Temporal and intra-population patterns in polar bear foraging ecology in western Hudson Bay

Abstract

Climate-mediated sea ice declines have led to alterations in ecosystem composition, phenology, and primary productivity, potentially altering trophic dynamics in Arctic marine ecosystems. As one of the Arctic's top predators and a species dependent on sea ice habitat for survival, polar bears Ursus maritimus are at risk of habitat loss from sea ice declines, and therefore have been used to monitor the effects of climate change in the Arctic. We used stable isotope (δ 15 N, δ 13 C) values of 806 hair samples collected from Western Hudson Bay (WH) polar bears from 1993−1994 and 2004−2016 to examine variations in isotopic niche size and diet within the population, patterns in foraging ecology over time, and the relationship between hair isotopic values and sea ice dynamics. We found significant variation in isotopic values between age-and sex-classes. Adult males had the highest δ 15 N and δ 13 C values and the largest isotopic niches, reflecting broader diets. In contrast, adult females (with and without cubs) and subadults differed isotopically from adult males, and their relatively smaller isotopic niches potentially make them more vulnerable to habitat change. Population δ 15 N values significantly increased over time from 1993 (mean ± SD: 18.8 ± 0.5 ‰) to 2016 (19.5 ± 0.7 ‰), while δ 13 C values significantly decreased from 1993 (−16.2 ± 0.3 ‰) until 2011 (−17.7 ± 0.2 ‰) before increasing until 2016 (−17.0 ± 0.3 ‰). Values of δ 13 C were significantly correlated with length of the open-water period, suggesting that this apex predator's foraging ecology was affected by climate change. We suggest that WH polar bears may have undergone a significant dietary niche shift and/or that the baseline isotopic values in this ecosystem may have changed over the last 25 yr in response to climate change.

Influence of sea ice dynamics on population energetics of Western Hudson Bay polar bears

“…Sea ice is the most important single factor influencing polar bear demographic responses in the changing Arctic marine ecosystem ( Stirling et al , 1999 ; Bromaghin et al , 2015 ; Lunn et al , 2016 ). Our results are consistent with the association between earlier breakup/later freeze-up and declining body condition ( Stirling et al , 1999 ; Obbard et al , 2016 ; Sciullo et al , 2016 ; Laidre et al , 2020 ), increased stress ( Boonstra et al , 2020 ), altered foraging ecology ( McKinney et al , 2009 ; Johnson et al , 2019 ), and reduced reproduction/survival rates and abundance ( Regehr et al , 2007 ; Rode et al , 2010 ; Lunn et al , 2016 ; Obbard et al , 2018 ) reported in various polar bear populations including WH, Southern Beaufort Sea, Southern Hudson Bay and Baffin Bay. Changes to energetic intake and expenditure in response to sea ice dynamics have consequences for energetic balances ( Pagano et al , 2018 ).…”

“…Adult females and juveniles are often vulnerable demographic groups and their condition can influence population trends by affecting reproduction and survival rates ( Lockyer, 1986 ; Miller et al , 2011 ; Fortune et al , 2013 ; Keay et al , 2018 ). The small body size, dietary constraints, energetic demands of growth, risk of kleptoparasitism from larger bears and inexperienced hunting skills of younger bears make them more vulnerable to reductions in sea ice and thus prey availability ( Stirling, 1974 ; Rode et al , 2010 ; Thiemann et al , 2011a ; Pilfold et al , 2016 ; Johnson et al , 2019 ; Laidre et al , 2020 ). In contrast, adult males can best buffer against sub-optimal conditions given their larger body size, broader diets, more effective hunting skills and kleptoparasitism from smaller bears ( Stirling, 1974 ; Regehr et al , 2007 ; Thiemann et al , 2011a ; Pilfold et al , 2016 ; Johnson et al , 2019 ).…”

“…The length of the open-water period (i.e. when bears are on land) was calculated as the date of freeze-up minus the date of breakup, then further subtracting 25 days due to the bears arriving onshore approximately 21 to 28 days after breakup ( Stirling et al , 1999 ; Castro de la Guardia et al , 2017 ; Johnson et al , 2019 ). In addition, the Arctic Oscillation winter index (AOw) and the North Atlantic Oscillation winter index (NAOw) were extracted for each year to examine broad climate variability.…”

Reconstructing the diet, trophic level, and migration pattern of Mysticete whales based on baleen isotopic composition

“…Dietary estimates reconstructed using bulk isotopic values reflect a mixture of influencing factors and often provide results that are inconclusive or muddled (28). This is especially true in systems where the isotopic baseline supporting production changes or the trophic level that animals feed at have shifted (29,30). The δ 15 N values of resources supporting primary production can shift substantially, both spatially and temporally, depending on the balance of biogeochemical processes affecting available N sources (NO3 -, NO2 -NH4 + , N2) within an ecosystem (31)(32)(33)(34).…”

The influence of sea ice dynamics on population energetics of Western Hudson Bay polar bears

“…10% for three consecutive days was used as the date of freeze-up (Etkin 1991;Stirling et al 1999;Lunn et al 2016). The length of the open water period (i.e., when bears are on land) was calculated as the date of freeze-up minus the date of breakup, then further subtracting 25 days due to the bears arriving onshore approximately 21 to 28 days after breakup (Stirling et al 1999;Castro de la Guardia et al 2017;Johnson et al 2019). In addition, the Arctic Oscillation winter index (AOw) and the North Atlantic Oscillation winter index (NAOw) were extracted for each year to examine broad climate variability.…”