Climate change and the increasing impact of polar bears on bird populations

Abstract

The Arctic is becoming warmer at a high rate, and contractions in the extent of sea ice are currently changing the habitats of marine top-predators dependent on ice. Polar bears (Ursus maritimus) depend on sea ice for hunting seals. For these top-predators, longer ice-free seasons are hypothesized to force the bears to hunt for alternative terrestrial food, such as eggs from colonial breeding birds. We analyzed time-series of polar bear observations at four locations on Spitsbergen (Svalbard) and one in east Greenland. Summer occurrence of polar bears, measured as the probability of encountering bears and the number of days with bear presence, has increased significantly from the 1970/80s to the present. The shifts in polar bear occurrence coincided with trends for shorter sea ice seasons and less sea ice during the spring in the study area. This resulted in a strong inverse relationship between the probability of bear encounters on land and the length of the sea ice season. Within, 10 years after their first appearance on land, polar bears had advanced their arrival dates by almost 30 days. Direct observations of nest predation showed that polar bears may severely affect reproductive success of the barnacle goose (Branta leucopsis), common eider (Somateria mollissima) and glaucous gull (Larus hyperboreus). Nest predation was strongest in years when the polar bears arrived well before hatch, with more than 90% of all nests being predated. The results are similar to findings from Canada, and large-scale processes, such as climate and subsequent habitat changes, are pinpointed as the most likely drivers in various parts of the Arctic. We suggest that the increasing, earlier appearance of bears on land in summer reflects behavioral adaptations by a small segment of the population to cope with a reduced hunting range on sea ice. This exemplifies how behavioral adaptations may contribute to the cascading effects of climate change.

Marine birds and mammals foraging in the rapidly deglaciating Arctic fjord - numbers, distribution and habitat preferences

“…However, reports on negative consequences predominate in the case of both seabirds (e.g. Kittlitz's murrelet and ivory gull; Kuletz et al 2003) and seals and polar bear (Kovacs et al 2011;Gilg et al 2012;Post et al 2013;Prop et al 2015;Descamps et al 2016). Our study demonstrates that the densities and species diversity of seabirds foraging in Hornsund are high compared with other studied Arctic fjords, suggesting that so far, they are benefitting from the current intensive glacier melting.…”

“…Pinnipeds and polar bears, which rely on the former as the main item in their diet, are strongly associated with sea ice as a breeding, hauling and hunting habitat. The rapid sea ice decline and deglaciation of recent decades in the Arctic is commonly thought to be having dramatic consequences for their population numbers, distribution and feeding habits (Moore and Huntington 2008;Kovacs et al 2011;Gilg et al 2012;Prop et al 2015;Descamps et al 2016). Also, several seabird species, including kittiwake, black guillemot and ivory gull, are definitely ice-associated and hence seriously threatened by ice-cover shrinking (Bradstreet and Cross 1982;Mehlum and Gabrielsen 1993;Gilchrist and Mallory 2005;Descamps et al 2016).…”

Polar Bear Foraging Behavior During the Ice-Free Period in Western Hudson Bay: Observations, Origins, and Potential Significance

“…These targets span multiple trophic levels and include vegetation, birds, eggs, ungulates, and marine mammals. Consuming food without an ice-based platform is not new, given polar bear foraging accounts reported much earlier in the natural history literature (e.g., Pedersen, 1962;Harrington, 1965;Perry, 1966) and has been documented in other subpopulations (e.g., Smith and Hill, 1996;Dyck and Romberg, 2007;Drent and Prop, 2008;Smith et al, 2010;Voorhees et al, 2014;Prop et al, 2015;Rogers et al, 2015). It is likely becoming more prevalent in WH because the bears are onshore longer.…”

Polar bears observed climbing steep slopes to graze on scurvy grass in Svalbard

“…However, according to modern taxonomic classification, the polar bear is a sister species to the opportunistic omnivorous brown bear (Ursus arctos) (Wozencraft 2005) and under some circumstances, for example when stranded on land, appear to be quite opportunistic. They will consume a wide variety of other foods, including ungulates like muskox (Ovibos moschatus) and reindeer (Rangifer tarandus), birds, eggs, rodents and crustaceans, and other polar bears (Lunn & Stirling 1985;Stempniewicz 1993Stempniewicz , 2006Donaldson et al 1995;Derocher et al 2002;Derocher 2012;Stempniewicz et al 2014;Prop et al 2015). However, none of these animals make up a significant part of their diet (Russell 1975;Derocher et al 2000;Brook & Richardson 2002).…”