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Linkages between large-scale climate patterns and the dynamics
of Arctic ungulate populations
Kyle Joly1,2, David R. Klein2, David L. Verbyla3, T. Scott Rupp3 and F. Stuart Chapin III2
1National
Park Service, Gates of the Arctic National Park and Preserve; 2 University of Alaska Fairbanks, Department of Biology and
Wildlife, Institute of Arctic Biology; 3 University of Alaska Fairbanks, School of Natural Resources and Agricultural Sciences
Caribou Population Growth Rates
Climate Patterns
We gathered existing estimates of the population sizes of the four Arctic caribou herds, the
Western Arctic Herd (WAH) depicted in blue, Teshekpuk Caribou Herd (TCH) in orange,
Central Arctic Herd (CAH) in purple and Porcupine Caribou Herd (PCH) in green. Records
with black stars are from Alaska Department of Fish and Game photocensuses, while
estimates in intervening years are based on equal rate changes between photocensuses. From
these estimates we calculated  (lamda), the population growth rate. We then compared  with
the intensity of regional climate patterns, both the AO and PDO, using linear regression.
Several indices have been developed to quantify the intensity of large-scale climate patterns,
including the North Atlantic Oscillation (NAO), Arctic Oscillation (AO), and the Pacific Decadal
Oscillation (PDO). The NAO, AO, and PDO indices are fundamentally similar in that each gauges
differences in oceanic temperatures and sea-level pressure. Each index can be divided into two
phases; “positive” or “negative”, whose effects depend on geographical location. The effects of
the NAO are most dramatic in Europe and in the eastern regions of North America. The AO has
stronger effects in the Arctic, especially northeastern Canada, though less so in northwestern
Canada and Alaska. The effects of the PDO are strong in the Pacific Northwest region of the
continental US and British Columbia, but also affect the interior and western regions of Alaska.
The PDO can have secondary effects in tropical regions, thus acting in a similar but opposite
manner of its more widely known counterpart, the El Nino-Southern Oscillation (ENSO). Unlike
the short-lived ENSO, the PDO and AO can remain in one phase for 20-30 years.
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Arctic Oscillation (AO) – Northeast Alaska
Negative Phase
Higher (+)
Lower (-)
Lower (-)
Lower (-)
Lower (-)
Lower (-)
* Different from PDO
* Different from PDO
Pacific Decadal Oscillation (PDO) – Northcentral Alaska
Variable
Sea-level Pressure
Air Temperature, Winter
Air Temperature, Summer
Annual Precipitation
Cloudiness, Winter
Cloudiness, Summer
Positive Phase
Lower (-)
Higher (+)
Higher (+)
Lower (-)
Higher (+)
Lower (-)
Negative Phase
Higher (+)
Lower (-)
Lower (-)
Higher (+)
Lower (-)
Higher (+)
* Different from AO
* Different from AO & Western AK
Pacific Decadal Oscillation (PDO) – Western Alaska
Positive Phase
Lower (-)
Higher (+)
Higher (+)
Higher (+)
Higher (+)
Lower (-)
Negative Phase
Higher (+)
Lower (-)
Lower (-)
Lower (-)
Lower (-)
Higher (+)
Western Arctic Herd
1.25
1.20
Growth Rate
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
-1.350
-0.850
-0.350
0.150
0.650
1.150
1.650
PDO
* Different from AO
* Different from Northcentral AK
• Positive correlation with
the “positive” phase of
the PDO
• Warmer, sunnier
summer conditions may
enhance growth of
forage, benefitting
caribou
• Growth of the herd was
associated with “positive”
phase values despite its
association with deeper
snows and warmer winter
temperatures that may
increase the number and
extent of rain-on-snow
and icing events
Caribou ranges and calving grounds in Arctic Alaska; Western Arctic Herd (WAH) depicted in blue, Teshekpuk
Caribou Herd (TCH) in orange, Central Arctic Herd (CAH) in purple and Porcupine Caribou Herd (PCH) in green.
Central Arctic Herd
Teshekpuk Caribou Herd
1.25
1.20
1.15
Growth Rate
Variable
Sea-level Pressure
Air Temperature, Winter
Air Temperature, Summer
Annual Precipitation
Cloudiness, Winter
Cloudiness, Summer
0
1.10
1.05
1.00
0.95
-1.100
-0.600
-0.100
0.400
0.900
1.400
1.900
PDO
• Negative correlation with the
“positive” phase of the PDO
• Warmer winter temperatures
increase the chances of
detrimental rain-on-snow and
icing events
•Decreased snow, more
summer wind, and warmer
summers temperatures cast
doubt on a number of
alternatives that could explain
the observed correlation
1.20
1.15
1.10
1.05
1.00
0.95
0.90
-50
0
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AO
• Negative correlation with the
“positive” phase of the AO
•Increased snow could restrict
access to winter forage,
increase vulnerability to
predators, and increase energy
expenditure while moving
• Warmer winter temperatures
increase the chances of
detrimental rain-on-snow and
icing events
• Cloudy, wet and cool
summers may retard growth of
summer forage species which
would be detrimental to
caribou
Porcupine Caribou Herd
1.09
1.07
1.05
Growth Rate
Positive Phase
Lower (-)
Higher (+)
Lower (-)
Higher (+)
Higher (+)
Higher (+)
Growth Rate
Variable
Sea-level Pressure
Air Temperature, Winter
Air Temperature, Summer
Annual Precipitation
Cloudiness, Winter
Cloudiness, Summer
Predation and winter range have often dominated debates about caribou
population dynamics. However, climate has direct and indirect impacts
on caribou populations across the Arctic. Linkages between large-scale
climate patterns and caribou populations have been documented in
Norway, Greenland, and Canada. We posited that similar linkages exist
in Alaska, so we analyzed the population growth rates of caribou herds
across arctic Alaska in relation to large-scale climate patterns, specifically
the Arctic Oscillation (AO) and the Pacific Decadal Oscillation (PDO).
We found that growth rates of all herds were significantly correlated with
these patterns. Positive and negative correlations were identified
depending on climate pattern and region. Climate may have negative
impacts of caribou populations through increased snow depths which
impede movement and access to forage in winter. Either of these effects
could affect predation rates. Under warming scenarios, increased
frequency and spatial extent of icing events or reduced lichen biomass, a
key winter forage, could also be detrimental to caribou. Warming may
however lengthen the growing season and enhance the growth of vascular
plants that are key summer forage species, benefiting caribou. We predict
that other wildlife species population dynamics in Alaska and the Asia are
also affected by large-scale climate patterns.
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Abstract
1.03
1.01
0.99
0.97
0.95
-50
-30
-10
10
30
50
70
90
110
AO
“Population growth rates for all four Arctic caribou herds were significantly
correlated with the intensity of large-scale climatic patterns”
• Negative correlation with the
“positive” phase of the AO
•Increased snow could restrict
access to winter forage,
increase vulnerability to
predators, and increase energy
expenditure while moving
• Warmer winter temperatures
increase the chances of
detrimental rain-on-snow and
icing events
• Cloudy, wet and cool
summers may retard growth of
summer forage species which
would be detrimental to
caribou