Thursday, 4 June 2009

The Herd Concept

Random: Am I the only one who finds the article headline "Palin, gas line supporters dismiss pipeline criticisms" incredibly obvious? Of course they disagree with people who disagree with them. That's the definition of supporter, eh? They support. Next, the DNM will publish a story saying "Critics continue to criticize."

I've got my work on deer off to my coauthors for their review, and I've effectively finished my work on them. I would be going back to the wolf stuff now, except I haven't heard any direction from ADF&G, so that'll continue to get put on the back of the stove. I'm back to caribou work, where we've went and raided our freezer for blood and tissue. So maybe it's a good time to talk about the herd concept.

When we talk about caribou, we tend to talk about caribou in herds. For management purposes, there's 32 recognized herds in the state of Alaska. Don't ask me to name them all. By and large, they're numerically concentrated in 7 herds of 10,000 caribou or greater, with the north slope herds being the largest. As a pure guess, I'd say 90% of the caribou in Alaska are in the Porcupine, Central Arctic, Western Arctic, Mulchatna, Nelchina, 40-Mile or Teshekpuk Lake caribou herds.

One of the defining features of caribou is that they exhibit the prehensity to seasonal movements. c.f. domestic reindeer, who tend to move fairly "randomly" through time. What happens is that we see caribou exhibit site fidelity among females for parturition (what normal people call "childbirth" or "calving"). Because they return to at least roughly the same area each cycle, we tend to treat all the animals who use that same space the same way as a management unit.

You can see site fidelity illustrated off to the right. The figure is from ANWR, based off of a radio collared female in 1987. You can see how she migrated to the calving ground, hung around a while (probably calving), before moving off over the course of a few months. The Western Arctic herd migration is very pronounced - whereas the PCH may migrate 200-300 miles, the WAH will move up to and over 400 miles from their summer grounds on the North Slope to their wintering grounds on the Seward Peninsula. You can see a nice animation of that at this link to the CARMA network's webpage. Beware, the file is about 3 MB, for those of you on slow connections. Here's a link to the PCH's page, where you can click on "Porcupine Caribou Animation."

So, it should be clear that the herds are somewhat demographically independent - that the number of caribou in an area is linked to how well the herd is doing, and not how well the herd nextdoor is doing. This makes herds a very useful concept for management, as we as managers want to manage how many animals there are (abundance). However, although useful for management, the herd concept might not be evolutionarily signifigant - it might not reflect the actual, biological reality of what are important groupings for the animals. This is because caribou occupy different space when they rut and when they calve. Rut is when all the sex is happening, and long term biological processes are chiefly concerend with who has the largest number of successful offspring.

Whether the herd concept is actually rooted real or not, we can't say yet. A previous researcher did a survey of the North Slope herds, and found that they weren't biologically discrete units. However, that researcher's research was filled with problems, and many scientists are reluctant to take it at any value. There's a number of people doing studies across the state, and in Canada, investigating just that. One finished project from Canada found a mixed bag - some herds were legitimate biological entities of evolutionary importance, and some were not. And some herds should have been considered multiple herds. It will be interesting, as time goes on, to see how the North Slope caribou herds are found to be in the future.

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