They need a certain habitat, and enough of it that’s contiguous to support a breeding population. If you drop one off by itself in a forest surrounded by city, it won’t last long. There’s not one long strip of suitable habitat that stretches from Alaska to Maine.
They apparently never made it east of about -100 degrees longitude (which covers some of the Dakotas, but not the rest). No doubt something about the plains (probably food resources) kept the grizzlies west of there.
Nowadays certain bipedal residents of those areas might constitute something of an obstacle to eastern progress.
It appears they’d at least have enough area: Some Googling produced a figure of 10,000 sq mi as the minimum size of sustainable grizzly habitat. Maine is about 3 times that large, and certainly the northwestern third has a very low human population.
The rangeseems to be largely centered on the Alasakan peninsula, branching from there.
There are very few animals that are truly cosmopolitan; no real reason the Brown Bear should be one of those few. I mean, what’s the largest animal that’s truly cosmopolitan, or nearly? Not counting human intervention, like cattle and horses. The wolf was pretty close to cosmopolitan in its day, no? Nowadays, what, nothing larger than a rat–which, come to think of it, probably owes its spread to man. The only other cosmopolitan animals I can think of are all birds and insects.
Grizzlies will move if they have to, but if they don’t have to why move? That’s most likely kept them were they are.
If there in an area which basically provides them with food and shelter, they won’t look. And then they have cubs and those cubs move to sparcer areas, then mama dies and baby cub grows up and now gets the good area.
Unless there is a large cyle where bears are forced to seek out new areas, they don’t go far, perfering to wait till their parents and grandparents die and taking over that area.
As for dropping off a population, it MIGHT work but usually introducing new animals to areas where they’ve not been before isn’t a good thing. For instance a flamingo got out of a zoo and lived in the Great Salt Lake, for years. A flamingo colony might be able to live there, but scientists are against putting non native species where they don’t exist.
But it happens other ways especially to birds, who for instance, often get blown off course in a storm.
We have a herron in the Chicago River. I don’t know how he got there but he’s been there since May, I don’t know how much longer he’ll stick around. Poor guy I don’t see any other herrons so he must be lonely by himself, unless he likes geese for company
This is not always true–brown bears (Ursus artcos) and the subspecies of Kodiak and grizzly bears (U. a. middendorffi and the unfortunately named U. a. horribilis)–will socialize to a degree around dense food sources, such as salmon spawns and such, but in general the males tend to roam across wide, overlapping zones, and the females tend to occupy somewhat smaller, dedicated areas. In some areas, like Katmai National Park and Preserve, the density of the bears can sometimes be as high as several dozen per square mile, though more typically during the summer they remain spread out through the mazes and dense brush of the island.
There are a few reasons that the range of brown bears is more restricted than blacks. For one, unlike American black bears (Ursus americanus) is smaller and more adaptable in diet and behavior. Mature blacks range from 200 lbs to about 600 lbs (although with a high protein and high fat diet blacks can approach 1000 lbs), and their omnivorous-leaning-to-herbivorous habits allow them vast range. In contrast, brown bears eat more protein and are generally more predatorial, though still not to the degree suggested by misguided public perception. Blacks are also more tolerant of varying climate–ranging all the way down to near-desert American Southwest and Mexico, as well as swamps of Florida–while the brown bear of North America and Eurasia sticks almost exclusively to sub-Arctic and mountainous forests. Black bears are also natural foragers and tend to become more readily acclimated to people and populated areas, while browns are (generally speaking) more retiring. The reproduction cycle of brown bears is somewhat more limited–the females tend to have fewer cubs, and tend to rear them for somewhat longer than blacks–so black bears are somewhat more prolific. It also doesn’t hurt that American black bears–used to far more often being prey than predator–are better at and more prone to running or climbing away from danger, while brown bears are somewhat more likely to stand ground, a fatal mistake against armed human hunters.
In short, American black bears are very adaptable, while browns tend toward a more restrictive, less prolific lifestyle. The comparison between coyotes and wolves is very apt in this situation; although the wolf is larger and ultimately closer to the domestic dog, the coyote is better adapted to living on the fringes of modern human civilization without being absorbed or domesticated.
Here’s a comparison of the historical and current range of the Grizzly.
While the former range in the lower 48 was mostly in the Great Plains and more open forest types in the west, in Canada the range includes boreal forest. It’s not clear what keeps (or kept) them spreading eastward through this habitat type.
The causes of range limits in many animals are obscure. Often we can’t tell why an animal ranges so far and no farther.
The coyote now is nearly cosmopolitan in North America. It wasn’t always so. It once had a range in the western US rather similar to the historical range of the Grizzly. It has now spread throughout the east, where it was formerly absent. It colonized Panama a few decades ago, and is still moving south.
On the money. A ranger at Katmai described the salmon feeding ground interaction as “forced socialization” (reminded me of our Foreign Service days). Even though thrown together because of the salmon spawn, grizzlies will still not willingly give ground, and fights are common. We saw several fights while there, and the salmon were almost thick enough to walk across the pond on.