Polar bears, snakes and caribou
Changes in terrain can greatly increase the time, effort, and danger involved in crossing. What's more, the terrain can quite literally change beneath their feet as the seasons cycle, leaving some routes only passable at specific points in the year, stranding wildlife in one place until the way clears.
Polar bears depend on sea ice in relatively shallow coastal areas to hunt their preferred prey, seals. Their massive bodies come with a massive appetite, and while they'll opportunistically eat a wide range of prey animals, seals are the only source of high-calorie blubber that can satisfy their metabolism's demands.
That makes the annual, seasonally-dependent formation and break-up of Arctic sea ice as critical to the timing of their migration as the blooming of nectar flowers is to that of birds and butterflies.
Polar bears live in multiple geographically dispersed populations that rarely interact, and the migratory patterns of each population differ in accordance with local geography and weather patterns. As sea ice recedes to the north each summer, some populations head for solid ground to select den sites near the shore, where they will wait for the ice to reform in autumn. Others follow the sea ice north, digging winter dens on pack ice.
Whether they spend the summer on dry land, or on pack ice over deep water, access to seals is greatly reduced, but those who remain on land have it especially rough. In Churchill, Manitoba, the summer ice-free period today is 3-4 weeks longer than it was 3 decades ago, which packs a double whammy for polars bears who must survive increasingly longer fasting seasons, with an increasingly shorter hunting season in which to pack on enough fat to do so.
In Greenland and Canada in the 1990s, many hunters mistakenly believed that polar bear populations were increasing and harvested more bears than ever. But in fact, the local population was already in decline. Hunters were simply seeing more of them because they were spending more time on shore waiting for the ice to return.
As polar bears linger on land longer each year and become increasingly desperate for food, contact with humans in some parts of Alaska, Canada, and Russia has increased in recent years. In Churchill, Manitoba, the self-proclaimed "Polar Bear Capital of the World," there's been an increase in polar bear encounters and — though they're still rare — attacks that has forced officials to step up efforts to deter the bears from entering town.
Most reptiles seem like unlikely candidates for migration. As ectotherms (cold-blood animals), their daily activities are tightly constrained by a temperature window outside of which they cannot effectively function, giving them limited time each day to cover ground. The amount of ground they can cover is likewise limited by their relatively small size and slower speeds.
Reptiles are also more likely to live in isolated ecological niches, surrounded by unsuitable habitat that effectively cuts them off from the rest of the world. For example, some U.S. and Mexican rattlesnakes that live high in the mountains may theoretically be able to thrive on a number of different mountaintops within a large mountain range, but would be unlikely to be able to make the journey through the lowlands in between to reach them.
But in Shawnee National Forest, the biannual crossing of Snake Road offers one of the most famous examples of snake migration. So many snakes — plus frogs, toads, and more reptiles and amphibians — cross the forest road that the U.S. Forest Services closes the road to traffic to protect them. These cold-blooded critters spend their summers in LaRue Swamp before crossing the road to reach the bluffs at the base of the forest's limestone cliffs on the other side, where most will tuck in for winter hibernation in the caves.
Before the Forest Service made the decision in 1972 to close the road to cars during the migration, countless snakes, frogs, toads, and turtles were struck by vehicles as they paused to bask in the sun on the warm asphalt. Thanks to their decision, and to the efforts of Scott Ballard, a District Heritage biologist and herpetologist who collected critically important data to support the closure, these sun-loving migrants are free to cross the road at their own pace as they complete their slow, hyper-local migration.
Roughly a quarter of a million caribou belong to the Western Arctic Herd, with a northwestern Alaska territory spanning 157,000 square miles — an area the size of the state of California. As they traverse the territory, driven by seasonal changes in weather and forage availability, these highly migratory animals can cover over 2,000 miles — on hoof — in a single year!
The herd once began heading south for the winter each September, but in recent decades warming has shifted their average departure time into late October and early November — and in some years, a majority of the herd stayed in their northern range all winter.
That shift has negative consequences for the Inupiaq hunters. Archaeological records reveal that for 10,000 years, Inupiaq hunters have met the migrating herd at Onion Portage, a caribou river crossing known in the Inupiaq tongue as Paatitaaq, meaning "wild onions" – which are especially abundant in the area. In 2017, when most of the herd skipped or delayed migrating south, hunters were unable to harvest what they needed.
Another potential threat looms for caribou, concerning their key winter food source, fruticose lichen. Because lichen grow very slowly in Alaska's black spruce stands, they're scarcely found in forests under 80 years old. If climate change drives more frequent wildfires in the area, the average age of remaining forests would decline, threatening to jeopardize the caribou's supply of this critically important winter food.