A Bayesian Network Modeling Approach to Forecasting the 21st Century Worldwide Status of Polar Bears - Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications - page 209

in Arctic waters [ Sakshaug , 2004]. Polar bears have been
shown to focus their annual activity areas over these regions
[ Stirling et al. , 1981; Amstrup and DeMaster , 1988; Stir-
ling , 1990; Stirling and Øritsland , 1995; Stirling and Lunn ,
1997; Amstrup et al. , 2000, 2004, 2005]. Ice over waters less
than 300 m deep is the most preferred habitat of polar bears
throughout the polar basin [ Durner et al. , 2008].
Polar bears inhabit regions with very different sea ice
characteristics. The southern reaches of their range includes
areas where sea ice is seasonal. There, polar bears are forced
onto land where they are food deprived for extended periods
each year. Other polar bears live in the harshest and most
northerly climes of the world where the ocean is ice covered
year-round. Still others occupy the pelagic regions of the
polar basin where there are strong seasonal changes in the
character and especially distribution of the ice. The common
denominator is that all polar bears regardless of where they
live make seasonal movements to maximize their foraging
time on sea ice that is suitable for hunting [ Amstrup , 2003].

considerable overlap in areas occupied by members of these
groups [ Amstrup et al. , 2004, 2005], they are thought to be
ecologically meaningful [ Aars et al. , 2006] and are managed
as subpopulations (Plate 1).
We recognized that many of the 19 subpopulations share
more similarities than differences and pooled them into four
ecological regions (Plate 1). We defined “ecoregions” on the
basis of observed temporal and spatial patterns of ice melt,
freeze, and advection, observations of how polar bears re-
spond to those patterns, and how general circulation models
(GCMs) forecast future ice patterns in each ecoregion.
The seasonal ice ecoregion (SIE) includes the two sub-
populations of bears which occur in hudson Bay, as well as
the bears of Foxe Basin, Baffin Bay, and Davis Strait. The
sum of the members of these five subpopulations is thought
to include about 7500 polar bears [ Aars et al. , 2006]. All five
share the characteristic that the sea ice, on which the polar
bears hunt, melts entirely in summer and bears are forced
ashore for extended periods of time during which they are
food deprived.
The archipelago ecoregion (AE) includes the channels be-
tween the Canadian Arctic Islands. This ecoregion includes
approximately 5000 polar bears representing six subpopu-
lations recognized by the PBSG [ Aars et al. , 2006]. These
subpopulations are Kane Basin, Norwegian Bay, Viscount-
Melville Sound, lancaster Sound, M'Clintock Channel, and
the Gulf of Boothia. Much of this region is characterized by
heavy annual and multiyear (perennial) ice that historically
has filled the interisland channels year-round. Polar bears re-
main on the sea ice, therefore, throughout the year.
In the polar basin as in the AE, polar bears mainly stay
on the sea ice year-round. In our analyses, we split the polar
basin into two ecoregions. This split was based upon the dif-
ferent patterns of sea ice formation and advection [ Rigor et
al. , 2002; Rigor and Wallace 2004; Maslanik et al. , 2007;
Meier et al. , 2007; Ogi and Wallace , 2007]. The polar basin
divergent ecoregion (PBDE) is characterized by extensive
formation of annual sea ice that is typically advected toward
the central polar basin, against the Canadian Arctic Islands
and Greenland, or out of the polar basin through Fram Strait.
The PBDE lies between ~127°W and 10°E and includes the
southern Beaufort, Chukchi, East Siberian-Laptev, Kara,
and Barents sea subpopulations. There are no population es-
timates for the kara Sea region. Assuming that 1000 bears
live in the kara Sea, this ecoregion could be home to ap-
proximately 8500 polar bears [ Aars et al. , 2006].
The polar basin convergent ecoregion (PBCE) is the re-
mainder of the polar basin including the east Greenland Sea,
the continental shelf areas adjacent to northern Greenland
and the Queen Elizabeth Islands, and the northern Beau-
fort Sea (Plate 1). There are thought to be approximately

2. METhODS

2.1. Overview

We used a Bayesian network (BN) model [ Marcot et al. ,
2006] to forecast future population status of polar bears in
each of four distinct ecoregions. The BN model incorpo-
rated projections of sea ice change as well as anticipated
likelihoods of changes in several other potential population
stressors. In the following sections, we provide detailed de-
scriptions of the four polar bear ecoregions. We describe the
process we used to make projections of the amount and dis-
tribution of future sea ice habitat. Finally, we provide details
of the BN population stressor model we used to project the
future status of polar bears.

2.2. Polar Bear Ecoregions

Polar bears are distributed throughout regions of the Arctic
and subarctic where the sea is ice covered for large portions
of the year. Telemetry studies have demonstrated spatial
segregation among groups or stocks of polar bears in differ-
ent regions of their circumpolar range [ Schweinsburg and
Lee , 1982; Amstrup 1986, 2000; Garner et al. , 1990, 1994;
Messier et al. , 1992; Amstrup and Gardner , 1994; Ferguson
et al. , 1999]. As a result of patterns in spatial segregation
suggested by telemetry data, survey and reconnaissance,
marking and tagging, and traditional knowledge, the Polar
Bear Specialist Group (PBSG) of the International Union
for the Conservation of Nature recognizes 19 partially dis-
crete polar bear groups [ Aars et al. , 2006]. Although there is