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201505Apr07:22

Polar bears unlikely to thrive on land-​based foods

Infor­ma­tion
pub­lished 05 April 2015 | mod­i­fied 05 April 2015
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Polar bear hudsonbayA team of sci­en­tists led by the U.S. Geo­log­i­cal Sur­vey (USGS) 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 behav­iour is wide­spread,” said Dr. Karyn Rode, lead author of the study. “In the regions where ter­res­trial feed­ing by polar bears has been doc­u­mented, polar bear body con­di­tion and sur­vival rates have declined.”

The authors detail their find­ings in a review arti­cle pub­lished in the April issue of the jour­nal Fron­tiers in Ecol­ogy and the Envi­ron­ment. 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
Dr. Karyn Rode, lead author, sci­en­tist with the USGS »

Rode con­tin­ues by point­ing out that “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 behav­iour is not yet com­mon, and is unlikely to have population-​level impacts on trends in body con­di­tion and sur­vival,” said Rode.

Few foods are as ener­get­i­cally dense as marine prey. Stud­ies sug­gest that polar bears con­sume the high­est lipid diet of any species, which pro­vides all essen­tial nutri­ents and is ideal for max­i­miz­ing fat depo­si­tion and min­i­miz­ing ener­getic require­ments. Poten­tial foods found in the ter­res­trial envi­ron­ment are dom­i­nated by high-​protein, low-​fat ani­mals and veg­e­ta­tion. Polar bears are not phys­i­o­log­i­cally suited to digest plants, and it would be dif­fi­cult for them to ingest the vol­umes that would be required to sup­port their large body size.

The reports of ter­res­trial feed­ing by polar bears pro­vide impor­tant insights into the ecol­ogy of bears on land,” said Rode. “In this paper, we tried to put those obser­va­tions into a broader con­text. Focused research will help us deter­mine whether ter­res­trial foods could con­tribute to polar bear nutri­tion despite the phys­i­o­log­i­cal and nutri­tional lim­i­ta­tions and the low avail­abil­ity of most ter­res­trial food resources. How­ever, the evi­dence thus far sug­gests that increased con­sump­tion of ter­res­trial foods by polar bears is unlikely to off­set declines in body con­di­tion and sur­vival result­ing from sea ice loss.”

The USGS is lead­ing stud­ies of polar bear response to sea ice loss through its Chang­ing Arc­tic Ecosys­tems Ini­tia­tive. Cur­rent stud­ies include exam­i­na­tion of polar bear nutri­tional and behavioural ecol­ogy, linked to population-​level con­se­quences. For fur­ther infor­ma­tion, visit the USGS Polar Bear Pro­gram.



(Source: USGS news release, 01.04.2015)


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