|
[ Return to Contents
| Post a Reply
| Post a new message
]
How bears densities are estimated
Posted by bushrat on Feb 02 2006
BRWNBR,
In the past, expensive Capture-Mark-Resight techniques were used to estimate bear densities. What we layman call population surveys are known in biological jargon as "density estimates." They usually have an error factor, what we know of as a plus or minus margin of error and what they call a "confidence interval." It's important to note the confidence interval of any study. Generally it is a 95%CI.
The CMR technique required helicopters and bios to dart and tranquilize bears (Capture), then install radio collars and/or ear tags (Mark) and then to fly back at another time to Resight these marked bears, as well as unmarked bears. CMR is used on everything from prairie chickens to bull trout to estimate densities and movements of wildlife. It is fairly accurate but very expensive on big game, and in the case of bears there was a learning curve that meant some bears died during the capture phase. Alaskan biologists and their counterparts in parts of Canada added much to the field and helped to alleviate future capture deaths from the knowledge they gained.
Newer technology allows studies to be done using hair follicles from bears that are "snagged" in "hair traps" and their dna analyzed. Bait stations are set up in a subunit, say, using scent lures and a strand of barbed wire around the bait on which bear hairs are snagged. This method also requires expensive helicopter time to access areas but we no longer have to actually harass, tranq, and mark the bears. The hair-snag studies require hundreds of "traps" to be set out over a large area. Biologists then revisit the hair-traps a couple of weeks later and collect any samples. In most studies, nearly 75% of hair-traps are "hit" by bears. A single guard hair from a bear is enough to determine with dna testing the gender, species,(black or brown bear) and individual bear, though it takes something like five hairs to further narrow things down with certainty.
So, picture these hundreds of hair-snag stations that are marked by gps coordinates, set up in a sub unit. Bios collect all the hair samples, send them to the lab, and are given the genotype results. I think the lab testing is rather spendy. This data is then used to determine bear densities and home ranges and movements.
Few CMR or hair-snag studies have been done in Alaska. The CMR studies are just too expensive and the hair-snag/dna studies are fairly new. Mostly, in the past, vast extrapolations have been done to determine bear densities. For instance, in unit 20E no CMR study was done but they "estimated" the bear densities there based on an intensive CMR bear study done in the foothills of the Alaska range 100 miles to the southwest in similar habitat. There are great dangers in extrapolating data over wide areas like this. What is true in one unit is not necessarily true in another.
Hope this helps. I could be wrong on some of it but that's the way I understand it. I'm sure you can google "capture mark resight" and "Hair snag sampling" and come up with a lot of stuff.
Best, Mark
Previous: bear counts BRWNBR Feb 01 2006
Next: OOPS!!! I'm a moron... B_M Feb 01 2006
 Message Thread:
|
