O R I G I N A L A R T I C L E
Andrew E. Derocher
Population ecology of polar bears at Svalbard, Norway
Received: 21 September 2004 / Accepted: 11 August 2005 / Published online: 30 September 2005
The Society of Population Ecology and Springer-Verlag Tokyo 2005
Abstract The population ecology of polar bears at
Svalbard, Norway, was examined from 1988 to 2002
using live-captured animals. The mean age of both fe-
males and males increased over the study, litter pro-
duction rate and natality declined and body length of
adults decreased. Dynamics of body mass were sugges-
tive of cyclical changes over time and variation in body
mass of both adult females and adult males was related
to the Arctic Oscillation index. Similarly, litter produc-
tion rate and natality correlated with the Arctic Oscil-
lation index. The changes in age-structure, reproductive
rates and body length suggest that recovery from over-
harvest continued for almost 30 years after harvest
ended in 1973 and that density-dependent changes are
perhaps being expressed in the population. However, the
variation in reproduction and body mass in the popu-
lation show a relationship between large-scale climatic
variation and the upper trophic level in an Arctic marine
ecosystem. Similar change in other polar bear popula-
tions has been attributed to climate change, and further
research is needed to establish linkages between climate
and the population ecology of polar bears.
Introduction
Polar bears (Ursus maritimus) are a highly specialised
predator of seals that live in the ice-covered seas of the
Arctic (DeMaster and Stirling
). Remote locations,
low density and high research costs for monitoring polar
bear populations result in an incomplete understanding
of their population dynamics. Researchers must piece
together a variety of elements to construct an insight
into the dynamics of a population: often with incomplete
time series or small sample sizes. Despite these limita-
tions, the population ecology of polar bears is reason-
ably well understood. Similar to other species of bears,
polar bears have delayed maturation, small litters and a
prolonged mother–offspring bond that results in low
population growth rates (Bunnell and Tait
). Pop-
ulation growth, however, is most sensitive to changes in
adult female survival rate (Taylor et al.
), but nat-
ural variation in survival of adults is low and treating
adult survival as a constant has been suggested (Eber-
hardt
; Amstrup and Durner
). Reproductive
rates, body size and condition are the most dynamic
components of polar bear populations and play a major
role in determining population dynamics (Derocher and
Stirling
; Stirling et al.
The Arctic sea ice habitat is a dynamic environment
and large temporal and spatial variation is common
(A˚dlandsvik and Loeng
; Shapiro et al.
; Bar-
ber and Iacozza
). Linkages between climate-driven
sea ice habitats and polar bears were first established
from hunting returns in Greenland (Vibe
). More
recent research has linked the dynamics of polar bear
populations to climatic variability that reduce produc-
tivity of their primary prey, ringed seals (Phoca hispida)
(Stirling et al.
; Stirling
). Changes in the pro-
ductivity of ringed seals affects polar bear natality and
offspring survival through changes in body condition
(Ramsay and Stirling
; Derocher and Stirling
;
Stirling
). Recent studies of several northern and
Arctic birds and mammals have linked the dynamics of
populations to climatic indices such as the Arctic
Oscillation and the North Atlantic Oscillation (Forch-
hammer et al.
; Aanes et al.
; Post and
Forchhammer
; Forchhammer and Post
However, no such linkages have been made for polar
bears.
A. E. Derocher
Department of Biological Sciences, University of Alberta,
Edmonton, T6G 2E9, Alberta, Canada
E-mail: derocher@ualberta.ca
Tel.: +1-780-4925570
Fax: +1-780-4929234
A. E. Derocher
Norwegian Polar Institute, Tromsø, 9296, Norway
Popul Ecol (2005) 47:267–275
DOI 10.1007/s10144-005-0231-2
The best studied polar bear populations are found
in the Beaufort Sea, north of Alaska and western
Canada, and in western Hudson Bay (Amstrup et al.
; Stirling et al.
; Stirling
). The dynamics
of reproduction in these two populations are substan-
tially different. Large declines in the condition and
reproductive rates of polar bears in western Hudson
Bay have been linked to possible density-dependent
responses following recovery from over-harvest (Der-
ocher and Stirling
) and more recently to
climate change (Stirling and Derocher
; Stirling
et al.
). In contrast, the dynamics of condition and
reproduction in the Beaufort Sea are more closely tied
to recovery from over-harvest and climatic variation
(Amstrup et al.
; Stirling
). Svalbard, north of
Norway, is the only other polar bear population with
a long-term ecological research program. The dynam-
ics of this population are only partially understood
and are related to a history of intensive harvest that
resulted in a severely depleted population (Larsen
).
Global concerns over rapidly increasing harvest levels
of polar bears in the 1960s culminated in the signing of
the International Agreement on Polar Bears in 1973 that
resulted in management regimes being imposed on most
populations (Prestrud and Stirling
). Interpretation
of the Agreement by the five member states varied
widely. Polar bears in Russia had already received total
protection from harvest in 1956 (Prestrud and Stirling
). Norway introduced a total ban on harvest in
1973, while Canada and the United States implemented
inventory programs and harvest monitoring. While
extensive research on polar bears has been conducted in
North America, all of these populations still undergo
extensive harvest, which is a significant source of mor-
tality (Amstrup et al.
; Taylor et al.
; Lee and
Taylor
; Amstrup and Durner
). In Norway,
the termination of hunting and the International
Agreement spurred research on the population (Larsen
; Wiig
; Mauritzen et al.
). Polar bears in
Svalbard are the only studied population without an
ongoing harvest.
No meaningful population estimates are available for
polar bears in the Svalbard–Barents Sea area but esti-
mates derived in the 1970s–1980s, when the population
was thought to be recovering from over-harvest (Larsen
), were of questionable reliability when produced
and with the passage of 20 years, have little relevance for
the current state of the population. Despite the lack of
harvest, conservation issues pertaining to polar bears in
the Barents Sea have centred upon the possible effects of
anthropogenic pollutants. In particular, levels of per-
sistent organic pollutants in polar bears were thought to
negatively influence the immune system (Bernhoft et al.
; Lie et al.
) and hormone homeostasis (Skaare
et al.
) and may have affected the population
(Derocher et al.
). Levels of pollutants declined in
the 1990s (Henriksen et al.
) but the effects on the
population remain unclear.
In this paper, I examine the population ecology of
polar bears in Svalbard and examine the age structure,
reproductive rates and dynamics and body size dynam-
ics. I also examine relationships between the population
parameters and the Arctic Oscillation climatic index.
Study area and methods
Polar bears were live-captured during late March to
mid-May in 1988–2002 in the Svalbard area (78
N,
20
E) eastward to the central Barents Sea (70N, 44E;
Fig.
). Polar bears in Svalbard are part of the Barents
Sea population and are linked genetically and through
movements to the bears in the western Russian Arctic
(Wiig
; Paetkau et al.
; Mauritzen et al.
In general, sampling was constrained to nearshore areas
due to helicopter range restrictions imposed by the dis-
tribution of fuel caches. Sampling intensity of the pop-
ulation varied between years due to do environmental
conditions and funding but the method of searching for
animals was similar over time. Efforts were made to
capture each animal observed unless the conditions were
deemed unsafe. The sample is believed to be represen-
tative of the population.
Polar bears (
‡1 year of age) were captured using a
helicopter and remote injection of the drug Zoletil
(Stirling et al.
). A vestigial premolar tooth was
extracted from all bears for age determination (Calvert
and Ramsay
). Ages were unavailable for 54 bears
and an estimated age was used for those analyses where I
could allocate the bear to a specific group (e.g., adult
females
‡4 years old with cubs-of-the-year) but these
animals were excluded from age-specific analyses. The
sex, reproductive status and a series of standardised
morphometric measure were collected from each bear.
Body length (cm) was measured as the dorsal straight-
line distance from the tip of the nose to the caudal end of
20 E
°
40 E
°
60 E
°
75 N
°
80 N
°
0
200
400
km
Svalbard
Barents Sea
Frans Josef
Land
Novaya
Zemlya
Arctic Ocean
Fig. 1 Map of the study area showing Svalbard, Norway, Franz
Josef Land and Novaya Zemlya, Russia, where the Barents Sea
polar bear population is located
268
the last tail vertebrae with bears lying sternally recum-
bent with the back legs straight behind and the front legs
flexed at the elbows with the forelegs forward and par-
allel to the body. Axillary girth (cm) was measured as
the circumference around the chest at the axilla with a
rope tightened with a tension of about 0.5 kg. Body
mass was estimated from a regression model developed
for the study population using axillary girth and body
length (Derocher and Wiig
Age-specific reproductive parameters were calculated
based on the methods of Stirling et al. (
) and
Ramsay and Stirling (
) and include cubs-of-the-year
of both sexes. Age-specific mean litter size (LS
x
) was
calculated from:
LS
x
¼
P
cubs-of-the-year with females age
x
P
females age
x
with cub-of-the year litters
:
In instances where no litters from a female of age x
were observed, LS
x
was assigned a value of 0.000 if fe-
males of that age class were captured. Age-specific rate
of litter production (LP
x
) was calculated as
LP
x
¼
P
females age
x
with cub-of-the-year litters
P
females age
x
:
An estimate of natality rate (N
x
), the product of LS
x
and LP
x
, was derived from:
N
x
¼
P
cubs-of-the-year with females age
x
P
females age
x
:
The above equations and notation were used to be
consistent with previous literature. However, because
the natality rate applies to a period roughly 4 months
after birth, it reflects recruitment to the spring.
I used SAS statistical software (SAS Institute
) for
all analyses. Statistical significance was set to P
£ 0.05.
Values are presented as means ±1SE. Some information
was not available for all animals, resulting in varying
sample sizes between analyses. Ages were log
10
trans-
formed for statistical analyses to normalise data.
To test for temporal trends in body length, I used the
year of birth so that trends would relate to the cohort
rather than the year of capture. Year of birth was deter-
mined from the age at capture subtracted from the year of
capture. Females attain 97% of their asymptotic body
length at 4.4 years of age in the Svalbard population
(Derocher and Wiig
) and to reduce age-related biases
I used females
‡5 years of age for body length analyses.
Males in the population attain 97% of their asymptotic
body length at 6.2 years of age (Derocher and Wiig
but growth is slower than in females, so males
‡8 years of
age were included in body length analyses.
I used multiple regression to explore the relation-
ships between the mean body mass for adult females
and adult males and the Arctic Oscillation index. The
Arctic Oscillation is a mode of climate variability in the
Northern Hemisphere north of 20
N related to sea-le-
vel pressure variations (Thompson and Wallace
The Arctic Oscillation shares some features with the
North Atlantic Oscillation but the Arctic Oscillation
has a more northern centre of action that includes most
of the Arctic (Tremblay
). Higher than normal sea-
level pressure over the Arctic results in weaker westerly
winds in the upper atmosphere and colder conditions in
northern areas. In contrast, lower sea-level pressure
over the Arctic results in a warming pattern and an
influx of warmer Atlantic water into the Arctic.
Monthly
values
for
the
Arctic
Oscillation
were
obtained from http://www.cpc.ncep.noaa.gov/products/
precip/CWlink/daily_ao_index/ao_index.html and were
fitted to mass data for adult females (
‡4 years old)
without cubs-of-the-year and adult males (
‡10 years
old). These age groups were selected to minimise age-
related and reproductive status-related variation in the
mass data. Adult females with cubs-of-the-year were
excluded from the analyses because they den over-
winter while lone females and females with older off-
spring remain active. The mean of body mass for each
sex was used as a single value for each year and was
considered as an index of condition.
To assess the role of climate on the population, I
created a spring and winter index of Arctic Oscillation. I
used the mean of the Arctic Oscillation index for April–
June for a spring index and used a winter index (Octo-
ber–January) and monthly values of Arctic Oscillation
for the 12 months preceding capture in the multiple
stepwise regressions. Variables were retained in the
model if significant at P
£ 0.05.
Results
Age structure
The age structure of the captured population was con-
structed from 553 females and 509 males and revealed
Fig. 2 Age structure of female and male polar bears sampled near
Svalbard, 1988–2002
269
that bears 2–5 years of age were under-represented in
the sample (Fig.
old, the age of independence) increased over 1988–2001
(linear regression, F
). A similar pattern was noted for males (
old) over 1990–2001 (linear regression, F
late parts of the study, the mean age of females (
‡3 years old) where the mean age from 1988–1993 was
‡1 year old), which was 8.0±0.6 years (n=84)
for 1988–1993 and rose to 10.8±0.4 years (n=230) for
1998–2001. For males (
‡1 year old), in 1988–1993 the
mean age was 8.4±0.9 years (n=54) and rose to
11.7±0.4 years (n=246) in 1998–2001.
Reproductive rates
The earliest age of first reproduction in females was
4 years of age (Table
=0.37) and was 1.72 cubs/litter (SE=0.05, n=106)
with annual means ranging over 1.71–2.00 cubs/litter
(Fig.
). Singleton litters comprised 32.1%, twins
=58) and varied significantly (ANOVA, F
=0.0019) between 1.00 yearlings/litter and 2.00 year-
lings/litter between years (Fig.
litters comprised 48.3% and twin yearling litters 51.7%
of the sample.
Age-specific litter size, litter production rates and
natality rates were determined from pooled data
(Table
natality rate declined (linear regression, F
Table 1 Age-specific litter size (LS
x
), litter production (LP
x
) and
natality (N
x
) rates for female polar bears captured in the Svalbard
area in 1993–2002. Sample size (n litters) refers to the number of
females of age x with cub-of-the-year litters and n females refers to
the number of females of age x of all reproductive classes
Age (years)
LS
x
n
litters
LP
x
N
x
n
females
4
1.00
1
0.143
0.143
7
5
1.50
2
0.100
0.150
20
6
1.62
13
0.650
1.050
20
7
1.84
19
0.559
1.029
34
8
1.70
10
0.400
0.680
25
9
1.58
12
0.444
0.704
27
10
2.00
6
0.375
0.750
16
11
1.86
7
0.333
0.619
21
12
2.20
5
0.357
0.786
14
13
1.33
3
0.273
0.364
11
14
1.62
8
0.444
0.722
18
15
2.00
6
0.462
0.923
13
16
2.00
2
0.333
0.667
6
17
–
0
0.000
0.000
10
18
1.00
2
0.250
0.250
8
19
1.60
5
1.000
1.600
5
20
1.00
1
0.200
0.200
5
21
1.67
3
0.375
0.625
8
22+
1.00
1
0.000
0.067
15
Overall
1.72
106
0.375
0.643
283
Fig. 3 Mean age (±SE) of female and male polar bears
‡3 years
old sampled in the Svalbard area by year of capture. Linear
regressions are shown as sloping lines
Fig. 4 Mean litter size of cubs-of-the-year and yearlings (±SE) for
polar bears in the Svalbard area, 1992–2002. The number of litters
sampled is indicated above each error bar for cubs and to the right
of the symbol for yearlings
270
Using multiple regression, litter production rate was
negatively correlated to the spring Arctic Oscillation
of the preceding year (F
1,8
=6.14, P=0.038, R
2
=0.43)
and natality rate was negatively correlated to the
Arctic Oscillation in May of the preceding year for
the same period (F
1,8
=5.34, P=0.049, R
2
=0.40;
Fig.
Dynamics of body size
Body mass of adult females (
‡4 years old) without
cubs-of-the-year varied between years (1990–2002;
ANOVA, F
12,229
=2.93, P<0.001; Fig.
the body mass of adult males (
Using multiple regression, the body mass of females
without cubs was positively related to the Arctic
Oscillation index in July (partial R
2
=0.59) and nega-
tively in December (partial R
2
=0.14; multiple regres-
sion, F
2,10
=13.42, P=0.0015, R
2
=0.73). The body mass
of adult males was positively related to the Arctic
Oscillation index in April (partial R
2
=0.60) and
negatively in September (partial R
2
=0.14; multiple
regression, F
2,10
=13.93, P=0.0013, R
2
=0.74).
There was a decrease in the body length of adult fe-
males (
‡5 years of age) by year of birth over the study
(F
1,322
=11.80, P<0.001, R
2
=0.04, slope=
0.23 cm/
year; Fig.
). Similarly, there was a significant negative
trend in body length of adult males
of birth (linear regression, F
Fig. 5 Dynamics of natality
rate (cubs/female per year) and
litter production rate
(proportion of reproductive
aged females with cubs) for
female polar bears at Svalbard,
Norway, from 1993 to 2002.
Solid
and dashed lines indicate
the linear regressions
Fig. 6 Relationship between
natality and litter production
and the Arctic Oscillation index
for polar bears in Svalbard,
Norway, 1990–2002. The mean
of April–June Arctic Oscillation
values were used for litter
production and May for
natality. Solid and dashed lines
indicate linear regressions
271
Discussion
Age structure
The population of polar bears at Svalbard has a long
history of harvest that dates back almost to the dis-
covery of the islands in 1596 (Conway
). In modern
times, the population was intensively harvested before
total protection in 1973 with an average of 320 bears/
year taken between 1945 and 1970 (Lønø
; Larsen
). Most of the harvest was conducted on land using
self-killing guns which were indiscriminate with respect
to sex or reproductive status and the majority of the
harvest occurred on the eastern side of Svalbard where
maternity dens are concentrated (Lønø
; Larsen
). Many cubs were captured alive suggesting that a
large number of adult females were killed and, in har-
vested populations, population trend is most sensitive to
the harvest of adult females (Taylor et al.
). The
population was thought to be in decline due to excessive
harvest at least up until 1970 but had begun to recover
after this time (Larsen
An earlier study that examined the population up to
1993 reported a scarcity of female polar bears >15 years
of age (Wiig
) suggesting that the population had
not fully recovered at that time. Three causes for the
rarity of older females were postulated: errors in ageing,
sampling bias and differential mortality with higher
mortality of older females (Wiig
). However, all
three of these issues were dismissed by Wiig (
) and
no clear hypothesis for the lack of older females was
offered. The issue of differential mortality was dismissed
because higher mortality rates of females in their teens
would be a substantial departure from other polar bear
populations where adult female survival rates were
estimated to be 0.952–0.983 and higher than younger
females (Amstrup and Durner
). In contrast, a
hypothesis was forwarded that, following the cessation
of hunting in Norway, a rapid rise in pollutant levels
may have slowed the recovery in the population through
reduced natality or higher mortality rates that would
have reduced the abundance of females in their mid-
teens (Derocher et al.
). The Svalbard population
was much more polluted than other polar bear popula-
tions with pollutant loads at or exceeding the level where
reproduction and survival has been affected in other
species (AMAP
; Norstrom et al.
). Some ma-
jor pollutant levels declined in the 1990s (Henriksen
et al.
) but the impacts, if any, on the population
remain speculative.
Changes in the age-structure of bears have been used
to assess the impacts of harvest (Paloheimo and Fraser
; Fraser et al.
; Bunnell and Tait
) and
increases in age following harvest controls have been
interpreted to represent reduced harvest mortality and
population recovery (Bunnell and Tait
; Amstrup
et al.
; Stirling
). Analyses show that the mean
age of adult females and males in the population has
increased. The mean age of bears in the Svalbard pop-
ulation (
‡1 year) in 1988–1992 (about 8 years old for
females and males) was similar to that found in a heavily
harvested population in the Beaufort Sea in western
Canada (Stirling
). This is further support that the
population had not recovered from over-harvest by the
early 1990s. In contrast to the Svalbard population,
recovery in the Beaufort Sea population was rapid fol-
lowing the introduction of harvest regulations and the
mean age of captured bears increased by about 2 years
in a 3-year period (Stirling
). The increase in the
mean age of females and males suggests that recovery in
the population beyond that noted by Larsen (
continued even after 30 years without harvest. The slow
recovery in the Svalbard population would support the
suggestion by Derocher et al. (
) that pollution may
have affected population growth.
Fig. 8 Mean body length (±SE) of adult female polar bears
(
‡5 years old) and adult males (‡8 years old) caught near Svalbard
between 1988 and 2002 by year of birth. Solid and dashed lines
indicate the linear regressions
Fig. 7 Mean mass (±SE) of adult female polar bears (
‡4 years old)
without cubs-of-the-year and adult males (
‡10 years old) caught
near Svalbard between 1990 and 2002
272
Reproduction
The mean litter size of cubs and yearlings, litter pro-
duction rate and natality rate found in this study were
similar to those reported from other populations (Stir-
ling et al.
; Ramsay and Stirling
; Derocher
). Litter production rate was estimated at 0.41 lit-
ters/female per year based on satellite telemetry obser-
vations for the period 1988–1996 (Wiig
), and this
rate was similar to the overall rate of 0.40 litters/female
per year for bears of the same ages for 1993–2002 from
capture data. Changes in the reproductive rates of polar
bears have been related to density-dependent changes
(Derocher and Stirling
) but in the absence of
population estimates for the Svalbard population this
hypothesis cannot be tested. Annual fluctuations in
natality rates (range 0.3–1.2 cubs/female per year) in the
population were higher than found in the Beaufort Sea
where values ranged between 0.2 cubs/female and
0.6 cubs/female per year (Stirling
). The increase in
mean age of females and males could be partially ex-
plained by a reduction in recruitment to the population,
which is suggested by the decline in litter production rate
and natality. Further research in the population is nee-
ded to determine whether the changes in the population
are part of longer-term fluctuations or directional
changes associated with density-dependence or climate
change. Decreases in the vital rates of polar bears have
been linked to climate warming in other populations
(Stirling et al.
; Derocher et al.
Reproductive rates for polar bears show both long-
and short-term variation (Derocher and Stirling
Stirling
), but this is the first study to link repro-
duction in polar bears to climate: the Arctic Oscillation
index. However, given that the population may be
showing density-dependent responses, it is not possible
to differentiate the climatic effects from population ef-
fects. However, a recent study on ringed seals has linked
the North Atlantic Oscillation and climatic parameters
to the recruitment of seal pups into the population
(Ferguson et al.
). Given that ringed seals are a
primary prey species of polar bears in the Barents Sea
(Derocher et al.
) and the linkages between body
condition and reproductive rates in polar bears (Der-
ocher and Stirling
), the linkage between polar
bear reproduction and the Arctic Oscillation may be
related to the availability of prey. Further research in the
population should allow a better understanding of the
relationship between climate and reproduction.
Dynamics of body size
Temporal, sex-specific and individual variation in body
size of mammals is often associated with nutritional
conditions during the growth period (e.g., Ralls and
Harvey
; Kingsley et al.
; Fowler
; Leberg
and Smith
). Long-term trends in polar bears have
only been documented in western Hudson Bay where
most age and sex classes have declined in mass and adult
females declined in body length (Derocher and Stirling
; Atkinson et al.
). Short-term variation in
body mass linked to sea ice conditions was noted in the
Beaufort Sea population (Kingsley
; Stirling
Similar to other species, the variation and changes in
body size or condition in polar bears ultimately link
back to the nutrition available to individuals or cohorts.
The linkages found between the body mass of adult
females and adult males and the Arctic Oscillation
suggest that, similar to many other ecosystems, climate
plays an important role in the dynamics of the popula-
tion. The stronger correlation between the Arctic
Oscillation index in the spring for both females and
males suggests that climate influences affect polar bears
more during this period. Spring is the period of hyper-
phagia for polar bears (Watts and Hansen
; Ramsay
and Stirling
) but the environmental conditions that
the Arctic Oscillation produces on the sea ice that are of
significance to polar bears are unclear. Polar bear
hunting success is correlated with snow cover over rin-
ged seal lairs, with lower success with greater snow
depths (Hammill and Smith
), but research is nee-
ded to determine whether snow depth on the sea ice in
polar bear habitat is correlated with the Arctic Oscilla-
tion. Body mass in both sexes is also correlated, but less
so, with the Arctic Oscillation in the autumn and winter
but the cause of these linkages may be more difficult to
determine because polar bear sea ice habitats are less
accessible at this time of year. Timing of ice formation or
winter severity may be areas of future investigations.
Unlike the western Hudson Bay population where
long-term declines in the body mass and condition of the
polar bear population have been noted (Derocher and
Stirling
; Stirling et al.
), the Svalbard popu-
lation shows no linear trend in body mass over time. The
suggestion of a cycle in the body mass of the bears
warrants further study with additional years of data but,
given that that polar bears accumulate extensive fat
stores with age (Derocher and Stirling
; Atkinson
and Ramsay
), it seems plausible that a year of poor
nutrition may result in a multi-year recovery in a pop-
ulation when measured as body mass.
Long-term changes in the distribution of sea ice are
well documented and have been correlated with climatic
change (Parkinson
; Comiso
). Within the Ba-
rents Sea, sea ice has decreased by approximately 2–8%
per decade and is related to the Arctic Oscillation (Liu
et al.
). The linkage between climate and polar bears
may be indirect and relate to changes in sea ice and
access to prey (Stirling and Derocher
; Derocher
et al.
). Alternatively, the linkage may be through
bottom-up processes affecting phytoplankton, zoo-
plankton and fish, as has been noted in the Barents Sea
(Ottersen and Stenseth
), and thence to the prey of
polar bears: ringed seals and bearded seals (Erignathus
barbatus
; Derocher et al.
Changes in the mean adult body length of polar bears
were noted in western Hudson Bay but only in adult
273
females (Atkinson et al.
). The lack of decline in
adult males was thought to result from lower power to
detect changes due to the prolonged growth period of
males (ibid.). In contrast, the decline in body length of
both adult females and adult males in the Svalbard area
may be due to a longer study period or more rapid de-
cline and a greater ability to detect changes. While
Atkinson et al. (
) were unable to determine the
cause of the reduction in body length, they hypothesized
that reduction in per capita nutrition may be involved.
Similarly, longer-term changes in prey abundance or per
capita availability to the bears in the Barents Sea may
have been involved in the decline in body length.
Lacking data on changes in population size over time,
it is not possible to conclude that the changes observed
in the population are related to density-dependent re-
sponses. However, increases in the mean age, decreases
in reproductive rates and a reduction in body length are
consistent with changes associated with increasing pop-
ulation density. That the population may have been slow
to recover from over-harvest due to the effects of pol-
lutants is still speculative and unlikely to be resolved
with the data that exists for this population.
Exploration of relationships between reproduction
and body mass and climatic indices should provide a
fruitful area for further research. However, as Forch-
hammer and Post (
) noted, large-scale climatic
indices only provide part of the information needed and
understanding local weather conditions is essential. Gi-
ven the remoteness of polar bear habitat, these linkages
may be difficult to establish. However, the rapid changes
in the sea ice of the Arctic that are linked to climate
warming suggest that research on the linkages between
climate and the upper trophic levels of Arctic marine
ecosystems warrant immediate attention.
Acknowledgements This study was supported by the Norwegian
Polar Institute, the Norwegian Research Council, the Norwegian
Ministry of the Environment, the Norwegian Department of Nat-
ure Management, the WWF Arctic Programme and the Natural
Sciences and Engineering Research Council of Canada. I am
grateful for the assistance provided by the Governor of Svalbard
and the staff of the Hopen Weather Station. I would like to thank
Øystein Wiig, Magnus Andersen, Pa˚l Prestrud, Mette Mauritzen,
Stanislav Belikov and Andrei Boltunov for their support and
assistance in this study. Animal handling methods were approved
by the National Animal Research Authority (P.O. Box 8147 Dep.,
0033 Oslo, Norway).
References
Aanes R, Sæther BE, Smith FM, Cooper EJ, Wookey PA, Ørits-
land NA (2002) The Arctic Oscillation predicts effects of climate
change in two trophic levels in a high-arctic ecosystem. Ecol
Lett 5:445–453
A˚dlandsvik B, Loeng H (1991) A study of the climatic system in the
Barents Sea. Polar Res 10:45–49
AMAP (1998) AMAP assessment report: Arctic pollution issues.
Arctic monitoring and assessment programme, Oslo
Amstrup SC, Durner GM (1995) Survival rates of radio-collared
female polar bears and their dependent young. Can J Zool
73:1312–1322
Amstrup SC, Stirling I, Lentfer JW (1986) Past and present status
of polar bears in Alaska. Wildl Soc Bull 14:241–254
Atkinson SN, Ramsay MA (1995) The effects of prolonged
fasting of the body composition and reproductive success
of female polar bears (Ursus maritimus). Funct Ecol 9:559–
567
Atkinson SN, Stirling I, Ramsay MA (1996) Growth in early life
and relative body size among adult polar bears (Ursus mariti-
mus
). J Zool 239:225–234
Barber DG, Iacozza J (2004) Historical analysis of sea ice condi-
tions in M’Clintock Channel and the Gulf of Boothia, Nun-
avut: implications for ringed seal and polar bear habitat. Arctic
57:1–14
Bernhoft A, Skaare JU, Wiig Ø, Derocher AE, Larsen HJS (2000)
Possible immunotoxic effects of organochlorines in polar bears
(Ursus maritimus) at Svalbard. J Toxicol Environ Health A
59:561–574
Bunnell
FL,
Tait
DEN
(1981)
Population
dynamics
of
bears—implications. In: Fowler CW, Smith TD (eds) Dynamics
of large mammal populations. Wiley, New York, pp 75–98
Bunnell FL, Tait DEN (1985) Mortality rates of North American
bears. Arctic 38:316–323
Calvert W, Ramsay MA (1998) Evaluation of age determination of
polar bears by counts of cementum growth layer groups. Ursus
10:449–453
Comiso JC (2002) A rapidly declining perennial sea ice cover in the
Arctic. Geophys Res Lett 29:1956
Conway M (1906) No man’s land: a history of Spitsbergen from its
discovery in 1596 to the beginning of the scientific exploration
of the country. Cambridge University Press, Cambridge
DeMaster DP, Stirling I (1981) Ursus maritimus. Mamm Spec
145:1–7
Derocher AE (1999) Latitudinal variation in litter size of polar
bears: ecology or methodology? Polar Biol 22:350–356
Derocher AE, Lunn NJ, Stirling I (2004) Polar bears in a warming
climate. Integr Comp Biol 44:163–176
Derocher AE, Stirling I (1992) The population dynamics of polar
bears in western Hudson Bay. In: McCullough DR, Barrett RH
(eds) Wildlife 2001: populations. Elsevier, London, pp 1150–
1159
Derocher AE, Stirling I (1994) Age-specific reproductive perfor-
mance of female polar bears. J Zool 234:527–536
Derocher AE, Stirling I (1995) Temporal variation in reproduction
and body mass of polar bears in western Hudson Bay. Can J
Zool 73:1657–1665
Derocher AE, Stirling I (1996) Aspects of survival in juvenile polar
bears. Can J Zool 74:1246–1252
Derocher AE, Stirling I (1998) Maternal investment and factors
affecting offspring size in polar bears (Ursus maritimus). J Zool
245:253–260
Derocher AE, Wiig Ø, Andersen M (2002) Diet composition of
polar bears in Svalbard and the western Barents Sea. Polar Biol
25:448–452
Derocher AE, Wiig Ø (2002) Postnatal growth in body length and
mass of polar bears (Ursus maritimus) at Svalbard. J Zool
256:343–349
Derocher AE, Wolkers H, Colborn T, Schlabach M, Larsen TS,
Wiig Ø (2003) Contaminants in Svalbard polar bear samples
archived since 1967 and possible population level effects. Sci
Total Environ 301:163–174
Eberhardt LL (1977) Optimal policies for conservation of large
mammals, with special reference to marine ecosystems. Biol
Conserv 4:205–212
Ferguson SH, Stirling I, McLoughlin P (2005) Climate change and
ringed seal (Phoca hispida) recruitment in western Hudson Bay.
Mar Mamm Sci 21:121–135
Forchhammer MC, Post E (2004) Using large-scale climate indices
in climate change ecology studies. Pop Ecol 46:1–12
Forchhammer MC, Post E, Stenseth NC (1998) Breeding phenol-
ogy and climate. Nature 391:29–30
Fowler CW (1990) Density dependence in northern fur seals
(Callorhinus ursinus). Mar Mamm Sci 6:171–196
274
Fraser D, Gardner JF, Kolenosky GB, Strathearn S (1982) Esti-
mation of harvest rate of black bears from age and sex data.
Wildl Soc Bull 10:53–57
Hammill MO, Smith TG (1991) The role of predation in the
ecology of the ringed seal in Barrow Strait, Northwest Terri-
tories, Canada. Mar Mamm Sci 7:123–135
Henriksen EO, Derocher AE, Gabrielsen GW, Skaare JU, Wiig Ø
(2001) Monitoring PCBs in polar bears: lessons learned from
recent data. J Environ Monit 3:493–498
Kingsley MCS (1979) Fitting the von Bertalanffy growth equation
to polar bear age-weight data. Can J Zool 57:1020–1025
Kingsley MCS, Nagy JA, Reynolds HV (1988) Growth in length
and weight of northern brown bears: differences between sexes
and populations. Can J Zool 66:981–986
Larsen T (1985) Polar bear denning and cub production in Sval-
bard, Norway. J Wildl Manage 49:320–326
Larsen T (1986) Population biology of the polar bear (Ursus
maritimus
) in the Svalbard area. Norsk Polarinst Skr 184:1–55
Leberg PL, Smith MH (1993) Influence of density on growth of
white-tailed deer. J Mamm 74:723–731
Lee J, Taylor M (1994) Aspects of the polar bear harvest in the
Northwest Territories, Canada. Int Conf Bear Biol Manag
9:237–243
Lie E, Larsen HJS, Larsen S, Johansen GM, Borgen T, Derocher
AE, Lunn NJ, Norstrom RJ, Wiig Ø, Skaare JU (2003) Does
high OC exposure impair the resistance to infection in polar
bears (Ursus maritimus)? Part I: effect of OC on the humoral
immunity. J Toxicol Environ Health A 67:555–582
Liu J, Curry JA, Hu Y (2004) Recent Arctic sea ice variability:
connections to the Arctic Oscillation and the ENSO. Geophys
Res Lett 31:L09211
Lønø O (1970) The polar bear (Ursus maritimus Phipps) in the
Svalbard area. Norsk Polarinst Skr 149:1–115
Mauritzen M, Derocher AE, Wiig Ø (2001) Space-use strategies of
female polar bears in a dynamic sea ice habitat. Can J Zool
79:1704–1713
Mauritzen M, Derocher AE, Wiig Ø, Belikov SE, Boltunov A,
Hansen E, Garner GW (2002) Using satellite telemetry to define
spatial population structure in polar bears in the Norwegian
and western Russian Arctic. J Appl Ecol 39:79–90
Norstrom RJ, Belikov SE, Born EW, Garner GW, Malone B,
Olpinski S, Ramsay MA, Schliebe S, Stirling I, Stishov MS,
Taylor MK, Wiig Ø (1998) Chlorinated hydrocarbon contam-
inants in polar bears from eastern Russia, North America,
Greenland and Svalbard. Arch Environ Contam Toxicol
35:354–367
Ottersen G, Stenseth NC (2001) Atlantic climate governs oceano-
graphic and ecological variability in the Barents Sea. Limnol
Ocean 46:1774–1780
Paetkau D, Amstrup SC, Born EW, Calvert W, Derocher AE,
Garner GW, Messier F, Stirling I, Taylor M, Wiig Ø, Strobeck
C (1999) Genetic structure of the world’s polar bear popula-
tions. Mol Ecol 8:1571–1585
Paloheimo JE, Fraser D (1981) Estimation of harvest rate and
vulnerability from age and sex data. J Wildl Manag 45:948–958
Parkinson CL (2000) Variability of Arctic sea ice: the view from
space, an 18-year record. Arctic 53:341–358
Post E, Forchhammer MC (2002) Synchronization of animal
population dynamics by large-scale climate. Nature 420:168–
171
Prestrud P, Stirling I (1994) The International Polar Bear Agree-
ment and the current status of polar bear conservation. Aquat
Mamm 20.3:113–124
Ralls K, Harvey PH (1985) Geographic variation in size and sexual
dimorphism of North American weasels. Biol J Linn Soc
25:119–167
Ramsay MA, Stirling I (1988) Reproductive biology and ecology of
female polar bears (Ursus maritimus). J Zool 214:601–634
SAS Institute (1989) SAS/STAT User’s guide, ver. 6, 4th edn. SAS
Institute, Cary, N.C.
Shapiro I, Colony R, Vinje T (2003) April sea ice extent in the
Barents Sea, 1850–2001. Polar Res 22:5–10
Skaare JU, Bernhoft A, Derocher A, Gabrielsen GW, Goksøyr A,
Henriksen E, Larsen HJ, Lie E, Wiig Ø (1999) Organochlorines
in top predators at Svalbard—occurrence, levels and effects.
Toxicol Lett 112:103–109
Stirling I (2002) Polar bears and seals in the eastern Beaufort Sea
and Amundsen Gulf: a synthesis of population trends and
ecological relationships over three decades. Arctic 55:59–76
Stirling I, Calvert W, Andriashek D (1980) Population ecology
studies of the polar bear in the area of southeastern Baffin Is-
land. Can Wildl Serv Occ Pap 44:1–31
Stirling I, Derocher AE (1993) Possible impacts of climatic
warming on polar bears. Arctic 46:240–245
Stirling I, Kingsley M, Calvert W (1982) The distribution and
abundance of seals in the eastern Beaufort Sea, 1974–1979. Can
Wildl Serv Occ Pap 47:1–25
Stirling I, Lunn NJ, Iacozza J (1999) Long-term trends in the
population ecology of polar bears in western Hudson Bay in
relation to climate change. Arctic 52:294–306
Stirling I, Spencer C, Andriashek D (1989) Immobilization of polar
bears (Ursus maritimus) with Telazol in the Canadian Arctic. J
Wildl Dis 25:159–168
Taylor MK, DeMaster DP, Bunnell FL, Schweinsburg RE (1987)
Modeling the sustainable harvest of female polar bears. J Wildl
Manag 51:811–820
Thompson DWJ, Wallace JM (1998) The Arctic Oscillation sig-
nature in the wintertime geopotential height and temperature
fields. Geophy Res Lett 25:1297–1300
Tremblay LB (2001) Can we consider the Arctic Oscillation inde-
pendently from the Barents Oscillation? Geophys Res Lett
28:4227–4330
Vibe C (1967) Arctic animals in relation to climatic fluctuations.
Meddelelser om Grønland 170:1–227
Watts PD, Hansen SE (1987) Cyclic starvation as a reproductive
strategy in the polar bear. Symp Zool Soc Lond 57:305–318
Wiig Ø (1995) Distribution of polar bears (Ursus maritimus) in the
Svalbard area. J Zool 237:515–529
Wiig Ø (1998) Survival and reproductive rates for polar bears at
Svalbard. Ursus 10:25–32
275
Document Outline
- Sec1
- Sec2
- Fig1
- Sec3
- Sec4
- Fig2
- Sec5
- Tab1
- Fig3
- Fig4
- Sec6
- Fig5
- Fig6
- Sec7
- Sec8
- Fig8
- Fig7
- Sec9
- Sec10
- Ack
- Bib
- CR1
- CR2
- CR3
- CR4
- CR5
- CR6
- CR7
- CR8
- CR9
- CR10
- CR11
- CR12
- CR13
- CR14
- CR15
- CR16
- CR17
- CR18
- CR19
- CR20
- CR21
- CR22
- CR23
- CR24
- CR25
- CR26
- CR27
- CR28
- CR29
- CR30
- CR31
- CR32
- CR33
- CR34
- CR35
- CR36
- CR37
- CR38
- CR39
- CR40
- CR41
- CR42
- CR43
- CR44
- CR45
- CR46
- CR47
- CR48
- CR49
- CR50
- CR51
- CR52
- CR53
- CR54
- CR55
- CR56
- CR57
- CR58
- CR59
- CR60
- CR61
- CR62
- CR63
- CR64
- CR65
- CR66
- CR67
- CR68
- CR69
Wyszukiwarka
Podobne podstrony:
polar bears in warming climate
polar bears make little use of Nieznany
possible impacts of climatic warming on polar bears
Jażdżewska, Iwona The Warsaw – Lodz Duopolis in the light of the changes in the urban population de
Variation in NSSI Identification and Features of Latent Classes in a College Population of Emerging
[Open Life Sciences] Genetic diversity and population structure of wild pear (Pyrus pyraster (L ) Bu
polar bears make little use of terrestial food web, Canada
autismo prevalence of disorders of the autism spectrum in a population cohort of children in south t
possible impacts of climatic warming on polar bears
terrestrial foraging by polar bears during ice free period in hudson bay
Dance, Shield Modelling of sound ®elds in enclosed spaces with absorbent room surfaces
Proteomics of drug resistance in C glabrata
Microstructures and stability of retained austenite in TRIP steels
MMA Research Articles, Risk of cervical injuries in mixed martial arts
Development of financial markets in poland 1999
Antigone Analysis of Greek Ideals in the Play
Analysis of Police Corruption In Depth Analysis of the Pro
więcej podobnych podstron