CARIBOU FROM SPACE!
Getting the big view of migration timing
SOMETIMES YOU HAVE TO GET REALLY FAR BACK FROM SOMETHING TO understand what is going on. Consider the timing of migratory caribou movement en masse each spring. The longest terrestrial migration on Earth, it is a highly complex and variable mass movement that has occurred for millennia on a continental scale. Humans have been studying and tracking caribou up close for almost that long. But a recent and very remote observation has led to a major reconsideration of why migrating caribou migrate when they do.
The migratory barren ground caribou (Rangifer tarandus groenlandicus) is a subspecies found mainly in Nunavut, the Northwest Territories and Greenland. Smaller and lighter-coloured than boreal woodland caribou, barren ground caribou travel in large migratory herds categorized by their calving grounds (for example, the Baffin Island herds, the Bathurst herd, and the Qamanirjuaq Lake herd to the west of Hudson Bay).
Conventional wisdom says the timing of leaving their winter ranges largely depends on two localized factors: snowmelt and forage availability. That seemed obvious. However, a 2019 paper based on research partly funded by NASA, the U.S. space agency, suggests the massive move is not based on local conditions at all. Analyzing satellite images compiled over more than 20 years (see them at earthobservatory.nasa.gov), biologists were able to simultaneously observe seven different herds’ behaviours, taking herd monitoring to a whole different height. And what they found amazed them: the caribou all head out at the same time — the annual spring start to the migration is synchronized right across the continent.
What the researchers do not yet know is why. Some speculate the caribou can sense in some way large-scale climatic cycles, triggering continent-wide behaviours. Much more work at micro and macro levels remains.
The research raised another fundamental question and postulated another amazing answer: if migrating caribou departures are synchronized, why some years do they arrive at their calving grounds sporadically and scattered, their timing uncoordinated? The research suggests weather is a prime factor, which is hardly surprising. But the weather when? It turns out the single best predictor of the arrival time of caribou at their spring grounds are the conditions the previous summer!
The ideal caribou summer is cold and windy, conditions that limit bug populations in the bogs, fens and wetlands where caribou congregate. That’s because, a bit like humans, caribou suffer in buggy conditions. Caribou will walk or run for hours and days and hundreds of kilometres fleeing the harassment. The physical toll can be huge, taxing the female caribou to exhaustion, affecting the health of next year’s calves. Mating among the scattered herd is fragmented and delayed. The change of timing puts them out of sync with food supply. Birth outcomes are poorer. Herds dwindle another year.
If there is any hope of saving caribou over the long term, understanding the mechanisms and connections that explain the species’ heightened sensitivity to both local and systemic climatic conditions is crucial in this time of global heating, when almost every wild population of the planet’s caribou and reindeer is in serious decline.
To learn about Canada’s non-migrating caribou populations, see Understory, p 18.