Geoscience Reference
In-Depth Information
with reduced body condition, reproduction, survival, and
population size for polar bears in parts of their range [
Stir-
ling
et al.
, 1999;
Obbard
et al.
, 2006;
Stirling and Parkin-
son
, 2006;
Regehr
et al.
, 2007b]. Observed [
Comiso
, 2006]
and projected [
Holland
et al.
, 2006] sea ice declines have led
to the hypothesis that the future welfare of polar bears may
be diminished worldwide and to the proposal by the U.S.
Fish and Wildlife Service (FWS) to list the polar bear as a
threatened species under the Endangered Species Act [
U.S.
Fish and Wildlife Service
, 2007].
Classification as “threatened” requires determination that
a species is likely to become “endangered” within the “fore-
seeable future” throughout all or a significant portion of its
range. An “endangered” species is any species that is in
danger of extinction throughout all or a significant portion
of its range. For polar bears, the “foreseeable future” was
defined as 45 years from now [
U.S. Fish and Wildlife Serv-
ice
, 2007]. here we describe a method for combining avail-
able information on polar bear life history and ecology with
projections of the future state of Arctic sea ice to project
the future worldwide status of polar bears. We present our
forecast in a “compared to now” setting where projections
for the decade of 2045-2054 are compared to the “present”
period of 1996-2006. For added perspective, we looked to
the nearer term as well as beyond the defined foreseeable
future by comparing projections for the periods 2020-2029,
2070-2079, and 2090-2099 to the present. Also, we looked
back to the period of 1985-1995. hence we examined six
time periods in total.
Our view of the present and past was based on sea ice con-
ditions derived from satellite data. Our future forecasts were
based on information derived from general circulation model
(GCM) projections of the extent and spatiotemporal distri-
bution of sea ice, our understanding of how polar bears have
responded to ongoing changes in sea ice, and projections
of how polar bears are likely to respond to future changes.
This paper synthesizes information in nine Administrative
Reports prepared by the U.S. Geological Survey and deliv-
ered to the FWS in 2007 (http://www.usgs.gov/newsroom/
special/polar_bears/) plus other recent literature.
Polar bears occur throughout portions of the Northern
hemisphere where the sea is ice covered for all or much of
the year. Polar bears are thought to have branched off of
brown bear (
Ursus arctos
) stocks as long ago as 250,000
years, but they appear in the fossil record no earlier than
120,000 years ago [
Talbot and Shields
, 1996;
Hufthamer
,
2001;
Ingolfsson and Wiig
, 2007]. Since moving offshore,
behavioral and physical adaptations have allowed polar
bears to increasingly specialize at hunting seals from the
surface of the ice [
Stirling
, 1974;
Smith
, 1980;
Stirling and
Øritsland
, 1995].
Over much of their range, polar bears are nutritionally de-
pendent on the ringed seal (
Phoca hispida
). Polar bears oc-
casionally catch belugas (
Delphinapterus leucas
), narwhals
(
Monodon monocerus
), walrus (
Odobenus rosmarus
), and
harbor seals (
P. vitulina
) [
Smith
, 1985;
Calvert
and Stirling
,
1990;
Smith and Sjare
, 1990;
Stirling and Øritsland
, 1995;
Derocher
et al.
, 2002]. Walruses can be seasonally important
in some parts of the polar bear range [
Parovshchikov
, 1964;
Ovsyanikov
, 1996]. Bearded seals (
Erignathus barbatus
) can
be a large part of their diet where they are common and are
probably the second most common prey of polar bears [
De-
rocher
et al.
, 2002]. The most common prey of polar bears,
however, is the ringed seal [
Smith and Stirling
, 1975;
Smith
,
1980]. The relationship between ringed seals and polar bears
is so close that the abundance of ringed seals in some areas
appears to regulate the density of polar bears, while polar
bear predation, in turn, regulates density and reproductive
success of ringed seals in other areas [
Hammill and Smith
,
1991;
Stirling and Øritsland
, 1995]. Across much of the
polar bear range, their dependence on ringed seals is close
enough that the abundances of ringed seals have been esti-
mated by knowing the abundances of polar bears [
Stirling
and Øritsland
, 1995;
Kingsley
, 1998]. Although polar bears
occasionally catch seals on land or in open water [
Furnell
and Oolooyuk
, 1980], they consistently catch seals and other
marine mammals only at the air-ice-water interface.
like all bears, polar bears are opportunistic and will take
a broad variety of foods when available. When stranded on
land for long periods polar bears will consume coastal marine
and terrestrial plants and other terrestrial foods [
Derocher
et
al.
, 1993]. Polar bears have been observed hunting caribou
[
Derocher
et al.
, 2000;
Brook and Richardson
, 2002], and
they rarely have been observed fishing [
Townsend
, 1911;
Dyck and Romber
, 2007]. They will eat eggs, catch flight-
less (molting) birds, take human refuse, and consume a va-
riety of plant materials [
Russell
, 1975;
Lunn and Stirling
,
1985;
Derocher
et al.
, 1993;
Smith and Hill
, 1996;
Stemp-
niewicz
, 1993, 2006]. Although individual bears may gain
short-term energetic rewards from alternate foods, available
data suggest that polar bears gain little benefit at the popula-
tion level from these sources [
Ramsay and Hobson
, 1991].
Maintenance of polar bear populations appears dependent
upon marine prey, largely ringed seals.
Although polar bears occur in most ice-covered regions
of the Northern hemisphere [
Stefansson
, 1921], they are not
evenly dispersed. They are observed most frequently in shal-
low water areas nearshore and in other areas, called polyn-
yas, where currents and upwellings keep the winter ice cover
from freezing solid. These shore leads and polynyas create
a zone of active unconsolidated sea ice that is small in geo-
graphic area but contributes ~50% of the total productivity
Search WWH ::
Custom Search