The press release (pdf here) issued by the American Museum of Natural History (AMNH) a few days ago, regarding several recent papers on polar bear consumption of terrestrial foods around Churchill, Manitoba (Western Hudson Bay), left a lot to be desired in terms of relaying accurate information.
21 August 2021 UPDATE: I have amended this post to correct some comments I made originally about the definition of ‘spring breakup’ of sea ice on Hudson Bay. While the ice indeed begins to break up in spring (usually defined for the Arctic as April-June, bears rarely come ashore before July 1 (summer being July-September). Confusion comes from the standard meteorological method of the defining ‘breakup’ for Western Hudson Bay as the date when ice coverage drops below 50% of the area, rather than the newer, more relevant date when most polar bears have actually come ashore after the date ice coverage reaches 30% (Castro de la Guardia et al. 2017). Using the modern method, the earliest breakup date was in 1999 (17 June), which was still the case in 2021).
Recent research on polar bear diets by Robert Rockwell and Linda Gormezano, it says, suggests that an increase in the consumption of caribou and snow geese since 1968 is a sign that the polar bears are nutritionally stressed due to recent sea ice changes in spring (blamed on global warming) but may be adapting by changing their summer/fall diet.
As the AMNH press release puts it:
“polar bears in the warming Arctic are turning to alternate food sources.”
However, the AMNH press release fails to mention – as the papers it publicizes certainly do – that Western Hudson Bay populations of caribou and snow geese have increased exponentially since a similar polar bear diet study was done in 1968/69. In other words, there were hardly any caribou or geese around back in the late 60s — little wonder polar bears weren’t eating any.
While evidence of polar bears consuming caribou and geese in recent years is certainly an “increase” over late 1960s levels, that fact says more about the status of caribou and geese populations than it does about polar bears and global warming. Media outlets that quoted the press release verbatim, as many do these days (e.g. here, here, and here), missed this essential part of the story – and of course, so did their readers.
In addition, the AMNH press release makes this erroneous statement that was picked up by every media outlet I saw except one (either exactly as written or reworded slightly):
“Climate warming is reducing the availability of their ice habitat, especially in the spring when polar bears gain most of their annual fat reserves by consuming seal pups before coming ashore for the summer.” [my bold]
The earliest breakup dates since 1991 for Western Hudson Bay, where this research was done, have been mid-June (according to the most recent study by Cherry et al. 2013) and for the last few years breakup has occurred in July.
Overall, the press release and resulting media reports seem to be further examples of hyping global warming at the expense of the actual science involved and I have to agree with Andrew Derocher’s interpretation of the significance of terrestrial food items for polar bears. See what you think.
The principle study conducted by Rockwell and Gormezano (reported in two papers, Gormezano and Rockwell, 2013a and 2013b, abstracts below) involved identification of plant and animal parts found in polar bear fecal samples (bear poop, aka “scat”). They compared their results to a previous scat study done in 1968 and 1969 by Richard Russell (Russell 1975).
[FYI, I’ve done lots of this kind of scat work myself — not for polar bears, but for seals, sea lions and river otters — so I know what it entails]
While they also did some observational work watching bears chase geese (reported in Iles et al., 2013; see abstract below), the primary scientific contribution was the scat analysis.
Despite what the press release implies, they found that the consumption of geese and caribou was pretty minimal relative to other terrestrial foods consumed in summer and fall while on shore: marine algae and Lyme grass were the most frequently consumed items (Gormezano and Rockwell, 2013b). Polar bears ate lots of marine algae and Lyme grass back in 1968/69 as well.
This is what Gormezano and Rockwell (2013a:3517-3518, pdf here) say in their paper about caribou and geese:
Russell did not report caribou or snow goose in polar bear fecal samples collected along the coast of the Hudson Bay Lowlands. In the 1960s, fewer than a hundred caribou were estimated for the population north of the Nelson River (C. Jonkel, S. Kearney, pers. comm.) and sparse groups of <50 animals were counted further south (Abraham and Thompson 1998). Caribou numbers have been increasing steadily (30- to 50-fold) since (Williams and Heard 1986; C. Jonkel, S. Kearney, and R. Brook, pers. comm.), while the animals are also expanding their summer range toward the coast (Abraham and Thompson 1998), thus increasing increasing potential interactions with arriving bears (Fig. 3). Similarly, snow goose abundance has increased 5- to 20-fold across the region since the 1960s (Hanson et al. 1972; Kerbes et al. 2006; Alisaukas et al. 2011), with the highest increase and geographic expansion being on the Cape Churchill Peninsula (Rockwell et al. 2009). [my bold; note, “Fig. 3” is my Fig. 1. above]
Polar bears seem to have taken advantage of the substantial increase in availability of both caribou and snow geese (Table 1). During the summer months, when the two species are raising their offspring, polar bears arriving onshore now regularly overlap herds of caribou and flocks of geese as the bears travel along the coast and move inland (Iles et al. 2013; L.J. Gormezano and R.F. Rockwell, unpubl. obs.). The increased co-occurrence of polar bears and the now plentiful caribou and snow geese facilitate opportunities for both predation as well as scavenging kills made by other predators (e.g., wolves, Canis lupus, [Brook and Richardson 2002]; grizzly bears, Ursus arctos, [Rockwell et al. 2008]. [my bold]
The snow goose population increase is considered an Arctic ecological disaster in the making. In an online report (“Snow goose population problem, Part 1” the US Geological Survey (February 2013) says this:
“The LaPerouse Bay population [Western Hudson Bay] has increased at eight percent per year for at least the last 30 years. Only 2000 pairs nested at LaPerouse Bay in 1968. By 1990, the colony had expanded to more than 22,500 pairs.”
See also the recent article: “Too many geese big honking problem for North: scientists” (Bob Weber, The Globe and Mail, June 23, 2013)
In addition, in the abstract of that paper (copied in full in the reference section below), Gormezano and Rockwell also make this ridiculous statement:
Under current climate trends, spring ice breakup in Hudson Bay is advancing rapidly, leaving polar bears (Ursus maritimus) less time to hunt seals during the spring when they accumulate the majority of their annual fat reserves. [my bold]
Sea ice breakup in Hudson Bay begins in spring but is completed in summer: to characterize it as a ‘spring’ phenomenon is misleading — breakup dates occur from early to mid-summer (rarely in June), and the timing of it has varied widely in recent years. In the text of the article, the authors do not even cite a reference to support this mis-characterization of sea ice breakup as a spring event: on pg 3516 they state: “If the trend in earlier spring breakup in Hudson Bay continues, polar bears will spend more time on shore during the summer…“. [my bold]
By “earlier spring breakup” they mean advancing from mid-June to perhaps early June: not from mid-June to April. Is that deliberately misleading or just sloppy wording? Either way, the fact that the press release repeats it is especially unfortunate.
The claim that the recent consumption of caribou, snow goose and other items by polar bears could be an adaptive response to “nutritional stress” [another way of suggesting that “polar bears are starving“], caused by reduced sea ice in spring, is not supported by their data.
These authors had no information on the body condition of the polar bears that left the scats they analyzed, so no conclusions can legitimately be made about whether or not the bears whose scats contained goose and/or caribou parts (or plant remains) were starving — or in good or very good condition. The authors simply do not know if there was any “nutritional stress” involved.
Polar bears are essentially in fasting mode while they are on shore during the summer: they metabolize stored fat to meet their energy needs. That said, polar bears also routinely consume plant material, and whatever else they might come across during their sojourn, whether out of boredom, for the vitamins, to relieve hunger pangs, or because they like the taste — perhaps a bit of each. But as far as we know, polar bears always done this.
For example, in 1874, the bears of St. Matthew Island in the Bering Sea (see previous post here) were described as “lazily sleeping in grassy hollows, or digging up grass and other roots, browsing like hogs.”
However, it has not been demonstrated that consumption of any of these terrestrial foods assists in polar bear survival. I have to agree with this part of what Andrew Derocher had to say to NBC News reporter John Roach (“As Arctic ice melts, polar bears switch diets to survive, studies say“ January 24, 2014):
“Polar bears will eat anything,” he told NBC News. “The question is: Does is it do them any good? And everything we can see from what bears eat when they are on land is it has a very, very minimal energetic return relative to the cost.” [my bold]
The claim made by Rockwell in the press release that their research results indicate polar bears are “more resilient [to global warming] than previously thought” is grandstanding nonsense. Polar bears are indeed resilient to changing sea ice conditions but not because of the odd bits of terrestrial foods they eat during the summer. On top of that, the misrepresentation of the sea ice breakup phenomenon in Hudson Bay is more than disappointing — with misinformation like this, it’s no wonder the public gets confused.
Cherry, S.G., Derocher, A.E., Thiemann, G.W., Lunn, N.J. 2013. Migration phenology and seasonal fidelity of an Arctic marine predator in relation to sea ice dynamics. Journal of Animal Ecology 82: 912-921.
Iles, D.T., Peterson, S.L., Gormezano, L.J., Koons, D.N. and R.F. Rockwell, R.F. 2013. Terrestrial predation by polar bears: not just a wild goose chase. Polar Biology 36:1373-1379. DOI: 10.1007/s00300-013-1341-5.
Behavioral predictions based on optimal foraging models that assume an energy-maximizing strategy have been challenged on both theoretical and empirical grounds. Although polar bears (Ursus maritimus) are specialist predators of seal pups on the Arctic ice pack, the use of terrestrial food sources during the ice-free period has received increased attention in recent years in light of climate predictions. Across a 10-day period of observation, we documented between four and six individual polar bears successfully capture at least nine flightless lesser snow geese (Chen caerulescens caerulescens) and engage in at least eight high-speed pursuits of geese. The observed predatory behaviors of polar bears do not support predictions made by energy-optimizing foraging models and suggest that polar bears may frequently engage in energy inefficient pursuits of terrestrial prey. Further study of the nutritional needs and foraging behaviors of polar bears during the ice-free period is warranted, given that polar bears are predicted to spend more time on land as climate change advances.
Gormezano, L.J. and Rockwell, R.F. 2013a. What to eat now? Shifts in polar bear diet during the ice-free season in western Hudson Bay. Ecology and Evolution 3: 3509-3523. DOI: 10.1002/ece3.740. [open access] pdf here.
Under current climate trends, spring ice breakup in Hudson Bay is advancing rapidly, leaving polar bears (Ursus maritimus) less time to hunt seals during the spring when they accumulate the majority of their annual fat reserves. For this reason, foods that polar bears consume during the ice-free season may become increasingly important in alleviating nutritional stress from lost seal hunting opportunities. Defining how the terrestrial diet might have changed since the onset of rapid climate change is an important step in understanding how polar bears may be reacting to climate change. We characterized the current terrestrial diet of polar bears in western Hudson Bay by evaluating the contents of passively sampled scat and comparing it to a similar study conducted 40 years ago. While the two terrestrial diets broadly overlap, polar bears currently appear to be exploiting increasingly abundant resources such as caribou (Rangifer tarandus) and snow geese (Chen caerulescens caerulescens) and newly available resources such as eggs. This opportunistic shift is similar to the diet mixing strategy common among other Arctic predators and bear species. We discuss whether the observed diet shift is solely a response to a nutritional stress or is an expression of plastic foraging behavior.
Gormezano, L.J. and Rockwell, R.F. 2013b. Dietary composition and spatial patterns of polar bear foraging on land in western Hudson Bay. BMC Ecology 13:51. DOI: 10.1186/1472-6785-13-51. [in press] draft copy is open access
Flexible foraging strategies, such as prey switching, omnivory and food mixing, are key to surviving in a labile and changing environment. Polar bears (Ursus maritimus) in western Hudson Bay are versatile predators that use all of these strategies as they seasonally exploit resources across trophic levels. Climate warming is reducing availability of their ice habitat, especially in spring when polar bears gain most of their annual fat reserves by consuming seal pups before coming ashore in summer. How polar bears combine these flexible foraging strategies to obtain and utilize terrestrial food will become increasingly important in compensating for energy deficits from lost seal hunting opportunities. We evaluated patterns in the composition of foods in scat to characterize the foraging behaviors that underpin the diet mixing and omnivory observed in polar bears on land in western Hudson Bay. Specifically, we measured diet richness, proportions of plant and animal foods, patterns in co-occurrence of foods, spatial composition and an index of temporal composition.
Scats contained between 1 and 6 foods, with an average of 2.11 (SE = 0.04). Most scats (84.9%) contained at least one type of plant, but animals (35.4% of scats) and both plants and animals occurring together (34.4% of scats) were also common. Certain foods, such as Lyme grass seed heads (Leymus arenarius), berries and marine algae, were consumed in relatively higher proportions, sometimes to the exclusion of others, both where and when they occurred most abundantly. The predominance of localized vegetation in scats suggests little movement among habitat types between feeding sessions. Unlike the case for plants, no spatial patterns were found for animal remains, likely due the animals’ more vagile and ubiquitous distribution.
Our results suggest that polar bears are foraging opportunistically in a manner consistent with maximizing intake while minimizing energy expenditure associated with movement. The frequent mixing of plant-based carbohydrate and animal-based protein could suggest use of a strategy that other Ursids employ to maximize weight gain. Further, consuming high rates of certain vegetation and land-based animals that may yield immediate energetic gains could, instead, provide other benefits such as fulfilling vitamin/mineral requirements, diluting toxins and assessing new foods for potential switching.
Russell, R.H. 1975. The food habits of polar bears of James Bay and southwest Hudson Bay in summer and autumn. Arctic 28: 117-129. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/2823/0 [open access]