As I was going through some old emails this morning, I stumbled on a 2013 paper that had been forwarded to me, but I never got around to reading. Moments ago, I finished perusing it and found it pretty darn fascinating, with some interesting potential for the future.
First a little background:
The Porcupine Caribou Herd is currently the second largest herd in Alaska, and growing fast. If trends continue, in the next year or so, it will have more individuals than the shrinking Western Arctic Herd. Caribou populations fluctuate up and down, even by the hundreds of thousands, and the Porcupine is currently on an upswing. The herd’s range encompasses most of the Arctic National Wildlife Refuge, as well as a large chunk in the northern Yukon Territories, just over the border.
The caribou spend their winters south of the Brooks Range, then usually (though not always) move east into Canada, before migrating north onto the coastal plain where the calves are born in June. For many years, particularly during the 1990s and early 2000s, they calved on the plains of the Arctic Refuge’s 1002 area, but then for one reason or another, moved their calving grounds east into Canada. The Refuge was still heavily used, but for post-calving foraging. Last year, they again dropped their young on the US side of the border, making us wonder if perhaps they were shifting their calving range westward again.
Female caribou, unlike every other species of ungulate, have antlers. Males, and females who aren’t pregnant, shed their antlers in the autumn just after mating. Pregnant females, however, shed their antlers within a few days of having their calves.
Those antlers can survive for centuries, even millenia on the surface of the cold, arctic tundra. Which means that by looking at where female antlers are distributed on the tundra, we can get an idea of where calving grounds have been over long periods of time, and even what specific habitats are selected for calving.
Neat right?
It is neat. Very. And that is exactly what this 2013 paper in Proceedings of the Royal Society, Biological Sciences, looked into. The researchers selected a study area along the Jago River on the coastal plain of the Arctic National Wildlife Refuge, an area known to have hosted the herd during calving in recent decades. Within the study area, they ran surveys to count antler sheds and other bones.
It turns out, that the caribou do not seem to use the habitats of the coastal plain equally. Openly vegetated, riparian terraces had disproportionately more shed antlers than nearby tussock-tundra habitats. In other words, caribou prefer to have their calves on exactly the kind of tundra that is pleasant to walk and camp upon: the dry benches above the river.
When I read that, I wasn’t surprised. First, more antlers are likely to be found on that short dry tundra because they are easier to spot and are not likely to sink or get covered up by the deep, wet grass. Second, and more pragmatically, if I were a caribou, I sure wouldn’t want my calf to drop immediately down into sopping wet, bumpy tussock tundra.
The authors considered the detectability issue, accounted for it, and still decided that the caribou used the dry terraces more often than tussock tundra. This is despite the fact that caribou select tussocks for post-calving habitats due to the rich forage tussocks provide.
This is all very cool stuff, and has the potential to shed light on preferred calving habitats and areas, and how those areas can change over time. I’m particularly interested to know how the usage patterns vary. It could be that a large herd like the Porcupine require multiple calving areas, the use of which is staggered, allowing the grounds to recover after many years of use by the foraging herds.
Understanding how caribou use the landscape over time (hundreds or thousands of years), is vital when the future of the Arctic National Wildlife Refuge is so uncertain. The more we learn about how the animals move across the arctic, the more it seems that the caribou need options: options for wintering, options for migration routes, options for calving grounds. With climate change rapidly changing the arctic, and oil and gas development knocking on the Refuge’s eastern boundary, preserving as many of those options as possible is even more important. Hopefully we will have the foresight to understand that.
Reference:
Miller, Joshua H., Patrick Druckenmiller, and Volker Bahn. 2013. Antlers on the Arctic Refuge: capturing multi-generational patterns of calving ground use from bones on the landscape. Proceedings of the Royal Society Biology. 2013 280, 20130275.
