Population ecology of polar bears in Davis Strait, Canada and Greenland

Abstract

Until recently, the sea ice habitat of polar bears was understood to be variable, but environmental variability was considered to be cyclic or random, rather than progressive. Harvested populations were believed to be at levels where density effects were considered not significant. However, because we now understand that polar bear demography can also be influenced by progressive change in the environment, and some populations have increased to greater densities than historically lower numbers, a broader suite of factors should be considered in demographic studies and management. We analyzed 35 years of capture and harvest data from the polar bear ( Ursus maritimus ) subpopulation in Davis Strait, including data from a new study (2005–2007), to quantify its current demography. We estimated the population size in 2007 to be 2,158 ± 180 (SE), a likely increase from the 1970s. We detected variation in survival, reproductive rates, and age-structure of polar bears from geographic sub-regions. Survival and reproduction of bears in southern Davis Strait was greater than in the north and tied to a concurrent dramatic increase in breeding harp seals ( Pagophilus groenlandicus ) in Labrador. The most supported survival models contained geographic and temporal variables. Harp seal abundance was significantly related to polar bear survival. Our estimates of declining harvest recovery rate, and increasing total survival, suggest that the rate of harvest declined over time. Low recruitment rates, average adult survival rates, and high population density, in an environment of high prey density, but deteriorating and variable ice conditions, currently characterize the Davis Strait polar bears. Low reproductive rates may reflect negative effects of greater densities or worsening ice conditions.