Post by warsaw on Feb 22, 2014 9:44:53 GMT -9
Demography of barren-ground grizzly bears
Philip D. McLoughlin, Mitchell K. Taylor, H. Dean Cluff, Robert J. Gau,
Robert Mulders, Ray L. Case, Stan Boutin, and François Messier
Grizzly bears (Ursus arctos), like many long-lived animals,
are highly susceptible to overexploitation. Late age at maturity,
small litter sizes, and long interbirth intervals maintain
low intrinsic rates of increase for the species. Because of this,
all populations of grizzly bears in Canada are classified as being
of “special concern” to the Committee on the Status of
Endangered Wildlife in Canada (COSEWIC 2002).
Barren-ground grizzly bears inhabiting the Arctic coastal
plain, however, may be particularly sensitive to overexploitation
because they live at low densities in an area of low productivity
and high seasonality (Ferguson and McLoughlin 2000;
McLoughlin et al. 2000). We would predict, relative to other
grizzly bear populations, low reproduction resulting from
delayed age at first parturition, longer birth and reproductive
intervals, and smaller litter sizes. Of all grizzly bear populations,
barren-ground populations may be most susceptible to
direct mortality associated with human activity...
Discussion
Ferguson and McLoughlin (2000) concluded that in areas
of high altitude (>1000 m) and high latitude (>65°N), populations
of grizzly bears respond to extremes in environmental
conditions with risk-spreading adaptations. For example,
seasonality explains 43% of the variation in age at maturity
for Arctic-interior populations of grizzly bears in North
America (Ferguson and McLoughlin 2000). Populations in
these extreme environments are limited by resources; hence,
life-history responses should limit reproductive effort. Reproductive
females allocate resources to offspring that reduce
the risk of cub mortality. If females allocate their resources
sequentially in reproductive bouts, they should allocate them
to a safer, less productive option in risky environments of
extreme variability. The Arctic is characterized by less predictable
year-to-year variation and greater interannual (i.e.,
seasonal) variation (Ferguson and Messier 1996; McLoughlin
et al. 2000). Changes in the timing of reproduction in life
history, such as a greater age at maturity, a longer interbirth
interval, greater longevity (Cohen 1970; Philippi and Seger
1989; Sajah and Perrin 1990), and reduced offspring size
and number (McGinley et al. 1987), minimize the effects of
a stochastic environment so the geometric fitness is greater
(Yoshimura and Jansen 1996).
The grizzly bear population in Canada’s central Arctic is
near the northern- and eastern-most extent of grizzly bear
range in North America. The population is characterized by
relatively low density and living in an area of low productivity
and high seasonality (Ferguson and McLoughlin 2000;
McLoughlin et al. 2000). We anticipated low reproduction
resulting from delayed age at first parturition, longer birth
and reproductive intervals, and smaller litter size.
As expected, age at first parturition was late compared
with that of other grizzly bear populations (Case and Buckland
1998; Ferguson and McLoughlin 2000); however, birth
and reproductive intervals were shorter than for most northern
populations, and similar to intervals of southern interior
populations. Further, litter sizes of this study were among
the largest recorded for grizzly bears in Canada and Alaska
(Case and Buckland 1998). Natality, which reflects both litter
size and birth interval, indicated that cub production in
the central Arctic was higher than in most other grizzly bear
populations, including southern populations (Case and Buckland
1998). These data suggest that factors other than adaptations
to low primary productivity and high seasonality are
governing the life history of grizzly bears in Canada’s central
Arctic.
Although it is not reflected by the relatively high survival
rates obtained for adult males, there has been a strongly
male-biased harvest (approximately 13.4 bears per year, of
which ~30% are females) in the study area for over 40 years
(McLoughlin and Messier 2001). As a result, it is possible
that male density in the study area is substantially lower than
female density. Female reproduction may be enhanced by reduced
numbers of males because of reduced risks from
intraspecific predation (Miller 1990; McLellan 1994), unless
the killing of adult males invites immigration of predatory
subadult males (Wielgus and Bunnell 2000). Reduced intraspecific
predation may directly affect life history through
changes to mortality schedules. Where resources are scarce
or unpredictable (i.e., the central Arctic), lower rates of
intraspecific predation may indirectly influence life history
by allowing females with cubs to exploit higher quality habitats
from which they were once excluded by predatory
males. Here, life-history traits such as adult female size, offspring
size, litter size, and reproductive interval may be affected.
Body mass of adult females (mean = 126 kg, n = 60;
from Ferguson and McLoughlin 2000) in the central Arctic
averages 10–20 kg more than that in adjacent barren-ground
grizzly bear populations (Ferguson and McLoughlin 2000).
An increase in body size may account for the larger litters
observed in this study, and potentially plays a role in shortening
reproductive intervals by resulting in larger offspring
at birth and (or) increasing milk production.
We believe the population of grizzly bears in the study
area to be stable or slightly increasing (λ = 1.033); however,
there is uncertainty about this estimate because of our inability
to adequately estimate all required parameters. Our estimated
subadult survival rate (0.831), which is the mean
between yearling and adult female survival rates (excluding
capture mortality) for ages 2–4, has the greatest potential for
error. Nonetheless, we believe this figure to be conservative:
the rate is at the low end of the range of subadult female survival
rates reported by Wielgus (2002) for grizzly bear populations
in the Rocky Mountains (mean = 0.868, SD = 0.054).
Although we note stability in the population under study,
we caution that there is a definite risk of future population
decline if annual harvest rates are increased from historic
levels. Modelling studies using data presented here show
that by only slightly increasing historic rates of harvest from
a mean of 13.4 bears per year precipitates a negative population
trajectory (McLoughlin and Messier 2001). Unreported
illegal mortality may already be contributing to a higher risk
Philip D. McLoughlin, Mitchell K. Taylor, H. Dean Cluff, Robert J. Gau,
Robert Mulders, Ray L. Case, Stan Boutin, and François Messier
Grizzly bears (Ursus arctos), like many long-lived animals,
are highly susceptible to overexploitation. Late age at maturity,
small litter sizes, and long interbirth intervals maintain
low intrinsic rates of increase for the species. Because of this,
all populations of grizzly bears in Canada are classified as being
of “special concern” to the Committee on the Status of
Endangered Wildlife in Canada (COSEWIC 2002).
Barren-ground grizzly bears inhabiting the Arctic coastal
plain, however, may be particularly sensitive to overexploitation
because they live at low densities in an area of low productivity
and high seasonality (Ferguson and McLoughlin 2000;
McLoughlin et al. 2000). We would predict, relative to other
grizzly bear populations, low reproduction resulting from
delayed age at first parturition, longer birth and reproductive
intervals, and smaller litter sizes. Of all grizzly bear populations,
barren-ground populations may be most susceptible to
direct mortality associated with human activity...
Discussion
Ferguson and McLoughlin (2000) concluded that in areas
of high altitude (>1000 m) and high latitude (>65°N), populations
of grizzly bears respond to extremes in environmental
conditions with risk-spreading adaptations. For example,
seasonality explains 43% of the variation in age at maturity
for Arctic-interior populations of grizzly bears in North
America (Ferguson and McLoughlin 2000). Populations in
these extreme environments are limited by resources; hence,
life-history responses should limit reproductive effort. Reproductive
females allocate resources to offspring that reduce
the risk of cub mortality. If females allocate their resources
sequentially in reproductive bouts, they should allocate them
to a safer, less productive option in risky environments of
extreme variability. The Arctic is characterized by less predictable
year-to-year variation and greater interannual (i.e.,
seasonal) variation (Ferguson and Messier 1996; McLoughlin
et al. 2000). Changes in the timing of reproduction in life
history, such as a greater age at maturity, a longer interbirth
interval, greater longevity (Cohen 1970; Philippi and Seger
1989; Sajah and Perrin 1990), and reduced offspring size
and number (McGinley et al. 1987), minimize the effects of
a stochastic environment so the geometric fitness is greater
(Yoshimura and Jansen 1996).
The grizzly bear population in Canada’s central Arctic is
near the northern- and eastern-most extent of grizzly bear
range in North America. The population is characterized by
relatively low density and living in an area of low productivity
and high seasonality (Ferguson and McLoughlin 2000;
McLoughlin et al. 2000). We anticipated low reproduction
resulting from delayed age at first parturition, longer birth
and reproductive intervals, and smaller litter size.
As expected, age at first parturition was late compared
with that of other grizzly bear populations (Case and Buckland
1998; Ferguson and McLoughlin 2000); however, birth
and reproductive intervals were shorter than for most northern
populations, and similar to intervals of southern interior
populations. Further, litter sizes of this study were among
the largest recorded for grizzly bears in Canada and Alaska
(Case and Buckland 1998). Natality, which reflects both litter
size and birth interval, indicated that cub production in
the central Arctic was higher than in most other grizzly bear
populations, including southern populations (Case and Buckland
1998). These data suggest that factors other than adaptations
to low primary productivity and high seasonality are
governing the life history of grizzly bears in Canada’s central
Arctic.
Although it is not reflected by the relatively high survival
rates obtained for adult males, there has been a strongly
male-biased harvest (approximately 13.4 bears per year, of
which ~30% are females) in the study area for over 40 years
(McLoughlin and Messier 2001). As a result, it is possible
that male density in the study area is substantially lower than
female density. Female reproduction may be enhanced by reduced
numbers of males because of reduced risks from
intraspecific predation (Miller 1990; McLellan 1994), unless
the killing of adult males invites immigration of predatory
subadult males (Wielgus and Bunnell 2000). Reduced intraspecific
predation may directly affect life history through
changes to mortality schedules. Where resources are scarce
or unpredictable (i.e., the central Arctic), lower rates of
intraspecific predation may indirectly influence life history
by allowing females with cubs to exploit higher quality habitats
from which they were once excluded by predatory
males. Here, life-history traits such as adult female size, offspring
size, litter size, and reproductive interval may be affected.
Body mass of adult females (mean = 126 kg, n = 60;
from Ferguson and McLoughlin 2000) in the central Arctic
averages 10–20 kg more than that in adjacent barren-ground
grizzly bear populations (Ferguson and McLoughlin 2000).
An increase in body size may account for the larger litters
observed in this study, and potentially plays a role in shortening
reproductive intervals by resulting in larger offspring
at birth and (or) increasing milk production.
We believe the population of grizzly bears in the study
area to be stable or slightly increasing (λ = 1.033); however,
there is uncertainty about this estimate because of our inability
to adequately estimate all required parameters. Our estimated
subadult survival rate (0.831), which is the mean
between yearling and adult female survival rates (excluding
capture mortality) for ages 2–4, has the greatest potential for
error. Nonetheless, we believe this figure to be conservative:
the rate is at the low end of the range of subadult female survival
rates reported by Wielgus (2002) for grizzly bear populations
in the Rocky Mountains (mean = 0.868, SD = 0.054).
Although we note stability in the population under study,
we caution that there is a definite risk of future population
decline if annual harvest rates are increased from historic
levels. Modelling studies using data presented here show
that by only slightly increasing historic rates of harvest from
a mean of 13.4 bears per year precipitates a negative population
trajectory (McLoughlin and Messier 2001). Unreported
illegal mortality may already be contributing to a higher risk
