Friday, 21 May 2010

Unimak and South West caribou instability

I was fairly surprised to see that there was a story about the Unimak Caribou Herd (UCH) in the Alaska Daily News this morning; I checked around and found the source of this new burst of public awareness, which was a press release posted on ADF&G's website. As someone who knows a few things about the South West and Alaska Peninsula herds in general, and about the situation specifically, I thought I might try to add meaningfully to the conversation.

To the right I have a figure of the area we're talking about. I'm using ADF&G's figure from their website, since it's far better than my own. In the region, there are 5 designated caribou herds, plus feral reindeer on Kodiak island (which is a hotbed of introduced species). Mulchatna (MCH) is a conglomeration of multiple herds, including the now defunct Killbuk, and a few others, which merged into the mass we have today. Nushagak is actually quite separate from MCH because it a) has separate calving grounds and b) is entirely man-made, made from transplanted Northern Alaska Peninsula Caribou (NAP) a couple decades ago.

The NAP has quite a bit of range overlap with MCH, and recent authors have used this to describe the situation down there as a "Meta-population," that is to say a population of populations, with patches of terrain gaining and losing caribou at some background rate. I'm not sure I agree with their assessment, for technical reasons. However, NAP and the Southern Alaska Peninsula Caribou (SAP) don't really mix, as SAP and NAP are constrained short of Henderson Bay (generally). Similarly, UCH and SAP don't really mix, because of the False Pass that the village is named for.

Unimak caribou probably descended from animals which somehow made it across False Pass, and settled on the island. The island itself is lightly inhabited, and has broad plains with a few major volcanos sticking out (It's the AP, volcanos are like seagulls. ;) ). The Alaska Peninsula and Unimak island are both fairly marginal caribou habitat, or at least appear to be as over the long term, caribou have a hard time finding any equilibrium census, and they tend to fluctuate wildly over time. It's tempting to blame this on poor lichen, human harvest, other predation, and severe weather events, but to be completely honest, any satisfactory explanation of population dynamics out there probably must include all those factors.

So that's to say the area isn't a stranger to precipitous caribou declines. In the late 40's, NAP is thought to have scrapped along at a mere 2,000 animals, before peaking again at 20,000 in 1984. However, even this 10 fold didn't come as low as SAP, which declined to 600 animals as recently as 2007. After intensive management, the calf survival rate was brought up significantly in 08-09, and the population appeared to inch its way up so slightly to a slightly higher census.

In UCH, the population is at a mere 400. Harvest has generally been 10 animals (more or less) for the past decade, with no harvest being allowed in 09-10 regulatory year. In general, the level of human harvest represented a negligible portion of the herd until the decline between 06 and 09 censuses. Hunting on the whole AP has been "Verboten" for the last regulatory year.

The UCH's problem is illustrated through two things. First, it's abnormally low Calves per 100 Cows ration. It's 3:100. Needless to say, that is abnormally low. SAP had a lower ratio until intensive management was instituted, so it's not to say it's unrecoverable. However, the second illustrative problem is the extreme sex ratio fluctuation that's occurred. Currently, there are 5 bulls for every 100 cows in UCH. While Caribou are harem breeders (One male breeds with many females), this ratio is probably contributing to the other, abysmally low Calf-Cow ratio. There  just isn't enough bulls to "Cover" all of the cows.

So, I hope I've convinced you that it's not hyperbole when one of my good friends in the agency said "We have concerns that in one to two years, the herd will be on a trajectory that will cause it to cease to exist." While I'd put this possibility on the low end of things (Maybe 5-10% chance), I have to agree with his assessment on the whole. Discussion up to this point has focused on rescue transplants of males. Hopefully, the thought goes, additional males could cover the females not currently getting covered. The long-term genetic consequences of this is worrisome, but the short-term demographic consequences of the extreme sex ratio is far greater, and we can mitigate the genetic effects through appropriate management.

This development relates to the other factor of the cow:calf ratio, which is survival-to-census. Wolves can be highly effective predators of caribou calves, as can bears (for a short period). The argument goes that wolves are at a "Population High," and therefore intensive management is warranted in order to preserve both caribou and wolves long term viability on Unimak island. Most people will generally agree that IM was somewhat effective in bringing SAP on an upward trajectory, and we'll see how effective it was long-term when those 2008 animals really get to breeding in 2010.

However, long-term, I think we need an over-all strategy. South-Western caribou will go through population fluctuations much greater percent-wise than other herds in the arctic. This is inevitable, unless climate change has the unforeseen boon of stabilizing caribou cycling. We have three long-term directions we could take. First, we could intervene and attempt to stabilize the system through clever pushing-of-the-buttons. This, I'm sure, will be expensive. Second, we could supplement Caribou to stabilize them, turning the AP into something akin to the National Elk Refuge. This would be potentially less expensive, but have some major eco-systems wide effects. Or, we could manage for natural ecosystems, which is a fancy way of saying "Take our hands off the wheel and let the truck go wherever." All three have their strengths and weaknesses as options. All three require value judgements. However, a consistent direction is far preferred to a reactionary approach.

Slides from the FWS presentation from ADF&G, and the table is from the 09 caribou inventory report.

No comments: