Species area relationships...
This is really a great simplification of the species/area relationship, but it's relevant to the situation today because species such as Grizzly Bears are isolated on these islands of habitat and the small ones are going to go extinct and even the larger ones are going to get smaller until they go extinct, unless dispersers can somehow reach those islands. So their survival depends upon the area, the size of the island, and the distance away from a possible source of dispersing individuals.
The yellow area on these maps shows the area historically occupied by grizzly bears at their maximum range expansion by about 1850. Then by 1920 grizzlies had been reduced to these smaller patches which are colored various shades of red. All of the smaller populations existing in 1920 have since gone extinct. Only the largest areas, the purple areas on the map, are the islands of habitat where the grizzlies have managed to persist.Today, there are only about 1,000 Grizzly Bears in the United States. All of these have small populations on relatively small islands of habitat so they're still at a fairly high risk of extinction. They can go extinct locally one at a time from some of these intrinsic factors, or they may be wiped out on a broader scale by extrinsic factors.