This is directed at one specific population of organisms. The Hudson Bay polar bears are the southernmost population of polar bears in the world. A warming trend (and specifically a change in sea ice dynamics) will affect them first and more drastically than polar bear populations in colder zones.
I think what you're saying is that other populations may expand their range. That's true and it's also an indicator. So I should have said that borderline populations will be stressed most by an environmental trend that is negative with respect to their survivability. Other borderline populations could be augmented by the same trend because it is positive with respect to their survivability. Right?