With a few big guns on one side and some fringe folk on the other, a dichotomy of opinion has developed amongst polar bear researchers regarding whether or not consumption of land-based foods is making a difference to polar bear survival (or is likely to in the future).
It’s been going on for a while now. Last year, two US Geological Survey members of the IUCN Polar Bear Specialist Group (PBSG) apparently got fed up with the scatter-shot of papers promoting terrestrial foods as a possible salvation for polar bears (e.g. Iles et al. 2013; Iverson et al. 2014; Rockwell and Gormezano 2013a, 2013b). In April, they published a canon volley of a paper meant to blow all that ‘polar-bears-might-survive’ talk out of the water once and for all (Rode et al. 2015).
Note that although the PBSG didn’t bother to update their website with the IUCN Red List assessment when it came out last November, they had a link up to the USGS terrestrial feeding paper within 16 days (with an editorial summary in the “NEWS” section as well as a listing of the paper in the “RECOMMENDED READING” section). As I suggest below, this difference in treatment may not be a coincidence.
However, another group of non-PBSG researchers haven’t given up just yet, as shown by the title chosen for the most recent news report (Alaska Dispatch News, 16 June 2016) on the just-finished International Bear Conference:
Want to know how a changing climate is affecting polar bears? Look at what they’re eating.
Apparently, Jeff Welker, a University of Alaska Anchorage researcher who is co-author on two recent papers on the subject of polar bear consumption of terrestrial foods (Tartu et al. 2016 and Rogers et al. 2015), gave a presentation at the conference that raised this topic yet again. His talk came on the heels of an earlier presentation by Todd Atwood, who suggested Southern Beaufort polar bears might benefit from staying onshore to eat left-over whale scraps instead of staying on the sea ice during the summer (even though he had no data to support that opinion – as I discussed in this post).
Here’s the question: is this dispute really about what polar bears eat or don’t eat when they’re on land – or is it symptomatic of the underlying politics of polar bears? [CBC documentary video by that name here]
From Friday’s newspaper piece about the presentation given by Welker [my bold]:
“We’re seeing bears almost act like scavengers. They’re really starting to show this ability to diversify their diet,” Jeff Welker, a biology professor at the University of Alaska Anchorage, said in a Wednesday presentation of the research. “This is something that may lead to their success, the ability to adapt to these new conditions and these other resources.”
Bears that come to the Beaufort Sea shore in autumn are feeding from the pile of bowhead scraps left by local Inupiat whale hunters, a contrast with Beaufort Sea bears that stay out on sea ice farther north, he said.
Around Svalbard, where ice conditions vary widely from year to year and from locality to locality, bears in low-ice areas have turned to eggs, birds, whales, walruses and even reindeer.
Compare that to the anti-terrestrial food USGS press release issued last year, with biologist Karen Rode as lead author (but co-authored by former USGS employee Steve Amstrup) that hit back hard on claims that land-based foods make a survival difference (see previous post here) [my bold]:
A team of scientists led by the U.S. Geological Survey found that polar bears, increasingly forced on shore due to sea ice loss, may be eating terrestrial foods including berries, birds and eggs, but any nutritional gains are limited to a few individuals and likely cannot compensate for lost opportunities to consume their traditional, lipid-rich prey — ice seals.
“Although some polar bears may eat terrestrial foods, there is no evidence the behavior is widespread,” said Dr. Karyn Rode, lead author of the study and scientist with the USGS.
The scientists noted that over much of the polar bear’s range, terrestrial habitats are already occupied by grizzly bears. Those grizzly bears occur at low densities and are some of the smallest of their species due to low food quality and availability. Further, they are a potential competitor as polar bears displaced from their sea ice habitats increasingly use the same land habitats as grizzly bears.
“The smaller size and low population density of grizzly bears in the Arctic provides a clear indication of the nutritional limitations of onshore habitats for supporting large bodied polar bears in meaningful numbers,” said Rode. “Grizzly bears and polar bears are likely to increasingly interact and potentially compete for terrestrial resources.”
The study found that fewer than 30 individual polar bears have been observed consuming bird eggs from any one population, which typically range from 900 to 2000 individuals. “There has been a fair bit of publicity about polar bears consuming bird eggs. However, this behavior is not yet common, and is unlikely to have population-level impacts on trends in body condition and survival,” said Rode.
What’s this all about, really?
It seems to me the battle lines on this issue pit the very powerful ‘polar-bears-are-all-going-to-die’ group, led by former USGS biologist Steve Amstrup (but now an employee of the activist group Polar Bears International), against a small ‘polar-bears-might-be-able-to-adapt’ fringe faction that is refusing to be silenced.
Both groups accept the premise that sea ice declines blamed on global warming are already a survival issue for polar bears (that will become increasingly worse in the future). But one side sees hope in the bears’ adaptive abilities and the other (led by Amstrup) sees hope only from political action to reduce human-generated fossil fuel emissions. Therein lies the rub.
Note that it may not mean much that PBSG members such as Jon Aars, Magnus Andersen, Gregory Thiemann, and Elizabeth Peacock are listed as co-authors on the two recent Welker terrestrial feeding papers mentioned above, since it is standard courtesy to list colleagues as co-authors if they contributed data they collected to a new analysis (Thiemann, Peacock) or if they assisted in collecting new data (Aars, Anderson). In other words, their presence as co-authors does not necessarily mean they accept the premise of the papers.
However, Amstrup’s inclusion as a co-author on the land-based food rebuttal paper (Rode et al. 2015) is not a courtesy add-on.
Amstrup is a very powerful member of the PBSG and he’s held tightly to the predictions of polar bear declines to extinction he made that resulted in a ‘threatened’ listing for polar bears under the ESA in 2008 (Amstrup et al. 2007, 2010). He has stuck with his opinions despite recent criticisms of his work by IUCN Red List Standard and Petitions Committee member (and mathematical modeling expert) Resit Akçakaya. In short, the IUCN would not allow the PBSG group that wrote the 2015 Red List assessment to build on Amstrup’s sub-standard science – and I expect that made Amstrup furious.
To add insult to injury, Amstrup is listed as a co-author of the Red List document (Wiig, Ø., Amstrup, S., Atwood, T., Laidre, K., Lunn, N., Obbard, M., Regehr, E. & Thiemann, G. 2015). [Is that the professional courtesy co-authorship thing again?]
But the more important question is whether or not the failure of the PBSG to acknowledge the 2015 Red List assessment on their website for more than 6 months is a result of Amstrup’s anger over the process.
Are he and his colleagues preparing a rebuttal to their own work or something similar? Are other PBSG members keeping quiet publicly over the damning details of the Red List assessment in order to escape Amstrup’s wrath? Will they eventually have to band together and throw him under the bus in order to escape his insistence that only his predictions portend the future properly?
I guess we’ll have to wait and see what the “revised” PBSG website has to say on the subject, if they address it at all. In the end, however, I see the bickering over the relative importance of terrestrial foods as symptomatic of the politics of polar bears that Amstrup takes so personally, in part because it’s now his job to do so. Impartial science it’s not.
Amstrup, S.C., DeWeaver, E.T., Douglas, D.C., Marcot, B.G., Durner, G.M., Bitz, C.M., and Bailey, D.A. 2010. Greenhouse gas mitigation can reduce sea-ice loss and increase polar bear persistence. Nature 468:955-958. http://www.nature.com/nature/journal/v468/n7326/abs/nature09653.html
Amstrup, S.C.,Marcot, B.G. and Douglas,D.C. 2007. Forecasting the rangewide status of polar bears at selected times in the 21st century. Administrative Report, US Geological Survey. Reston, Virginia.
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.
Iverson, S.A., Gilchrist, H.G., Smith, P.A., Gaston, A.J. and Forbes, M.R. 2014. Longer ice-free seasons increase the risk of nest depredation by polar bears for colonial breeding birds in the Canadian Arctic. Proceedings of the Royal Society B 281(1779):20133128 doi:10.1098/rspb.2013.3128. Open Access. http://rspb.royalsocietypublishing.org/content/281/1779/20133128.abstract
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. 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.
Rode, K.D., Robbins, C.T., Nelson, L. and Amstrup, S.C. 2015. Can polar bears use terrestrial foods to offset lost ice-based hunting opportunities? Frontiers in Ecology and the Environment 13(3): 138–145, doi:10.1890/140202 http://www.esajournals.org/doi/abs/10.1890/140202 [paywalled]
Increased land use by polar bears (Ursus maritimus) due to climate-change-induced reduction of their sea-ice habitat illustrates the impact of climate change on species distributions and the difficulty of conserving a large, highly specialized carnivore in the face of this global threat. Some authors have suggested that terrestrial food consumption by polar bears will help them withstand sea-ice loss as they are forced to spend increasing amounts of time on land. Here, we evaluate the nutritional needs of polar bears as well as the physiological and environmental constraints that shape their use of terrestrial ecosystems. Only small numbers of polar bears have been documented consuming terrestrial foods even in modest quantities. Over much of the polar bear’s range, limited terrestrial food availability supports only low densities of much smaller, resident brown bears (Ursus arctos), which use low-quality resources more efficiently and may compete with polar bears in these areas. Where consumption of terrestrial foods has been documented, polar bear body condition and survival rates have declined even as land use has increased. Thus far, observed consumption of terrestrial food by polar bears has been insufficient to offset lost ice-based hunting opportunities but can have ecological consequences for other species. Warming-induced loss of sea ice remains the primary threat faced by polar bears.
Rogers, M.C., Peacock, E., Simac, K., O’Dell, M.B. and Welker, J.M. 2015. Diet of female polar bears in the southern Beaufort Sea of Alaska: evidence for an emerging alternative foraging strategy in response to environmental change. Polar Biology 38(7):1035-1047. http://link.springer.com/article/10.1007/s00300-015-1665-4 [paywalled]
Polar bear (Ursus maritimus) diet may become more variable in some Arctic regions due to climate warming and altered sea ice habitat. We surveyed carbon and nitrogen stable isotope profiles of five polar bear tissues sampled from adult females in the Southern Beaufort Sea of Alaska in order to assess inter-tissue isotopic variability and to determine whether any dietary shifts are occurring in this population. We did not detect any significant shifts from historical means in population-level tissue stable isotope values. A number of sectioned hair samples, however, were significantly depleted in 15N relative to the mean. We hypothesized that lower hair δ15N values were due to the consumption of bowhead whale (Balaena mysticetus) tissue. Telemetry data showed that polar bears with 15N-depleted hair sections were located on multiple dates near known subsistence-harvested bowhead whale bone piles and had spent 90 % of the prior year within 50 km of the shore. Bears with hair section δ15N values at or above the mean spent no time near bowhead whale bone piles and less than half of the year nearshore. An isotopic mixing model estimation of diet proportions determined that bowhead whale comprised approximately 50–70 % of fall diet for bears with lower hair δ15N values. We conclude that these results offer emergent evidence of an alternative foraging strategy within this population: ‘coastal’ bears, which remain near to shore for much of the year and use bowhead whale bone piles when they are present. In contrast, ‘pelagic’ bears follow a more typical strategy and forage widely on sea ice for seals.
Tartu, S., Bourgeon, S., Aars, J., Andersen, M., Ehrich, D., Thiemann, G.W., Welker, J.M. and Routti, H. 2016. Geographical area and life history traits influence diet in an Arctic marine predator. PLOS ONE 11(5):e0155980 doi.org/10.1371/journal.pone.0155980 Open access
Global changes are thought to affect most Arctic species, yet some populations are more at risk. Today, the Barents Sea ecoregion is suffering the strongest sea ice retreat ever measured; and these changes are suspected to modify food access and thus diet of several species. Biochemical diet tracers enable investigation of diet in species such as polar bears (Ursus maritimus). We examined individual diet variation of female polar bears in Svalbard, Norway, and related it to year, season (spring and autumn), sampling area and breeding status (solitary, with cubs of the year or yearlings). Sampling areas were split according to their ice cover: North-West (less sea ice cover), South-East (larger amplitude in sea ice extent) and North-East/South-West (NESW) as bears from that zone are more mobile among all regions of Svalbard. We measured fatty acid (FA) composition in adipose tissue and carbon (δ13C) and nitrogen (δ15N) stable isotopes in plasma and red blood cells. Females feeding in the North-West area had lower δ15N values than those from the NESW. In South-East females, δ13C values were lower in autumn compared to spring and females seemed less selective in their diet as depicted by large variances in stable isotope values. Considering the differences in FA composition and stable isotope values, we suggest that females from the North-West and South-East could ingest a higher proportion of avian prey. With regard to breeding status, solitary females had higher δ15N values and smaller variance in their stable isotopic values than females with cubs, suggesting that solitary females were more selective and prey on higher trophic level species (i.e. seals). Overall, our results indicate that prey availability for Svalbard polar bears varies according to geographical area and prey selectivity differs according to breeding status. Our findings suggest that complex changes in sea ice and prey availability will interact to affect Svalbard polar bear feeding patterns and associated nutrition.
Wiig, Ø., Amstrup, S., Atwood, T., Laidre, K., Lunn, N., Obbard, M., Regehr, E. & Thiemann, G. 2015. Ursus maritimus. The IUCN Red List of Threatened Species 2015: e.T22823A14871490. Published online here; PDF here.
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