AbstractPredation 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.

Climate PatternsSeveral 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.

Caribou Population Growth RatesWe 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.

Linkages between large-scale climate patterns and the dynamics of Arctic ungulate populations

“Population growth rates for all four Arctic caribou herds were significantly correlated with the intensity of large-scale climatic patterns”

PDO PDO AO AO

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

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• 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

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Western Arctic Herd • 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

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Arctic Oscillation (AO) – Northeast Alaska

Variable Positive Phase Negative PhaseSea-level Pressure Lower (-) Higher (+)Air Temperature, Winter Higher (+) Lower (-)Air Temperature, Summer Lower (-) Lower (-) * Different from PDOAnnual Precipitation Higher (+) Lower (-)Cloudiness, Winter Higher (+) Lower (-)Cloudiness, Summer Higher (+) Lower (-) * Different from PDO

Pacific Decadal Oscillation (PDO) – Northcentral Alaska

Variable Positive Phase Negative PhaseSea-level Pressure Lower (-) Higher (+)Air Temperature, Winter Higher (+) Lower (-)Air Temperature, Summer Higher (+) Lower (-) * Different from AOAnnual Precipitation Lower (-) Higher (+) * Different from AO & Western AKCloudiness, Winter Higher (+) Lower (-)Cloudiness, Summer Lower (-) Higher (+)

Pacific Decadal Oscillation (PDO) – Western Alaska

Variable Positive Phase Negative PhaseSea-level Pressure Lower (-) Higher (+)Air Temperature, Winter Higher (+) Lower (-)Air Temperature, Summer Higher (+) Lower (-) * Different from AOAnnual Precipitation Higher (+) Lower (-) * Different from Northcentral AKCloudiness, Winter Higher (+) Lower (-)Cloudiness, Summer Lower (-) Higher (+)

Porcupine Caribou Herd

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• 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

Teshekpuk Caribou Herd

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• 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

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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