Transient benefits of climate change for a high‐Arctic polar bear (Ursus maritimus) subpopulation

Abstract

Kane Basin (KB) is one of the world's most northerly polar bear (Ursus maritimus) subpopulations, where bears have historically inhabited a mix of thick multiyear and annual sea ice year-round. Currently, KB is transitioning to a seasonally ice-free region because of climate change. This ecological shift has been hypothesized to benefit polar bears in the near-term due to thinner ice with increased biological production, although this has not been demonstrated empirically. We assess sea-ice changes in KB together with changes in polar bear movements, seasonal ranges, body condition, and reproductive metrics obtained from capture-recapture (physical and genetic) and satellite telemetry studies during two study periods (1993-1997 and 2012-2016). The annual cycle of sea-ice habitat in KB shifted from a year-round ice platform (~50% coverage in summer) in the 1990s to nearly complete melt-out in summer (<5% coverage) in the 2010s. The mean duration between sea-ice retreat and advance increased from 109 to 160 days (p = .004). Between the 1990s and 2010s, adult female (AF) seasonal ranges more than doubled in spring and summer and were significantly larger in all months. Body condition scores improved for all ages and both sexes. Mean litter sizes of cubs-of-the-year (C0s) and yearlings (C1s), and the number of C1s per AF, did not change between decades. The date of spring sea-ice retreat in the previous year was positively correlated with C1 litter size, suggesting smaller litters following years with earlier sea-ice breakup. Our study provides evidence for range expansion, improved body condition, and stable reproductive performance in the KB polar bear subpopulation. These changes, together with a likely increasing subpopulation abundance, may reflect the shift from thick, multiyear ice to thinner, seasonal ice with higher biological productivity. The duration of these benefits is unknown because, under unmitigated climate change, continued sea-ice loss is expected to eventually have negative demographic and ecological effects on all polar bears.

Accelerated sea ice loss in the Wandel Sea points to a change in the Arctic’s Last Ice Area

“…Currently, little is known about marine mammal densities and biological productivity in the WS and the broader LIA. Recent studies indicate there may be some transient benefits for polar bears in areas transitioning from thick multi-year ice to thinner first year ice 24,25 , as biological productivity in the system increases 26 . However, this is largely the case in shallow water <300 m in depth and it is unclear if this will occur in multi-year ice regions elsewhere.…”

Marine mammal conservation: over the horizon

“…by shifting their range or diet; Moore & Huntington 2008, Evans et al 2010, Schumann et al 2013, Ramp et al 2015. In some cases, the impacts of climate change may be initially positive but later unknown (Moore & Reeves 2018, Laidre et al 2020b. For example, evidence of a number of positive impacts linked to thinner sea-ice (range expansion, improved body condition and stable reproductive performance) has been identified for a subpopulation of polar bears (Laidre et al 2020b).…”

“…In some cases, the impacts of climate change may be initially positive but later unknown (Moore & Reeves 2018, Laidre et al 2020b. For example, evidence of a number of positive impacts linked to thinner sea-ice (range expansion, improved body condition and stable reproductive performance) has been identified for a subpopulation of polar bears (Laidre et al 2020b). However, the rapid pace of climate change and the large number of potential cumulative and synergistic stressors make predicting specific impacts challenging (Laidre & Heide-Jørgensen 2005, Burek et al 2008, Moore 2008, Patyk et al 2015.…”

“…Although some sub-populations of polar bears may initially benefit from the effects of climate change on sea-ice thickness (Laidre et al 2020b), rising temperatures pose severe risks to the species as a whole. A reduction in sea-ice leads to diminished access to prey and lower reproductive success (Laidre et al 2020a), as well as increased disturbance from humans due to the opening up of new shipping routes (Gross 2018).…”

Survival and abundance of polar bears in Alaska’s Beaufort Sea, 2001–2016

“…The Chukchi Sea subpopulation, for example, appears to remain healthy despite experiencing considerable summer sea ice loss, presumably because ecosystem productivity is high and access to prey remains adequate (Regehr et al., 2018 ; Rode et al., 2014 , 2018 , 2021 ). Indeed, some have speculated that a reduction in the prevalence of thick, multiyear ice may increase ecosystem productivity and thereby provide short‐term benefit to high‐Arctic subpopulations (Derocher et al., 2004 ), although evidence regarding that hypothesis is limited (Florko et al., 2021 ; Laidre et al., 2020b ). Even so, the long‐term effects of climate warming for the species can only be negative (Atwood, Marcot, et al., 2016 ; Molnár et al., 2020 ; Stirling & Derocher, 2012 ).…”