Tuesday, 16 December 2008

Glacier NP Bears redux!

Remember Kendall? I wrote a little something about her methods on the big Glacier Grizzly Bear project a while back. It's going to be a classic `how to` for non-invasive DNA mark-recapture population estimates, even if that's a mouthful. Let's talk about what she found, now.

To get an appreciation how big of a project it was, she had 5, 14 day sessions. That's ten weeks of pure field work, plus breakdown and set up times. In the lab, the research group had 5,562 samples from between 626 and 633 snag sites, with a 28% return on the hair traps. They checked between 660 and 829 hair rubs, and collected a total of 3,985 samples from them (11.7% return on the rubs).

Hell, I have a hard time keeping track of 500 samples.

When they analyzed the samples genetically, they couldn't determine the species of about 9% of the hair trap samples - so those go right out. They're no good. 63.5% were black bears, and 27.5% where grizzly bears. When it comes to the tree rubs, 12.2% couldn't have the species be determined, 36.9% where black bear, 50.9% were brown bear. I can't help but wonder if the difference in the `failure to identify` rate is significant. With that high of a sample size, it's quite possible.

Luckily, mixed samples - if two bears rubbed against the same tree, or same trap - where rare, with only about 2% from both traps and rubs. That 2% wasn't analysed, because there's no way to sort out which hairs belonged to which bear.

For the technical minded Ho was 0.71, and the P-ID was 6E-6. P-SIB was 7E-3. That's some good battery of ┬Ásats for such a low population large carnivore! When all ┬Ásats were considered, each bear differed at at least 4 markers, or more. Things that make you go `hmmm.`

In total, they found 185 unique bears in one year, and 22 in the other - 58% female at the hair traps. That sex ratio seems a little high in favour of males. Maybe someone can tell me if that's unusual, for Brown Bears, since they're not typically my forte. Using a mathematical mark-recapture model, they calculated 241 bears in 1998 (95% CI=202-303) and 241 in 2000 (95% CI=205-304). Mean bear density from this was 30 bears per 1000 km^2 (95% CI=27-35).

What does this all mean?

First, Kendall says `huzzah! Our first rigorous estimate! Cake for everyone!` And she's right. This is the first time we have a scientifically valid estimate of how many bears there are for the area. The density is comparable to a number of other studies in interior NA, though the authors throw in a healthy dose of `take that with a grain of salt.` Third, it supports the notion that the habitat in Glacier NP is much, much better for bears than what's going on around GNP. This isn't a trivial difference, either.

What's notable in their discussion is what isn't there. This data will go to court to try and support a de-listing. Probably anticipating it, the authors made zero policy recomendations as to whether they should unprotect the Grizzly. If you read between the lines, and look at some of the other data they show in multi-use land (16 bears / 1000 km^2), it seems like the authors are leaning towards not. While GNP remains a core of strong bear populations, the area around it drops off quickly. By allowing collection on the fringes, you could quickly exterpait edge of the GNP population, due to the large home ranges of the animals. There's really insufficient connectivity between GNP and the nearest patch of grizzly habitat to allow for some population resilience outside the park - that is to say, fragmentation is an issue, here.

But that's just my two cents. With additional data, I could find I'm wildly wrong on that.

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