I wrote about this issue in January (January – and journalists are still pushing it).
This month, the New York Times (September 22, 2014 James Gorman, “For Polar Bears, a Climate Change Twist”) is pushing it big-time (and so it’s been picked up elsewhere, like by the Anchorage Daily News).
Myths and misinformation about this phenomenon dispelled below.
Polar bears eat more snow geese and snow goose eggs (and other sea birds) now than they did in 1968 because there are more caribou and geese, and because there are also more polar bears.
Polar bears have always eaten terrestrial foods – but exactly how much they ate in the past was not studied in detail, so there really isn’t any reliable data with which to compare these new studies (Russell 1975; Rockwell and Gormezano 2013a and 2013b, abstracts below).
Is “climate change” a factor in the recent phenomenon of goose-eating polar bears? Saying so may get these Hudson Bay researchers more grant funds but that does not mean the study results support such a conclusion.
Does eating geese and eggs make any difference to the long term survival of polar bears or their cubs? None of the research referred to even attempted to do so. No one has demonstrated that consumption of any of these terrestrial foods assists in polar bear survival.
These researchers don’t know if the bears were ‘nutritionally stressed’ when the bears ate terrestrial foods or very fat, so they cannot claim the foods were relieving nutritional stress due to lack of fat reserves.
From the NY Times story:
“One goose or one nest may not seem like much. But polar bears are gluttons. Dr, Rockwell described one case in which a bear ate about 1,200 eggs — of eider ducks, in this case — in four days. He said Dr. Gormezano had calculated that a clutch of four eggs would amount to 825 calories, the equivalent of one and a half Big Macs. Three hundred four-egg clutches would be 247,500 calories, or about 10 percent of a bear’s yearly nutritional needs.”
Perhaps the bear that ate all those goose eggs at one sitting was a fat bear that just got even fatter — these researchers could not say one way or another.
More importantly, I’ve criticized polar bear researchers for presenting such anecdotal information as if it was scientific evidence, and I’ll say it again here: one example, at one point in time, means nothing. Anecdotes are useless for scientific purposes.
Rockwell is making a mountain out of a minor research molehill and journalists are lapping it up indiscriminately. Polar bears are resilient to changing sea ice conditions but not because of the odd bits of terrestrial foods they eat during the summer but because they put on fat during the spring/early summer feeding period.
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]