wildly ineffective rebuttal of my scientific conclusion that Amstrup’s 2007 polar bear survival model has failed miserably.

This is Part 2 of my expose, see Part 1 here.

Quote #2 from my op-ed to which Amstrup and Derocher responded:

“For example, Canadian polar bear biologist Ian Stirling learned in the 1970s that spring sea ice in the southern Beaufort Sea periodically gets so thick that seals depart, depriving local polar bears of their prey and causing their numbers to plummet. But that fact, documented in more than a dozen scientific papers, is not discussed today as part of polar bear ecology.”

Amstrup responded, in part:

“Both Ian Stirling and I have published on the inter-annual and even multi-annual variation in sea ice extent, etc.  …Ian specifically mentioned in some early publications that there seemed a nearly decadal oscillation in ice thickness, etc. …Regardless, as the world has warmed and ice continued to thin, evidence of any such cycle in the Beaufort has disappeared. We did not see a crush of heavier ice in the middle of the first or second decades of the 2000s. …Why would researchers spend much time now discussing a pattern in the sea ice that no longer occurs?”

Derocher responded:

“Heavy sea ice conditions are largely a past issue for ringed seals. …It is loss of sea ice habitat as a whole that is negatively affecting ringed seals and thus polar bears.” [cited papers: Bromaghin et al (2015) and Hunter et al (2010) ]

My response to both comments on Quote 2:

Both Amstrup and Derocher claim that thick ice in spring was a thing of the past by the turn of the century, a phenomenon that no longer occurs in the Southern Beaufort now that sea ice in summer has declined. This is patently untrue.

Even though Stirling and colleagues argued in their 2008 paper that the thick spring ice conditions in 2004, 2005 and 2006 (but not those in 1974-1976) were caused by storms initiated or intensified by greater amounts of open water in previous summers, they did not deny that thick-ice conditions were present from 2004-2006 that were as severe as those documented for 1974-1975.

This is what Ian Stirling (Stirling et al. 2008:15-16) had to say about the 2004-2006 event in the eastern half of the Southern Beaufort (corroborated by seal biologists (Harwood et al. 2012, 2015):

“During our study, sea ice conditions in the southeastern Beaufort Sea showed some major differences from past years (Stirling et al., 1993 and unpubl. data). From 2003 through 2006, large areas of the annual landfast ice from northeast of Atkinson Point to the Alaska border (Fig. 1) were compressed into high pressure ridges interspersed with extensive areas of rafted floes and rubble (especially in 2005; Fig. 2). In some places, these areas extended offshore from the mainland coast for tens of kilometres. Such heavy ice reduces the availability of low consolidated ridges and refrozen leads with accompanying snowdrifts typically used by ringed seals for birth and haul-out lairs (Smith and Stirling, 1975, 1978). Although we were unable to make a quantified comparison, our subjective impression is that in 12 previous spring field seasons surveying the same area for polar bears (1971 – 79, 1985 – 87) only once, in 1974, did we observe similarly extensive areas of rubble, pressure ridges, and rafted floes. [my bold]

Since there is evidence for such thick ice conditions about every decade from the 1960s to the mid-2000s, this makes five decades worth of data on this phenomenon: certainly enough to deserve comment by biologists and to warrant inclusion in future survival models, especially given previous catastrophic effects on polar bear health and survival.

Derocher’s cited papers (Bromaghin et al. 2015; Hunter et al. 2010) are not evidence that less summer ice was the cause of declining polar bear populations in the first decade of the 2000s in the Southern Beaufort: there is correlation with less ice, to be sure. But that correlation is meaningless given that thick ice conditions were known to have been present in 2004-2006, conditions described as at least as severe as mid-1970s events, about which much is known (e.g. Burns et al. 1975; DeMaster 1980; Harwood et al. 2000; Ramseier et al. 1975; Smith 1987; Smith and Stirling 1978; Stirling 2002; Stirling et al. 1975a, 1975b, 1981, 1982, 2008; Stirling and Lunn 1997).

Quote #3 from my op-ed to which Amstrup and Derocher responded:

“…many scientists were surprised when other researchers subsequently found that ringed and bearded seals (the primary prey of polar bears) north of the Bering Strait especially thrived with a longer open-water season, which is particularly conducive to fishing”

Amstrup responded:

“The Chukchi sea is essentially all continental shelf and is probably the most productive of the Arctic Seas. This is in contrast to the Beaufort Sea which, beyond the very narrow continental shelf, is very unproductive. Recent research has shown that this tremendous productivity and the fact that, although ice has significantly retreated, bears there still have fewer ice free days over the shelf than in the Beaufort, can explain why Chukchi Sea polar bears have not yet declined like those in the Beaufort.”

Derocher responded:

Both ringed seals and bearded seals are sea ice obligate species: there are significant conservation concerns about both species across the Arctic. The basis of the statement that the seals are thriving is unfounded in the peer-reviewed literature. Both species are listed under the US Endangered Species Act.

The polar bears living north of the Bering Strait have not shown the same loss in body condition, survival, and reproduction noted in the neighboring Beaufort Sea because the ecosystems are vastly different in the distribution of continental shelf habitat: huge area in the Chukchi Sea, a narrow band in the Beaufort. Polar bear populations respond to local changes, and with 19 polar bear populations, there will be 19 different scenarios playing out over time. Loss of sea ice in the Chukchi Sea in winter 2017/18 may change the situation there.”

My response to both comments on Quote 3:

This quote is about ringed and bearded seals thriving in the Chukchi Sea due to a longer ice-free season that was contrary to predicted effects (Crawford and Quakenbush 2013; Crawford et al. 2015). Amstrup presents a strawman argument when he discusses Beaufort Sea seals, as does Derocher in describing Beaufort Sea polar bears. Derocher’s comment about ESA ‘threatened’ status for ringed and bearded seals is also a strawman argument. While what he says is true (USFWS 2012a, 2012b), it is also true that no other Arctic nation or the ICUN is concerned about any Arctic seal species, including ringed and bearded seals (Kovacs 2015, 2016a, 2016b; Lowry 2016): they are all listed as ‘least concern’ on the IUCN Red List.

Quote #4 from my op-ed to which Amstrup and Derocher  responded:

“…while it’s true that studies in some regions show polar bears are lighter in weight than they were in the 1980s, there is no evidence that more individuals are starving to death or becoming too thin to reproduce because of less summer ice.”

Amstrup responded:

“We know that polar bears depend on the ice surface to catch their prey. We know that increasing numbers of ice free days have resulted in poorer body condition in some areas (e.g. Southern Beaufort, Western and Southern Hudson Bay), we know that poorer cub survival has followed both declining ice and poorer body condition, and all the evidence suggests these things are linked. Perhaps this is not “proof” that less available summer ice is the cause (correlation does not necessarily imply causation), but I am not aware of evidence for any other explanation. And I don’t think the female polar bears are intentionally having cubs but not feeding them.” [my bold]

Derocher responded:

“There is evidence. Bromaghin et al 20151 and Hunter et al 20102 examine this issue.

Bromaghin et al state, “Low survival from 2004 through 2006 led to a 25–50% decline in abundance. We hypothesize that low survival during this period resulted from (1) unfavorable ice conditions that limited access to prey during multiple seasons; and possibly, (2) low prey abundance. For reasons that are not clear, survival of adults and cubs began to improve in 2007 and abundance was comparatively stable from 2008 to 2010, with ~900 bears in 2010 (90% CI 606–1212). However, survival of subadult bears declined throughout the entire period. Reduced spatial and temporal availability of sea ice is expected to increasingly force population dynamics of polar bears as the climate continues to warm. However, in the short term, our findings suggest that factors other than sea ice can influence survival.”

Hunter et al stated, “Deterministic models projected population growth in years with more extensive ice coverage (2001-2003) and population decline in years with less ice coverage (2004-2005). … The resulting stochastic population projections showed drastic declines in the polar bear population by the end of the 21st century. These projections were instrumental in the decision to list the polar bear as a threatened species under the U. S. Endangered Species Act.”  References cited: (1. Bromaghin et al (2015); 2. Hunter et al (2010).”

My response to both comments on Quote 4:

Neither one of these responses provide evidence that more bears are starving to death or have become too thin to reproduce due to reduced summer sea ice, which was the point of my statement.

Neither of the references cited by Derocher provide such evidence: the effects cited from the Southern Beaufort, as explained in my paper (Crockford 2017) and the State of the Polar Bear Report 2017 (Crockford 2018), and reiterated in my response to Quote 2 comments above, are explained not by less summer sea ice but by thick ice in the springs of 2004-2006 (Stirling et al. 2008). Continued poor survival of subadults after 2007 may have been correlated with less summer ice but on its own is not evident of causation.

Amstrup raises the issue of poor cub survival and reduced weight of females in Western and Southern Hudson Bay. But he should know that these effects can be caused by conditions over the winter and spring that affect prey condition or availability (Crockford 2015; Ferguson et al. 2005; Ramsay and Stirling 1988; Stirling 2005), even more so than a longer ice-free season. Much has been written about them.

Previous work by Derocher and Stirling make clear that weights of bears and cub surival declined markedly in the late 1980s and early 1990s in Western Hudson Bay (Derocher and Stirling 1992; 1995a; 1995b; 1996; Stirling 2005).

For example, Derocher and Stirling, writing in the early 1990s (1992:1155), state:

“Over the last 10 years, the productivity of female polar bears in western Hudson Bay has decreased. Coinciding with the trend in reproductive output was a decline in the mean weight of females with cubs. A decline in weight was found in most other segments of the population (unpublished data). The two-fold increase in the number of cubs lost from a litter between spring and autumn during the 1980’s represents a substantial increase in cub mortality. We believe that the decline in weight of adult females was responsible for the decrease in cub survival. From the early to late 1980’s, there was a three-fold increase in the proportion of females that lost their whole litter in the spring to autumn period. These changes resulted in reduced reproductive output and changes in the reproductive status of females in autumn.” [my bold]

Similarly, Derocher and Stirling (1995b:1657), state:

“Reproduction rates declined in the late 1980s from higher levels in 1966- 1984. Litter production decreased from 0.48 to 0.34 litters per female each year over the study. Recruitment to autumn declined from 0.75 to 0.52 cubs per female each year, in part because of cub mortality between spring and autumn, which increased from 25.0% in 1980- 1984 to 50.9% in 1987- 1992. Spring and autumn litter sizes of females with cubs did not vary annually, but litter sizes of females with yearlings varied between years. The proportion of yearlings independent of their mother in autumn dropped from 81 % prior to 1980 to 34 % in 1980- 1992. After 1986, offspring remained with their mothers longer, resulting in the birth interval increasing from 2.1 to 2.9 years. Body mass of most age-classes of females and males declined in the 1980s. Unlike earlier studies that found reproduction rates to be higher in western Hudson Bay than in higher latitude populations, reproduction rates in 1986- 1992 in western Hudson Bay were similar to those of other populations. Insufficient information was available to determine the cause of declines in reproduction and body mass.” [my bold]

In other words, weight declines and poor cub survival are not exclusive to recent years in Western Hudson Bay, as implied, which  means these symptoms can have other causes besides a longer-than-usual ice-free season.

Quote #5 from my op-ed to which Amstrup and Derocher  responded:

“The failure of the 2007 polar bear survival model is a simple fact that explodes the myth that polar bears are on their way to extinction.”

Amstrup responded:

“Multiple papers published subsequent to my work in 2007 have corroborated the outcomes we projected. However, the accuracy or failure of my work to inform the Secretary of Interior* cannot be evaluated until mid century. And as the figure above shows, we are not there yet.  Reference cited: Amstrup et al (2010).”

[Derocher did not respond to this last quote]

My response to Amstrup’s comments on Quote 5:

Amstrup claims that many papers published subsequent to his work in 2007 have corroborated the outcomes he and his colleagues projected yet he does not bother to cite  a single one.

Sea ice hit mid-century-like levels before the ink was dry on their 2007 reports (Crockford 2017, 2018) and stayed at those levels. Yet polar bear numbers not only failed to decline as predicted, they have increased slightly.

Amstrup again tries to use recent sea ice and/or polar bear survival model predictions to suggest my critique of his 2007 model (based on 2006 sea ice predictions) is incorrect or premature. However, none of these papers are relevant.

Note that the model is often considered Amstrup’s work (by himself and others) because his opinion alone was used to predict how polar bears would respond to various levels of sea ice decline (Amstrup et al. 2007). As a consequence, he takes any criticism of the model as personal criticism and responds defensively (see emails in this post).

It turns out some of the assumptions Amstrup made for his 2007 model were wrong (Crockford 2017, 2018). He was wrong to ignore the Southern Beaufort thick spring ice phenomena in his model and wrong to assume that ringed and bearded seals would suffer gravely with less summer ice.

My overall response to these comments on my op-ed:

This “analysis” by Derocher and Amstrup was a weak effort at rebutting the major points of my op-ed, which was a summary of my State of the Polar Bear Report (Crockford 2018). Most of the points (Quotes 1, 2, 3, and 5) were also essential components of my 2017 critique of Amstrup and colleague’s 2007 survival model used to support listing polar bears as ‘threatened’ with extinction in the US (Crockford 2017).

There was little except bluster and misdirection in the comments made by Derocher and Amstrup. Insisting – repeatedly – that models developed after Amstrup and colleagues 2007 USGS reports were published provide evidence that my critique is premature might fool the general public but it won’t fool fellow scientists.

It would have been more appropriate for Amstrup and Derocher to have properly reviewed my 2017 PeerJ Preprint paper. However, now we know why they passed: they didn’t have scientifically valid arguments to make.

The premise of my 2017 paper, and the focus of my State of the Polar Bear Report, is that the model prediction made by Amstrup and colleagues in 2007 failed, based on observations of sea ice and polar bear population sizes estimated since then. Nothing Amstrup and Derocher have said undermines that conclusion. Polar bears are thriving because the assumptions Amstrup made about how the bears would respond to much reduced summer sea ice conditions were wrong.


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