They die at temperatures that are a sustained sub-65 degrees or sustained 100+ degrees (which you can demonstrate with a live ferret, although I wouldn't recommend it).
The blackfooted ferret (endangered) is native to California. The claim that domestic sable ferrets would be invasive is ridiculous on it's face.
> Nature is too important to be left to the caprices of pet owners.
The CA fish and game commission is completely ideologically captured as they do not want to engage in any sort of scientific study for the non-existence of existing process (beyond a simple decree) or legal jeopardy that they may become embroiled in, after the fact. They much rather enjoy their salaries mediating the relatively safe cases of licensing and managing bureaucratic rules around existing fish and game.
There's also the uncomfortable truth that people do possess ferrets in CA. Pet stores stock goods for them and vets treat them. People all around CA let their ferrets roam outside and come back in when it gets colder at night (and to get food/water). Young adult/College dorms and apartments are rife with ferret owners. Again, the claims about ferrets being a "potentially" invasive species are specious, at best.