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Dynamics in Caribou Migration Patterns

Researchers at the University of Maryland have found an important dynamic underlying changes to Caribou (American reindeer, Rangifer tarandus) migration and population sizes. In recent decades, many herd populations in North America have experienced significant decline. Based on a large scale study that included data found from 80% of all migrating caribou over 20 years, researchers found that the amount of food resources available (ie, lichen) was not a driving factor in changing migration habits and that migration patterns showed significant changes only when it came to seasonal weather conditions and likely effects thereafter. This article discusses those underlying changes to an arctic keystone species as an example within the context of global climate change.

On dasher!, on dancer!, on prancer!,…… on dasher 25!, on dancer 25!…

Movin’ On Up: From Boreal Forest to Arctic Tundra

Caribou constitute the largest migrating mammal population in the world1. Their migrations surpass even the famous blue wildebeest migration in the savannas of Africa in terms of land distance covered as they migrate from wintering grounds in the boreal forest to summer calving grounds in the Canadian tundra to reproduce. Like many arctic animals, caribou have experienced significant decline in population as the climate has warmed. The lack of understanding of the specific connection between a warming climate and declining population have driven this research.

1. Kcutlip. “Who Goes Farthest? The World’s Longest Wild Animal Terrestrial Migrations And Movements.” College of Computer, Mathematical, and Natural Sciences, 25 Oct. 2019,
Surprisingly, an annual global migration of 9 caribou on December 24 – 25 was considered an outlier for this study and thus was not considered for the data set.

Availability of food resources can be a climactic and biotic factor in driving migration patterns. This was one suspected cause of these changes and subsequent declines of caribou from the current literature. As temperatures in the arctic have warmed significantly, forage is available at earlier times, plants can develop chemical defenses earlier, inedible shrubs can replace lichen in the tundra, and insects can harass large herbivores for longer periods of time that can lead to loss of foraging time2. These known factors led to hypotheses surrounding forage as an expected important dynamic in caribou decline.

2 Pearson, R. G., S. J. Phillips, M. M. Loranty, P. S. A. Beck, T. Damoulas, S. J. Knight, and S. J. Goetz. 2013. Shifts in Arctic vegetation and associated feedbacks under climate change. Nature Climate Change 3:673–677.
Caribou herds and migration locales

Calm and Warm Lead to Poor Maternal Health

Because of the large and varied data generated from collared animals from multiple herds and attempting to impress individual organism statistics to a local population, the researchers developed a complex model to test factors on altered migrations. The model tested factors such as: regional climactic patterns including ocean driven climate cycles (ie. North Atlantic Oscillation), seasonal snow coverage, and other weather data (ie. wind speed and temperature). The model weighed these variables compared to when in the spring the caribou left their wintering grounds (mainly in the boreal forest or taiga) to migrate to calving areas in the northern tundra of Canada and Alaska.

The scientists model showed inconsistent results from snow cover and vegetation when compared to migration times; an unexpected result meaning that earlier loss of snow cover or earlier emergence of vegetation could not account for earlier migration times. What did show significant effect was the weather in the previous summer on the migration times in the following spring meaning that something occurring during the warmer months on the tundra was causing delayed or earlier departure times from wintering grounds for migration.

Based on the study, researchers identified that maternal (cow) health was a primary factor in a changing arrival at calving grounds. The likely culprit for delayed arrivals and poor maternal health? Pesky insects – namely mosquitos, black flies, and bot flies.

The significant effects proposed by the authors shown in dotted red lines

The researchers model results showed how warmer weather during the summer combined with calm wind conditions resulted in a delayed migration time for caribou populations the following spring. The warmer temperatures and decreased wind speed would enhance insect harassment on mother caribou. A decreased ability to forage and build healthy tissue during the summer due to the insect attacks would likely delay migration starts the following spring because of decreased energy reserves and poor health of calves. Reinforcing this analysis, where weather data showed cooler summer temperatures and higher average wind speed, the following spring migration tended to be earlier and not delayed – a likely function of decreased insect harassment and improved maternal health.

Limited Say

What can we say with this study regarding changing migrations? While this study certainly constitutes a large-scale look at effects on caribou migration, the model the researchers used leaves many possibilities still frozen in the permafrost, so to speak. While their research was inconclusive when it came to forage availability, there were many unexplored tracts to this point. For example, snow cover was examined only from satellite data and leaves snow quality from the review which means that perhaps tramped down or iced over snow from warmer than usual weather events would be a contributing factor to altered migration times*. In addition, the specific rationale for earlier migration is still unconfirmed and should be duly tested. Perhaps this would involve a measure of caribou maternal body mass as a quantitative data point to connect previous summer weather with spring migration times. Specific tracking of individual cows and their experience surrounding insect harassment and measured health factors could add further support to the researchers findings.

*iced over or tramped down snow is easier to walk long distances on despite the decreased ability to forage underneath

Phenological Tracking and Plasticity

The significance of the changing caribou migration is just one example of how a keystone* species is being affected by a changing climate. Interestingly, the authors note how the migration time across the continent is relatively plastic for caribou; that is, having the capacity to change migration times strategically based on conditions. This capacity for changing migration times may help caribou “surf the snow line” (follow the emerging forage or stable ice for ease of commuting) but its ability to retain healthy herd populations with warming conditions remains to be tested as the arctic continues to rapidly warm and climactic conditions trend towards instability.

The rapid effects of anthropogenic climate change are driving alarming trends in what is known as phenological mismatch3 most prominently seen in arctic biomes. Warmer and earlier spring arrivals are causing producers (plants) to flower at earlier times often before the arrival of pollinators which can lead to population decrease as plants would lack their coevolution partner in reproduction and pollinators would miss an important food source. The effects on producers may in turn have consequences for larger herbivores such as the caribou.

*Caribou are keystone species in that many species in multiple trophic levels in the arctic ecosystem depend on them either as a food source, source of dispersal.
3 “Phenological Mismatch with Abiotic Conditions Implications for Flowering in Arctic Plants.” Ecology, U.S. National Library of Medicine, Mar. 2015,

Sources and Further Reads:

Gurarie, Eliezer, et al. “Tactical Departures and Strategic Arrivals: Divergent Effects of Climate and Weather on Caribou Spring Migrations.” The Ecological Society of America, John Wiley & Sons, Ltd, 12 Dec. 2019,

Kcutlip. “Caribou Migration Linked to Climate Cycles and Insect Pests.” College of Computer, Mathematical, and Natural Sciences, 12 Dec. 2019,

Saalfeld, Sarah T, et al. “Phenological Mismatch in Arctic-Breeding Shorebirds: Impact of Snowmelt and Unpredictable Weather Conditions on Food Availability and Chick Growth.” Ecology and Evolution, John Wiley and Sons Inc., 16 May 2019,, Helen C, et al. “Phenological Mismatch with Abiotic Conditions Implications for Flowering in Arctic Plants.” Ecology, U.S. National Library of Medicine, Mar. 2015,


Published by eprileson

I am a historian and writer who wants to bring to light current events through a historical perspective. It is difficult to understand today's current events without having a grasp of what has occurred before. This is a running thread to help keep people informed about the present and remind everyone to not forget their past. Enjoy and please comment!

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